11.4: Nerve impulses (2023)

generate nerve impulses

ONEnerve impulse, like lightning, is an electrical phenomenon. A nerve impulse occurs due to a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge occur? 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 rest, the sodium-potassium pump maintains a charge differential across the cell membrane of the neuron. The sodium-potassium pump is an active transport mechanism that moves sodium ions out of cells and potassium ions into cells. The sodium-potassium pump moves both ions from areas of lower to higher concentration using energy in ATP and transport proteins in the cell membrane. Figure \(\PageIndex{3}\)shows in more detail how the sodium-potassium pump works. Sodium is the main ion in the fluid outside the cells and potassium is the main ion in the fluid inside the cells. These concentration differences create an electrical gradient across the cell membrane, calledresting potential🇧🇷 Tight control of the resting membrane potential is crucial for the transmission of nerve impulses.

action potential

ONEaction potential, also called a nerve impulse, is an electrical charge that travels along the membrane of a neuron. It can be produced when the membrane potential of a neuron is altered by chemical signals from a nearby cell. During an action potential, the cell membrane potential rapidly changes from negative to positive as sodium ions flow into the cell through ion channels while potassium ions flow out of the cell, as shown in figure \( \PageIndex{3}\).

The change in membrane potential causes the cell to become depolarized. An action potential works according to the all-or-nothing principle. That is, the membrane potential must reach a certain level of depolarization, called the threshold, or an action potential will not fire. This threshold potential varies, but is generally about 15 millivolts (mV) more positive than the resting membrane potential of the cell. If membrane depolarization does not reach threshold, no action potential occurs. You can see in figure \(\PageIndex{4}\) that two depolarizations did not reach the -55 mV threshold.

The first channels to open are sodium ion channels, which allow sodium ions to enter the cell. The resulting increase in positive charge within the cell (up to about +40 mV) triggers the action potential. This is called membrane depolarization. Potassium ion channels then open, allowing potassium ions to flow out of the cell, ending the action potential. The inside of the membrane again becomes negative. This is called membrane repolarization. Both ion channels then close and the sodium-potassium pump restores the resting potential to -70 mV. The action potential travels down the axon to the synapse like a wave would travel along the surface of water. Figure \(\PageIndex{4}\) shows the change in axon membrane potential during an action potential. The nerve undergoes a brief refractory period before reaching resting potential. No further action potentials can be generated during the refractory period.

In myelinated neurons, ionic fluxes occur only in the nodes of Ranvier. As a result, the action potential signal "hops" from node to node along the axon membrane rather than spreading evenly across the membrane, as it does in axons without myelin sheath. This is due to the accumulation of Na+ and K+ ion channels in Ranvier's nodes. Unmyelinated axons lack nodes of Ranvier, and ion channels in these axons are distributed over the entire surface of the membrane.

transmission of nerve impulses

The place where an axon terminal meets another cell is called theSynapse.It is here that a nerve impulse is transmitted to another cell. 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, making direct electrical connections between neurons. However, most synapses are chemical synapses. The transmission of nerve impulses across chemical synapses is more complex.

chemical synapses

In a chemical synapse, both the presynaptic and postsynaptic regions of cells are filled with the molecular machinery involved in transmitting nerve impulses. As shown in figure \(\PageIndex{5}\), the presynaptic area contains many tiny spherical vessels called synaptic vesicles, which are filled with so-called chemicalsneurotransmitter🇧🇷 When an action potential reaches the axon terminal of the presynaptic cell, it opens channels that allow calcium to enter the terminal. Calcium causes the synaptic vesicles to fuse with the membrane and release their contents into the narrow space between the presynaptic and postsynaptic membranes. This area is called the synaptic cleft. Neurotransmitter molecules travel across the synaptic cleft and bindreceivers, which are proteins embedded in the membrane of the postsynaptic cell.

The effect of a neurotransmitter on a postsynaptic cell depends primarily on the type of receptors it activates, allowing a given neurotransmitter to have different effects on different target cells. A neurotransmitter can stimulate one set of target cells, inhibit others, and have complex modulatory effects on still others, depending on the nature of the receptors. However, some neurotransmitters have relatively consistent effects on other cells.

review

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 is generated.
4. Describe how a signal is transmitted from a presynaptic cell to a postsynaptic cell at a chemical synapse.
5. In general, what determines the effect of a neurotransmitter on a postsynaptic cell?
6. Identify three general types of effects that neurotransmitters can 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 what kind of ions into the neuron results in an action potential?
   1. How do these ions enter the cell?
   2. What does this flow of ions do to the relative charge inside the neuron compared to the outside?
9. The sodium-potassium pump:
   1. is activated by an action potential
   2. need energy
   3. does not require energy
   4. pumps potassium ions out of cells
10. Right or wrong.Some action potentials are larger than others, depending on the strength of the stimulation.
11. Right or wrong.Synaptic vesicles from the presynaptic cell enter the postsynaptic cell.
12. Right or wrong.Eventually, an action potential in a presynaptic cell can result in inhibition of the postsynaptic cell.
13. Name three neurotransmitters.

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assignments

1. Adapted from Mandeep GrewalLincoln Blitzthrough theUS navy photofellow photographer 2nd class Aaron Ansarov; Public domain: Wikimedia Commons
2. Schematic of the sodium-potassium pumpthrough thelady of hatsMariana Ruiz Villarreal, published no.public domainAbout or Wikimedia Commons
3. action potentiallicensedCC-BY 3.0por OpenStax
4. action potentialthrough thecris 73, licensedCC-BY 3.0About or Wikimedia Commons
5. Chemical scheme of severed synapseFile created by Looie496, USANational Institute of Health, originally compiled by the National Institute on Aging, released into the public domain via Wikimedia Commons
6. Text adapted fromhuman biologythrough theCK-12licensedCC-BY-NC 3.0