80 8.4 Nerve Impulses

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 8.4.1 Lightning strikes due to a difference in electrical charge, and results in an electrical current.

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 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

, 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 , which are electrically-charged  or .

Resting Potential

When a is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the maintains a difference in charge across the of the neuron. The sodium-potassium pump is a mechanism of 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 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 . 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 . It begins when the neuron receives a chemical signal from another cell or some other type of .  If the stimulus is strong enough to reach , 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.

Action Potential
Figure 8.4.2 An action potential speeds along an axon in milliseconds. Sodium ions flow in and cause the action potential, and then potassium ions flow out to reset the resting potential.

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 . This is where the transmission of a nerve impulse to another cell occurs. The cell that sends the nerve impulse is called the , and the cell that receives the nerve impulse is called the .

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  that are packed with chemicals called . When an 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 . The neurotransmitter molecules travel across the synaptic cleft and bind to receptors, which are proteins embedded in the membrane of the postsynaptic cell.

Chemical Synapse
Figure 8.4.3 This diagram shows how an action potential transmits a signal across a synapse to another cell by neurotransmitter molecules. The inset diagram shows in detail the structures and processes occurring at a single axon terminal and synapse.

 

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 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 or .
  • 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, and (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 from the immune system blocking receptors for the neurotransmitter 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 , 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 is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.
  • The  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 of the neuron.
  • An 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 . At a chemical synapse, 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

  1. Define nerve impulse.
  2. What is the resting potential of a neuron, and how is it maintained?
  3. Explain how and why an action potential occurs.
  4. Outline how a signal is transmitted from a presynaptic cell to a postsynaptic cell at a chemical synapse.
  5. What generally determines the effects of a neurotransmitter on a postsynaptic cell?
  6. Identify three general types of effects that neurotransmitters may have on postsynaptic cells.
  7. Explain how an electrical signal in a presynaptic neuron causes the transmission of a chemical signal at the synapse.
  8. 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?
  9. 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).

References

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/GFy_h8cu@10.53: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

 

License

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

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