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.
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.
