Neuroscience For Kids
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 move the squid's tail. 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
When a neuron is not
sending a signal, it is "at rest." When a neuron is at rest, 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 by an exchange of ions across 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! |
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| 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 © 1996-2010, Eric H. Chudler All Rights Reserved.
