spinal cord(redirected from Spinal cord segment)
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spinal cord,the part of the 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.
..... Click the link for more information. occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal columnspinal column,
bony column forming the main structural support of the skeleton of humans and other vertebrates, also known as the vertebral column or backbone. It consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments.
..... Click the link for more information. , technically known as the vertebral column. Extending from the first lumbar vertebra to the medulla at the base of the brainbrain,
the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function
..... Click the link for more information. , the spinal cord of a human adult is about 18 in. (45 cm) long. Structurally, the cord is a double-layered tube, roughly cylindrical in cross section. The outer layer consists of white matter, i.e., myelin-sheathed nerve fibers. These are bundled into specialized tracts that conduct impulses triggered by pressure, pain, heat, and other sensory stimuli or conduct motor impulses activating muscles and glands. The inner layer, or gray matter, is primarily composed of nerve cell bodies. Within the gray matter, running the length of the cord and extending into the brain, lies the central canal through which circulates the cerebrospinal fluid. Three protective membranes, the meningesmeninges
, three membranous layers of connective tissue that envelop the brain and spinal cord (see nervous system). The outermost layer, or dura mater, is extremely tough and is fused with the membranous lining of the skull.
..... Click the link for more information. , wrap the spinal cord and cover the brain—the pia mater is the innermost layer, the arachnoid lies in the middle, and the dura mater is the outside layer, to which the spinal nerves are attached. Connecting with the cord are 31 pairs of these spinal nerves, which feed sensory impulses into the spinal cord, which in turn relays them to the brain. Conversely, motor impulses generated in the brain are relayed by the spinal cord to the spinal nerves, which pass the impulses to muscles and glands. The spinal cord mediates the reflex responses to some sensory impulses directly, i.e., without recourse to the brain, as when a person's leg is tapped producing the knee jerk reflex. Nerve fibers in the spinal cord usually do not regenerate if injured by accident or disease.
(medulla spinalis), in humans and other vertebrates, the part of the central nervous system located in the vertebral canal. The spinal cord has retained the features of the primitive chordate neural tube more than any other part of the central nervous system.
The spinal cord is cylindrical and contains a central canal. It is surrounded by three meninges: the pia mater (the innermost meninx), the arachnoid (the middle meninx), and the dura mater (the outermost meninx). It is held firmly in place by ligaments extending from the meninges to the inner wall of the bony canal. The space between the arachnoid, the pia mater, and the spinal cord itself is filled with cerebrospinal fluid, which also fills the central canal of the spinal cord. The anterior (upper) end of the spinal cord is continuous with the medulla oblongata, and the posterior (lower) end, with the terminal filament.
The spinal cord is conventionally divided into segments according to the number of vertebrae present. In humans there are 31–33 segments: eight cervical, 12 thoracic, five lumbar, five sacral, and one to three coccygeal segments.
A group of nerve fibers—the root filaments of the spinal nerves—extend from each segment and combine to form the roots of the spinal nerves. Each pair of roots corresponds to a vertebra and departs from the vertebral canal through an intervertebral foramina, or opening. In adult animals and humans, the spinal cord is shorter than the vertebral canal, and as a result the roots of the lower segments extend downward in the form of a bundle and are transmitted from the vertebral canal through the intervertebral foramina. The dorsal (posterior) roots of spinal nerves contain sensory (afferent, or centripetal) nerve fibers along which impulses are transmitted to the spinal cord from receptors in the skin, muscles, tendons, joints, and internal organs. The ventral (anterior) roots of spinal nerves contain motor (efferent, or centrifugal) nerve fibers along which impulses are transmitted from the motor or sympathetic-nerve cells of the spinal cord to the periphery, to skeletal muscles to the smooth muscles of the blood vessels, and to the internal organs. The posterior and anterior roots join before entering the intervertebral foramina, forming mixed nerve roots upon emerging from the spine.
