PROPAGATION OF ACTION POTENTIAL
The trigger zone, also known as the axon hillock, is a specialized region of the neuron where the action potential is generated. Once this membrane potential reaches the threshold level (-55 to -50 millivolts), the voltage-gated sodium channels open, initiating the depolarization phase.
As sodium ions enter the cell, the depolarization spreads along the axon due to local circuit currents. This influx of positive charge depolarizes the adjacent regions of the axon membrane, triggering the opening of next voltage-gated sodium channel. This process continues down the length of the axon with the uniform strength and speed all along. After an action potential propagates, the region of the axon becomes temporarily refractory to further stimulation. This prevents the action potential from propagating backward and ensures that the signal moves in one direction along the axon.8.13.1 Conduction Speed of Action Potential
The conduction speed of an action potential is influenced by axon diameter, myelination, and temperature. The faster conduction of action potential in larger-diameter axons is attributed to the less internal resistance to the flow of ions as the action potential travels along a wider cross-sectional area. In myelinated axons it is attributed to saltatory conduction wherein the action potentials “leap” between the gaps called nodes of Ranvier, where the axon membrane is exposed. This saltatory conduction increases the speed of propagation by allowing the action potential to “skip” along the axon, rather than propagating continuously along its entire length. Higher temperatures in optimum range generally increase the conduction speed of action potentials as the elevated temperatures increase the rate of diffusion and enzymatic reactions involved in the propagation of the action potential.
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