Intro | Axon | Axon Hillock | Dendrites | Myelin Sheath | Nodes of Ranvier | Soma | Synapse | Terminal Buttons
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Part 3: Multiple-Choice Self-Test
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The Myelin Sheath of a neuron consists of fat-containing cells that insulate the axon from electrical activity. This insulation acts to increase the rate of transmission of signals. A gap exists between each myelin sheath cell along the axon. Since fat inhibits the prop agation of electricity, the signals jump from one gap to the next. Multiple sclerosis is characterized by patches of demyelination (destruction or loss of the myelin sheath) in the central nervous system. The symptoms that result from this demyelination are determined by the functions normally contributed by the affected neurons. Disruption of muscle control, speech and visual disturbances are common.
The myelin sheath (a tubular case or envelope) gives the whitish appearance to the white matter of the brain. Myelin cells are included in the category of Gail cells. Glial cells function to support the processes of neurons in a variety of ways. The glial cells forming myelin sheaths are called oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The gaps (approximately 1 micrometer wide) formed between myelin sheath cells long the axons are called Nodes of Ranvier. Since fat serves as a good insulator, the myelin sheaths speed the rate of transmission of an electrical impulse along the axon. The electrical impulse jumps from one node to the next at a rate as fast as 120 meters/second. This rapid rate of conduction is called saltatory conduction. Myelinated axons are rare in the autonomic nervous system. Invertebrates do not have myelinated sheaths.