The Nervous System
Lesson 9, Page 3 of 19

The synapse refers to the gap between nerve cells, through which the nerve cells communicate with each other.

The Myelin Sheath

In humans and other vertebrates specialized cells called Schwann cells wrap around the axons of some neurons forming a lipid and cholesterol-rich insulating layer called the myelin sheath. This sheath electrically insulates the axon, kind of like the plastic coating around a copper electrical wire. Neurons with myelin sheaths are said to be myelinated. They make up myelinated nerves which make up the majority of nerves in the peripheral nervous system (the nerves outside the brain and spinal cord.)


Schwann cells cover most of the axon but leave bare sections at regular intervals called nodes of Ranvier. Nerve impulses which travel along a myelinated axon jump between these gaps. This process allows myelinated neurons to conduct impulses at twice the speed of unmyelinated neurons.

Without the insulating effect of myelin, nerve impulses passing along one axon can generate impulses in nearby neurons, some of which stimulate muscle contraction. This can cause shivering and jerky movements of the muscles. People whose myelin sheaths are damaged, as a result of multiple sclerosis for example, have difficulties in controlling their muscles.

Nerve cells that have a sheath of Schwann cells around their axons, for insulation and faster conduction, are known as ___________ nerves.


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