PHYSIOLOGY OF NERVE AND MUSCLE CELLS lecture 4

O B J E C T I V E S :
AT THE END OF THE LECTURE YOU SHOULD BE ABLE TO
•Explain the properties of mixed nerves regarding appearance of recorded potential.
•Classify neurons on the basis of their structure and function.
•Illustrate some nerve function abnormalities.
Saltatory Conduction
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Conduction in myelinated axons depends upon a similar pattern of circular current flow. However, myelin is an effective insulator, and current flow through it is negligible. Instead, depolarization in myelinated axons jumps from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize to the firing level the node ahead of the action potential. This jumping of depolarization from node to node is called saltatory conduction. It is a rapid process, and myelinated axons conduct up to 50 times faster than the fastest unmyelinated fibers.
Orthodromic & Antidromic Conduction
• An axon can conduct in either direction. When an action potential is initiated in the middle of it, two impulses traveling in opposite directions are set up by electrotonic depolarization on either side of the initial current sink. In a living animal, impulses normally pass in one direction only, ie, from synaptic junctions or receptors along axons to their termination. Such conduction is called orthodromic. Conduction in the opposite direction is called antidromic. Since synapses, unlike axons, permit conduction in one direction only, any antidromic impulses that are set up fail to pass the first synapse they encounter and die out at that point.