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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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
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
Farlex Partner Medical Dictionary © Farlex 2012
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.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
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).
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
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
Medical Dictionary, © 2009 Farlex and Partners
myelin sheathSee MYELIN.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
medullated nerve fibrea 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.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
The cover that surrounds many nerve cells and helps to increase the speed by which information travels along the nerve.
Mentioned in: Acoustic Neuroma
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.