medullary sheath

n. See myelin sheath.

medullary sheath

n 1. (Anatomy) anatomy a myelin layer surrounding and insulating certain nerve fibres 2. (Botany) a layer of thick-walled cells surrounding the pith of the stems of some higher plants

med′ullary sheath′

n. 1. a narrow zone made up of the innermost layer of woody tissue immediately surrounding the pith in plants. 2. myelin sheath. [1840–50]

Thesaurus

Noun

medullary sheath - a layer of myelin encasing (and insulating) the axons of medullated nerve fibersmyelin sheathsheath, case - an enveloping structure or covering enclosing an animal or plant organ or partaxolemma - outer membrane covering an axonmedullated nerve fiber, myelinated nerve fiber - a nerve fiber encased in a sheath of myelinnodes of Ranvier, Ranvier's nodes - small gaps in the myelin sheath of medullated axonsoligodendria, oligodendroglia - tissue consisting of glial cells with sheetlike processes that form the myelin sheath of nerve fibersmedulla, myelin, myeline - a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers

medullary sheath

sheath

[shēth] a tubular case or envelope.arachnoid sheath the delicate membrane between the pial sheath and the dural sheath of the optic nerve.carotid sheath a portion of the cervical fascia enclosing the carotid artery, internal jugular vein, vagus nerve, and sympathetic nerves supplying the head.connective tissue sheath of Key and Retzius endoneurium.crural sheath femoral sheath.dural sheath the external investment of the optic nerve.femoral sheath the fascial sheath of the femoral vessels.Henle's sheath endoneurium.lamellar sheath the perineurium.medullary sheath myelin sheath.myelin sheath (nerve sheath) the sheath surrounding the axon of myelinated nerve cells, consisting of concentric layers of myelin formed in the peripheral nervous system by the plasma membrane of Schwann cells, and in the central nervous system by the plasma membrane of oligodendrocytes. It is interrupted at intervals along the length of the axon by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerve impulses (see conduction" >saltatory conduction).pial sheath the innermost of the three sheaths of the optic nerve.root sheath the epidermic layer of a hair follicle.sheath of Schwann neurilemma.synovial sheath synovial membrane lining the cavity of a bone through which a tendon moves.tendon sheath a lubricated fibrous or synovial layer of tissue in which the tendon is housed and through which it moves.

my·e·lin sheath

the lipoproteinaceous envelope in vertebrates surrounding most axons larger than 0.5-mcm diameter; it consists of a double plasma membrane wound tightly around the axon in a variable number of turns and supplied by oligodendroglia cells (in the brain and spinal cord) or Schwann cells (in peripheral nerves); unwound, the double membrane would appear as a sheetlike cell expansion devoid of cytoplasm but for a few narrow cytoplasmic strands corresponding to apparent interruptions of the regular myelin structure, the incisures of Schmidt-Lanterman. The myelin sheath of each axon is composed of a fairly regular longitudinal sequence of segments, each corresponding to the length of sheath supplied by a single oligodendroglia or Schwann cell; in the short interval between each two neighboring segments, the nodes of Ranvier, the axon is unmyelinated even though it is enclosed by complex, fingerlike, membranous expansions of the neighboring oligodendroglia or Schwann cells. Synonym(s): medullary sheath