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DIFFERENCES IN PROPAGATION OFACTION POTENTIAL IN MYELINATEDAND UNMYELINATED AXONS
The action potential in an unmyelinated neuron trav-els slowly along the axon because every region of the axon has sodium and potassium channels. In a myelinated cell, the myelin sheath serves as insulators, preventing movement of ions through the membrane. Ions move only through the numerous channels lo-cated in the nodes and the action potential is propa- gated from one node of Ranvier to another, literally jumping from node to node across the myelin. Hence, propagation is rapid. This is known as saltatory con-duction. It should be noted that jumping is only ametaphor. Actually, the action potential in one node depolarizes the membrane at the next node to thresh-old and a new action potential is produced there. Ac-tion potential is also faster in thicker axons. The rate of conduction ranges from 1.0 m/sec in thin, un-myelinated fibers to 100 m/sec (225 miles per hour) in thick, myelinated fibers.
When the action potential reaches a synapse, it causes the release of neurotransmitters into the synaptic cleft. The neurotransmitters, in turn, pro-duce electrical changes in the postsynaptic neuron.
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