myelin sheath

the insulating layer around many axons that increases the speed of conduction of nerve impulses. It consists of myelin and is laid down by glia, which wrap themselves around adjacent axons. The myelin sheath is interrupted by small gaps, called nodes of Ranvier, which are spaced about every millimeter along the axon. Also called medullary sheath.

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Psychology term of the day

January 19th 2025

spinal cord

the part of the central nervous system that extends from the lower end of the medulla oblongata, at the base of the brain, through a canal in the center of the spine as far as the lumbar region. In transverse section, the cord consists of an H-shaped core of gray matter (see periaqueductal gray; anterior horn; dorsal horn) surrounded by white matter consisting of tracts of long ascending and descending nerve fibers on either side of the cord that are linked by the white commissure. The spinal cord is enveloped by the meninges and is the origin of the 31 pairs of spinal nerves. See also spinal root.