Anatomy

The spine is a very complex mechanical structure that is highly flexible yet very strong and stable. In the normal spine, regardless of your position or activity, including sleeping, there is always some type of physical demand being placed on it.

The primary functions of the spine include:

  • Protect the spinal cord, nerve roots, and internal organs
  • Provide flexibility of motion
  • Provide structural support and balance for upright posture. The spine bears the load of the head, shoulders and arms, and upper body. The upper body weight is then distributed to the hips and legs. The spine attempts to keep the body's weight balanced evenly over the pelvis. This reduces the amount of work required by the spinal muscles and can eliminate muscle fatigue and back pain.

The normal adult spine is balanced over the pelvis, requiring minimal workload on the muscles to maintain and upright posture.

Loss of spinal balance can result in strain to the spinal muscles and deformity of the spine as it attempts to maintain an upright posture.

Regions of the Spine

There are 33 vertebrae (bones) in the spine. Anatomically, the spine is divided into four regions:

  • The top 7 vertebrae that form the neck are called the cervical spine and are labeled C1-C7.
  • The upper back, or thoracic spine, has 12 vertebrae, labeled T1-T12.
  • The lower back, or lumbar spine has 5 vertebrae, labeled L1-L5.
  • The sacrum and coccyx (tailbone) are made up of 9 vertebrae that are fused together to form a solid bone. The sacrum is labeled S1.

Curves of the Spine

When viewed from the front or back, the normal spine is in a straight line, with each vertebra sitting directly on top of the other. A side-to-side curve in the spine is called a scoliosis.

When viewed from the side, the normal spine has three gradual curves:

  • The neck has a lordosis; it curves toward the back.
  • The thoracic spine has a kyphosis; it curves toward the front.
  • The lumbar spine also has a lordosis.

These curves help the spine to support the load of the head and upper body, and maintain balance in the upright position.

Vertebrae

Although the vertebrae have slightly different appearances as they range from the cervical spine to the lumbar spine, they all have the same basic structures, and the structures have the same names. Only the first and second cervical vertebrae are structurally different in order to support the skull.

Each vertebra has an anterior arch and a posterior arch, which form a hole, called a foramen. The spinal cord passes through foramen of each vertebra.

The anterior arch is called the vertebral body. Discs connect one vertebral body to another to allow motion of the spine and cushion it against heavy loads. Together, the vertebral bodies and discs bear about 80 percent of the load to the spine.

The posterior arch consists of the pedicles, laminae, and processes.

The pedicles are two short cylinders of bone that extend from the vertebral body. Nerve roots branch off the spinal cord and exit to the body between the pedicles of two vertebrae. If the spine becomes unstable, the pedicles may compress the nerve root, cause pain or numbness.

Laminae are two flattened plates of bone that form the walls of the posterior arch. Over time, the laminae may thicken, a process called stenosis. This thickening compresses the spinal cord and/or nerves causing pain or numbness.

The articular, transverse, and spinous processes project off the laminae. Ligaments and tendons attach to the processes. The articular processes join one vertebra to another posteriorly.

The transverse processes extend out on either side of the laminae. The spinous process is the bony projection that can be felt through the back of someone's skin.

Intervertebral Discs

Intervertebral discs are located between each vertebra from C2-C3 to L5-S1. Combined, they make up one fourth the height of the spinal column. The discs act as shock absorbers to the loads placed on the spine and allow movement of the spine. Movement at a single disc level is limited, but all of the vertebrae and discs combined allow for a significant range of motion.

The intervertebral disc is made up of two components: the annulus fibrosis and the nucleus pulpous. The annulus fibrosis is the outer portion of the disc. It is composed of layers of collagen and proteins, called lamellae. The fibers of the lamellae slant at 30-degree angles, and the fibers of each lamella run in a direction opposite the adjacent layers. This creates a structure that is exceptionally strong, yet extremely flexible.

The nucleus pulpous is the inner gel material surrounded by the annulus fibrosis. It makes up about 40 percent of the disc. This ball-like gel is contained within the lamellae. The nucleus is composed primarily of loose collagen fibers, water, and proteins. The water content of the nucleus is about 90 percent at birth and decreases to about 70 percent by the fifth decade.

Injury or aging of the annulus fibrosis may allow the nucleus pulpous to be squeezed through the annulus fibers either partially, causing the disc to bulge, or completely, allowing the disc material to escape the disc. The bulging disc or nucleus material may compress the nerves or spinal cord, causing pain.

In the early years of life, the discs have a blood supply that nourishes them. In the second and third decades, discs gradually lose this blood supply, until they are avascular. At this point, the disc begins to degenerate, or age. By the age of 50, over 95 percent of all people will have disc degeneration. The disc begins to lose water content and shrinks. The spine's range of motion and shock-absorbing ability are decreased. This may result in injury to the nerves and vertebrae, and the aging disc itself may generate pain.

Spinal Cord and Nerve Roots

The brain and spinal cord together make up the central nervous system. The spinal cord is located immediately below the brain stem. It extends through the foramen magnum, a hole at the base of the skull.

The spinal cord functions as a sophisticated network that carries information from the outer elements of the body (skin, muscles, ligaments, joints) through the sensory tracts, to the central "computer," the brain. Data are processed there, and new information such as muscle control is sent out through the motor tracts of the spinal cord.

The spinal cord ends as the conus medullar is at the L1 vertebral level, where it branches into the caudal equine, a collection of nerves that extend from the conus medlars to the sacrum. The conus medlars nerves float freely in spinal fluid, making it possible to pass a needle safely into the area to draw a sample of spinal fluid or inject drugs, anesthetics, or radiologic substances for x-ray, MRI or CT scan.

