Name: how nerve signals travel in axons
Uploaded: 2025-10-23T11:47:13.000Z
Duration: 1 min 41 s
Channel: JoVE

The action potential, a specific change in charge across the cell membrane, is the primary way that electrical signals are propagated in the nervous system.

Neurons typically have a resting potential of about negative 70 millivolts. When they receive signals, such as neurotransmitters, their membrane potential can hyperpolarize, decrease or depolarize, increase.

When a neuron is depolarized to the threshold potential, the point where an action potential is triggered, voltage-gated sodium channels open, because of the activity of the sodium potassium pump. There is a greater concentration of sodium ions outside of neurons and a greater concentration of potassium ions inside. Therefore when the sodium channels open, sodium rushes in, down its gradient.

The influx of positive charge rapidly increases the membrane potential to around plus 40 millivolts, the peak of the action potential. Sodium channels then inactivate, preventing more sodium from flowing in. Also, voltage-gated potassium channels now open, allowing potassium to flow out down its gradient, reducing the membrane potential.&#160;The membrane becomes briefly hyperpolarized, called the refractory period, greatly reducing the chance of a new action potential until the resting potential is restored.

In myelinated axons the action potential is regenerated at each node of Ranvier, allowing the signal to be reliably and rapidly transmitted over long distances.

action potentials

Action Potentials

<h4>Overview</h4>

Neurons communicate by firing action potentials&#8212;the electrochemical signal that is propagated along the axon. The signal results in the release of neurotransmitters at axon terminals, thereby transmitting information in the nervous system. An action potential is a specific &#8220;all-or-none&#8221; change in membrane potential that results in a rapid spike in voltage.

<h4>Membrane Potential in Neurons</h4>

Neurons typically have a resting membrane potential of about -70 millivolts (mV). When they receive signals&#8212;for instance, from neurotransmitters or sensory stimuli&#8212;their membrane potential can hyperpolarize (become more negative) or depolarize (become more positive), depending on the nature of the stimulus.

If the membrane becomes depolarized to a specific threshold potential, voltage-gated sodium (Na+) channels open in response. Na+ has a higher concentration outside of the cell as compared to the inside, so it rushes in when the channels open, moving down its electrochemical gradient. As positive charge flows in, the membrane potential becomes even more depolarized, in turn opening more channels. As a result, the membrane potential quickly rises to a peak of around +40 mV.

At the peak of the action potential, several factors drive the potential back down. The influx of Na+ slows because the Na+ channels start to inactivate. As the inside of the cell becomes more positive, there is less electrical attraction driving Na+ inwards. The initial depolarization also triggers the opening of voltage-gated potassium (K+) channels, but they open more slowly than the Na+ channels. Once these K+ channels open&#8212;around the peak of the action potential&#8212;K+ rushes out, down its electrochemical gradient. The reduced influx of positive charge from Na+ combined with efflux of positive charge from K+ rapidly lowers the membrane potential.

For a brief period after an action potential, the membrane is hyperpolarized compared to the resting potential. This is called the refractory period because, during this time, the cell is incapable of producing a new action potential, thus preventing the action potential from moving backward in a cell.

<h4>Myelin Sheath Increases Conductivity</h4>

Specialized glial cells&#8212;oligodendrocytes in the CNS and Schwann cells in the PNS&#8212;extend long processes that wrap around neuronal axons. This wrapping provides insulation, preventing leakage of the current as it travels along the axon. Additionally, electrical signals are propagated down myelinated axons by passive, positive current flow in the myelinated regions. Voltage-gated Na+and K+ channels are only found in the gaps between the myelin, at the nodes of Ranvier, triggering regeneration of the action potential at each node. In this way, the action potential appears to &#8220;jump&#8221; down the axon at the nodes&#8212;a process called saltatory conduction.

<h4>The Giant Nerves of the Squid</h4>

John Z. Young, a zoologist and neurophysiologist, discovered that the squid has nerve cells with axons much wider than mammalian neurons. These nerves control a rapid escape maneuver that is facilitated by the faster action potentials that are only possible in the larger axons. The larger diameter of the axons enabled the initial studies and descriptions of the ionic mechanisms involved in an action potential. This work was pioneered in the 1950s by Alan Hodgkin and Andrew Huxley while working on the giant nerve of the Atlantic squid. Together, they described the permeability of axonal membranes to sodium and potassium ions and were able to quantitatively reconstruct the action potential based on their electrode recordings.

Overview

Neurons communicate by firing action potentials&#8212;the electrochemical signal that is propagated along the axon. The signal results in the ...

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KEY TERMS AND CONCEPTS

how nerve signals travel in axons

Neurons send signals through action potentials, the rapid electrical changes that move along the axon. These signals lead to neurotransmitter release at axon terminals, which passes information through the nervous system. An action potential is an all-or-none change in membrane potential, the voltage across a cell membrane.

A neuron usually has a resting membrane potential of about -70 millivolts. When it receives signals from neurotransmitters or sensory input, the membrane can become more negative or more positive. If the membrane depolarizes to threshold, voltage-gated sodium (Na+) channels open.

Sodium ions are more concentrated outside the cell, so they rush in when these channels open. This inward flow makes the membrane even more positive and opens more Na+ channels. The voltage quickly rises to a peak of about +40 millivolts.

At the peak, the membrane starts to return toward resting potential. Na+ channels begin to inactivate, so less sodium enters the cell. At the same time, voltage-gated potassium (K+) channels open more slowly, and K+ moves out of the cell down its electrochemical gradient.

The loss of positive charge from K+ and the reduced Na+ entry quickly lower the membrane potential. For a short time after the spike, the membrane becomes more negative than the resting level. This is the refractory period, when the neuron cannot make a new action potential, so the signal does not travel backward.

Myelin sheath also helps action potentials move faster. Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system wrap around axons and act as insulation. This reduces current leakage as the signal travels.

In myelinated axons, voltage-gated Na+ and K+ channels are found only at the nodes of Ranvier, the gaps between myelin. The action potential is regenerated at each node, so it seems to jump along the axon. This is called saltatory conduction.

The giant nerves of the squid helped scientists study these signals. John Z. Young found that squid axons are much wider than mammalian axons, which supports very fast action potentials for the squid escape response. In the 1950s, Alan Hodgkin and Andrew Huxley used the giant nerve of the Atlantic squid to measure sodium and potassium permeability and to rebuild the action potential from electrode recordings.

How Nerve Signals Travel in Axons

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Neurons send signals through action potentials, the rapid electrical changes that move along the axon. These signals lead to neurotransmitter release at axon terminals, which passes information through the nervous system. An action potential is an all-or-none change in membrane potential, the voltage across a cell membrane. A neuron usually has a resting membrane potential of about -70 millivolts. When it receives signals from neurotransmitters or sensory input, the membrane can become more...

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key terms and concepts

Learn how action potentials travel along axons, trigger neurotransmitter release, and move faster with myelin, nodes of Ranvier, and saltatory conduction.

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How Nerve Signals Travel in Axons

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