Spinal cord

Definition

The spinal cord is the elongated bundle of nervous tissue that carries nerve impulses between the brain and the rest of the body. It lies in the vertebral canal of the vertebral column .

Description

The spinal cord lies within the vertebral canal, which is the hollow part of the vertebral column, or spine, that consists of 33 bones called vertebrae. The canal is formed by the stacked vertebrae which all contain a central vertebral foramen, or hole. The spinal cord extends from the lowest part of the brain, called the brainstem, through a hole located at the base of the skull , the foramen magnum, and continues down the vertebral canal to the twenty-first vertebra of the spine.

Like the brain, the spinal cord is protected by three layers of membranes, called meninges. The inner meninge that makes direct contact with the spinal cord is called the pia mater. It is separated from the second layer by a space called the subarachnoid space. This space is filled with cerebrospinal fluid (CSF), the colorless fluid that bathes the entire brain and spinal cord. The second layer is the thin and spider web-like arachnoid mater and it is separated from the outermost layer by a space called the subdural space. The outermost layer is the dura mater, a protective sheath made of tough fiber. Between the dura mater and the bone of the vertebral canal is a space, called the epidural space, which contains a small amount of fatty tissue and blood vessels . The spinal dura mater

prolongs the dura mater that lines the skull cavity and extends to the sacrum, the second to last bone of the vertebral column. It also covers each of the spinal nerves as they leave the vertebral canal. Both the arachnoid and pia mater also prolong the arachnoid and pia surrounding the brain, but unlike the arachnoid, which continuously follows the dura mater, the pia ends where the spinal cord ends. A stringy extension of the pia mater, called the filum terminale joins the end of the spinal cord to the end of the dura mater. Additionally, the pia mater contains thin projections called denticulate ligaments, that connect the spinal cord to the dura mater.

Function

The major function of the spinal cord is to carry nerve impulses between the brain and the rest of the body. Together, the brain and the spinal cord constitute the central nervous system (CNS). The other nerves of the nervous system, that is the motor and sensory nerves, constitute the peripheral nervous system (PNS).

The spinal cord consists of a core of grey nervous tissue surrounded by a thicker section of white tissue. The grey matter looks like a butterfly with outspread wings and the upper and lower sections of these wings are called the posterior and anterior horns. The tissues of the spinal cord are full of nerve cells, also called neurons. Neurons with large cell bodies that are located in the anterior horns give rise to motor nerve fibers that connect to spinal nerves which pass out of the cord to skeletal muscle. The grey matter of the spinal cord also contains other neurons that connect together to form nerve pathways and the white matter contains nerves that are wrapped in myelin sheaths and form nerve tracts. The tracts that conduct sensory impulses from the body to the brain are called ascending tracts and those that conduct motor impulses from the brain to muscles and glands are called descending tracts.

Thirty-one pairs of spinal nerves emerge from the spinal cord. They are all mixed nerves, meaning that they provide a two-way communication system for sensory and motor information exchange between the spinal cord and the rest of the body. Spinal nerves are numbered according to the vertebral column level from which they stem. There are eight pairs of cervical nerves, C1 to C8, twelve pairs of thoracic nerves, T1 to T12, five pairs of lumbar nerves, L1 to L5, five pairs of sacral nerves, S1 to S5, and one pair of coccygeal nerves.

Role in human health

The spinal cord is an extremely important component of the CNS because it provides the crucial link between the brain and the spinal nerves that connect to the individual muscles and organs of the body. The role of the spinal cord in human health however, is not only to carry this sensory and motor information. It also carries a great deal of other crucial information as well, having to do with involuntary and automatic body functions. For example, the regulation of the chemical contents of the blood and body fluids is carried out by an automatic feedback control system that involves the spinal cord and its attached network of peripheral nerves. The regulation the heart , stomach , and intestines are other examples. These are all vital body functions of which we are unaware of and that all proceed with the involvement of the spinal cord nervous tissues.

KEY TERMS

Arachnoid mater —One of three meninges covering the central nervous system (CNS) the others are the dura and pia maters. The dura mater encloses the arachnoid which in turn covers the pia mater.

Brain stem —Lowest part of the brain that connects with the spinal cord. It is a complicated neural center with several neuronal pathways between the cerebrum, spinal cord, cerebellum, and motor and sensory functions of the head and neck. It consists of the medulla oblongata, the part responsible for cardiac and respiratory control, the midbrain, which is involved in basic, involuntary body functions, and the pons, where some cranial nerves originate.

