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This positive spike constitutes the action potential: the electrical signal that typically moves from the cell body dlown the axon to the axon terminals. The electrical signal moves down the axon like a wave; at each 84 Ch apter 3 Biopsycholo gy point, some of the sodium ions that enter the cell diffuse to the neJ1.'t section of the axon, raising the charge past the threshold of excitation and triggering a new influx of sodium ions.
The axons are analog to digital converters and transmitters. The axon initial segment produces a digital action potential (spike) when the summation of the voltages within the neuron are above a threshold. The action potential is a one-tenth of a volt electrical signal that travels down an axon at speeds up to about 100 meters per second. Myelin sheaths cover many axons; this increases the speed a signal can travel down an axon. The myelinated axons make up the white matter of the brain.