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Spinal Cord

Spinal Cord

The spinal cord is a bundle of nerve fibers, no thicker than the human thumb, that links the brain with the rest of the body. The spinal cord is protected by the vertebral column, and together with the brain it comprises the central nervous system. The nerves that enter and exit the spinal cord form the peripheral nervous system.

Some nerves enter the spinal cord on its dorsal surface (which is closest to the back). These nerves carry sensory information to the spinal cord and are called afferent nerves. For example, they allow a person to determine if the pan on the stove is hot or cold, or if one's hand is touching rough sandpaper or smooth silk. In contrast, the nerves that exit the ventral surface (closest to the stomach) of the spinal cord carry information from the spinal cord to the rest of the body. These nerves enable a person to jerk his or her hand away from a hot pan or throw a dog a ball. The term for nerves that conduct commands from the spinal cord to muscles and organs is efferent.

The afferent and efferent nerves are associated with an H-shaped area of gray matter in the center of the spinal cord. The gray matter is separated into dorsal and ventral horns. The ventral horn contains the cell bodies of efferent neurons that control muscles and organs. The sensory nerves enter the dorsal horn where they make connections with nerve cells that travel to the brain.

The gray matter is surrounded by white matter, which contains the long projections of nerve cells (called axons ) that carry information to other parts of the nervous system. Axons that carry similar information are grouped into bundles or tracts. Many tracts start in the dorsal horn and carry sensory information from the cord to the brain (for example, the message that the hand is touching silk instead of sandpaper). Since these neurons are traveling "up" the cord, they are often referred to as ascending tracts. In addition to ascending tracts, the white matter also contains descending tracts. As the name implies, these tracts begin in the brain and travel down the spinal cord to make connections with neurons in the ventral horn. They provide a person with voluntary control of his or her muscles, as well as the involuntary control over internal organs.

In short, the spinal cord carries all of the information that enters and exits the brain. Therefore, it is not surprising that when this flow of information is blocked by injury, the consequences are devastating. Patients suffer paralysis and loss of sensation in their legs (paraplegia) if the lower part of the cord is damaged, or in their arms and legs (quadriplegia) if the injury is in the upper regions of the cord. In addition, control over urination, defecation, and sometimes respiration is lost depending on the level and extent of the damage. Once the spinal cord has been injured, the damage is usually permanent. Physical therapy can enable a patient to regain a small amount of movement over time, but the medical field has yet to discover a way to reconnect the severed nerve cells to produce normal function.

see also Brain; Central Nervous System; Neuron; Peripheral Nervous System

Sheri L. Boyce

Bibliography

Martini, Frederic H., and Edwin F. Bartholomew. Essentials of Anatomy and Physiology, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Martini, Frederic H., Edwin F. Bartholomew, and Kathleen Welch. The Human Body in Health and Disease. Upper Saddle River, NJ: Prentice Hall, 2000.

McDonald, John. "Repairing the Damaged Spinal Cord." Scientific American 280 (1999): 6573.

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spinal cord

spinal cord The spinal cord extends down from the brain stem at the base of the skull, enclosed in the vertebral canal; brain and spinal cord in continuity comprise the central nervous system. Like the brain, the cord is ensheathed by membranes (meninges), and bathed by cerebrospinal fluid. In the spinal cord are tracts of white matter, nerve fibres carrying information to and from the brain as well as between different levels of the cord itself; and a core of grey matter, containing nerve cells and synapses that mediate motor, sensory, and reflex functions. The substance of the cord is continuous, but functional segments are marked by the series of nerve roots at intervals down its length. At each level, two nerve roots (dorsal or posterior carrying ingoing nerve impulses; ventral or anterior carrying outgoing impulses) join to form a spinal nerve on each side. The uppermost emerges between the skull and the uppermost cervical vertebra; the rest emerge between two adjacent vertebrae, and between the segments of the sacrum. There are 8 cervical nerves, and below this the nerves are named according to the vertebra above their point of exit: thus there are 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve. The spinal canal is longer, however, than the spinal cord, which ends in the lumbar part of the canal. Therefore the distance that a spinal nerve must travel to reach its point of exit increases from above downwards, from zero for the first cervical nerve to about 20 cm for the lowest sacral and coccygeal. In the canal below the end of the cord, there is therefore a sheaf of descending spinal nerves that becomes progressively smaller as the nerves leave; this is known as the horse's tail — the cauda equina. This arrangement has consequences for the effects of spinal injury at different vertebral levels. Anywhere above the second lumbar vertebra, it is the spinal cord that is damaged; below this, it is spinal nerves. Spinal cord damage leaves uncontrolled motor neurons below the level of the lesion; voluntary movement is lost, but after recovery from an initial period of spinal shock, the muscles can and do contract, spontaneously and reflexly: a spastic paralysis. Damage to the spinal nerves in the cauda equina, by contrast, separates the affected muscles from their spinal motor neurons; voluntary movement is lost and the muscles remain relaxed: a flaccid paralysis followed by wasting. In either case paralysis is accompanied by loss of sensation.

Sheila Jennett


See nervous system.See also central nervous system; meninges; motor neurons; paralysis; reflexes; spinal shock.

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spinal cord

spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column. Extending from the first lumbar vertebra to the medulla at the base of the brain, 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 meninges, 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.

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spinal cord

spinal cord The part of the vertebrate central nervous system that is posterior to the brain and enclosed within the vertebral column. It consists of a hollow core of grey matter (H-shaped in cross section) surrounded by an outer layer of white matter; the central cavity contains cerebrospinal fluid. The white matter contains numerous longitudinal nerve fibres organized into distinct tracts: ascending tracts consist of sensory neurons, conducting impulses towards the brain; descending tracts consist of motor neurons, transmitting impulses from the brain. Paired spinal nerves arise from the spinal cord.

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spinal cord

spinal cord Tubular, central nerve cord, lying within the spine. It carries sensory information to the brain. With the brain, it makes up the central nervous system. It gives rise to the 31 pairs of spinal nerves, each of which has sensory and motor fibres, and these connect to various parts of the body.

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spinal cord

spinal cord n. the portion of the central nervous system enclosed in the vertebral column, consisting of nerve cells and bundles of nerves connecting all parts of the body with the brain. It extends from the medulla oblongata in the skull to the level of the second lumbar vertebra.

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spinal cord

spi·nal cord • n. the cylindrical bundle of nerve fibers and associated tissue that is enclosed in the spine and connects nearly all parts of the body to the brain, with which it forms the central nervous system.

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spinal cord

spinal cord In vertebrates, a tube containing neurones and bundles of nerve fibres, many of which connect to the brain, enclosed within the spinal column.

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