Central nervous system (CNS) —One of two major divisions of the nervous system. The CNS consists of the brain, the cranial nerves and the spinal cord.

Cerebrospinal fluid (CSF) —A clear colorless fluid that circulates in the brain and in the subarachnoid spaces surrounding the brain and spinal cord. The CSF lies between the spinal cord and the arachnoid mater thereby suspending the spinal cord in fluid.

Cervical vertebrae —Vertebrae of the neck.

Epidural space —This space lies between the dura mater and the walls of the vertebral canal, it contains loose connective tissue, blood vessels and some fatty tissue.

Foramen —A hole in a bone usually for the passage of blood vessels and/or nerves.

Foramen magnum —Large opening at the base of the skull that allows passage of the spinal cord.

Intervertebral disk —Disk-shaped pads of fibrous cartilage interposed between the vertebrae of the vertebral column that provide cushioning and join the vertebrae together.

Meninges —The membranes that surround and protect the brain and spinal cord. There are three layers: the dura mater (outermost), arachnoid membrane (middle) and the pia mater (innermost).

Nervous system —The entire system of nerve tissue in the body. It includes the brain, the brainstem, the spinal cord, the nerves and the ganglia and is divided into the peripheral nervous system (PNS) and the central nervous system (CNS).

Paraplegia —Paraplegia is permanent paralysis of the trunk and lower limbs. It is caused by injury or disease affecting the spinal cord below the chest or waist.

Peripheral nerves —The nerves outside of the brain and spinal cord, including the autonomic, cranial, and spinal nerves. These nerves contain cells other than neurons and connective tissue as well as axons.

Peripheral nervous system (PNS) —One of the two major divisions of the nervous system. The PNS consists of the somatic nervous system (SNS), that controls voluntary activities and of the autonomic nervous system (ANS), that controls regulatory activities. The ANS is further divided into sympathetic and parasympathetic systems.

Quadraplegia —Quadraplegia is permanent paralysis of the trunk, lower and upper limbs. It is caused by injury or disease affecting the spinal cord at the neck level.

Sacrum —The triangular-shaped bone found between the fifth lumbar vertebra and the coccyx. It consists of five fused vertebrae and it articulates on each side with the bones of the pelvis (ilium), forming the sacroiliac joints.

Sensory nerve —A nerve that receives input from sensory cells, such as the skin or muscle receptors.

Skull —All of the bones of the head.

Spinal cord —Elongated part of the central nervous system (CNS) that lies in the vertebral canal of the spine and from which the spinal nerves emerge.

Vertebra —Flat bones that make up the vertebral column. The spine has 33 vertebrae.

Vertebral canal —Hollow part of the vertebral column formed by the vertebral foramina of the stacked vertebrae. It encloses the spinal cord.

Vertebral foramen —The opening formed in vertebrae that allows passage of the spinal cord.

Common diseases and disorders

Spinal cord injuries are usually the result of trauma to the vertebral column. When dislocations and fractures of the spine occur, the vertebrae may press on the spinal cord, thus compressing the nerves. Pressure applied to the spinal cord may result in muscle weakness or paralysis . It could also cause abnormal sensations, such as pain , tingling, or burning. In severe cases, the cord might even be torn or severed, and the function of the spinal cord risks being seriously impaired if not altogether destroyed. A damaged spinal cord results in loss of sensation and/or motor function below the level of the injury. Thus, injuries to the cord at the chest or waist level may result in paraplegia, which is paralysis of the legs and/or part of the trunk. Damage to the cord in the neck region may result in paralysis of all four limbs and the trunk, a condition called quadriplegia, and it can be fatal. Other disorders of the spinal cord include:

• Epidural abscesses. Infections that occur in the epidural space around the dura mater. These create pockets of infected fluid that affect the spinal nerve roots and generate enough pressure to impair neurological function.
• Foraminal stenosis. Normally, nerve roots have enough room to easily slip through the foramina of the spine. However, with age and disease, they may become clogged and blocked, thus trapping and compressing the nerves.
• Pinched nerve. The two nerves most commonly pinched in the spinal cord are L5 and S1. The L5 nerve supplies the nerves to the muscles that raise the foot and big toe, and a pinched L5 may lead to weakness in these muscles. Likewise, a pinched S1 may lead to weakness with the large muscle in the back of the calf.
• Sciatica. The compression of the spinal roots of the sciatic nerve. It is characterized by pain in the low back region that radiates down the back of the thigh, the leg and into the foot. It results from diseased sciatic nerve roots or can be caused by a tumor, or intervertebral disc displacement resulting from injury or inflammation.
• Spinal stenosis. A narrowing of spaces in the spine that results in pressure on the spinal cord and nerve roots. This disorder usually involves the narrowing of one or more of three areas of the spine: the vertebral canal, the canals at the base or roots of nerves branching out from the spinal cord, and the vertebral foramina. It is usually a degenerative disorder caused by old age, but may also be an inherited disease.

