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Section 4: Neuron Action Potential
When the cell membrane of neurons or muscle fibers is depolarized enough, they propagate a nondecremental wave of electrical polarization called an action potential. Neurons use these action potentials to quickly transmit information across distance. Muscle fiber action potentials result in a mechanical contraction of the fiber, resulting in the production of force and motion.
Lots of cells have a membrane potential. What makes nerve and muscle cells unique is that they are electrically active: their membrane potential varies, and the cells react in characteristic ways to changes in the membrane potential.
Very small changes in voltage don't produce much in the way of interesting behavior from the neuron. But when a neuron membrane is depolarized beyond its threshold potential, a chain reaction occurs in the cell membrane. A wave of strongly depolarizing current starts at the point where the cell was depolarized, and runs down the neuron's axon, setting off other events.
What makes these changes possible are voltage-gated ion channels: special membrane channel proteins that are voltage-sensitive. There are many. for the purposes of this course, the most important one to understand is the voltage-gated sodium channel.
The voltage-gated sodium channel is a protein channel that lies in the cell membrane. At the resting potential, it is closed, and no ions pass through it. When the membrane is depolarized, it undergoes a conformational change, and allows sodium ions to pass through it. This change is short-lived, however, and soon the protein undergoes a second conformational change, and becomes inactivated. Before it will open again, the membrane must first return to the resting potential, and the channel must return to the closed state.
The axon membrane is chock full of voltage-gated sodium channels. When something depolarizes the axon membrane, some voltage-gated sodium channels open up, and sodium ions begin to flow through. Specifically, they flow in to the cell. This happens because the concentration of sodium is much higher outside the cell than inside, and the sodium ions flow down this concentration gradient. Another reason this happens is that the positive ions flow toward the negative interior of the cell along the voltage gradient.
Whenever there is a net flow of charged particles, there is an electrical current. And this electrical current results in a voltage change: the cell membrane is further depolarized. If the membrane is depolarized enough (above the threshold potential), other nearby voltage-gated potassium channels will open, and increase the influx of sodium ions, further depolarizing the membrane. The result is a chain reaction, causing a wave of strongly depolarizing current to flow down the entire length of the axon membrane. This wave of depolarization is called the action potential, and it is crucial to the functioning of nerve and muscle cells. It allows neurons to carry information long distances without degradation. In a muscle cell, it is what triggers the events that cause the cell to contract.
There are other important voltage-gated ion channels. For example, the voltage-gated potassium channel also plays a role in the action potential. For the curious, I refer you to Principles of Neural Science by Kandel et al. (ISBN: 0071390111).
Neurons can use electrical potentials to send signals from one part of the neuron to another, but to communicate with other neurons (or muscle cells or other target cells), they usually use chemical signals. These chemical signals are transmitted across specialized connections between cells called synapses.
Terminology:
By the end of this section, make certain that you understand what each of these terms mean, and can apply them appropriately.
- Action Potential
- All-or-None
- Nondecremental
- Voltage-gated Na+ channel
- Nodes of Ranvier
- Absolute Refractory Period
- Relative Refractory Period
- Myelin
- Saltatory conduction
Section 1: Cells are Batteries
Section 2: Why Are Cells Batteries?
Section 4: The Neuron Action Potential
Section 5: Muscle Fiber Action Potential
Section 6: Synapses and the Neuromuscular Junction (NMJ)
If you have any questions regarding this section, please ask them in the Neuroanatomy User Forum, or in the comments section at the bottom of this page.
