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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 ?
Intracellular EPSPs from longitudinal and transverse muscle fibers in the octopus arm were correlated with relatively large single units in the dorsal nerve roots of the axial nerve cord.
Here we found that the core of both slow and fast muscle activation is limited to the region where the dorsal nerve root enters the muscles.
We have shown here that the axial nerve cord contains networks that can generate a phase lag between the activity in adjacent ventral and dorsal nerve roots without requiring feedback (Figs.
The flexion-induced responses were recorded from the dorsal nerve of the 3rd nerve root of the 6th thoracic ganglion with a tungsten hook electrode insulated by addition of mineral oil.
A controlled stretch stimulus was thus delivered to the receptor muscle in the relevant segment, and the resulting responses were obtained from the dorsal nerve of the 3rd nerve root, the dorsal nerve was cut distally and introduced into a suction electrode made of a glass capillary.
The 3rd nerve root of each thoracic ganglion branches complexly: the dorsal nerve in this 3rd root provides a common pathway both for the central projection of the axons of stretch receptor cells and for efferents to the extensor muscle (Fig.
The loss of startle-induced bradycardia after dorsal nerve cutting indicates that the cardioinhibitor nerves are the causative inputs for this behavior.
The dorsal nerves approach the heart along the dorsal surface of the posterior dorsal alary ligaments and traverse the posterior half of the ventricle in the connective tissue covering the heart before penetrating to the cardiac ganglion.
Role of dorsal nerves in heart response to startle and exercise
However, the dorsal nerves are presumed to act as sensory nerves supply to the penile shaft, glans and anterior urethra without an autonomic role.

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