The spinal cord consists of two symmetrical halves joined by a thin transverse band of nervous substance—the commissure. Nerve cells and their short processes (dendrites) form the gray matter that encircles the central canal; in cross section the gray matter of the spinal cord resembles a butterfly with outstretched wings. The nerve fibers that comprise the ascending and descending tracts of the spinal cord form the white matter of the spinal cord along the edges of the gray matter. Projections of the gray matter—the anterior, posterior, and lateral horns—divide the white matter into the anterior, posterior, and lateral funiculi of the spinal cord. The boundaries between the funiculi are the sites where the dorsal and ventral roots emerge.
The nerve cells, or neurons, of the gray matter are usually found in groups (nuclei) and arranged unevenly such that each part of the gray matter is characterized by a specific type of nerve cell. Most important are the nuclei of the anterior horn, which contain motor neurons (motoneurons). Long motoneuronal processes (axons) depart through the ventral root and innervate the skeletal musculature. The intermediate substance of the spinal cord contains a nucleus whose cells have short axons that form synaptic junctions with other neurons of the spinal cord. This nucleus contains intercalary cells (interneurons) that are joined in a chain of varying complexity.
A nucleus containing preganglionic cells of the sympathetic nervous system is situated in the intermediate substance between the thoracic and upper lumbar segments. The axons of these cells emerge from the spinal cord through the ventral roots and continue toward peripheral ganglia, where synaptic junctions are formed with postganglionic neurons; these neurons innervate the muscles and secretory apparatus of the internal organs. The apex of the posterior horn contains an accumulation of nerve cells called the substantia gelatinosa, the processes of which interweave to form a network (the neuropil). The sensory fibers that enter the spinal cord through the dorsal roots pass through the substantia gelatinosa and form synaptic junctions primarily in interneuronal nuclei; very few of the fibers are in direct contact with motoneurons.
The nerve fibers that are conveyed in the funiculi of the white matter serve as tracts for the transmission of signals to and from the brain. The ascending sensory fibers are processes of cells of the spinal ganglia (columns of Burdach and Goll in the posterior funiculi) or of the cells of the intermediate substance (cerebellospinal columns of Gowers and Flechsig, the spinothalamic column in the lateral funiculi).
The descending motor fibers originate from the cells of various nuclei of the brain, including the red and vestibular nuclei and the nuclei of the reticular formation. The fibers transmit motor signals to the cells of the spinal cord. They also join to form various columns, for example, the rubrospinal, vestibulospinal, and reticulospinal columns. A special descending tract begins at the pyramidal neurons of the motor area of the cortex of the cerebral hemispheres. The fibers of the descending tracts establish synaptic links with various intercalary and motor neurons of the spinal cord.
The action of the spinal cord is reflexive in nature. Reflexes originate under the action of afferent signals that reach the spinal cord from receptors (the initial points of a reflex arc). They may also be effected by signals that proceed initially to the brain and then descend to the spinal cord along descending tracts. Specific muscular reflexes, defense reflexes, flexor and extensor reflexes, the contraction of blood vessels, and several other reflexes of the internal organs are maintained when the spinal cord is cut and the connection to the brain severed. The reflexes, however, are maintained in a weakened form as a result of the development of spinal shock. The most complex reflex reactions of the spinal cord are controlled by various brain centers. In these reactions the spinal cord serves as a link in the transmission of signals from the brain to effectors. These signals are modified by the intercalary neurons of the spinal cord and combined with signals arriving simultaneously to the spinal cord from peripheral receptors. The major role in the integrative function of the spinal cord is played by excitatory and inhibitory synaptic processes that are produced in nerve cells under the influence of impulses that arrive along various nerve tracts. The summation of excitatory synaptic processes is the basis for the mutual fortification of functionally unidirectional reflex reactions. The coincidence of functionally opposite reflexes, for example, flexor and extensor reflexes, leads to their mutual inhibition.
