What are spine, vertebrae, and disks?

Structure and Functions

The spinal column undergoes developmental changes from infancy to the adult state and then degenerative changes with aging. The alignment of the spinal column at birth is in the shape of a C curve, the fetus having been curled up. This forward-tilted curve is retained except in the neck and lower back. As the infant raises its head and attempts to see things, head control and vision gradually require the head to tilt back, resulting in a posterior curve at the neck. As children progress to standing and walking, the lower spine also develops a posterior curve (a normal lumbar lordosis). As individuals increasingly use the predominant upper extremities, these muscles become stronger and increase the pull on that side of the spine, resulting in a lateral curve (a normal scoliosis).

Humans generally have thirty-three vertebrae, but abnormalities can occur in their numbers, shape, alignment, density, and maturation. There are seven vertebrae in the neck or cervical area, twelve in the chest or thoracic area, five in the lower back or lumbar area, five usually joined together in the pelvic region to form the sacrum, and three or four rudimentary vertebrae partially fused to form the coccyx. The typical vertebra develops embryologically from two bone growth centers. The front of one bone growth center becomes the body and part of the arch, while the other bone growth center evolves into the spinous processes, the transverse processes, the articular processes, and the back portion of the arch.

The thirty-three vertebrae of the spine have various defined parts and characteristics. The oval or round vertebral body has a spongy center surrounded by a dense bone. Above and below it are layers of cartilage. Lack of the mineral calcium reduces the density of the bone, resulting in osteoporosis. This condition may occur in postmenopausal women, causing pain, fractures, and kyphosis (a thoracic humpback). The pedicle portions of this anterior arch have an upper and lower notch called the intervertebral foramina, through which the spinal nerves exit from the spinal canal. At the junction of pedicle and lamina are projections upward and downward that form the articular processes, or joints. The alignment and direction of these joints vary with the spinal region and serve to control spinal motion. The transverse processes assist in the movement of muscles and at the chest level act as links between the ribs. There are generally twelve thoracic vertebrae and twelve ribs. On occasion, an extra cervical rib may be present; this extra rib narrows the exit space at the neck and can pinch the cervical nerves and blood vessels.

The upper two cervical processes are different from all other vertebrae. The first cervical vertebra supports the head and is called the atlas. Instead of a body, it has an enlarged anterior arch and a groove on which the head rests. Its transverse processes are long, but its spinous process is a little knob. The second cervical vertebra is also unique, with an upward projection called the dens that is like a pole fitting into the ring of the first cervical vertebral arch. It represents the body of the first cervical vertebra. This pole and ring permit the head to rotate.

The cervical transverse processes or lateral projections have holes, the transverse foramina, through which the vertebral arteries send blood to the brain. As one progresses downward, the vertebrae become larger and bear more weight. The thoracic vertebrae have their joint surfaces in the frontal plane, and the transverse processes are solid. The ribs share half of the joint surfaces with the adjoining vertebrae, except for the first, tenth, eleventh, and twelfth vertebrae; these vertebrae join only their corresponding ribs. As one continues down the spine, the spinous processes become more slender and project downward, almost touching one another. The lower thoracic vertebrae are closer to the lumbar vertebrae in appearance.

The lumbar vertebrae are more massive, carry more stress, and support the weight of the body. The transverse processes in this region are thinner but longer, increasing the leverage action for muscles. The joint surfaces tilt upward, backward, and toward the center, opening the joint space when one bends forward. This action permits side bending, increasing the side pressures on the disks. The five sacral vertebrae are fused, with small ridges representing the sites of fusion. Small openings allow the upper four sacral nerves to exit the spine. The sacrum contributes the back portion of the pelvis. The coccyx, or tailbone, is composed of three or four rudimentary vertebrae, with the first distinct and separate from the other two or three, which are fused. The back of the spine is layered with muscles going to the upper extremities, with long vertical muscles underneath, intermediate-length muscles under these, and the deepest muscle groups closest to the spine and only two or three vertebrae in length.

Vertebrae are separated by washers called disks. The central portion of a disk is a gel-like substance, the nucleus pulposus, which offers hydraulic cushioning and allows some movement. This substance is enclosed in a ligamentous covering called the annulus fibrosis. Disks contribute approximately one-fourth of body length, can add about eight degrees in motion per vertebra, and can alter their shape to accommodate the solid vertebrae when the body is bent. The nucleus pulposus is 70 to 80 percent water, and about 14 kilograms (30 pounds) of pressure must be exerted constantly by ligaments to maintain the shape of the disk. Dehydration, aging, compressive forces, and simply a day of normal activity can temporarily reduce the amount of the watery nucleus pulposus, increase the bulging of the disks, and shorten one’s stature.

