do nerve impulses travel along neurons

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Polarization is established by maintaining an excess of sodium ions (Na  + ) on the outside and an excess of potassium ions (K  + ) on the inside. A certain amount of Na  +  and K  +  is always leaking across the membrane through leakage channels, but Na  + /K  +  pumps in the membrane actively restore the ions to the appropriate side.

The main contribution to the resting membrane potential (a polarized nerve) is the difference in permeability of the resting membrane to potassium ions versus sodium ions. The resting membrane is much more permeable to potassium ions than to sodium ions resulting in slightly more net potassium ion diffusion (from the inside of the neuron to the outside) than sodium ion diffusion (from the outside of the neuron to the inside) causing the slight difference in polarity right along the membrane of the axon.

Other ions, such as large, negatively charged proteins and nucleic acids, reside within the cell. It is these large, negatively charged ions that contribute to the overall negative charge on the inside of the cell membrane as compared to the outside.

In addition to crossing the membrane through leakage channels, ions may cross through  gated channels.  Gated channels open in response to neurotransmitters, changes in membrane potential, or other stimuli.

The following events characterize the transmission of a nerve impulse (see Figure 1):

• Resting potential.  The resting potential describes the unstimulated, polarized state of a neuron (at about –70 millivolts).
• Graded potential.  A graded potential is a change in the resting potential of the plasma membrane in the response to a stimulus. A graded potential occurs when the stimulus causes Na  +  or K  +  gated channels to open. If Na  +  channels open, positive sodium ions enter, and the membrane depolarizes (becomes more positive). If the stimulus opens K  +  channels, then positive potassium ions exit across the membrane and the membrane hyperpolarizes  (becomes more negative). A graded potential is a local event that does not travel far from its origin. Graded potentials occur in cell bodies and dendrites. Light, heat, mechanical pressure, and chemicals, such as neurotransmitters, are examples of stimuli that may generate a graded potential (depending upon the neuron).

Figure 1.Events that characterize the transmission of a nerve impulse.

The following four steps describe the initiation of an impulse to the “resetting” of a neuron to prepare for a second stimulation:

• Action potential.  Unlike a graded potential, an action potential is capable of traveling long distances. If a depolarizing graded potential is sufficiently large, Na  +  channels in the trigger zone open. In response, Na  +  on the outside of the membrane becomes depolarized (as in a graded potential). If the stimulus is strong enough—that is, if it is above a certain threshold level—additional Na  +  gates open, increasing the flow of Na  +  even more, causing an action potential, or complete depolarization (from –70 to about +30 millivolts). This in turn stimulates neighboring Na  +  gates, farther down the axon, to open. In this manner, the action potential travels down the length of the axon as opened Na  +  gates stimulate neighboring Na  +  gates to open. The action potential is an all‐or‐nothing event: When the stimulus fails to produce depolarization that exceeds the threshold value, no action potential results, but when threshold potential is exceeded, complete depolarization occurs.
• Repolarization. In response to the inflow of Na  + , K  +  channels open, this time allowing K  +  on the inside to rush out of the cell. The movement of K  +  out of the cell causes repolarization by restoring the original membrane polarization. Unlike the resting potential, however, in repolarization the K  +  are on the outside and the Na  +  are on the inside. Soon after the K  +  gates open, the Na  +  gates close.
• Hyperpolarization.  By the time the K  +  channels close, more K  +  have moved out of the cell than is actually necessary to establish the original polarized potential. Thus, the membrane becomes hyperpolarized (about –80 millivolts).
• Refractory period. With the passage of the action potential, the cell membrane is in an unusual state of affairs. The membrane is polarized, but the Na  +  and K  +  are on the wrong sides of the membrane. During this refractory period, the axon will not respond to a new stimulus. To reestablish the original distribution of these ions, the Na  +  and K  +  are returned to their resting potential location by Na  + /K  +  pumps in the cell membrane. Once these ions are completely returned to their resting potential location, the neuron is ready for another stimulus.

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11.41: Nerve Impulse

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What do nerve  cells  look like?

Note that like most other  cells , these nerve cells have a  nucleus . They also have other  organelles . However, the long, threadlike extensions of the nerve cells are unique. This is where the nerve impulses are transmitted.

Neurons and Nerve Impulses

The  nervous system  is made up of nerves. A  nerve  is a bundle of nerve  cells . A nerve cell that carries messages is called a  neuron  (Figure below). The messages carried by neurons are called  nerve impulses . Nerve impulses can travel very quickly because they are electrical impulses.

Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside the body.

What Does a Neuron Look Like?

A neuron has a special shape that lets it pass signals from one cell to another. A neuron has three main parts (Figure above):

• The cell body.
• Many dendrites.

The  cell body  contains the  nucleus  and other  organelles . Dendrites and axons connect to the cell body, similar to rays coming off of the  sun .  Dendrites  receive nerve impulses from other cells.  Axons  pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells but only one axon. The axon is covered with a  myelin sheath , a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The  node of Ranvier  is any gap within the myelin sheath exposing the axon, and it allows even faster transmission of a signal.

Types of Neurons

Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons.

• Sensory neurons  carry nerve impulses from sense organs and internal organs to the  central nervous system .
• Motor neurons  carry nerve impulses from the  central nervous system  to organs, glands, and muscles—the opposite direction.

Both types of neurons work together. Sensory neurons carry information about  the environment  found inside or outside of the body to the  central nervous system . The central nervous system uses the information to send messages through motor neurons to tell the body how to respond to the information.

The  Synapse

The place where the axon of one neuron meets the dendrite of another is called a  synapse . Synapses are also found between neurons and other types of cells, such as muscle cells. The axon of the sending neuron does not actually touch the dendrite of the receiving neuron. There is a tiny gap between them, the synaptic cleft (Figure below).

The following steps describe what happens when a  nerve impulse  reaches the end of an axon.

• When a  nerve impulse  reaches the end of an axon, the axon releases chemicals called  neurotransmitters .
• Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron.
• Neurotransmitters bind to the membrane of the dendrite.
• The binding allows the nerve impulse to travel through the receiving neuron.

Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline.

You can watch an animation of nerve impulses and neurotransmitters at  http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php .

Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives.

• Neurons, or nerve cells that carry nerve impulses, are made up of the cell body, the axon, and several dendrites.
• Signals move across the synapse, the place where the axon of one neuron meets the dendrite of another, using chemicals called neurotransmitters.

Explore More

Use the resource below to answer the questions that follow.

• Neuroscience For Kids  at  http://faculty.washington.edu/chudler/cells.html
• What are the three types of neurons?
• What neurons are most abundant in the central nervous system?
• What is the function of sensory neurons?
• What is the function of motor neurons?
• What is the role of interneurons?
• Describe a neuron and identify its three main parts.
• Distinguish between dendrites and the axon.
• Distinguish between sensory and motor neurons.
• Explain how one neuron transmits a nerve impulse to another neuron.

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8.4 Nerve Impulses

Created by CK-12 Foundation/Adapted by Christine Miller

When Lightning Strikes

This amazing cloud-to-surface lightning occurred when a difference in electrical charge built up in a cloud relative to the ground. When the buildup of charge was great enough, a sudden discharge of electricity occurred. A nerve impulse is similar to a lightning strike. Both a nerve impulse and a lightning strike occur because of differences in electrical charge, and both result in an electric current.

Generating Nerve Impulses

A  nerve impulse , like a lightning strike, is an electrical phenomenon. A nerve impulse occurs because of a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves  ions , which are electrically-charged atoms  or molecules .

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane of the neuron. The sodium-potassium pump is a mechanism of active transport that moves sodium ions (Na+) out of cells and potassium ions (K+) into cells. The sodium-potassium pump moves both ions from areas of lower to higher concentration, using energy in ATP and carrier proteins in the cell membrane. The video below, “Sodium Potassium Pump” by Amoeba Sisters, describes in greater detail how the sodium-potassium pump works. Sodium is the principal ion in the fluid outside of cells, and potassium is the principal ion in the fluid inside of cells. These differences in concentration create an electrical gradient across the cell membrane, called resting potential .  Tightly controlling membrane resting potential is critical for the transmission of nerve impulses.