Trauma or a pathological process in the spinal cord leads to a weakening in corresponding motor or vegetative functions (paralyses). Sensibility may be affected, including sensibility to stimulations of pain and temperature and to stimulations produced by mechanical forces and the sensibility of the motor apparatus and several internal organs. Depending on the nature of the injury, dysfunctions of the spinal cord may be general or selective. Because the various ascending tracts are mutually independent, damage to one-half of the spinal cord leads to the impairment of discriminatory and joint sensation on the corresponding side with the retention of temperature and pain sensation (Brown-Séquard syndrome). Injury to the descending tracts of the spinal cord may lead, in addition to the cessation of voluntary movements, to the retention and even strengthening of reflex contractions in response to peripheral stimulation (spastic paralysis).
REFERENCESBekhterev, V. M. Provodiashchie puti spinnogo i golovnogo mozga, 2nd ed., part 1. St. Petersburg, 1896.
Villiger, E. Golovnoi i spinnoi mozg. Moscow-Leningrad, 1930. (Translated from German.)
Beritov, I. S. Obshchaia fiziologiia myshechnoi i nervnoi sistem, 2nd ed., vol. 2. Moscow-Leningrad, 1948.
Kostiuk, P. G. Struktura ifunktsiia niskhodiashchikh sistem spinnogo mozga. Leningrad, 1973.
Granit, R. Osnovy reguliatsii dvizhenii. Moscow, 1973. (Translated from English.)
The Interneuron. Edited by M. A. Brazier. Berkeley-Los Angeles, 1969. Page 177.
Diseases of the spinal cord may be caused by many factors. Thus, hereditary diseases of the spinal cord, for example, Friedreich’s ataxia, are often accompanied by signs of injury to the spinal cord. Many neural infections are characterized by damage to the substance of the spinal cord, to the spinal meninges, and to the roots of spinal nerves (meningitis, myelitis, poliomyelitis, radiculitis). Syndromes caused by injury to the spinal cord are characteristic of such diseases caused by syphilis as tabes dorsalis and of several chronic diseases of the nervous system, including syringomyelia, amyotrophic lateral sclerosis, and multiple sclerosis. Spinal tumors may be primary or metastatic. Primary tumors may be extramedullary, developing mainly from the spinal meninges and roots, or intramedullary, developing in the substance of the cord primarily from glia cells.
Abscesses of the spinal cord may develop as complications of injuries to the spine as a result of the spread of infection with the blood flow or by contact. They may be situated on or outside the dura mater (epidural abscesses) or beneath the dura mater (subdural abscesses). Tuberculous spondylitis is accompanied by spinal disturbances in 10 to 15 percent of the cases. Spinal disturbances also sometimes accompany herniation of an intervertebral disk and degenerative processes in the spine, including osteochondrosis and spondylosis. Spinal circulatory disturbances result from changes in the spine and diseases of the thoracic and abdominal aorta and the arteries that directly supply blood to the spinal cord. These disturbances may lead to spinal-cord infarction.
Traumatic injuries to the spinal cord, including concussion, contusion, compression, and hemorrhaging into the meninges and substance, occur both as isolated incidences and in conjunction with fractures of the spine. With closed fractures, dislocations, and stab and gunshot wounds, damage to the meninges, white matter, and gray matter are common, with the spinal cord being completely severed in many instances.
Afflictions of the spinal cord are treated using conservative and surgical methods, depending on the cause and nature of the disease or injury.
REFERENCESDavidenkov, S. N. Nasledstvennye bolezni nervnoi sistemy, 2nd ed. Moscow, 1932.
Razdol’skii. I. Ia. Opukholispinnogo mozga ipozvonochnika. Leningrad, 1958.
Tsuker, M. B. Klinicheskaia nevropatologiia detskogo vozrasta. Moscow, 1972.
Bogorodinskii, D. K., and A. A. Skoromets. lnfarkty spinnogo mozga. Leningrad, 1973.
Ugriumov, V. M., and E. I. Babichenko. Zakrytye povrezhdeniia pozvonochnika i spinnogo mozga. Leningrad, 1973.
V. B. GEL’FAND
spinal cord[′spīn·əl ‚kȯrd]