When the action potential reaches the end of the axon terminals What causes the release of ?

Up to this point we have tried to explain how the membrane potential is maintained. This balance changes, however, when the cell is depolarized sufficiently to trigger an action potential. A transient depolarizing potential, such as an excitatory synaptic potential, causes some voltage- gated Na channels to open,and the resultant increase in membrane Na permeability allows Na influx to overpass the K efflux. Thus a net influx of positive charge flows through the membrane, and positive charges accumulate inside the cell, causing further depolarization.

The increase in depolarization causes more voltage gated Na channels to open, resulting in a greater influx of positive charge, which accelerates the depolarization still further.

This positive feedback cycle, develops exponentially driving the membrane potential toward the positive values. On the other hand, K efflux continues through the K channels, and a slight diffusion of Cl into the cell also counteracts the depolarizing tendency of the Na influx. Nevertheless so many voltage-gated Na channels open during the rising phase of the action potential that the permeability to Na is much greater than that of Cl and K. There are two processes that repolarize the membrane, terminating the action potential. First , as the depolarization continues, it slowly turns off, or inactivates, the voltage gated Na channels. This is so, because Na channels have tow types of gating mechanisms: activation, which rapidly opens that channel in response to depolarization, and inactivation which closes the channel if the depolarization is maintained.

The second repolarization process results from the delayed opening of voltage gated K channels. As K channels begin to open, K efflux increases. The delayed increase in K efflux combined with a decrease in Na influx to produce a net efflux of positive charge from the cell, which continues until the cell has repolarized to its resting value.

Note that this process results in a wave of depolarization/repolarization that can propagate all over the surface of the cell's axon (which is the cell structure actually devoted to the generation of the action potential). Once the depolarization/repolarization wave reaches the axon terminal it turns on the mechanisms of synaptic transmission which results in the necessary communication between nerve cells.

Thus the action potential is used by the nerve cell as a propagating signal. The most common site of initiation of the action potential is the axon hillock where the highest concentration of voltage gated ion channels is found.

The whole picture of electric signaling can be summarized as follows:

I.-The interaction of a neurotransmitter (or , alternatively, the activation of a sensory receptor system) with receptor molecules on the external membranes of the neuron (which occur mainly on the dendritic tree or the cell soma), initiates a synaptic potential. The synaptic potential spreads passively (there are no voltage gated ion channels involved in this process) to the axon hillock or initial segments of the axon.

II.- At the trigger zone, the synaptic potential may or may not (depending on whether the actual amplitude of the arriving potential exceeds the threshold of the voltage gated channels) initiate an action potential that propagates actively to the terminal of the neuron.

III.- Finally, at the terminal, the action potential causes transmitter release, which triggers a synaptic potential in the target cell.

Lights, Camera, Action Potential

This page describes how neurons work. I hope this explanation does not get too complicated, but it is important to understand how neurons do what they do. There are many details, but go slow and look at the figures.

Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to help the squid move. How giant is this axon? It can be up to 1 mm in diameter - easy to see with the naked eye.

Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable.

Resting Membrane Potential

A neuron is at rest when it is not sending an an electrical signal. During this time, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-) and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

Action Potential

The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired - this is the "ALL OR NONE" principle.

Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.

And there you have it...the Action Potential

Try it!

Do you like interactive word search puzzles? Try this Action Potential Puzzle Hear some action potentials in the Sounds of Neuroscience gallery. Read about the physical factors behind the action potential. Nerve Signaling - from NobelPrize.org BrainU Animations

Did you know?

The giant axon of the squid can be 100 to 1000 times larger than a mammalian axon. The giant axon innervates the squid's mantle muscle. These muscles are used to propel the squid through the water.

Copyright © 1996-2020, Eric H. Chudler All Rights Reserved.

When the action potential reaches the end of the axon terminals it causes the release of?

When the action potential reaches the end of the axon terminals it causes the release of quizlet?

What happens when the action potential reaches the end of the axon at the axon terminals How does one neuron communicate with another neuron and complete the circuit?

What happens when the action potential reaches the end of the axon terminal?