Sodium Potassium Pump, Amoeba Sisters, 2020.

Action Potential

A nerve impulse is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. The reversal of charge is called an  action potential . It begins when the neuron receives a chemical signal from another cell or some other type of stimulus .  If the stimulus is strong enough to reach threshold , an action potential will take place is a cascade along the axon.

This reversal of charges ripples down the axon of the neuron very rapidly as an electric current, which is illustrated in the diagram below (Figure 8.4.2). A nerve impulse is an all-or-nothing response depending on if the stimulus input was strong enough to reach threshold. If a neuron responds at all, it responds completely. A greater stimulation does not produce a stronger impulse.

In neurons with a myelin sheath on their axon, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential appears to jump along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which the action potential travels.

Transmitting Nerve Impulses

The place where an axon terminal meets another cell is called a  synapse . This is where the transmission of a nerve impulse to another cell occurs. The cell that sends the nerve impulse is called the  presynaptic cell , and the cell that receives the nerve impulse is called the  postsynaptic cell .

Some synapses are purely electrical and make direct electrical connections between neurons. Most synapses, however, are chemical synapses. Transmission of nerve impulses across chemical synapses is more complex.

Chemical Synapses

At a chemical synapse, both the presynaptic and postsynaptic areas of the cells are full of molecular machinery that is involved in the transmission of nerve impulses. As shown in Figure 8.4.3, the presynaptic area contains many tiny spherical vessels called synaptic vesicles  that are packed with chemicals called  neurotransmitters . When an action potential reaches the axon terminal of the presynaptic cell, it opens channels that allow calcium to enter the terminal. Calcium causes synaptic vesicles to fuse with the membrane, releasing their contents into the narrow space between the presynaptic and postsynaptic membranes. This area is called the  synaptic cleft . The neurotransmitter molecules travel across the synaptic cleft and bind to  receptors , which are proteins embedded in the membrane of the postsynaptic cell.

Neurotransmitters and Receptors

There are more than a hundred known neurotransmitters, and more than one type of neurotransmitter may be released at a given synapse by a presynaptic cell. For example, it is common for a faster-acting neurotransmitter to be released, along with a slower-acting neurotransmitter. Many neurotransmitters also have multiple types of receptors to which they can bind. Receptors, in turn, can be divided into two general groups: chemically gated ion channels and second messenger systems.

• When a chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the cell membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory .
• When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell. This may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.

The effect of a neurotransmitter on a postsynaptic cell depends mainly on the type of receptors that it activates, making it possible for a particular neurotransmitter to have different effects on various target cells. A neurotransmitter might excite one set of target cells, inhibit others, and have complex modulatory effects on still others, depending on the type of receptors. However, some neurotransmitters have relatively consistent effects on other cells. Consider the two most widely used neurotransmitters, glutamate and GABA (gamma-aminobutyric acid). Glutamate receptors are either excitatory or modulatory in their effects, whereas GABA receptors are all inhibitory in their effects in adults.

Problems with neurotransmitters or their receptors can cause neurological disorders. The disease myasthenia gravis , for example, is caused by antibodies from the immune system blocking receptors for the neurotransmitter acetylcholine in postsynaptic muscle cells. This inhibits the effects of acetylcholine on muscle contractions, producing symptoms, such as muscle weakness and excessive fatigue during simple activities. Some mental illnesses (including depression ) are caused, at least in part, by imbalances of certain neurotransmitters in the brain. One of the neurotransmitters involved in depression is thought to be serotonin , which normally helps regulate mood, among many other functions. Some antidepressant drugs are thought to help alleviate depression in many patients by normalizing the activity of serotonin in the brain.

8.4 Summary

• A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.
• The  sodium-potassium pump  maintains an electrical gradient across the plasma membrane of a neuron when it is not actively transmitting a nerve impulse. This gradient is called the resting potential of the neuron.
• An action potential is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. It begins when the neuron receives a chemical signal from another cell or some other type of stimulus. The action potential travels rapidly down the neuron’s axon as an electric current and occurs in three stages: Depolarization, Repolarization and Recovery.
• A nerve impulse is transmitted to another cell at either an electrical or a chemical synapse . At a chemical synapse, neurotransmitter chemicals are released from the presynaptic cell into the synaptic cleft between cells. The chemicals travel across the cleft to the postsynaptic cell and bind to receptors embedded in its membrane.
• There are many different types of neurotransmitters. Their effects on the postsynaptic cell generally depend on the type of receptor they bind to. The effects may be excitatory, inhibitory, or modulatory in more complex ways. Both physical and mental disorders may occur if there are problems with neurotransmitters or their receptors.

8.4 Review Questions

• Define nerve impulse.
• What is the resting potential of a neuron, and how is it maintained?
• Explain how and why an action potential occurs.
• Outline how a signal is transmitted from a presynaptic cell to a postsynaptic cell at a chemical synapse.
• What generally determines the effects of a neurotransmitter on a postsynaptic cell?
• Identify three general types of effects that neurotransmitters may have on postsynaptic cells.
• Explain how an electrical signal in a presynaptic neuron causes the transmission of a chemical signal at the synapse.
• The flow of which type of ion into a neuron results in an action potential? How do these ions get into the cell? What does this flow of ions do to the relative charge inside the neuron compared to the outside?
• Name three neurotransmitters.

8.4 Explore More

Action Potentials, Teacher’s Pet, 2018.

TED Ed| What is depression? – Helen M. Farrell, Parta Learning, 2017.

5 Weird Involuntary Behaviors Explained!, It’s Okay To Be Smart, 2015.

Attributions

Figure 8.4.1

Lightening/ Purple Lightning, Dee Why   by Jeremy Bishop on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 8.4.2

Action Potential by CNX OpenStax, Biology on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.

Figure 8.4.3

Chemical_synapse_schema_cropped by Looie496 created file (adapted from original from US National Institutes of Health, National Institute on Aging) is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Amoeba Sisters. (2020, January 29). Sodium potassium pump. YouTube. https://www.youtube.com/watch?v=7NY6XdPBhxo&feature=youtu.be

CNX OpenStax. (2016, May 27) Figure 4 The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes [digital image]. In Open Stax, Biology (Section 35.2). OpenStax CNX.  https://cnx.org/contents/[email protected]:cs_Pb-GW@6/How-Neurons-Communicate

It’s Okay To Be Smart. (2015, January 26). 5 Weird involuntary behaviors explained! YouTube. https://www.youtube.com/watch?v=ZE8sRMZ5BCA&feature=youtu.be

Mayo Clinic Staff. (n.d.). Depression (major depressive disorder) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/depression/symptoms-causes/syc-20356007

Mayo Clinic Staff. (n.d.). Myasthenia gravis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/myasthenia-gravis/symptoms-causes/syc-20352036

National Institute on Aging. (2006, April 8). Alzheimers disease: Unraveling the mystery.  National Institutes of Health. https://www.nia.nih.gov/ ( archived version )

Parta Learning. (2017, December 8). TED Ed| What is depression? – Helen M. Farrell. YouTube. https://www.youtube.com/watch?v=rBcU_apy0h8&t=291s

Teacher’s Pet. (2018, August 26). Action potentials. YouTube. https://www.youtube.com/watch?v=FEHNIELPb0s&feature=youtu.be

A signal transmitted along a nerve fiber.

An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.

The smallest particle of an element that still has the properties of that element.

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.

A functional unit of the nervous system that transmits nerve impulses; also called a nerve cell.

A solute pump that pumps potassium into cells while pumping sodium out of cells, both against their concentration gradients. This pumping is active and occurs at the ratio of 2 potassium for every 3 calcium.

The semipermeable membrane surrounding the cytoplasm of a cell.

The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.

A complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer.

The difference in electrical charge across the plasma membrane of a neuron that is not actively transmitting a nerve impulse.

Reversal of electrical charge across the membrane of a resting neuron that travels down the axon of the neuron as a nerve impulse.

Something that triggers a behavior or other response.

The critical level to which a membrane potential must be depolarized to initiate an action potential.

The place where the axon terminal of a neuron transmits a chemical or electrical signal to another cell.

The cell that sends the nerve impulse.

The cell that receives the nerve impulse.

These membrane-bound organelles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell.

A type of chemical that transmits signals from the axon of a neuron to another cell across a synapse.

A space that separates two neurons. It forms a junction between two or more neurons and helps nerve impulse pass from one neuron to the other.

A protein on a cell membrane or inside of a cell that binds with a hormone, neurotransmitter, or other chemical signal to produce a response.

A neurotransmitter that will have excitatory effects on the neuron, meaning it will increase the likelihood that a neuron will fire an action potential.

A neurotransmitter that decreases the likelihood that a neuron will fire an action potential.

A chemical that nerve cells use to send signals to other cells. It is by a wide margin the most abundant excitatory neurotransmitter in the vertebrate nervous system.

A naturally occurring amino acid that works as a neurotransmitter in your brain. Neurotransmitters function as chemical messengers. GABA is considered an inhibitory neurotransmitter because it blocks, or inhibits, certain brain signals and decreases activity in your nervous system.

An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses.

An organic chemical that functions in the brain and body of many types of animals (and humans) as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells, such as neurons, muscle cells and gland cells.

A neurotransmitter. It has a popular image as a contributor to feelings of well-being and happiness, though its actual biological function is complex and multifaceted, modulating cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction.

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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26.2 How Neurons Communicate

In this section, you will explore the following questions:

• What is the basis of the resting membrane potential?
• What are the stages of an action potential, and how are action potentials propagated?
• What are the similarities and differences between chemical and electrical synapses?
• What is long-term potentiation and long-term depression, and how do both relate to transmission of impulses across synapses?

Connection for AP ® Courses

The neuron is a great example of a structure-function relationship at the cellular level. Information flow along a neuron is usually from dendrite to axon and from neuron to neuron or from neuron to a cell of a target organ. Like other eukaryotic cells, neurons consist of a cell membrane, nucleus, and organelles, including mitochondria. Action potentials propagate impulses along neurons. When an axon is at rest, the membrane is said to be polarized; that is, there is an electrochemical gradient across it, with the inside of the membrane being more negatively charged than the outside. We explored the formation of electrochemical gradients using H + when we studied photosynthesis and cellular respiration. The neuron, however, uses Na + and K + to establish a gradient. It is also important to recall that ions cannot diffuse across the lipid bilayer of the cell membrane and must use transport proteins; in this case, the transport proteins are voltage-gated Na + and K + channels.

At rest, the Na + /K + pump, powered by ATP, maintains this gradient, known as resting membrane potential . In response to a stimulus, such as an odorant molecule, membrane potential changes, and an action potential is generated along the membrane as the voltage-gated Na + and K + channels open sequentially, causing the membrane to depolarize. In depolarization , the inside of the membrane becomes more positive than the outside as Na + flows to the inside. Repolarization occurs when K + flows across the membrane to the outside. In myelinated neurons, action potentials “jump” between gaps of unmyelinated axons (nodes of Ranvier), a phenomenon called saltatory conduction.

Transmission of a nerve impulse from one neuron to another or to another type of cell such as a muscle cell occurs across a junction called a synapse. Synaptic vesicles at the axon terminal of the presynaptic neuron release chemical messengers called neurotransmitters into the junction; neurotransmitters then bind to receptors embedded in the membrane of the postsynaptic neuron. Neurotransmitters may be either excitatory (such as acetylcholine or epinephrine) or inhibitory (such as serotonin or GABA) as they either increase or decrease the change of an action potential in the postsynaptic neuron. Many drugs, including both pharmaceuticals and drugs of abuse, can induce changes in synaptic transmission; for example, tetrahydrocannabinol (more commonly known as THC) in marijuana binds to a naturally occurring neurotransmitter important to short-term memory.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

All functions performed by the nervous system—from a simple motor reflex to more advanced functions like making a memory or a decision—require neurons to communicate with one another. While humans use words and body language to communicate, neurons use electrical and chemical signals. Just like a person in a committee, one neuron usually receives and synthesizes messages from multiple other neurons before “making the decision” to send the message on to other neurons.

Nerve Impulse Transmission within a Neuron

For the nervous system to function, neurons must be able to send and receive signals. These signals are possible because each neuron has a charged cellular membrane (a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons and environmental stimuli. To understand how neurons communicate, one must first understand the basis of the baseline or ‘resting’ membrane charge.

Neuronal Charged Membranes

The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations: open, closed, and inactive, as illustrated in Figure 26.9 . Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential .

Link to Learning

This video discusses the basis of the resting membrane potential.

Resting Membrane Potential

A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell, as shown in Table 26.1 . The difference in the number of positively charged potassium ions (K + ) inside and outside the cell dominates the resting membrane potential ( Figure 26.10 ). When the membrane is at rest, K + ions accumulate inside the cell due to the activity of the Na/K pump, driving both ions against their concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium potassium pump help to maintain the resting potential, once established. Recall that sodium potassium pumps brings two K + ions into the cell while removing three Na + ions per ATP consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid. It should be noted that chloride ions (Cl – ) tend to accumulate outside of the cell because they are repelled by negatively-charged proteins within the cytoplasm.

Action Potential

A neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream neurons. Transmission of a signal between neurons is generally carried by a chemical called a neurotransmitter. Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the resting membrane potential called an action potential. When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, ion channels open. At excitatory synapses, this opening allows positive ions to enter the neuron and results in depolarization of the membrane—a decrease in the difference in voltage between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV). Na + channels in the axon hillock open, allowing positive ions to enter the cell ( Figure 26.10 and Figure 26.11 ). Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. Action potentials are considered an "all-or nothing" event, in that, once the threshold potential is reached, the neuron always completely depolarizes. Once depolarization is complete, the cell must now "reset" its membrane voltage back to the resting potential. To accomplish this, the Na + channels close and cannot be opened. This begins the neuron's refractory period , in which it cannot produce another action potential because its sodium channels will not open. At the same time, voltage-gated K + channels open, allowing K + to leave the cell. As K + ions leave the cell, the membrane potential once again becomes negative and repolarizes. The diffusion of K+ out of the cell actually continues for a short period of time past the time of the achievement of the resting potential, and the membrane hyperpolarizes , in that the membrane potential becomes more negative than the cell's normal resting potential. This is the result of the slow closing of the K+ channels. At this point, the sodium channels will return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the threshold potential. Eventually all the K + channels close, and the cell returns back to its resting membrane potential.

Visual Connection

This video presents an overview of action potential.

Myelin and the Propagation of the Action Potential

For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential conduction. In demyelinating diseases like multiple sclerosis, action potential conduction slows because current leaks from previously insulated axon areas. The nodes of Ranvier, illustrated in Figure 26.13 are gaps in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na + and K + channels. Flow of ions through these channels, particularly the Na + channels, regenerates the action potential over and over again along the axon. This ‘jumping’ of the action potential from one node to the next is called saltatory conduction . If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly since Na + and K + channels would have to continuously regenerate action potentials at every point along the axon instead of at specific points. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon.

Synaptic Transmission

The synapse or “gap” is the place where information is transmitted from one neuron to another. Synapses usually form between axon terminals and dendritic spines, but this is not universally true. There are also axon-to-axon, dendrite-to-dendrite, and axon-to-cell body synapses. The neuron transmitting the signal is called the presynaptic neuron, and the neuron receiving the signal is called the postsynaptic neuron. Note that these designations are relative to a particular synapse—most neurons are both presynaptic and postsynaptic. There are two types of synapses: chemical and electrical.