Resources

BOOKS

Byrne, T. N., Benzel, E. C. and S. G., Waxman. Diseases of the Spine and Spinal Cord. Oxford: Oxford University Press, 1999.

Cramer, G. D. and S.A. Darby. Basic and Clinical Anatomy of the Spine, Spinal Cord, and ANS. St. Louis: Mosby, 1995.

Palmer, S., Harris, K., and J. Kriegsman. Spinal Cord Injury: A Guide for Living (Johns Hopkins Press Health Book). Baltimore: Johns Hopkins University Press, 2000.

Reeves, C. Still Me. New York: Ballantine Books, 1999.

ORGANIZATIONS

American Paraplegia Society (APS) 75-20 Astoria Blvd., Jackson Heights, NY 11370. (718) 803-3782. <http://www.apssci.org/contactAPS.htm>.

National Spinal Cord Injury Association 701 Democracy Boulevard, Suite 300-9, Bethesda, MD 20817. (301) 588-6959; (800) 352-9424. <http://www.spinalcord.org/>.

Spinal Cord Society 19051 County Highway 1, Fergus Falls, MN 56537-7609. (218) 739-5252; (218) 739-5261. <http://members.aol.com/scsweb>.

OTHER

The Spinal Cord Injury Resource Center. <http://www.spinalinjury.net/>.

"The Spinal Cord or Medulla Spinalis." Bartleby.com edition of Gray's Anatomy of the Human Body.<http://www.bartleby.com/107/185.html>.

Monique Laberge, Ph.D.

Spinal Cord

Definition

The spinal cord is the elongated bundle of nervous tissue that carries nerve impulses between the brain and the rest of the body. It lies in the vertebral canal of the vertebral column.

Description

The spinal cord lies within the vertebral canal, which is the hollow part of the vertebral column, or spine, that consists of 33 bones called vertebrae. The canal is formed by the stacked vertebrae which all contain a central vertebral foramen, or hole. The spinal cord extends from the lowest part of the brain, called the brainstem, through a hole located at the base of the skull, the foramen magnum, and continues down the vertebral canal to the twenty-first vertebra of the spine.

Like the brain, the spinal cord is protected by three layers of membranes, called meninges. The inner meninx that makes direct contact with the spinal cord is called the pia mater. It is separated from the second layer by a space called the subarachnoid space. This space is filled with cerebrospinal fluid (CSF), the colorless fluid that bathes the entire brain and spinal cord. The second layer is the thin and spider web-like arachnoid mater and it is separated from the outer-most layer by a space called the subdural space. The outermost layer is the dura mater, a protective sheath made of tough fiber. Between the dura mater and the bone of the vertebral canal is a space, called the epidural space, which contains a small amount of fatty tissue and blood vessels. The spinal dura mater prolongs the dura mater that lines the skull cavity and extends to the sacrum, the second to last bone of the vertebral column. It also covers each of the spinal nerves as they leave the vertebral canal. Both the arachnoid and pia mater also prolong the arachnoid and pia surrounding the brain, but unlike the arachnoid, which continuously follows the dura mater, the pia ends where the spinal cord ends. A stringy extension of the pia mater, called the filum terminale joins the end of the spinal cord to the end of the dura mater. Additionally, the pia mater contains thin projections called denticulate ligaments, which connect the spinal cord to the dura mater.

Function

The major function of the spinal cord is to carry nerve impulses between the brain and the rest of the body. Together, the brain and the spinal cord constitute the central nervous system (CNS). The other nerves of the nervous system, that is the motor and sensory nerves, constitute the peripheral nervous system (PNS).

