How Neurons Work: Part 2 of 2

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An important feature of the action potential is its all-or-none property: Once the sodium channels open, the entire electrochemical sequence occurs. Furthermore, when the sodium channels dose and the potassium channels open, it is impossible to initiate a second action potential (at that point on the neuron) until the potassium channels close. This is cabled the absolute refractory period. It is possible, but more difficult, to trigger a second action potential during the period when both types of channels have closed and the ionic pump is restoring the resting potential. This is called the relative refractory period. These refractory periods limit the number of nerve impulses a neuron can transmit in a fixed period of time (i.e., pulses per second) since some minimum time must elapse between successive pulses.

Why do nerve impulses seem to travel along a neuron? If an action potential occurs at one point. it triggers off another action potential next to itself, which in turn triggers another one next to itself, and so on. The whole series moves like a wave along the neuron. A new action potential is triggered only in front of the traveling nerve impulse because the region just behind it is still in its absolute refractory period. This, then, is how information in the form of nerve impulses (waves of action potentials), is carried along a neuron.

While nerve impulses travel faster along thicker axons really rapid travel occurs only along axons that are partially enclosed and insulated by fatty cells called marlin. Action potentials on myelinated neurons only occur   at unenclosed junctions called nodes between these fatty cells, Nodes are found every 1 or 2 millimeters along the axon When an action potential   occurs at one junction (node) it quickly triggers one at the next junction,   instead of moving continuously along the axon as it does in the absence of the myelin.  This rapid jumping from node to node along a myelinated neuron is called saltatory conduction (saltation   means ”leaping”). Thus, depending on the thickness and myelinetion   of a neuron its speed of information transmission varies from about 1/5 to 120 meters per second.

Until now we have considered only how a nerve impulse once trip gored. moves along a neuron. We shall now consider how this triggering normally occurs and how the information traveling along one neuron is carried over to another.

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