Name: how nerve signals travel down axons
Uploaded: 2025-08-28T15:01:35.000Z
Duration: 1 min 14 s
Channel: JoVE

An action potential is the rapid change in a neuron&#8217;s resting membrane potential that propagates electrical signals in the nervous system.The typical resting potential of a neuron is about &#8211;70 millivolts, which can be altered by signals, such as neurotransmitters.When the membrane potential becomes less negative, it depolarizes beyond the threshold potential value, triggering an action potential.As a result, voltage-gated sodium channels open and sodium ions rush into the cell down their electrochemical gradient.The positive ion influx raises the membrane potential to about +30 millivolts, marking the peak of the action potential.The sodium channels then inactivate, and voltage-gated potassium channels open.Potassium ions move down their concentration gradient. The membrane becomes less positive and repolarizes.Briefly, the membrane potential dips below the resting potential, decreasing the chance of another action potential until the resting state is regained.

action potential

Action Potential

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 to the nervous system. An action potential is a specific "all-or-none" change in membrane potential that results in a rapid spike in voltage.
Membrane potential in neurons
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 the 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.
Myelin sheath increases conductivity
Specialized glial cells&#8212;oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (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 "jump" down the axon at the nodes&#8212;a process called saltatory conduction.
The giant nerves of the squid
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.

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

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Cell Biology

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Channels and the Electrical Properties of Membranes

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KEY TERMS AND CONCEPTS

how nerve signals travel down axons

Nerve signals travel down axons as action potentials, which are electrochemical pulses that let neurons communicate. This signal moves to the axon terminals and triggers neurotransmitter release, passing information through the nervous system. An action potential is an all-or-none change in membrane potential that produces a fast spike in voltage.

Neurons usually rest at about -70 millivolts. When they receive input from neurotransmitters or sensory stimuli, the membrane potential can become more negative or more positive. If depolarization reaches a threshold, voltage-gated sodium channels open. Sodium ions are more concentrated outside the cell, so they rush in and make the membrane depolarize even more.

As positive charge enters, more sodium channels open and the voltage rises quickly to about +40 millivolts. At the peak, sodium channels begin to inactivate, so sodium influx slows. The initial depolarization also opens voltage-gated potassium channels, but these channels open more slowly. Once they open, potassium ions move out of the cell, and the membrane potential drops back down.

For a short time after the spike, the membrane becomes more negative than the resting level. This is the refractory period. During this time, the neuron cannot start a new action potential. That delay helps keep the signal from moving backward along the cell.

Myelin sheath makes signal travel faster along many axons. Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system wrap their long processes around axons. This wrapping insulates the axon and reduces current loss. Voltage-gated sodium and potassium channels are concentrated at the nodes of Ranvier, the gaps between myelin. The action potential is regenerated at each node, so the signal appears to jump along the axon. This is called saltatory conduction.

The giant nerves of the squid helped scientists study these signals. Squid nerve cells have axons much wider than mammalian neurons, and those large axons support a rapid escape response. Their size also made it possible to measure the ionic basis of the action potential. In the 1950s, Alan Hodgkin and Andrew Huxley used the Atlantic squid giant nerve to show how sodium and potassium permeability shape the action potential. Their electrode recordings let them quantitatively reconstruct the signal.

How Nerve Signals Travel Down Axons

aquaporins

Aquaporins and Membrane Transport

Aquaporins are membrane proteins that move water across cell membranes. Some aquaporins, called aquaglyceroporins, also transport glycerol. A few aquaporins may also help move volatile substances such as carbon dioxide and ammonia. These channels are found in many different tissues. Water-selective aquaporins are important in the kidneys, exocrine glands, and central nervous system. Aquaporins that may act as gas channels are often highly expressed in red blood cells, epithelial cells, and...

non-gated ion channels

Leak Channels in Resting Membranes

Leak channels are ion channels that let charged ions move across the plasma membrane without a gate. These proteins help maintain membrane potential, which is essential for cell survival. They can move more than a thousand ions in milliseconds during a cellular event. Unlike gated ion channels, non-gated channels have no opening trigger. They are also called leakage or passive channels. Instead, they switch between open and closed states at an intrinsic rate, so ions keep “leaking” through...