The spinal cord consists of a core of grey nervous tissue surrounded by a thicker section of white tissue. The grey matter looks like a butterfly with outspread wings and the upper and lower sections of these wings are called the posterior and anterior horns. The tissues of the spinal cord are full of nerve cells, also called neurons. Neurons with large cell bodies that are located in the anterior horns give rise to motor nerve fibers that connect to spinal nerves which pass out of the cord to skeletal muscle. The grey matter of the spinal cord also contains other neurons that connect together to form nerve pathways and the white matter contains nerves that are wrapped in myelin sheaths and form nerve tracts. The tracts that conduct sensory impulses from the body to the brain are called ascending tracts and those that conduct motor impulses from the brain to muscles and glands are called descending tracts.

Thirty-one pairs of spinal nerves emerge from the spinal cord. They are all mixed nerves, meaning that they provide a two-way communication system for sensory and motor information exchange between the spinal cord and the rest of the body. Spinal nerves are numbered according to the vertebral column level from which they stem. There are eight pairs of cervical nerves, C1 to C8, twelve pairs of thoracic nerves, T1 to T12, five pairs of lumbar nerves, L1 to L5, five pairs of sacral nerves, S1 to S5, and one pair of coccygeal nerves.

Role in human health

The spinal cord is an extremely important component of the CNS because it provides the crucial link between the brain and the spinal nerves that connect to the individual muscles and organs of the body. The role of the spinal cord in human health, however, is not only to carry this sensory and motor information. It also carries a great deal of other crucial information as well, having to do with involuntary and automatic body functions. For example, the regulation of the chemical contents of the blood and body fluids is carried out by an automatic feedback control system that involves the spinal cord and its attached network of peripheral nerves. The regulation the heart, stomach, and intestines are other examples. These are all vital body functions of which we are unaware and that all proceed with the involvement of the spinal cord nervous tissues.

Common diseases and disorders

Spinal cord injuries are usually the result of trauma to the vertebral column. When dislocations and fractures of the spine occur, the vertebrae may press on the spinal cord, thus compressing the nerves. Pressure applied to the spinal cord may result in muscle weakness or paralysis. It could also cause abnormal sensations, such as pain, tingling, or burning. In severe cases, the cord might even be torn or severed, and the function of the spinal cord risks being seriously impaired if not altogether destroyed. A damaged spinal cord results in loss of sensation and/or motor function below the level of the injury. Thus, injuries to the cord at the chest or waist level may result in paraplegia, which is impairment of the legs and/or part of the trunk. Damage to the cord in the neck region may result in impairment of all four limbs and the trunk, a condition called quadriplegia, and it can be fatal. Other disorders of the spinal cord include:

• Epidural abscesses. Infections that occur in the epidural space around the dura mater. These create pockets of infected fluid that affect the spinal nerve roots and generate enough pressure to impair neurological function.
• Foraminal stenosis. Normally, nerve roots have enough room to easily slip through the foramina of the spine. However, with age and disease, they may become clogged and blocked, thus trapping and compressing the nerves.
• Pinched nerve. The two nerves most commonly pinched in the spinal cord are L5 and S1. The L5 nerve supplies the nerves to the muscles that raise the foot and big toe, and a pinched L5 may lead to weakness in these muscles. Likewise, a pinched S1 may lead to weakness with the large muscle in the back of the calf.
• Sciatica. The compression of the spinal roots of the sciatic nerve. It is characterized by pain in the low back region that radiates down the back of the thigh, the leg and into the foot. It results from diseased sciatic nerve roots or can be caused by a tumor, or intervertebral disc displacement resulting from injury or inflammation.
• Spinal stenosis. A narrowing of spaces in the spine that results in pressure on the spinal cord and nerve roots. This disorder usually involves the narrowing of one or more of three areas of the spine: the vertebral canal, the canals at the base or roots of nerves branching out from the spinal cord, and the vertebral foramina. It is usually a degenerative disorder caused by old age, but may also be an inherited disease.

KEY TERMS

Arachnoid mater— One of three meninges covering the central nervous system (CNS). The others are the dura and pia maters. The dura mater encloses the arachnoid, which in turn covers the pia mater.

Brain stem— Lowest part of the brain that connects with the spinal cord. It is a complicated neural center with several neuronal pathways between the cerebrum, spinal cord, cerebellum, and motor and sensory functions of the head and neck. It consists of the medulla oblongata, the part responsible for cardiac and respiratory control, the midbrain, which is involved in basic, involuntary body functions, and the pons, where some cranial nerves originate.

