Medical Physiology - Distinctive Features of Signal Transmission in Nerve Trunks

Medical Physiology - Distinctive Features of Signal Transmission in Nerve Trunks
Large nerve fibers are myelinated, whereas small nerve fibers are unmyelinated. The axon constitutes the central core of the fiber, and its membrane facilitates the conduction of the action potential. A thick myelin sheath, formed by Schwann cells, encases the bigger axons. The sheath comprises several layers of cellular membrane that include the lipid sphingomyelin, an effective insulator. At the interface between two consecutive Schwann cells, a brief noninsulated segment measuring only 2 to 3 mm in length persists, allowing ions to move freely between the extracellular fluid and the axon. This region is the node of Ranvier. Saltatory conduction transpires in myelinated fibers. Although ions cannot traverse the thick sheaths of myelinated neurons considerably, they can pass with relative ease through the nodes of Ranvier.

Consequently, the nerve impulse traverses from node to node along the fiber, which is the basis for the term "saltatory." Saltatory conduction is advantageous for two reasons: • Enhanced speed. This method enhances the pace of nerve transmission in myelinated fibers by facilitating the depolarization process to leap across extensive intervals along the nerve fiber axis, achieving increases of 5 to 50 times. • Energy conservation. Secondly, saltatory conduction conserves energy for the axon as only the nodes undergo depolarization, resulting in a significantly reduced ion loss—potentially a hundredfold less—than would be required otherwise, thereby necessitating minimal energy to restore the sodium and potassium concentration gradients across the membrane following a sequence of nerve impulses. The conduction velocity is highest in large, myelinated nerve fibers.

The conduction velocity of action potentials in nerve fibers ranges from a minimum of 0.25 m/sec in diminutive unmyelinated fibers to a maximum of 100 m/sec in substantial myelinated fibers. The velocity increases roughly with fiber diameter in myelinated nerve fibers and about with the square root of fiber diameter in unmyelinated fibers.