Chemical Synapse

When an action potential reaches the axon terminal it depolarizes the membrane and opens voltage-gated Na + channels. Na + ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca 2+ channels to open. Calcium ions entering the cell initiate a signaling cascade that causes small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with the presynaptic membrane. Synaptic vesicles are shown in Figure 26.14 , which is an image from a scanning electron microscope.

Fusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic cleft , the extracellular space between the presynaptic and postsynaptic membranes, as illustrated in Figure 26.15 . The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane.

The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open. Neurotransmitters can either have excitatory or inhibitory effects on the postsynaptic membrane. For example, when acetylcholine is released at the synapse between a nerve and muscle (called the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic Na + channels to open. Na + enters the postsynaptic cell and causes the postsynaptic membrane to depolarize. This depolarization is called an excitatory postsynaptic potential (EPSP) and makes the postsynaptic neuron more likely to fire an action potential. Release of neurotransmitter at inhibitory synapses causes inhibitory postsynaptic potentials (IPSPs) , a hyperpolarization of the presynaptic membrane. For example, when the neurotransmitter GABA (gamma-aminobutyric acid) is released from a presynaptic neuron, it binds to and opens Cl - channels. Cl - ions enter the cell and hyperpolarizes the membrane, making the neuron less likely to fire an action potential.

Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the postsynaptic membrane can “reset” and be ready to receive another signal. This can be accomplished in three ways: the neurotransmitter can diffuse away from the synaptic cleft, it can be degraded by enzymes in the synaptic cleft, or it can be recycled (sometimes called reuptake) by the presynaptic neuron. Several drugs act at this step of neurotransmission. For example, some drugs that are given to Alzheimer’s patients work by inhibiting acetylcholinesterase, the enzyme that degrades acetylcholine. This inhibition of the enzyme essentially increases neurotransmission at synapses that release acetylcholine. Once released, the acetylcholine stays in the cleft and can continually bind and unbind to postsynaptic receptors.

Electrical Synapse

While electrical synapses are fewer in number than chemical synapses, they are found in all nervous systems and play important and unique roles. The mode of neurotransmission in electrical synapses is quite different from that in chemical synapses. In an electrical synapse, the presynaptic and postsynaptic membranes are very close together and are actually physically connected by channel proteins forming gap junctions. Gap junctions allow current to pass directly from one cell to the next. In addition to the ions that carry this current, other molecules, such as ATP, can diffuse through the large gap junction pores.

There are key differences between chemical and electrical synapses. Because chemical synapses depend on the release of neurotransmitter molecules from synaptic vesicles to pass on their signal, there is an approximately one millisecond delay between when the axon potential reaches the presynaptic terminal and when the neurotransmitter leads to opening of postsynaptic ion channels. Additionally, this signaling is unidirectional. Signaling in electrical synapses, in contrast, is virtually instantaneous (which is important for synapses involved in key reflexes), and some electrical synapses are bidirectional. Electrical synapses are also more reliable as they are less likely to be blocked, and they are important for synchronizing the electrical activity of a group of neurons. For example, electrical synapses in the thalamus are thought to regulate slow-wave sleep, and disruption of these synapses can cause seizures.

Signal Summation

Sometimes a single excitatory postsynaptic potential (EPSP) is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. This process is called summation and occurs at the axon hillock, as illustrated in Figure 26.16 . Additionally, one neuron often has inputs from many presynaptic neurons—some excitatory and some inhibitory—so IPSPs can cancel out EPSPs and vice versa. It is the net change in postsynaptic membrane voltage that determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential. Together, synaptic summation and the threshold for excitation act as a filter so that random “noise” in the system is not transmitted as important information.

Everyday Connection

Brain-computer interface.

Amyotrophic lateral sclerosis (ALS, also called Lou Gehrig’s Disease) is a neurological disease characterized by the degeneration of the motor neurons that control voluntary movements. The disease begins with muscle weakening and lack of coordination and eventually destroys the neurons that control speech, breathing, and swallowing; in the end, the disease can lead to paralysis. At that point, patients require assistance from machines to be able to breathe and to communicate. Several special technologies have been developed to allow “locked-in” patients to communicate with the rest of the world. One technology, for example, allows patients to type out sentences by twitching their cheek. These sentences can then be read aloud by a computer.

A relatively new line of research for helping paralyzed patients, including those with ALS, to communicate and retain a degree of self-sufficiency is called brain-computer interface (BCI) technology and is illustrated in Figure 26.17 . This technology sounds like something out of science fiction: it allows paralyzed patients to control a computer using only their thoughts. There are several forms of BCI. Some forms use EEG recordings from electrodes taped onto the skull. These recordings contain information from large populations of neurons that can be decoded by a computer. Other forms of BCI require the implantation of an array of electrodes smaller than a postage stamp in the arm and hand area of the motor cortex. This form of BCI, while more invasive, is very powerful as each electrode can record actual action potentials from one or more neurons. These signals are then sent to a computer, which has been trained to decode the signal and feed it to a tool—such as a cursor on a computer screen. This means that a patient with ALS can use e-mail, read the Internet, and communicate with others by thinking of moving his or her hand or arm (even though the paralyzed patient cannot make that bodily movement). Recent advances have allowed a paralyzed locked-in patient who suffered a stroke 15 years ago to control a robotic arm and even to feed herself coffee using BCI technology.

Despite the amazing advancements in BCI technology, it also has limitations. The technology can require many hours of training and long periods of intense concentration for the patient; it can also require brain surgery to implant the devices.

Watch this video in which a paralyzed woman use a brain-controlled robotic arm to bring a drink to her mouth, among other images of brain-computer interface technology in action.

Synaptic Plasticity

Synapses are not static structures. They can be weakened or strengthened. They can be broken, and new synapses can be made. Synaptic plasticity allows for these changes, which are all needed for a functioning nervous system. In fact, synaptic plasticity is the basis of learning and memory. Two processes in particular, long-term potentiation (LTP) and long-term depression (LTD) are important forms of synaptic plasticity that occur in synapses in the hippocampus, a brain region that is involved in storing memories.

Long-term Potentiation (LTP)

Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection. LTP is based on the Hebbian principle: cells that fire together wire together. There are various mechanisms, none fully understood, behind the synaptic strengthening seen with LTP. One known mechanism involves a type of postsynaptic glutamate receptor, called NMDA (N-Methyl-D-aspartate) receptors, shown in Figure 26.18 . These receptors are normally blocked by magnesium ions; however, when the postsynaptic neuron is depolarized by multiple presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions are forced out allowing Ca 2+ ions to pass into the postsynaptic cell. Next, Ca 2+ ions entering the cell initiate a signaling cascade that causes a different type of glutamate receptor, called AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, to be inserted into the postsynaptic membrane, since activated AMPA receptors allow positive ions to enter the cell. So, the next time glutamate is released from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because the binding of glutamate to these AMPA receptors will allow more positive ions into the cell. The insertion of additional AMPA receptors strengthens the synapse and means that the postsynaptic neuron is more likely to fire in response to presynaptic neurotransmitter release.

Long-term Depression (LTD)

Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic connection. One mechanism known to cause LTD also involves AMPA receptors. In this situation, calcium that enters through NMDA receptors initiates a different signaling cascade, which results in the removal of AMPA receptors from the postsynaptic membrane, as illustrated in Figure 26.18 . The decrease in AMPA receptors in the membrane makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP. The weakening and pruning of unused synapses allows for unimportant connections to be lost and makes the synapses that have undergone LTP that much stronger by comparison.

Science Practice Connection for AP® Courses

Don’t Eat the Fugu: Understanding the Neuron. Create a model of a neuron to explain how the vertebrate nervous system detects signals and transmits information. Then use the model to predict how abnormal cell structure, drugs, and toxins (such as tetrodotoxin found in fugu/pufferfish) can affect impulse transmission.