ligand-gated ion channels

How Ligand-Gated Ion Channels Work

Ligand-gated ion channels are membrane proteins that open when a chemical signal binds to them. They have a channel for ions to pass through and a ligand-binding site. When the ligand attaches, the channel opens and ions can move across the cell membrane. These channels are grouped into three subfamilies. The Cys-loop group includes nicotinic acetylcholine receptors, gamma-aminobutyric acid (GABA) receptors, glycine receptors, and 5-hydroxytryptamine receptors. The pore-loop group includes...

voltage-gated ion channels

How Voltage Gated Channels Control Signals

Voltage-gated ion channels control electrical signals in neurons, heart cells, and muscle cells. These transmembrane proteins open and close when the membrane potential changes. They sit in the cell membrane of electrically excitable cells and help ions move across it. Each channel includes a voltage-sensing domain that spans the lipid bilayer. Charged residues in this sensor shift when the membrane potential changes. That movement opens the channel and allows ions to pass through. Different...

mechanically-gated ion channels

Touch and Heat Sensors in Cell Membranes

Mechanically-gated ion channels are membrane proteins that open when cells experience physical stress. Tension, compression, swelling, and shear stress can change the shape of the protein. That change opens a transmembrane pore and lets ions pass to help transmit signals. In eukaryotes, these channels are found in several body regions, including neurons, lungs, skin, bladder, and heart. They support many normal physiological processes. They are also involved in disease when their function...

neuron structure

Neuron Signaling and Synapses

Neurons are the main cells of the nervous system that generate and pass along electrochemical signals. They communicate with one another mainly through neurotransmitters at special connections called synapses. Neurons can have different shapes, and those shapes often match their function. Most, however, share three main parts: a cell body, dendrites, and an axon. The cell body, or soma, contains the nucleus and the organelles needed for cell activity. Dendrites extend from the soma and usually...

resting membrane potential

How Neurons Keep a Negative Voltage

Neurons keep a negative voltage across their membrane when they are at rest. This membrane potential is the voltage difference between the inside and outside of a cell. In a resting neuron, it is usually around -70 millivolts, which means the inside is more negative than the outside and the cell is polarized. This negative charge depends on two main factors. The membrane is selectively permeable, so most ions and molecules cannot cross the lipid bilayer without help from ion channel proteins.

resting potential decay

Resting Membrane Potential Maintenance

The resting membrane potential stays near -70 mV in a neuron because the membrane allows ions to move in a selective way. At rest, the membrane is slightly permeable to sodium ions (Na+) and chloride ions (Cl-) and highly permeable to potassium ions (K+). Ion channels and pumps keep the concentration differences between the inside and outside of the cell in place. Potassium leak channels let K+ move out of the cell at rest. This outward movement makes the inside of the cell more negative. A...

Nerve signals travel down axons as action potentials, which are electrochemical pulses that let neurons communicate. This signal moves to the axon terminals and triggers neurotransmitter release, passing information through the nervous system. An action potential is an all-or-none change in membrane potential that produces a fast spike in voltage. Neurons usually rest at about -70 millivolts. When they receive input from neurotransmitters or sensory stimuli, the membrane potential can become...

channel rhodopsins

Light-Gated Ion Flow in Channel Rhodopsins

Channel rhodopsins are light-sensitive ion channels found in some organisms. They belong to the rhodopsin family, which also includes photoreceptors in bacteria, plants, and the vertebrate retina. These proteins use bound chromophores, such as retinal in rhodopsins and bilin in phytochromes, to respond to light. Rhodopsins are a group of cell surface proteins called G-protein coupled receptors. These proteins help move signals across biological membranes. Over evolution, the rhodopsin family...