The spinal cord
connects the peripheral structures of the body to the brain and the nerve cells in the spinal cord itself. Until about the third month of life, the spinal cord occupies the entire length of the spinal canal. The vertebrae grow faster than the spinal cord, however, and later in life it occupies two-thirds of the length of the spinal canal (approximately 42 to 45 centimeters) and ends at about the second lumbar vertebra. The coverings on the cord continue as the filum terminale, which is attached to the sacral vertebrae.

There are three coverings of the spinal cord within the spinal canal. The outermost, toughest, and fibrous covering, called the dura, is separated from the bony surfaces by fat and blood vessels. The spinal cord is covered by a thin layer called the pia that enters into the spinal cord and separates various portions of the cord. Surrounding the pia is a protective fluid, the spinal fluid, which is held in place by the arachnoid layer; this fluid can be aspirated for analysis, as in a lumbar puncture, or spinal tap. The nerves exiting from the spinal cord are also segmental. There are only seven cervical vertebrae but eight cervical nerves. The eighth cervical nerve exits below the seventh cervical vertebra, and thereafter all roots from the vertebrae leave below the corresponding vertebra. These segmental nerves then join, divide, send messages to the brain and other organs, and control bodily functions and motion.

Ligaments run vertically in front of the vertebral bodies (the anterior longitudinal ligament) and in back of the vertebral bodies and within the spinal canal (the posterior longitudinal ligament). These ligaments support the vertebrae and disks and limit excessive motion forward and backward. The posterior longitudinal ligament ceases at about the fourth lumbar vertebra, and thus farther down the spine the fibrous rings about the disks are weaker, permitting herniations. Such leakages from the disks may press on nerve roots exiting at these levels and cause pain. These ruptures of the disks occur mostly when lifting with the trunk bent forward and sideways. There are additional ligaments between the spinous processes at the tips (the supraspinous ligament) and along the length of the spinous processes (the interspinous ligament). The portion of the arch between the spinous processes and the transverse processes is the lamina. Between the laminae are the ligamentum flavum. The interspinous ligament, ligamentum
flavum, and nucleus pulposus have no pain fibers. The laminae and disks may be removed when disk surgery is performed, and pieces of bone from the hip may be used to fuse and reduce motion at this level.

Motion between each vertebra is about eight degrees. Motion is enhanced by the facets, vertical bony projections with joint surfaces. They are structured like the other joints of the body, and thus are subject to irritation and arthritic changes. Flexion of the trunk opens the spaces in the lumbar joints and allows side bending and rotation in the same direction. Extension of the trunk compresses and limits motion in these facets.

The blood supply to the spinal cord and the surrounding tissues diminishes progressively downward. Excessive activity can lead to insufficient circulating blood to these nerves and give rise to symptoms in the lower extremity called intermittent claudication. The front portion of the spinal cord contains motor nerve cells, leading to the trunk and the muscles of the extremities, that can be inhibited or activated by impulses. The lateral areas of the spinal region contain messenger tracts and the autonomic nervous system. The back portion of the spinal cord has sensory structures that carry messages of pain, temperature, position, and touch to the brain. The different levels within the spinal cord connect to different segmental structures of the body. The fifth cervical nerve down to the first thoracic nerve connect to the upper extremities, and the second lumbar to second sacral roots connect to the lower extremities. Compression of these nerve roots at the spine may cause symptoms in areas distant from the spine.

Disorders and Diseases

The anatomy of the trunk regulates the erect, or standing, posture. It is influenced by the structure of the vertebrae and extremities and even by the tilt of the head. It is also influenced by cultural factors, emotion, habits, and occupation. Good posture involves standing straight with the head up, the shoulders back and up, the stomach in, and the hips and knees straight. This position also is most efficient in energy expenditure, since it requires the least amount of muscle activity in order to balance the weights in the front and back of the trunk, as well as the weights on the left and right sides of the body. Looking at the front of the body, the central gravitational, or weight-bearing, line falls in line with the nose and between the pelvis, knees, and ankles. From the back, the gravitational line follows the center of the head, the spinous processes, the gluteal fold between the buttocks, and between the knees and ankles. The weight stresses should be balanced between the right side and the left side of the body and between the front and the back. The head should be maintained so that the eyes and the labyrinth in the inner ear are level. Any deviations cause
imbalance in the trunk, resulting in muscle strain, ligament sprains, pain, and possibly deformities. Abnormalities in posture and gait may be attributable to muscle imbalance, deformities at birth, problems in development, disease, or surgical procedures.

There are several birth defects that affect the spine. Occasionally, during embryonic development
, the two bone growth regions that form a vertebra do not fuse together, causing an opening in the arch in the back called spina bifida. Spondylolysis occurs when the articular process remains unattached. Ligaments, muscles, and tendons are unable to attach to these areas securely, and the result is weakness in the vertebra and a reduction in stability. Occasionally, this weakness permits the upper vertebra to slide forward, a condition called spondylolisthesis. The misalignment and narrowing of the spinal canal can compress the nerve structures within it, a condition known as spinal stenosis. The lower cervical and lower lumbar regions, which need more nerve tissue to supply the extremities, are especially vulnerable to pressure symptoms.