Think About It

Potassium channel blockers, such as procainamide, are often used to treat abnormal activity in the heart. These channel blocks impede the movement of K + through voltage-gated K + channels. What is the likely effect(s) of these medications on action potentials?

Teacher Support

• The activity is an application of AP ® Learning Objective 3.47 and Science Practice 1.1 because students are creating a model to explain how abnormal cell structure, drugs, and toxins can interfere with normal functioning of neurons
• Students should model how tetradoxin binds to voltage-gated sodium channels, preventing them from opening during an action potential. This is how tetradoxin can result in paralysis.
• The Think About It question is an application of AP ® Learning Objective 3.45 and Science Practice 1.2 because students are describing how action potentials transmit (or do not transmit) information along neurons.
• Potassium channel blockers prevent potassium from entering the neuron. This slows repolarization, and increases the effective refractory period. This will, therefore, reduce the number of action potentials a heart nerve cell generates.

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An Overview of the Different Parts of a Neuron

From Dendrites to the Terminal Buttons Found at the End of Axons

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Adah Chung is a fact checker, writer, researcher, and occupational therapist. 

Neurons are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. Each part of the neuron, from the dendrite to the terminal buttons found at the end of the axon, plays a role in communicating information throughout the body.

Neurons carry messages throughout the body, including sensory information from external stimuli and signals from the brain to different muscle groups in the body. In order to understand exactly how a neuron works, it is important to look at each individual part of the neuron. The unique structures of the neuron allow it to receive and transmit signals to other neurons as well as other types of cells.

Dendrites are tree-like extensions at the beginning of a neuron that help increase the surface area of the cell body. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma. Dendrites are also covered with synapses.

Characteristics

• Have many dendrites, or only one dendrite
• Are short and highly branched
• Transmit information to the cell body

Most neurons possess these branch-like extensions that extend outward away from the cell body. These dendrites then receive chemical signals from other neurons, which are then converted into electrical impulses that are transmitted toward the cell body.

Some neurons have very small, short dendrites, while other cells possess very long ones. The neurons of the central nervous systems have very long and complex dendrites that then receive signals from as many as a thousand other neurons.

If the electrical impulses transmitted inward toward the cell body are large enough, they will generate an action potential. This results in the signal being transmitted down the axon.​

The soma, or cell body, is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional.  

• Contains numerous organelles involved in a variety of cell functions
• Contains a cell nucleus that produces RNA that directs the synthesis of proteins
• Supports and maintains the functioning of the neuron

Think of the cell body as a small factory that fuels the neuron.

The soma produces the proteins that the other parts of the neuron, including the dendrites, axons, and synapses, need to function properly.

The support structures of the cell include mitochondria, which provide energy for the cell, and the Golgi apparatus, which packages products created by the cell and dispatches them to various locations inside and outside the cell. 

Axon Hillock

The axon hillock is located at the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal (known as an action potential ) down the axon.

The axon hillock acts as something of a manager, summing the total inhibitory and excitatory signals. If the sum of these signals exceeds a certain threshold, the action potential will be triggered and an electrical signal will then be transmitted down the axon away from the cell body. This action potential is caused by changes in ion channels which are affected by changes in polarization.

• Acts as something of a manager, summing the total inhibitory
• Possesses an internal polarization of approximately -70mV in a normal resting state

When a signal is received by the cell, it causes sodium ions to enter the cell and reduce polarization. If the axon hillock is depolarized to a certain threshold, an action potential will fire and transmit the electrical signal down the axon to the synapses.

It is important to note that the action potential is an all-or-nothing process and that signals are not partially transmitted. The neurons either fire or they do not.

The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it transmits information.

Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.

• Most neurons have only one axon
• Transmit information away from the cell body
• May or may not have a myelin covering
• Range dramatically in size, from 0.1 millimeters to over 3 feet long  

The myelin surrounding the neurons protects the axon and aids in the speed of transmission. The myelin sheath is broken up by points known as the nodes of Ranvier or myelin sheath gaps. Electrical impulses are able to jump from one node to the next, which plays a role in speeding up the transmission of the signal.

Axons connect with other cells in the body including other neurons, muscle cells, and organs. These connections occur at junctions known as synapses.

The synapses allow electrical and chemical messages to be transmitted from the neuron to the other cells in the body.

Terminal Buttons and Synapses

Terminal buttons are found at the end of the axon, below the myelin sheath, and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse.

Neurotransmitters carry signals across the synapse to other neurons. When an electrical signal reaches the terminal buttons, neurotransmitters are then released into the synaptic gap.

• Contain vesicles holding the neurotransmitters
• Convert electrical impulses into chemical signals
• Cross the synapse where they are received by other nerve cells
• Responsible for the reuptake of any excessive neurotransmitters released during this process

A Word From Verywell

Neurons serve as basic building blocks of the nervous system and are responsible for communicating messages throughout the body.

Knowing more about the different parts of the neuron can help you to better understand how these important structures function as well as how different problems, such as diseases that impact axon myelination, might impact how messages are communicated throughout the body.

Luengo-Sanchez S, Bielza C, Benavides-Piccione R, Fernaud-Espinosa I, DeFelipe J, Larrañaga P. A univocal definition of the neuronal soma morphology using Gaussian mixture models .  Front Neuroanat . 2015;9:137. doi:10.3389/fnana.2015.00137

Miller AD, Zachary JF. Nervous System . In: Zachary JF, ed. Pathologic Basis of Veterinary Disease . St. Louis, MO: Mosby, Inc.; 2017. doi:10.1016/B978-0-323-35775-3.00014-X

Debanne D, Campana E, Bialowas A, Carlier E, Alcaraz G. Axon Physiology .  Psychol Rev. 2011;91(2):555-602 .  doi:10.1152/physrev.00048.2009

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Neuron Anatomy, Nerve Impulses, and Classifications

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Neurons are the basic unit of the nervous system and  nervous tissue . All cells of the nervous system are comprised of neurons. The nervous system helps us to sense and respond to our environment and can be divided into two parts: the  central nervous system  and the  peripheral nervous system .

The central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of sensory and motor nerve cells that run throughout the rest of the body. Neurons are responsible for sending, receiving, and interpreting information from all parts of the body.

Parts of a Neuron

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A neuron consists of two major parts: a cell body and nerve processes.

Neurons contain the same cellular components as other body cells . The central cell body is the process part of a neuron and contains the neuron's  nucleus , associated cytoplasm, organelles, and other cell structures. The cell body produces proteins needed for the construction of other parts of the neuron.

Nerve Processes

Nerve processes are "finger-like" projections from the cell body that are able to conduct and transmit signals. There are two types:

• Axons  typically carry signals away from the cell body. They are long nerve processes that may branch out to convey signals to various areas. Some axons are wrapped in an insulating coat of  glial cells  called oligodendrocytes and Schwann cells. These cells form the myelin sheath which indirectly assists in the conduction of impulses as myelinated nerves can conduct impulses quicker than unmyelinated ones. Gaps between the myelin sheath are called Nodes of Ranvier. Axons end at junctions known as synapses.
• Dendrites  typically carry signals toward the cell body. Dendrites are usually more numerous, shorter, and more branched than axons. They have many synapses in order to receive signal messages from nearby neurons.

Nerve Impulses

Encyclopaedia Britannica / Getty Images

Information is communicated among nervous system structures through nerve signals. Axons and dendrites are bundled together into what are called nerves. These nerves send signals between the brain, spinal cord, and other body organs via nerve impulses. Nerve impulses, or action potentials, are electrochemical impulses that cause neurons to release electrical or chemical signals that initiate an action potential in another neuron. Nerve impulses are received at neuronal dendrites, passed through the cell body, and are carried along the axon to the terminal branches. Since axons can have numerous branches, nerve impulses can be transmitted to numerous cells. These branches end at junctions called synapses.

