Impulse conduction

1. Impulse Conduction
&
The Synapse

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

9. 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.