Propagation and Transmission

Unlike the passive conduction processes of the den-drites, action potentials are active processes that are essentially regenerated locally as the response moves across the cell body and along the axon. This allows the action potential to propagate for long distances without significant degradation of response amplitude. In some neurons the axons are sheathed with an insulating material called myelin. The sheath is interrupted periodically at nodes where membrane proteins responsible for excitation are concentrated. Excitation in myelinated axons appears to hop from node to node at rates considerably faster than transmission in uninsulated axons.

Because a patch of membrane is insensitive or refractory for a period after firing and given the stereotyped spatial pattern of dendritic integration, action potentials normally propagate in one direction from the cell body to the far reaches of the axon. Electrical simulation can elicit backward—antidromic—transmission, and there is evidence that similar processes may operate in the recovery of dendrites. Electrophysiological transmission in some specialized neural tissue is mediated by direct conductive interconnections (gap junctions), but most interneuron transmission is mediated by electrochemical transmission at specialized sites known as synapses. At the presynaptic terminal, the arrival of an action potential causes ionic changes (including an influx of calcium) that lead to the release of a chemical neurotransmitter into the synaptic cleft. The neurotransmitter molecules diffuse across the synapse to bind to receptor proteins at the postsynaptic terminal. These receptors in turn activate ionic channels, which alter the local ionic composition and/or membrane potential within the postsynaptic dendrite, leading to another cycle of integration, excitation, and transmission.

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