It is at the synapse where chemical or electrical impulses must cross a gap and be carried to the dendrites of adjacent cells. At electrical synapses, ions and other molecules pass through gap junctions allowing for the passive transmission of electrical signals from one cell to the other. At chemical synapses, chemical signals called neurotransmitters are released which cross the gap junction to stimulate the next neuron. This process is accomplished by exocytosis of the neurotransmitters. After crossing the gap, neurotransmitters bind to receptor sites on the receiving neuron and stimulate an action potential in the neuron. 

Nervous system chemical and electrical signaling allow for quick responses to internal and external changes. In contrast, the endocrine system , which uses hormones as its chemical messengers, is typically slow-acting with effects that are long-lasting. Both of these systems work together to maintain homeostasis .

Neuron Classification

Stocktrek Images / Getty Images

There are three main categories of neurons. They are multipolar, unipolar, and bipolar neurons.

• Multipolar neurons are found in the central nervous system and are the most common of the neuron types. These neurons have a single axon ​and many dendrites extending from the cell body.​​
• Unipolar neurons have one very short process that extends from a single cell body and branches into two processes. Unipolar neurons are found in spinal nerve cell bodies and cranial nerves .
• Bipolar neurons are sensory neurons consisting of one axon and one dendrite that extend from the cell body. They are found in retinal cells and olfactory epithelium.

Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information from the central nervous system to  organs , glands, and  muscles . Sensory neurons send information to the central nervous system from internal organs or from external stimuli. Interneurons relay signals between ​motor and sensory neurons.​

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• Functions of the Central Nervous System
• Spinal Cord Function and Anatomy
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• What Is Lateral Inhibition? Definition and Examples
• The Peripheral Nervous System and What It Does
• What You Need to Know About Neurotransmitters
• White Matter and Your Brain
• What Is an Action Potential?
• Types of Cells in the Human Body
• Overview of the Medulla Oblongata
• Divisions of the Brain: Forebrain, Midbrain, Hindbrain
• How Do Insects Smell?
• Overview of the Five Senses
• A Definition and Explanation of the Steps in Exocytosis
• The 12 Animal Organ Systems

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13.19: Nerve Impulses

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How does a nervous system signal move from one cell to the next?

It literally jumps by way of a chemical transmitter. Notice the two cells are not connected, but separated by a small gap. The synapse. The space between a neuron and the next cell.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions , which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane (see Figure below ). It uses energy in ATP to pump positive sodium ions (Na + ) out of the cell and potassium ions (K + ) into the cell. As a result, the inside of the neuron is negatively charged compared to the extracellular fluid surrounding the neuron. This is due to many more positively charged ions outside the cell compared to inside the cell. This difference in electrical charge is called the resting potential.

The sodium-potassium pump maintains the resting potential of a neuron.

Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current (see Figure below ).

An action potential speeds along an axon in milliseconds.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The place where an axon terminal meets another cell is called a synapse . The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below ). When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter . The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell.

At a synapse, neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.

• A nerve impulse begins when a neuron receives a chemical stimulus.
• The nerve impulse travels down the axon membrane as an electrical action potential to the axon terminal.
• The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.
• Define resting potential and action potential.
• Explain how resting potential is maintained
• Describe how an action potential occurs.
• What is a synapse?

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Nerve Impulse

Introduction, continuous conduction , saltatory conduction , resting membrane potential, action potential , polarization , depolarization, repolarization , refractory period , electrical synapses , chemical synapses , cns and nerve impulse, myelin sheath, axon diameter, temperature, what is a nerve impulse, how is a nerve impulse produced, what is the refractory period, what are saltatory impulses.

Nerve impulse was discovered by British Scientist Lord Adrian in the 1930s. Owning to the importance of this discovery, he was awarded Noble Prize in 1932. Nerve Impulse is a major mode of signal transmission for the Nervous system. Neurons sense the changes in the environment and as a result, generate nerve impulses to prepare the body against those changes.

A nerve Impulse is defined as a wave of electrical chemical changes across the neuron that helps in the generation of the action potential in response to the stimulus. This transmission of a nerve impulse across the neuron membrane as a result of a change in membrane potential is known as Nerve impulse conduction.

It is a change in the resting state of the neuron. Due to nerve impulses, the resting potential is changed to an action potential to conduct signals to the target in response to a stimulus. The stimulus can be a chemical, electrical, or mechanical signal. 

The action potential is a result of the movement of ions in and out of the cell. Particularly the ions included in this process are sodium and potassium ions. These ions are propagated inside and outside the cell through specific sodium and potassium pumps present in the neuron membrane. The transmission of a nerve impulse from one neuron to another neuron is achieved by a synaptic connection (synapse) between them. It is thus a mode of communication between different cells.

The speed of nerve impulse propagation varies in different types of cells. The rate of transmission and generation of nerve impulses depends upon the type of cell. Besides, Myelin Sheath also helps in accelerating the rate of signal conduction (about 20 times). Generally, the speed of nerve impulses is 0.1-100 m/s.

Mechanism of Nerve Impulse Conduction

Nerve impulse conduction is a major process occurring in the body responsible for organized functions of the body. So, for the conduction of nerve impulses, there are two mechanisms:

• Continuous conduction
• Saltatory conduction

Continuous nerve impulse conduction occurs in non-myelinated axons. The action potential travels along the entire length of the axon. Hence, more time is taken in generating and then transmit nerve impulses during an action potential.

Continuous conduction requires more energy to transmit impulses and is a slower process (approximately 0.1 m/s). It delays the process of conducting signals because it uses a higher number of ion channels to alter the resting state of the neuron.

Saltatory is faster than continuous conduction and occurs in myelinated neurons. In myelinated neurons, myelinated sheaths are present. Between these myelinated sheaths, unmyelinated gaps are presently known as the nodes of Ranvier. Nerve impulse propagates by jumping from one node of Ranvier to the next. This makes the process of nerve impulse faster as the nerve impulse does not travel the entire length of the axon ( this happens in the case of continuous conduction). The nerve impulse travels at a speed of 100 m/s in saltatory conduction.

The number of channels utilized in saltatory conduction is less than in continuous conduction due to which delay of nerve impulse does not occur. This mode of nerve impulse transmission utilizes less energy as well.

If you consider the axon as an electrical wire or loop, nerve impulse that travels along the axon as current, and the charged particles ( sodium and potassium ions) as the electron particles then the process can be understood quite easily. As the flow of current in a wire occurs at a specific voltage only, similarly the conduction of nerve impulse occurs when a stimulus has a maximum threshold value of -55 millivolts. This is essential for altering the resting membrane state to action membrane potential.

When the voltage has the required number of electron particles it conducts current. Similarly, in the case of nerve impulse conduction, the neurons of the stimulus must have a threshold value for causing the movement of ions across the length of the axon (for conducting nerve impulse) by opening the voltage-gated ion channels.

Process of transmission of Nerve Impulse

For the transmission of a nerve impulse, the stages are below:

• Polarization
• Repolarization
• Refractory Period

Before going into the details of the process of nerve impulse transmission, let’s first discuss action and resting potential states.

The resting membrane potential refers to the non-excited state of the nerve cell at rest when no nerve impulse is being conducted. The resting membrane potential of the nerve cell is -70 mV. It is a static state and both the sodium and potassium channels are closed during this state maintaining a high concentration of sodium ions outside and high potassium ions concentration inside the cell.

An action potential occurs when the nerve cell is in an excited state while conducting nerve impulses. In this situation, sodium channels open and potassium channels are closed. This results in a huge influx of sodium ions inside the cells which trigger the nerve impulse conduction. The action potential is +40 mV.

Polarization is the situation in which the membrane is electrically charged but non-conductive. It means it doesn’t conduct nerve impulses in this state. During polarization, the membrane is in a resting potential state. The concentration of sodium ions is about 16 times more outside the axon than inside. In contrast, the concentration of potassium ions is 25 times more inside the axon than outside.

