A spinal cord injury (SCI) is damage to the spinal cord that results in a loss of function, such as mobility and/or feeling. Frequent causes of spinal cord injuries are trauma (car accident, gunshot, falls, etc.) or disease (polio, spina bifida, Friedreich’s ataxia, etc.).
The spinal cord is a bundle of nerve cells and fibers wrapped together extending down from the brain stem to the lower back. The cord is protected by a kind of bone tunnel made up of vertebrae which are separated by membranes called discs. The brain sends electrical signals through the spinal cord, giving instructions to the legs, arms, and other areas of the body.
The spinal column is separated into 5 specific functional areas:
- Cervical / C 1-7
- Thoracic / T 1 – 12
- Lumbar / L 1 – 5
There are 33 vertebrae that make up the bone structure of the spinal column, with the last four being fused together to make the tailbone.
Each vertebrae is separated by a soft bone substance, called a disc, which acts as a cushion and a seal at the same time.
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, a body and 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.
In the chest region the thoracic spine attaches to the ribs. 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. Those with a history of spinal injuries, arthritis and even obesity, are often at higher risk of developing thoracic spinal stenosis.
The motion that occurs in the thoracic spine is mostly rotation. The ribs prevent bending to the side. A small amount of movement occurs in bending forward and backward.
The lumbar vertebrae are large, wide, and thick. There are five vertebrae in the lumbar spine. The lowest lumbar vertebra, L5, 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.
Just like the spinal column is divided into cervical, thoracic, and lumbar regions, so is the spinal cord. 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 nerve 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.
There are many nerve pathways that transmit signals up and down the spinal cord. Some supply sensation from the skin and outer portions of the body. Others supply sensation from deeper structures such as the organs in the belly, the pelvis, or other areas. Other nerves transmit signals from the brain to the body. Still others work at the level of the spinal cord and act as “go betweens” in the signal transmission process.
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. 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 and hopefully into the work place.
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
Another type of special nerves are the autonomic nerves. In spinal cord injuries, they are very important. The autonomic nerves are divided into two types: the sympathetic and parasympathetic nerves.
The autonomic nervous system influences the activities of involuntary (also known as smooth) muscles, the heart muscle, and glands that release certain hormones. It controls cardiovascular, digestive, and respiratory systems. These systems work in a generally “involuntary” fashion. The primary role of the autonomic nervous system is to maintain a stable internal environment within the body. The heart and blood vessels are controlled by the autonomic nervous system. The sympathetic nerves help to control blood pressure based on the physical demands placed on the body. It also helps to control heart rate. The sympathetic nerves, when stimulated, cause the heart to beat faster.
The sympathetic nerves also cause constriction of the blood vessels throughout the body. When this happens, the amount of blood that is returned to the heart increases. These effects will increase blood pressure. Other effects include an increase in sweating and increased irritability or a sensation of anxiety.
When 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 simulates the sympathetic nerves can cause them to become overactive. This over-activity of the sympathetic nerves is what is called autonomic dysreflexia.
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 parasympathetic 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 parasympathetic nerves arise from two areas. 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.
Complete vs Incomplete
A spinal cord injury (SCI) is categorized as either “complete” or “incomplete”. A “complete” SCI means a total loss of function and sensation below the affected vertebrae, and an “incomplete” SCI means only partial loss of function or sensation.