Physiology 3120 Lecture 9: Lecture 9

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Myelin is an insulating material composed mainly of lipids (80%) and proteins (20%) Lecture 9 which wraps around and insulates the axon. Comes from schwann cells (pns) and oligodendrocytes (cns: both are glial cells of the nervous system. Myelin increases resistance across membrane by a factor of 5,000 no ion leakage out of axon. Electrical cord insulated with rubber = axon insulated with myelin. Nodes of ranvier gaps between myelin sheath: distance between each node is 1mm. Aps will only occur at the nodes of ranvier in myelinated nerves. Saltatory conduction does not involve membrane permeability changes all along the membrane, only at the nodes. Propagation of ap down myelinated nerve is similar to unmyelinated nerve. At the peak of ap, the inside of the membrane is positive (about +40 mv) with respect to the outside. This causes a region of negativity outside the membrane which draws of positive charges from the membrane ahead of ap.

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