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see nervous systemnervous system,
network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system.
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the cordlike association of nerve tissues that links the brain and nerve ganglia by innervation to the other organs and tissues of the body.

A nerve primarily consists of nerve fibers. In vertebrates many nerves converge to form a bundle that is surrounded by a connective tissue sheath, the perineurium; the thin interstitial layers of connective tissue that separate the individual fibers deep within the bundle constitute the endoneurium. Finally, the entire nerve trunk, comprising several bundles, is covered by an additional sheath, the epineurium.

Nerves can be sensory (also called afferent or centripetal) or motor (also called efferent or centrifugal). Some nerves, for example, those innervating the skeletal muscles, mainly include myelinated, or medullated, fibers; others, for example, the sympathetic nerves, largely consist of unmyelinated, or unmedullated, fibers.

In reptiles, birds, mammals, and man 12 pairs of cranial nerves branch from the brain: the olfactory (cranial nerve I), the optic (cranial nerve II), the oculomotor (cranial nerve III), the trochlear (cranial nerve IV), the trigeminal (cranial nerve V), the abducent (cranial nerve VI), the facial (cranial nerve VII), the acoustic (cranial nerve VIII), the glossopharyngeal (cranial nerve IX), the vagus (cranial nerve X), the accessory (cranial nerve XI), and the hypoglossal (cranial nerve XII). Only the first ten pairs are present in fish and amphibians.

In man there are 31 pairs of spinal nerves: eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal. Each pair innervates the effectors and receptors of a certain part of the body. The spinal nerves branch from the spinal cord into two roots—the posterior, or sensory, and the anterior, or motor. Both roots then combine to form a common trunk that consists of both sensory and motor fibers.

Several adjacent nerves can be combined into nerve plexuses, where an exchange of fibers between different nerves can take place. Three large plexuses are distinguished: the cervical, the brachial, and the lumbosacral. Each nerve plexus is the origin of several pairs of nerves; for example, the sacral portion of the lumbosacral plexus gives rise to the sciatic nerves.

Nerves that originate in the ganglia, trunks, and plexuses of the autonomic nervous system constitute a specific group. The optic nerve is remarkable for its large number of fibers; there are more than 1 million in the human optic nerve. Usually, however, there are 103 -104 fibers in a nerve. In invertebrates certain nerves are known to consist of only a few fibers. The peripheral nervous system in animals and man consists of aggregations of nerves.



A bundle of nerve fibers or processes held together by connective tissue.


Any one of the ribs of a groined vault, but esp. a rib which forms one of the sides of a compartment of the groining.


1. any of the cordlike bundles of fibres that conduct sensory or motor impulses between the brain or spinal cord and another part of the body
2. a large vein in a leaf
3. any of the veins of an insect's wing
References in periodicals archive ?
The different parts of the brachial plexus were swollen, but we found no segmental defects in the various nerve trunks that we explored, and we observed no neuroma.
These nerve trunks control the sweating of the hands and underarms and are located in front of the ribs on the back of the chest cavity.
A recent morphometric study[16] indicated that most aganglionic segments contain nerve trunks greater than 40 [micro]m in diameter, but that no fibers of that diameter were found in healthy segments of bowel or in normal rectal biopsies.
The classic hypoglossal nerve-facial nerve transposition graft is an end-to-end neurorrhaphy between the proximal end of the ligated ipsilateral hypoglossal nerve and the distal facial nerve trunk.
1] First, the tumor was clearly associated with a nerve trunk, and morphologic evidence of a neurofibroma component was present.
6) The most marked histologic finding in neurosarcoidosis is lymphoplasmacytic infiltration of the nerve trunk, sometimes accompanied by well-defined granulomas.
Schwannomas are encapsulated, and they grow eccentrically away from the nerve trunk; neurofibromas exhibit diffuse proliferation, and they grow within the nerve trunk.
22, 1992, provides several methods of enhancing the regeneration of injured nerves to deal with problems surrounding growth of nerve cells across the zone of injury, whether the injury results in total severance of a nerve trunk, or the condition known as "neuroma in continuity" where damage is caused by crushing, bruising, or partial laceration of the nerve.
Neuromuscular choristoma (NMC) is an uncommon tumor that usually involves a large nerve trunk.
The concept of the anastomosis between the two facial nerves, in cases of facial nerve paralysis in which the nerve trunk is not accessible proximal to the site of the injury, remains as sound today as it was when first presented as a preliminary report at the Second International Symposium on Facial Nerve Surgery held in Osaka, Japan, September 27-3 0, 1970.
Immediate nerve repair has been successful in cases of laceration injuries, but nerve grafting techniques are typically delayed when it is clear that direct suturing to the nerve trunk cannot be achieved without tension.