Central nervous system (CNS)— One of two major divisions of the nervous system. The CNS consists of the brain, the cranial nerves, and the spinal cord.

Cerebrospinal fluid (CSF)— A clear colorless fluid that circulates in the brain and in the subarachnoid spaces surrounding the brain and spinal cord. The CSF lies between the spinal cord and the arachnoid mater, thereby suspending the spinal cord in fluid.

Cervical vertebrae— Vertebrae of the neck.

Epidural space— This space lies between the dura mater and the walls of the vertebral canal. It contains loose connective tissue, blood vessels, and some fatty tissue.

Foramen— A hole in a bone usually for the passage of blood vessels and/or nerves.

Foramen magnum— Large opening at the base of the skull that allows passage of the spinal cord.

Intervertebral disk— Disk-shaped pads of fibrous cartilage interposed between the vertebrae of the vertebral column that provide cushioning and join the vertebrae together.

Meninges— The membranes that surround and protect the brain and spinal cord. There are three layers: the dura mater (outermost), arachnoid membrane (middle), and the pia mater (innermost).

Nervous system— The entire system of nerve tissue in the body. It includes the brain, the brainstem, the spinal cord, the nerves, and the ganglia and is divided into the peripheral nervous system (PNS) and the central nervous system (CNS).

Paraplegia— Paraplegia is permanent impairment of the trunk and lower limbs. It is caused by injury or disease affecting the spinal cord below the chest or waist.

Peripheral nerves— The nerves outside of the brain and spinal cord, including the autonomic, cranial, and spinal nerves. These nerves contain cells other than neurons and connective tissue as well as axons.

Peripheral nervous system (PNS)— One of the two major divisions of the nervous system. The PNS consists of the somatic nervous system (SNS), which controls voluntary activities and of the autonomic nervous system (ANS), which controls regulatory activities. The ANS is further divided into sympathetic and parasympathetic systems.

Quadraplegia— Quadraplegia is permanent impairment of the trunk, lower and upper limbs. It is caused by injury or disease affecting the spinal cord at the neck level.

Sacrum— The triangular-shaped bone found between the fifth lumbar vertebra and the coccyx. It consists of five fused vertebrae and it articulates on each side with the bones of the pelvis (ilium), forming the sacroiliac joints.

Sensory nerve— A nerve that receives input from sensory cells, such as the skin or muscle receptors.

Skull— All of the bones of the head.

Spinal cord— Elongated part of the central nervous system (CNS) that lies in the vertebral canal of the spine and from which the spinal nerves emerge.

Vertebra— Flat bones that make up the vertebral column. The spine has 33 vertebrae.

Vertebral canal— Hollow part of the vertebral column formed by the vertebral foramina of the stacked vertebrae. It encloses the spinal cord.

Vertebral foramen— The opening formed in vertebrae that allows passage of the spinal cord.

Resources

BOOKS

Byrne, T. N., E. C. Benzel, and S. G. Waxman. Diseases of the Spine and Spinal Cord. Oxford: Oxford University Press, 1999.

Cramer, G. D., and S. A. Darby. Basic and Clinical Anatomy of the Spine, Spinal Cord, and ANS. St. Louis: Mosby, 1995.

Palmer, S., K. Harris, and J. Kriegsman. Spinal Cord Injury: A Guide for Living (Johns Hopkins Press Health Book). Baltimore: Johns Hopkins University Press, 2000.

Reeves, C. Still Me. New York: Ballantine Books, 1999.

ORGANIZATIONS

American Paraplegia Society (APS). 75-20 Astoria Blvd., Jackson Heights, NY 11370. (718) 803-3782. 〈http://www.apssci.org/contactAPS.htm〉.

National Spinal Cord Injury Association. 701 Democracy Boulevard, Suite 300-9, Bethesda, MD 20817. (301) 588-6959; (800) 352-9424. 〈http://www.spinalcord.org/〉.

Spinal Cord Society. 19051 County Highway 1, Fergus Falls, MN 56537-7609. (218) 739-5252; (218) 739-5261. 〈http://members.aol.com/scsweb〉.

OTHER

The Spinal Cord Injury Resource Center. 〈http://www.spinalinjury.net/〉.

"The Spinal Cord or Medulla Spinalis." Bartleby.com edition of Gray's Anatomy of the Human Body. 〈http://www.bartleby.com/107/185.html〉.

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): 65–73.

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

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.