Impulse Conduction&

The Synapse

Higher vertebrates have specialized cells called Schwaan cells, in the peripheral nervous system, and oligodendrocytes, in the central nervous system, which wrap concentric layers of fatty membranes, called myelin, tightly around the axon. The sections of myelin are discontinuous, so that periodically along the length of the axon there are regions not covered by myelin sheaths, called the Nodes of Ranvier. The tight wrapping of myelin prevents any ionic exchange or spread of electric current, therefore the action potential can only occur at the nodes.

The nerve impulse or action potential will “jump” from node to node greatly increasing the speed of nerve transmission. This node to node transmission, called saltatory conduction, can produce transmission speeds of up to 200 meters per second and explains the speed at which we can react to potentially harmful stimuli.

When the action potential is present at one node, the influx of Na+ ions causes the displacement of K+ ions down the axon (like charges repel). This diffusion of K+ down the axon makes the next node more positive and depolarizes it to threshold. Thus the action potential jumps from node to node. Saltatory conduction, from “saltare,” meaning “to jump,” literally means jumping conduction.

Saltatory conduction achieves a considerable increase in conduction velocity, of the action potential, for a small increase in diameter. It has enabled mammalian nerves to be smaller and yet achieve the same conduction velocity.

Saltatory conduction is also very energy efficient, as only a small part of the axon is involved in the exchange of ions, much fewer ions need to be pumped back after the action potential has passed.

Impulses are transmitted along chains of neurons, but there is no direct contact between neurons. The point at which two neurons meet (but don’t touch) is called a synapse.

A gap, or synaptic cleft, separates the afferent process (dendrite) of the receptor neuron from the efferent process (axon) of the transmitter neuron. Chemical messengers, called neurotransmitters, carry the signal across the synapse from axon to dendrite.

Remember, this is a specific, ONE-WAY, flow of messages along a neuron: impulses go from dendrite ---> cell body —> axon -—> dendrite —-> cell body ---> axon, etc.