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Lights, Camera, Action Potential

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

Much of what weknow about how neurons work comes from experiments on the giant axon ofthe squid. This giant axon extends from the head to the tail of the squidand is used to move the squid's tail. How giant is this axon? It can beup to 1 mm in diameter - easy to see with the naked eye.

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

Resting Membrane Potential

When a neuron is notsending a signal, it is 'at rest.' When a neuron is at rest, the insideof the neuron is negative relative to the outside. Although theconcentrations of the different ions attempt to balance out on both sidesof the membrane, they cannot because the cell membrane allows only someions 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 moredifficult time crossing. The negatively charged protein molecules(A^-) inside the neuron cannot cross the membrane. In addition to these selective ionchannels, there is a pump that uses energy to move threesodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in thevoltage between the inside and outside of the neuron is measured, you havethe resting potential. The resting membranepotential of a neuron is about -70 mV (mV=millivolt) - this means that theinside of the neuron is 70 mV less than the outside. At rest, there arerelatively more sodium ions outside the neuron and more potassium ionsinside that neuron.

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Action Potential

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Theresting 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 ofelectrical activity that is created by a depolarizingcurrent. This means that some event (a stimulus) causes theresting potential to move toward 0 mV. When the depolarization reachesabout -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this criticalthreshold level, then no action potential will fire. Also, when thethreshold level is reached, an action potential of a fixed sized willalways fire..for any given neuron, the size of the action potential isalways the same. There are no big or small action potentials in one nervecell - all action potentials are the same size. Therefore, the neuroneither does not reach the threshold or a full action potential isfired - 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 theSounds of Neuroscience gallery.
Read about the physical factors behind the action potential.
Nerve Signaling - from NobelPrize.org

Did you know?

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

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