Inequality in leg lengths or pain in the lower extremities, upper extremities, abdomen, chest, or neck can lead to compensatory reactions in the trunk. These may then cause misalignment in the spine with side deviations (scoliosis), backward deviations (kyphosis, or humpback, in the thoracic area), and forward deviations (lordosis in the low back). Other contributing factors are overuse, muscle spasm or weakness, birth defects, developmental abnormalities, diseases, aging, and trauma. Some misalignments occur after surgery on the chest, back, or abdomen. Idiopathic scoliosis (scoliosis of unknown cause) may occur in adolescent girls; when it is severe, lung and heart functions can be impaired. Temporarily increased lordosis can occur in pregnancy, to compensate for the extra weight in the abdomen. Lordosis may be permanent because of structural changes in the spine, pendulous abdomen and/or breasts, or poor muscle balance. Therefore, back problems may be caused by poor posture, behavior, occupation, and structural, neurologic, or muscular factors. Pain located only
in the back is generally attributable to poor posture, trauma, or inflammation. If pain extends below the knees, the cause may be nerve pressure at the fifth lumbar or first sacral nerve roots. Bladder and rectal sphincter disturbances may indicate sacral nerve or cord involvement and may require immediate surgical care.

The body’s center of gravity is in the front portion of the second lumbar vertebra. Thus, the compressive force of body weight is increased when one lifts an object with the arms in front of the trunk. The greater the distance of the object from the center of the body, the greater the need for the back muscles to contract. Lifting a 45-kilogram (100-pound) object may require a 545-kilogram (1,200-pound) muscle pull and compressive force. The posterior longitudinal support for the disks that is provided by ligaments narrows in the sides at the level of the fourth and fifth lumbar vertebrae. Herniations through the fibrous disk ring at these levels are more likely, and a ruptured disk can pinch the nerves exiting at these levels.

Stresses that bear on the spine are categorized as compressive, shearing, elongating, or rotational. Compressive forces may involve the muscles, disks (possible ruptures), vertebrae (fractures), and the facet joints. Shearing stress may lead to forward slippage of the fourth or fifth vertebra from the segment below. This is most common in individuals with incomplete bony union of vertebrae. Rotatory stress may affect the joints, the facets of the spine, the short spine muscles, or the ligaments. On rare occasions, an elongating stretch or traction may strain muscles and sprain or tear ligaments.

Perspective and Prospects

Examinations of the spine usually include the history of the problem: whether it is sudden or gradual and whether it is influenced by certain activities or climates. The physician checks the patient’s posture for abnormal curvatures and determines whether they are fixed or functional. Functional curves disappear when the patient is lying down. The gait should be evaluated for symmetry, balance, and deviations. The physician will measure leg lengths for inequalities, the circumference of the chest for rib flare, and the circumferences of extremities for swelling or shrinkage. Next, the trunk’s range of motion will be evaluated, followed by palpation to discover tender spots and muscle spasms.

With spinal problems, a full examination of the entire body is indicated since pain can be referred to the back from other organs. Changes in function, sensation, and motor strength can indicate spinal cord or nerve involvement. Simple X rays of the spine will indicate alignment of the vertebrae, bone density, fracture lines, extraneous bone growth, cartilage thickness, and unusual soft tissue densities caused by hemorrhage or calcification. For problems other than bone and cartilage tissue involvement, a computed tomography (CT) scan or magnetic resonance imaging (MRI) may be needed; these techniques provide images of the area to be examined that are at different levels or depths. In 2013, reserachers at the University of Zurich published a study showing that spinal cord injury can cause irreversible tissue loss within forty days of sustaining injury.

Bibliography

"Concussion Damage Looks Much Like Early Alzheimer's: Study." MedlinePlus. June 18, 2013.

Fine, Judylaine. Conquering Back Pain: A Comprehensive Guide. Rev. ed. Englewood Cliffs, N.J.: Prentice Hall, 1987.

Jenkins, David B. Hollinshead’s Functional Anatomy of the Limbs and Back. 9th ed. Philadelphia: Saunders/Elsevier, 2009.

"NIH scientists discover molecule triggers sensation of itch." National Institutes of Health. May 23, 2013.

Palastanga, Nigel, Derek Field, and Roger Soames. Anatomy and Human Movement: Structure and Function. 5th ed. New York: Butterworth Heinemann/Elsevier, 2007.

Preidt, Robert. "Spinal Injury Causes Permanent Damage Within Weeks: Study." MedlinePlus. July 3, 2013.

Scott, Judith. Good-bye to Bad Backs: Stretching and Strengthening Exercises for Alignment and Freedom from Lower Back Pain. 3d ed. New York: Princeton Book, 2002.

Tortora, Gerard J., and Bryan Derrickson. Principles of Anatomy and Physiology. 12th ed. Hoboken, N.J.: John Wiley & Sons, 2009.