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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 ?
In this study, within 30 min postapplication, a significant degeneration was observed in the spinal nerve roots, especially in myelinated nerve fibers, in electron microscopy.
Durand, "Modeling of mammalian myelinated nerve for functional neuromuscular electrostimulation," in Proceedings of the 97th Annual Conference of IEEE Engineering in Medicine and Biology Society (EMBC '97), vol.
Conduction in a normally myelinated nerve is made possible by the presence of sodium channels, which are primarily responsible for the sodium ion shifts that occur with depolarization.
It showed an increased number of flattened endoneurial fibroblast-like cells, with long cytoplasmic processes that wrapped the regenerating unmyelinated or thinly myelinated nerve fibres, in most of the specimens.
Furthermore, the muscle and nerve biopsy revealed typical fiber Type II grouping atrophy, significantly decreased myelinated nerve fibers with abundantly thin myelinated fibers and onion bulbs changes.
The above changes decrease the activity of Na+K+ATP-ase (sodium potassium ATP-ase) thought to be located primarily in the nodal and paranodal regions of large myelinated nerve fibers resulting in increased intra-axonal Na+ concentration, reduced nodal Na+ permeability causing diminished conduction velocity.
This was in agreement with a previous study that showed different vulnerabilities between the unmyelinated and myelinated nerve fibers [23, 24].
Hypoglycemia causes Wallerian-type axonal degeneration of large myelinated nerve fibers in the peripheral nerve of insulin-treated diabetic animal models [19, 20].
(6) Recovery Ratio of Quantity, Diameter, and Myelin Sheath Thickness in Regenerated Myelinated Nerve Fibers.
Nerve biopsy revealed the focal loss of large myelinated nerve fibres without inflammation, granuloma, vasculitis or AFB.