Transmission of nerve impulse along the axon
Transmission of a nerve impulse along an axon
A nerve cell is like a receiver, transmitter and transmission line with the task of passing a signal along from its dendrites to the axon terminal bundle.
The stimulus triggers an action potential in the cell membrane of the nerve cell, and that action potential provides the stimulus for a neighboring segment of the cell membrane. When the propagating action potential reaches the axon, it proceeds down that "transmission line" by successive excitation of segments of the axon membrane.
Just the successive stimulation of action potentials would result in slow signal transmission down the axon. The propagation speed is considerably increased by the action of the myelin sheath.
The myelin sheath around the axon prevents the gates on that part of the axon from opening and exchanging their ions with the outside environment. There are gaps between the myelin sheath cells known as the Nodes of Ranvier. At those uncovered areas of the axon membrane, the ion exchange necessary for the production of an action potential can take place. The action potential at one node is sufficient to excite a response at the next node, so the nerve signal can propagate faster by these discrete jumps than by the continuous propagation of depolarization/repolarization along the membrane. This enhanced signal transmission is called saltatory conduction (from the Latin saltare, to jump or hop).
Tuzynski and Dixon offer some quantification of the sizes involved in these nerve cells. The axon is made up of connected segments of length about 2 mm and diameter typically 20 mm. This diameter compares to about 100 mm for the diameter of a human hair. Axon diameters may vary from 0.1 mm to 20 mm and may be up to a meter long. The much-studied squid has a giant axon of about a millimeter in diameter. The myelin sheaths are about 1mm in length. The action potential travels along the axon at speeds from 1 to 100 m/s.
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