patch clamp

Recording Ion Channel Currents with Patch Clamp

Patch clamp is a lab method used to record electrical currents across a cell membrane. It helps scientists study how ions move through channels in the membrane. These electrical signals are important for functions such as muscle contraction and nerve transmission. The method uses a glass micropipette filled with electrolyte solution. The pipette is sealed tightly to a small part of the cell membrane. This creates an electrically isolated patch, so ions moving through the channels flow into the...

electrical synapses

Gap Junctions in Electrical Signaling

Electrical synapses use gap junctions to pass signals directly between neurons and other cells. The presynaptic and postsynaptic membranes sit very close together, about 3.5 nm apart. Channel proteins connect the two cells and create a direct path for current. Unlike a chemical synapse, an electrical synapse does not rely on neurotransmitters crossing a synaptic cleft. The gap junction is built from two hexameric connexin hemichannels, also called connexons. Each adjacent cell contributes one...

chemical synapses

How Neurons Signal at Synapses

Chemical synapses let neurons pass signals to another neuron, a muscle cell, a gland cell, or a sensory cell. They use neurotransmitter molecules released from synaptic vesicles to carry the message. Because of that process, there is a short delay of about one millisecond between the arrival of the action potential and the opening of ion channels on the postsynaptic cell. Chemical synapses also send information in one direction only. A synaptic cleft separates the presynaptic and postsynaptic...

excitatory and inhibitory effects of neurotransmitters

excitatory-and-inhibitory-effects-of-neurotransmitters

How Neurotransmitters Trigger or Block Signals

Neurotransmitters can either trigger or block signals at a chemical synapse. When an action potential reaches the presynaptic axon terminal, it causes the neuron to release neurotransmitters into the synaptic cleft. The released molecule then binds to specific receptors on the postsynaptic cell membrane. A molecule is usually identified as a neurotransmitter when it is present in the presynaptic neuron, released in response to strong presynaptic depolarization, and matched with receptors on...

muscle contraction

How Muscles Receive Signals to Contract

Muscle contraction begins when a nerve signal reaches a skeletal muscle fiber. At the motor endplate, a motor neuron releases acetylcholine, a neurotransmitter that carries the signal to the muscle cell. Acetylcholine binds to receptors on the sarcolemma, the muscle cell membrane, and lets sodium ions enter the cell. This starts an action potential in the muscle fiber and helps pass electrical signals from the brain to the muscle. The signal must also be controlled so the muscle does not stay...

the role of ion channels in neuronal computation

the-role-of-ion-channels-in-neuronal-computation

How Ion Channels Shape Neuron Firing

Ion channels help neurons decide when to fire an action potential. A postsynaptic neuron often receives signals from many presynaptic neurons, and the axon hillock adds those inputs together to judge whether firing is likely. In some cases, one strong EPSP, or excitatory postsynaptic potential, is enough to trigger a spike. In many cases, several EPSPs must arrive close together for the cell to reach threshold. This signal adding process is called summation. It can be temporal when one...

long-term potentiation

How Synapses Strengthen During Learning

Long-term potentiation, or LTP, is a way the brain strengthens synapses over time. A synapse is the chemical connection between neurons. LTP is one form of synaptic plasticity, which means changes in synapse strength. It works opposite to long-term depression, or LTD, which weakens synapses. Together, LTP and LTD are major mechanisms behind learning and memory. LTP can happen when a presynaptic neuron repeatedly fires and stimulates a postsynaptic neuron. This is called Hebbian LTP. It follows...

long-term depression

Synaptic Weakening in Memory Circuits

Long-term depression, or LTD, is a form of synaptic weakening in the brain. It is one way synaptic plasticity, or changes in the strength of chemical synapses, can occur over time between presynaptic and postsynaptic neurons. LTD works opposite long-term potentiation, or LTP, which strengthens synapses. Together, LTD and LTP help support learning and memory. LTD also helps keep the brain flexible. If synapses stayed maximally strengthened for too long, the brain would become less efficient and...

key terms and concepts

Learn how action potentials move along axons, open sodium and potassium channels, and jump faster through myelin and nodes of Ranvier.

How Nerve Signals Travel Down Axons | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

How Nerve Signals Travel Down Axons

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