Spine Anatomy 101: Canadian Paraplegic Association

Resources >> SCI Facts

Spine Anatomy 101

Everything you wanted to know about the spine and spinal cord.

The spine works as the main support for the spinal cord and the nerve pathways that carry information from the arms, legs, and rest of the body, and carries signals from the brain to the body. This article will outline the general anatomy and functions of the spine.

Bony Anatomy

There are three 'regions' to the spinal column, cervical, thoracic and lumbar.

Cervical Spine: There are seven cervical bones or vertebrae. The cervical bones are designed to allow flexion, extension, bending, and turning of the head. They are smaller than the other vertebrae, which allows a greater amount of movement.

Each cervical vertebra consists of two parts: 1) a body 2) a protective arch for the spinal cord called the neural arch. Fractures or injuries can occur to the body, lim pedicles, or processes. Each vertebra articulates with the one above it and the one below it.

Thoracic Spine: The thoracic spine attaches to the ribs in the chest region. There are 12 vertebrae in the thoracic region. The spinal canal in the thoracic region is relatively smaller than the cervical or lumbar areas. This makes the thoracic spinal cord at greater risk if there is a fracture. The motion that occurs in the thoracic spine is mostly rotation (the ribs prevent bending to the side). Only a small amount of movement occurs in bending forward and backward.

Lumbosacral Spine: The lumbar vertebrae are large, wide, and thick. There are five vertebrae in the lumbar spine. The lowest lumbar vertebra, LS, articulates with the sacrum. The sacrum attaches to the pelvis. The main motions of the lumbar area are bending forward and extending backwards. Bending to the side also occurs.

Neuroanatomy

The spinal cord is also divided into cervical, thoracic and lumbar regions. Each portion of the spinal cord is divided into specific neurological segments.

The cervical spinal cord is divided into eight levels. Each level contributes to different functions in the neck and the arms. Sensations from the body are similarly transported from the skin and other areas of the body from the neck, shoulders, and arms up to the brain.

In the thoracic region, the nerves of the spinal cord supply muscles of the chest that help in breathing and coughing. This region also contains nerves in the sympathetic nervous system.

The lumbosacral spinal cord and nerves supply legs, pelvis, and bowel and bladder. Sensations from the feet, legs, pelvis, and lower abdomen are transmitted through the lumbosacral nerves and spinal cord to higher segments and eventually the brain.

Nerve Pathways

There are many nerve pathways that transmit signals up and down the spinal cord. Some of the functions of these nerve pathways are:

supplying sensation from the skin and outer portions of the body;

supplying sensation from deeper structures such as the organs in the belly, the pelvis, or other areas;

transmitting signals from the brain to the body.

The Motor Neuron

The upper motor neuron refers to injuries that are above the level of the anterior horn cell. This results in a spastic type of paralysis. Conversely, the lower motor neuron injury refers to an injury at or below the anterior horn cell that results in the flaccid type paralysis. This is usually seen in nerve root injuries or in the cauda equina syndrome that was mentioned previously.

The terms 'neurogenic bowel' and 'neurogenic bladder' are used to describe abnormal bowel and bladder function and can be classified as either an upper motor neuron or lower motor neuron type of problem. In general, those patients with an upper motor neuron paralysis will have an upper motor neuron bowel and bladder, and those with lower motor neuron injuries will have a lower motor neuron picture of the bowel and bladder. Adequate bowel and bladder management is critical for adequate reintegration of the patient/client into the community.

Sensory Pathways

Feelings from the body such as hot, cold, pain, and touch, are transmitted to the skin and other parts of the body to the brain where sensations are "felt." These pathways are called the sensory pathways. Once signals enter the spinal cord, they are sent up to the brain.

Different types of sensation are sent in different pathways, called "tracts." The tracts that carry sensations of pain and temperature to the brain are in the middle part of the spinal cord. These tracts are called the "spinothalamic." Other tracts carry sensation of position and light touch. These nerve impulses are carried along the back part of the spinal cord in what are called "dorsal columns" of the spinal cord.

Autonomic Nerve Pathways

In spinal cord injuries autonomic nerves are important because their primary role is to maintain a stable internal environment within the body. The automatic nervous system controls the cardiovascular, digestive and respiratory systems.

Sympathetic Nerves

The sympathetic nerves help to control blood pressure and heart rate, based on the physical demands placed on the body. They accomplish this by constricting the blood vessels throughout the body. When stimulated, these nerves also cause the heart to beat faster.

If a spinal cord injury is at or above the T6 level the sympathetic nerves below the injury become disconnected from the nerves above. They continue to operate automatically once the period of spinal shock is over. Anything that stimulates the sympathetic nerves can cause them to become overactive. This over-activity of the sympathetic nerves is what is called autonomic dysreflexia.

Parasympathetic Nerves

The parasympathetic nerves act in an opposite manner to the sympathetic nerves. These nerves tend to dilate blood vessels and slow down the heart. The most important nerve that carries sympathetic fibers is the vagus nerve. This nerve carries parasympathetic signals to the heart to decrease heart rate. Other nerves supply the blood vessels to the organs of the abdomen and skin.

The fibers that supply the organs of the abdomen, heart, lungs, and skin above the waist begin at the level of the brain and very high spinal cord. The nerves that supply the reproductive organs, pelvis, and leg, begin at the sacral level, or lowest part of the spinal cord. After a spinal cord injury, the parasympathetic nerves that begin at the brain continue to work, even during the phase of spinal shock. When dysreflexia occurs, the parasympathetic nerves attempt to control rapidly increasing blood pressure by slowing down the heart.