myelin sheath


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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 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

myelin sheath

n.
The insulating envelope of myelin that surrounds the core of a nerve fiber or axon and facilitates the transmission of nerve impulses. In the peripheral nervous system, the sheath is formed from the membranes of Schwann cells, and in the central nervous system it is formed from oligodendrocytes. Also called medullary sheath.

myelin sheath

a segmented fatty lamination composed of myelin that wraps the axons of many nerves in the body. The usual thickness of the myelin sheath is between 200 and 800 μm. Various diseases such as multiple sclerosis can destroy myelin wrappings.
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Myelin sheath

my·e·lin sheath

(mī'ĕ-lin shēth)
The lipoproteinaceous envelope in vertebrates surrounding most axons of more 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).
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MYELIN SHEATH

myelin sheath

Layers of the cell membrane of Schwann cells (peripheral nervous system) or oligodendrocytes (central nervous system) that wrap nerve fibers, providing electrical insulation and increasing the velocity of impulse transmission.
Synonym: Schwann sheath See: nerve fiber; neuron; illustration
See also: sheath

myelin sheath

See MYELIN.
Myelin sheathclick for a larger image
Fig. 226 Myelin sheath . Transverse section of a myelinated nerve fibre.

myelin sheath

or

medullated nerve fibre

a sheath of fatty substance associated with protein which surrounds larger nerve fibres of vertebrates. see DENDRITE. The white lipid coating is produced by SCHWANN CELLS. The sheath permits a greater current flow and thus speeds the transmission of nervous impulses. Constrictions in the myelin sheath, known as nodes ofRanvier, indicate the division between one Schwann cell and another. Nonmyelinated fibres occur commonly in invertebrates, internally in the spinal cord of vertebrates, and in the AUTOMATIC NERVOUS SYSTEM of vertebrates. Myelinated fibres, white in appearance, occur outside the spinal cord of vertebrates.

Myelin sheath

The cover that surrounds many nerve cells and helps to increase the speed by which information travels along the nerve.
Mentioned in: Acoustic Neuroma

myelin sheath

lipoproteinous envelop (formed by central nervous system oligodendroglia cells, and peripheral nervous system Schwann cells) surrounding nerve axons of >0.5μm diameter; presence of the myelin increases nerve conduction rates (it facilitates saltatory conduction); non-myelinated nerve fibres (e.g. autonomic and C fibres) conduct impulses at ~ 1m/s, large myelinated nerve fibres (e.g. Aβ [mechano-] and Aα [motor] fibres) conduct impulses at ~100m/s
References in periodicals archive ?
This enhanced lipid production and partially reversed myelin sheath formation defects in Lbk1 mutant Schwann cells.
In a recent study, we identified activated complement (C3d and C9neo) on fragmenting myelin sheaths within 58/58 active lesions derived from 20 patients with relapsing MS; (25) and in the outer actively demyelinating bands of Balo-type lesions.
Cells of the immune system misidentify the myelin sheath surrounding the peripheral motor neurons and destroy it.
MS develops as a result of damage to the myelin sheath of the nerves which interferes with their normal function and leads to loss of muscle control.
Hippocampus Brain blood supply Myelin sheath on neurons Brain stem Cerebral cortex
In some instances, attempted remyelination was suggested by the formation of a secondary thin myelin sheath around an axon that had abnormal myelin (figure 2, D).
In MS, the protective myelin sheath that surrounds nerve fibers is damaged, causing symptoms such as numbness, coordination problems, and muscle weakness and stiffness.
Multiple sclerosis is a chronic degenerative disease of the central nervous system in which patches of the myelin sheath around nerve fibres are lost.
The immune system begins attacking the myelin sheath which protects the nerves, thereby interrupting the impulses from the brain to another area of the body.
They have clinical data showing that taking an antiviral drug 2 times a day virtually eliminates the destruction of the myelin sheath.
It is characterized by the destruction of the myelin sheath that insulates the nerve fiber (Muller-Rohland, 1987).
The attacks lead to inflammation and injury to the myelin sheath, which can result in multiple areas of scarring (sclerosis).