The polarization state is also known as the “Unstimulated or non-conductive state”. Due to the difference in the concentration of ions inside and outside the membrane, a potential gradient is established ranging between -20-200mV ( in the case of humans, the potential gradient in the polarized state is near -70mV). In the polarized state, the axon membrane is more permeable to potassium ions instead of sodium ions and as a result, it causes rapid diffusion of potassium ions.

In the resting state, the membrane potential becomes electro-negatively charged due to the movement of positively charged potassium ions outside the cell and the presence of electro-negative proteins in the intracellular space.

It refers to a graded potential state because a threshold stimulus of about -55mV causes a change in the membrane potential. The threshold stimulus must be strong enough to change the resting membrane potential into action membrane potential.

This results in the alternation in the electro-negativity of the membrane because the stimulus causes the influx of sodium ions (electropositive ions) by 10 times more than in the resting state. For this, sodium voltage-gated channels open. The action potential state is based on the “All or none” method and has two possibilities:

If the stimulus is not more than the threshold value, then there will be no action potential state across the length of the axon.

If the stimulus is more than the threshold value, then it will generate a nerve impulse that will travel across the entire length of the axon.

It is a condition during which the electrical balance is restored inside and outside the axon membrane. Due to the high concentration of sodium ions inside the axoplasm, the potassium channels will open. During the repolarization state, the efflux of potassium ions through the potassium channel occurs. As a result of the opening of potassium voltage-gated channels, sodium voltage-gated channels will be closed. Thus, no sodium ions will move inside the membrane. Therefore, repolarization helps in maintaining or restoring the original membrane potential state.

Until potassium channels close, the number of potassium ions that have moved across the membrane is enough to restore the initial polarized potential state. As a result of this, the membrane becomes hyperpolarized and has a potential difference of -90 mV.

The refractory phase is a brief period after the successful transmission of a nerve impulse. During this period, the membrane prepares itself for the conduction of the second stimulus after restoring the original resting state. It persists for only 2 milliseconds.

During this, the sodium ATPase pump allows the re-establishment of the original distribution of sodium and potassium ions. The sodium and potassium ATPase pump, driven by using ATP, helps to restore the resting membrane state for the conduction of a second nerve impulse in response to the other stimulus. It causes the movement of ions against the concentration gradient. For every two potassium ions that move inside the cell, three sodium ions are transported outside. This process requires ATP because the movement of ions is against the concentration gradient of both ions.

The process of transmission of a nerve impulse from one neuron to the other, after reaching the axon’s synaptic terminal, is known as synapse. This transmission of the nerve impulse by synapses involves the interaction between the axon ending of one neuron (Presynaptic neuron) to the dendrite of another neuron (Postsynaptic neuron). There is space between the pre-synaptic neuron and post-synaptic neuron which is known as synaptic cleft or synaptic gap.

After transmitting from one neuron to another, the nerve impulse generates a particular response after reaching the target site. If somehow the synaptic gap doesn’t allow the passage of nerve impulse, the transmission of nerve impulse will not occur and consequently required response too.

Read more about the Myelin Sheath

Types of synapses 

There are two types of synapses:

• Electrical synapses
• Chemical synapses

In electrical synapses, two neurons are connected through channel proteins for transmitting a nerve impulse. The nerve impulse travels across the membrane of the axon in the form of an electrical signal. The signal is transmitted in the form of ions and therefore it is much faster than chemical synapses.

In electrical synapses, the synaptic gap is about 0.2nm which also favours faster nerve impulse conduction.

In chemical synapses, the conduction of nerve impulses occurs through chemical signals. These chemical signals are neurotransmitters. In this type of nerve impulse conduction, the synaptic gap is more than electrical synapses and is about 10-20 nm. Due to this, the transmission of nerve impulses is slower than electrical synapses.

Neurons help in transmitting signals in the form of nerve impulses from the Central nervous system to the peripheral body parts. Neurons are a complex network of fibres that transmit information from the axon ending of one neuron to the dendrite of another neuron. The signal finally reaches the target cell where it shows a response.

In conducting nerve impulses, the following play a major role:

• Axon- Helps in the propagation of nerve impulses to the target cell.
• Dendrites- Receive the signals from the axon ends.
• Axon Ending- Acts as a transmitter of signals.

Axon plays a major role in the process by transmitting signals in the form of nerve impulses via synapses to the target cells. The neuron is responsible for transferring signals to three target cells:

• Another neuron

And this results in the contraction of muscle, and secretion by glands and helps neurons to transmit action potential.

Factors Affecting the Speed of Nerve Impulse 

The following are some major factors that affect the speed of nerve impulses:

Myelin sheath is present around the neuron and functions as an electrical insulator. Due to this sheath, an action potential is not formed on the surface of the neuron. This Myelin sheath has regular gaps, where it is not present, called nodes of Ranvier. An action potential can form at these gaps and impulse will jump from node to node by saltatory conduction. This can be a factor in increasing the speed of nerve impulses from about 30-1 m/ to 90-1 m/s.  

As the axon diameter increase, the speed of nerve impulses increases as well. This is because a larger axon diminishes the ion leakage out of the axon. This helps in maintaining the membrane potential and thus favours faster nerve impulses.

Temperature cause changes in the rate of diffusion of ions across the neuron membrane. Temperature directly correlates with the transmission of nerve impulses. If the temperature is higher, the rate of diffusion of sodium and potassium ions will be high and the axon will become depolarized quickly which will cause a faster nerve impulse conduction.  

A nerve impulse is thus an important signal transduction mode for triggering a response in major body parts due to a strong stimulus. Any distraction in this process can have drastic effects on the body. 

Frequently Asked Questions

A nerve impulse is a wave of electrochemical changes that travel across the plasma membrane and helps in the generation of an action potential. Signals are propagated along the nerve fibres in the form of nerve impulses. 

A nerve impulse is produced when a stimulus acts on the nerve fibre, resulting in electrochemical changes across the nerve membrane. These electrochemical changes cause depolarization of the membrane resulting in the generation of nerve impulses.

It is a short duration of time during which a new nerve impulse cannot be generated in a neuron, after initiation of a previous action potential. This period occurs at the end of action potential and limits the speed at which nerve impulses can be generated in a nerve fibre. 

These are nerve impulses that jump from one node to another and are seen only in myelinated nerve fibres. Saltatory conduction increases the speed at which a nerve signal is conducted down the length of an axon.

•   Lodish, H; Berk, A; Kaiser, C; Krieger, M; Bretscher, A; Ploegh, H; Amon, A (2000). Molecular Cell Biology (7th ed.). New York, NY: W. H. Freeman and Company. p. 695.
• Marieb, E. N., & Hoehn, K. (2014).  Human anatomy & physiology.  San Francisco, CA: Pearson Education Inc.

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

StatPearls [Internet].

Neuroanatomy, neuron action potential.

Isaac Chen ; Forshing Lui .

Affiliations

Last Update: August 14, 2023 .

• Introduction

Neurons are electrically excitable, reacting to input via the production of electrical impulses, propagated as action potentials throughout the cell and its axon. These action potentials are generated and propagated by changes to the cationic gradient (mainly sodium and potassium) across their plasma membranes. These action potentials finally reach the axonal terminal and cause depolarization of neighboring cells through synapses. This action is the way these cells can interact with each other, i.e., at synapses via synaptic transmission. Normally, the cell’s interior is negative, compared to its outside. This state is the resting membrane potential of about -60mV. A neuronal action potential gets generated when the negative inside potential reaches the threshold (less negative). This change in membrane potential will open voltage-gated cationic channel (sodium channel) resulting in the process of depolarization and generation of the neuronal action potential. Neuronal action potentials are vital for propagation of impulses along any nerve fiber even at a distance. They also are crucial for communication among neurons through synapses. Disruption of this mechanism can have drastic effects resulting in lack of impulse generation and conduction, illustrated by various neurotoxins and demyelinating disorders. [1] [2]

• Structure and Function

The neuron’s membrane potential gets generated via a difference in the concentration of charged ions. The lipid bilayer of the neuronal cell membrane acts as a capacitor, the transmembrane channels as resistors. This resting (steady-state) potential is critical for the neuron’s physiological state, maintained by an unequal distribution of ions across the cellular membrane and established by ATP-dependent pumps--most notably, sodium-potassium antiporters. These exchangers are responsible for pumping sodium out of the cells into the extracellular space, potassium into the intracellular compartment. When opened, various channels allow permeable ions to flow down their electrochemical gradients, thereby altering the membrane potential. The gating of these channels is by second messengers, neurotransmitters, or voltage changes. Voltage-gated cationic channels are the main channels used in the generation and propagation of neuronal action potential.

