nerve sheath

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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.

nerve sheath

Etymology: L, nervus, nerve; AS, scaeth
any of several types of coatings or coverings for nerve fibers and nerve tracts. Kinds of nerve sheaths include endoneurial, medullary, myelin, and neurilemma.


1. A covering structure of connective tissue, usually of an elongated part, such as the membrane covering a muscle.
2. An instrument introduced into a vessel during angiographic procedures when multiple catheter changes are anticipated. It facilitates ease of change and decreases morbidity at the puncture site.

Patient care

The sheath introduced into the femoral artery, the preferred vascular access route for percutaneous coronary intervention, is a 4 to 6 French (1.35 to 2 mm) in size. The sheath remains in place after completion of the procedure and removal of the catheter until anticoagulation is reversed or anticoagulants are below peak action. The sheath is connected to high-pressure tubing and a flushing system; manual or automatic flushing keeps the line patent. A stopcock connected to the system permits drawing of blood samples.

axon sheath

A myelin sheath or a neurilemma.
See: myelin sheath

carotid sheath

The portion of cervical or pretracheal fascia enclosing the carotid artery, interior jugular vein, and vagus nerve.

crural sheath

The fascial covering of femoral vessels.

dural sheath

A fibrous membrane or external investment of the optic nerve.

femoral sheath

The fascia covering the femoral vessels.

sheath of Henle

See: Henle sheath

sheath of Hertwig

See: Hertwig's root sheath

sheath of Key and Retzius


lamellar sheath

A connective tissue sheath covering a bundle of nerve fibers. Synonym: nerve sheath; perineurium

medullary sheath

An obsolete term for myelin sheath.
Enlarge picture

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

nerve sheath

Lamellar sheath.

periarterial lymphoid sheath

The tissue composed of T lymphocytes that surrounds each arteriole in the spleen. The sheaths are attached to lymphoid follicles containing B cells and make up much of the white pulp. See: spleen

pial sheath

An extension of the pia that closely invests the surface of the optic nerve.

sheath of Schweigger-Seidel

The thickened wall of a sheathed artery of the spleen.

rectus sheath

A strong fibrous sleeve in which the rectus abdominis and pyramidalis muscles contract. The sheath is formed from the aponeuroses of the abdominal wall muscles as they meet in the linea alba at the abdominal midline.

root sheath

1. One of the layers of a hair follicle derived from the epidermis. It includes the outer root sheath, which is a continuation of the stratum germinativum, and the inner root sheath, which consists of three layers of cells that closely invest the root of the hair. See: hair
2. The epithelial covering that induces root formation in teeth. Also called Hertwig's root sheath.

synovial sheath

Synovial tendon sheath.

tendon sheath

A dense fibrous sheath that confines a tendon to an osseous groove, converting it into an osteofibrous canal. It is found principally in the wrist and ankle.
See: synovial sheath


a macroscopic cordlike structure of the body, comprising a collection of nerve fibers that convey impulses between a part of the central nervous system and some other body region. For a complete list of the named nerves of the body, see Table 14.
Depending on their function, nerves are known as sensory, motor or mixed. Sensory nerves, or afferent nerves, carry information from the periphery of the body to the brain and spinal cord. Sensations of heat, cold, pressure and pain are conveyed by the sensory nerves. Motor nerves, or efferent nerves, transmit impulses from the brain and spinal cord to the periphery, especially the muscles. Mixed nerves are composed of both motor and sensory fibers, and transmit messages in both directions.
Together, the nerves make up the peripheral nervous system, as distinguished from the central nervous system, which consists of the brain and spinal cord. There are 12 pairs of cranial nerves, which carry messages to and from the brain. Spinal nerves arise from the spinal cord and pass out between the vertebrae. The various nerve fibers and cells that make up the autonomic nervous system innervate the glands, heart, blood vessels and involuntary muscles of the internal organs. For a complete list of nerves, see Table 14.