Cranial anatomy is made up of the nervous system which is the central communication network for most body actions and the skull which protects the control center of the nervous system the brain.

The Skull

The skull is made up of 22 bones: the cranium includes eight bones that surround and protect the brain and 14 bones that form the face. In adults, all but one of the skull bones are fused together by immovable joints called sutures. The sutures lock the edges of the skull bones together, like pieces in a puzzle, to form a structure that is both rigid and strong. The mandible, or lower jaw, the only moveable skull bone, allows the mouth to open and close.

In newborns, the skull bones are not completely fused; they are linked by fibrous membrane called fontanels. Fontanels allow the skull to be compressed slightly during birth and accommodate growth of the brain during early infancy. By one-and-a-half years of age, the skull sutures have formed and the fontanels have disappeared.

Cranial Bones

The frontal bone forms the forehead. Two parietal bones form the sides of the cranial roof. Two temporal bones form the lower cranial sides. The occipital bone forms the cranial rear and floor. The ethmoid bone forms part of the nasal cavity. Shaped like a butterfly, the sphenoid bone forms the middle part of the cranial floor.

Facial Bones

The 14 facial bones provide the structure of the face and form the openings through which food, water, and air enter the body. Each of the following facial bones are paired: the maxillae form the upper jaw and front of the hard palate; the zygomatic bones form the cheeks; the nasal bones form the bridge of the nose; the lacrimal bones form part of the orbit, or eye socket; the palatine bones form the rear of the hard palate; and the inferior nasal conchae divide the nasal cavity. The vomer is a single bone that makes up part of the nasal septum, which divides the nostrils, and the mandible forms the lower jaw. The maxillae and mandible secure the teeth.

Small holes in the skull bones, called foraminae, and canals enable blood vessels, such as the carotid arteries and nerves, to enter and leave the skull. The spinal cord passes through a largest hole, called the foramen magnum, in the base of the cranium to join the brain. The occipital condyles on either side of the foramen magnum articulate with the first vertebra (C1) of the spine to permit up-and-down movement of the head.

The Nervous System

The nervous system is a communication network that controls and coordinates most body actions. People are not generally conscious of some of these activities as they occur, such as regulating body temperature, breathing, or heart rate. Others, we consciously control, such as movement, talking, eating, and thinking.

The nervous system has two main parts: the central and the peripheral nervous system. The brain and spinal cord make up the central nervous system, or CNS. The nerves branching off the central nervous system make up the peripheral nervous system, or PNS. The PNS consists of nerves bundles made up of sensory and motor neurons.

Neurons

The nervous system is made up of cells called neurons. Neurons are long, thin cells that transmit electrical impulses. Neurons have many branched endings, called dendrites, which receive impulses from other neurons. An axon, or nerve fiber, carries nerve impulses to other neurons or to muscle. Neurons do not touch, but are separated by a tiny gap called a synapse. When an impulse arrives at the end of an axon, it releases chemicals that generate an impulse in the dendrites of the neighboring neuron.

There are three types of neurons: sensory, motor, and association. Sensory neurons transmit nerve impulses from sense organs (eyes, ears, nose, tongue, and touch) to the brain. They also carry nerve impulses to the brain and spinal cord. Motor neurons transmit nerve impulses from the brain and spinal cord to a specific area of the body. A nerve impulse to a muscle, for example, may cause it to contract. Association neurons make up 90 percent of all neurons and are found only in the brain and spinal cord.

Nerves

Nerves consist of bundles of both sensory and motor neurons.

The Brain

The brain is the control center of the nervous system. It enables us to think, feel, and move. The brain constantly receives information and sends out instructions to the body through the spinal cord and the body's vast network of nerves.

There are 12 pairs of cranial nerves branching off the brain. These nerves relay impulses from the sensory organs, such as the eyes or ears. Thirty-one pairs of spinal nerves branch off the spinal cord, exiting between each level of vertebrae. These nerves relay impulses to and from the rest of the body.

The largest part of the brain is the cerebrum, which controls the most sophisticated functions, such as thought, imagination, memory, emotion, speech, and sensory perception. The human cerebrum is quite large. It has two halves, or hemispheres. A band of nerve tissue, called the corpus callosum, links two halves to allow them to exchange information. Each hemisphere is covered by a layer of gray tissue, called the cerebral cortex, which is responsible for the higher functions of the brain, including conscious thought. The cortex is composed of sulci (folds) and gyri (bulges), which together provide a large surface area in the limited space inside the skull.

The cortex of each hemisphere has four lobes: the occipital, temporal, parietal, and frontal lobes. The occipital lobe controls vision. The temporal lobe controls sound and speech. The parietal lobe controls movement, touch, and recognition. And the frontal lobe controls thinking and planning.

The brain stem and hypothalamus control automatic processes, such as breathing and heartbeat.

The cerebellum acts as a "mini brain" that coordinates body balance, posture, and movement.

Brain tissue is soft and delicate, and requires protection. In addition to the protection provided by the skull, the brain is surrounded by: the connective tissue membranes, or meninges; the dura mater; the arachnoid layer; and the pia mater. Beneath the arachnoid matter is a wide space filled with cerebrospinal fluid (CSF). This fluid forms a liquid cushion that reduces the weight of the brain and protects it from knocks and jolts. CSF also helps the brain to receive nourishment.