There are 100 billion neurons in the human brain, and there are a quadrillion synapses in the human brain. Any neuron will have on average of 1000 synapses which influence the electrical potential of the membrane. When the resting membrane potential (-60mV) becomes less negative, it depolarizes. When it is more negative, it hyperpolarizes. Upon collating the various movements of ions, particularly the entering of sodium, the cell may have sufficient signals to reach the threshold potential and achieves this threshold by sufficient positively charged ions entering the cell, i.e., terminating the polarity in what is called depolarization. At normal body temperature, the equilibrium potential for sodium is +55 mV, -103 mV for potassium. There are three stages in the generation of the action potential: (1) depolarization, changing the membrane’s potential from -60 mV to +40 mV primarily caused by sodium influx; (2) repolarization, a return to the membrane’s resting potential, primarily caused by potassium efflux; and (3) after-hyperpolarization, a recovery from a slight overshoot of the repolarization. [3] (see table below)  As mentioned, stage 1 is guided by an increased membrane permeability to sodium. Accordingly, the removal of extracellular sodium, or inactivation of sodium channels, prevents the generation of action potentials. [4]  Immediately after an action potential generates, the neuron cannot immediately generate another action potential; this is the absolute refractory period. At this moment, the sodium channels are inactivated and remain closed, whereas the potassium channels are still open. This state is followed by the relative refractory period when the neuron may only generate an action potential with a much higher threshold. Thie opens when some of the sodium channels are ready to be opened, and many are still inactivated, whereas some potassium channels are still open as well. The duration of the refractory periods will determine how fast an action potential may be generated and propagated. The propagation of the action potential continues until termination at a synapse, where it can either cause the release of neurotransmitters or conduction of ionic currents. The latter occurs at electrical synapses, whereby presynaptic and postsynaptic cells connect and avoid the use of neurotransmitters. [5]  Neurotransmitters are the norm, however, and get released at chemical synapses and neuromuscular junctions. [6]  

Local currents created by depolarization along a portion of the neuronal membrane, if sufficiently strong, can depolarize neighboring segments of the membrane to the threshold, thereby propagating the action threshold down the membrane and along the neuron’s axon. The determining factor in the speed of this propagation is primarily the extent to which the initial local currents first spread before creating further depolarizations. Factors influencing this speed include the membrane’s electrical resistance and internal contents of the axon. Wider axons have lower internal resistance, and having more voltage-gated sodium channels in the membrane decreases membrane resistance as well. Higher internal resistance and lower membrane resistance contribute to slower action potential propagations. Because the body does not have enough space, instead of making large axons, the nervous system, to maximize propagation velocity, employs glial cells, specifically oligodendrocytes and Schwann cells, to wrap themselves around axons, creating myelin sheaths. These sheaths contribute to greater membrane resistance, patching up areas where channels would otherwise leak. Still, the action potential can only propagate so far before requiring more sodium channels to perpetuate the potential, creating gaps in the myelin sheath called nodes of Ranvier. These nodes have high concentrations of those channels to restart the action potential along the axon, termed saltatory conduction. [1]

Neuron Action Potential - see the table in media below.

• Clinical Significance

The rapid depolarization or the upstroke of the neuronal action potential occurs as a result of the opening of the voltage-gated sodium channels. These channels are large transmembrane proteins with different subunits encoded by ten mammalian genes. Problems with these channels are collectively called channelopathies. The channelopathies may affect any excitable tissues, including neurons, skeletal, and cardiac muscles resulting in multiple different diseases. The neurological channelopathies present more commonly in different muscle diseases and the brain. Paramyotonia congenita results from mutations in the gene coding for the alpha-1 subunit of the sodium channel. Sodium channelopathies in the brain result in various forms of refractory epilepsy disorders.

There is a variety of neurotoxins that can block the action potential. One such deadly toxin is tetrodotoxin (TTX), which inhibits sodium channels. [7]  The naturally occurring toxin is normally ingested orally from pufferfish, a part of Japanese cuisine, and its incidence has spread beyond Southeast Asia to the Pacific and Mediterranean, as well as finding this toxin in many other species. By binding to sodium channels and inactivating them, tissues affected are rendered immobile and insensitive. The onset/severity of symptoms arising from TTX correlates on how much an individual consumes, and patients may first present with paraesthesias of the tongue/lips. This presentation is associated with or followed by headache/vomiting that may become muscle weakness and ataxia. Other symptoms include diarrhea, dizziness, and loss of reflexes. Death can occur from respiratory and/or heart failure. Of some clinical significance, however, TTX has some analgesic activity that has been the topic of study in treating pain, and a low dose may reduce heroin craving. Unfortunately, TTX has no cure and is often fatal, with observation and supportive care being the only treatment. Respiratory support comes in the form of endotracheal intubation or mechanical ventilation to support breathing. Early stages of poisoning can be treated with activated charcoal to adsorb the toxin before gastric absorption and with gastric lavage to reduce symptom severity. [8]

Ciguatoxin is a potent sodium channel blocker that causes a rapid onset of numbness, paraesthesia, dysaesthesia, and muscle paralysis. Ciguatoxins (CTX) are marine neurotoxins that are produced by the dinoflagellates. CTX works by blocking the voltage-gated sodium channels. Humans are exposed to CTX by ingestion of carnivore coral reef fishes, including grouper, red snapper, and barracuda, which feed on fish that have consumed the dinoflagellates.

Saxitoxin and its derivatives are known as paralytic shellfish toxins (PSTs). Found in marine and freshwater environments among dinoflagellates, PSTs act similarly to TTX and CTX, i.e., binding to voltage-gated sodium channels and blocking the movement of nerve impulses, as well as some degree of targeting the potassium and calcium channels. As such, just like TTX, sodium cannot enter through the inactivated sodium channels, preventing membrane depolarization. Because of the similarity of its mechanism of action to TTX, PSTs share similar consequences. Severe exposure can cause severe hypotension and general paralysis, and death can occur from respiratory failure/hypotension. [9] [10]

To illustrate the importance of myelin for saltatory conduction, different demyelinating diseases that destroy myelin can have varying degrees of severity because they reduce the conduction velocity of action potentials. [11]  Multiple sclerosis (MS) destroys oligodendrocytes, which help to maintain the fatty layer of the myelin sheath, preventing the effective carriage of electrical signals. Eventually, this causes total loss of myelin and breakdown of neuronal axons. [12]  MS commonly presents in white young adult females and can result in a plethora of signs/symptoms, physical, mental, and psychiatric, e.g., diplopia, blindness, muscle weakness, speech problems, tremors, incontinence, and vertigo. Diagnosis can be aided with testing for oligoclonal bands of IgG in cerebrospinal fluid on electrophoresis, found in many MS patients. [13] [14] [15] [16]

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Anatomy of Neurons. A. Two connected neurons. Neurons have a soma that contains a nucleus, an axon, and a dendritic tree. A single synapse (red circle) is formed at the point where an axon's neuron (black) connects to another neuron's (more...)

A neuronal action potential. The dashed line represents the threshold voltage. Used with permission from OpenStax under the Creative Commons Attribution 4.0 International license.

Neuron action potential - ionic movements Contributed by Forshing Lui MD

Disclosure: Isaac Chen declares no relevant financial relationships with ineligible companies.

Disclosure: Forshing Lui declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Chen I, Lui F. Neuroanatomy, Neuron Action Potential. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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