accelerator n's
the cardiac sympathetic nerves, which, when stimulated, accelerate the heart rate.
nerve biopsy
specimens taken from representative nerves by separation and removal of fascicles may provide useful information in the investigation of neuromuscular disorders or neuropathies. Consideration must be given to any resulting motor or sensory deficits that might result from the procedure. In dogs, the common peroneal, ulnar and tibial nerves are the usual sources.
nerve cuff
device used in the surgical repair of nerves to protect the site of anastomosis from an in-growth of connective tissue and to promote linear regeneration of neural elements.
depressor nerve
1. an inhibitory nerve whose stimulation depresses a motor center.
2. a nerve that lessens activity of an organ.
dermal nerve network
the organization of sensory nerve fibers to the dorsal root ganglia found in the dermis.
nerve endings
comprise afferent and efferent endings. Afferent endings transform sensations into acceptable stimuli by the CNS; include diffuse-free endings, free, modified free or encapsulated (e.g. tactile corpuscles, Krause's endbulbs, Golgi-Mazzoni corpuscles, genital corpuscles, lamellated corpuscles, Herbst corpuscles, Uffini corpuscles). Efferent endings transform nerve impulses into stimuli delivered to effector end organs; they include neuromuscular spindles, Golgi tendon organs.
encapsulated nerve endings
see nerve endings (above).
excitor nerve
one that transmits impulses resulting in an increase in functional activity.
excitoreflex nerve
a visceral nerve that produces reflex action.
nerve fiber
a process of a neuron, especially the long slender axon which conducts nerve impulses away from the cell. It may be medullated or nonmedullated.
free nerve endings
see nerve endings (above).
fusimotor n's
those that innervate the intrafusal fibers of the muscle spindle.
gangliated nerve
any nerve of the sympathetic nervous system. Called also ganglionated.
nerve gas
organophosphorus compounds specially selected for their toxicity to humans and used in chemical warfare.
nerve growth factor
a protein dimer composed of two identical polypeptide chains secreted by nerve cells and necessary for the growth and survival of certain classes of nerve cells during development.
nerve impulses
the physicochemical change in a nerve fiber's membrane which is caused by stimulation, e.g. from a stretch receptor, and which transmits a record of the sensation, or, in another case, of a motor instruction to an effector organ.
inhibitory nerve
one that transmits impulses resulting in a decrease in functional activity.
medullated nerve
myelinated nerve.
modified free nerve endings
see nerve endings (above).
myelinated nerve
one whose axons are encased in a myelin sheath.
pelvic n's
nerves of the parasympathetic outflow. See Table 14.
peripheral nerve
any nerve outside the central nervous system. Injury to a nerve causes pain initially and if tissue is destroyed, loss of function follows; signs are weakness or paralysis, atrophy, lower temperature and depressed reflexes.
pilomotor n's
those that supply the arrector muscles of hair.
pressor nerve
an afferent nerve whose impulses stimulate a vasomotor center and increases intravascular tension.
retinal nerve fiber layer
layer number 9 of the retina; axons of ganglion cells, make up bundles of nerve fibers and pass to the optic disk and lamina cribrosa; from there on they become the optic nerve.
secretory nerve
an efferent nerve whose stimulation increases glandular activity.
nerve sheath
nerve sheath tumor
neurilemmoma or schwannoma.
somatic n's
the sensory and motor nerves supplying skeletal muscle and somatic tissues.
somatic afferent n's
sensory neurons whose cell bodies reside in spinal and cranial nerve ganglia.
somatic efferent n's
motor neurons originating in ventral gray columns of the spinal cord and certain parts of the brain and are connected to striated muscles derived from embryonic somites.
spinal nerve
a segmental nerve which consists of afferent and efferent axons from its dorsal and ventral roots.
splanchnic n's
those of the blood vessels and viscera, especially the visceral branches of the thoracic, lumbar and pelvic parts of the sympathetic trunks.
nerve stimulator
an electrical device used to deliver a short stimulus to a peripheral nerve as a test of its function. It can be used to assess the effects of a neuromuscular blocking agent during clinical anesthesia.
sudomotor n's
those that innervate the sweat glands.
sympathetic n's
1. see sympathetic trunk.
2. any nerve of the sympathetic nervous system.
nerve terminal
nerve ending.
trophic nerve
one concerned with regulation of nutrition.
nerve trunk
the main body of a nerve; subsequently divides into branches.
unmyelinated nerve
one whose axons are not encased in a myelin sheath.
vasoconstrictor nerve
one whose stimulation causes narrowing of blood vessels.
vasodilator nerve
one whose stimulation causes dilatation of blood vessels.
vasomotor nerve
one concerned in controlling the caliber of vessels, whether as a vasoconstrictor or vasodilator.
vasosensory nerve
any nerve supplying sensory fibers to the vessels.
visceral afferent n's
nerves with cell bodies in spinal and cranial ganglia and which provide sensory innervation to thoracic and abdominal tissues.
visceral efferent n's
the parasympathetic component of the autonomic nervous system.
References in periodicals archive ?
Imaging plays an important role in diagnosis of malignant peripheral nerve sheath tumors.
Neurofibroma is also a benign nerve sheath tumor of heterogeneous origin (schwann cells and perineural fibroblasts).
We report a case of a patient who required CVC and had extensive involvement of BP caused by the nerve sheath tumour, which resembled the internal jugular vein and external carotid artery, as observed on US of the neck.
Schwannomas, previously referred to as neurilemmomas, typically derive from Schwann cells of the peripheral nerve sheath (1).
KEYWORDS: Optic nerve sheath fenestration, Papilledema, Idiopathic Intracranial hypertension.
Schwannomas (neurilemmomas) are benign tumours derived from the Schwann cells of the peripheral nerve sheath.
In their study, scientists found that inhibiting the action of a protein called BRD4 caused cancer cells to die in a model of malignant peripheral nerve sheath tumors (MPNSTs).
Abstract: Peripheral nerve sheath tumors are a heterogeneous group of neoplasms that comprise neurofibromas, schwannomas, neurilemmomas, and perineuromas.
Global Markets Direct's, 'Peripheral Nerve Sheath Tumor (Neurofibrosarcoma) - Pipeline Review, H1 2016', provides an overview of the Peripheral Nerve Sheath Tumor (Neurofibrosarcoma) pipeline landscape.