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Spinal Cord Injury: A Brief Explanation

The central nervous system has two major divisions: the brain and the spinal cord. In the case of a spinal cord injury (SCI) the damage has been done to the spinal cord. Spinal cord injuries effects each individual differently because the cord is usually not damaged in the same way.

Structures of the Nervous System

The spinal cord is an extension of the brain. It is a bundle of nerve fibers and cells from which spinal nerves arise to connect your brain with your muscles, skin and internal organs. Operating much like a telegraph line, it is a transmitter of two-way communication between the brain and body parts.

From the spinal cord, 31 pairs of spinal nerves branch out to all parts of the body at various levels. From the upper part of the cord, some of these nerves branch and rebranch, leading to the upper torso, arms and hands. From the lower part of the cord, other nerves branch and rebranch, leading to the pelvis, thighs, calves and feet. These 31 pairs of spinal nerves act as the major pathways leading to and from the spinal cord. Branching and dividing ever further from the spinal cord are the peripheral nerves that reach every millimeter of your skin surface, every muscle, every blood vessel, every bone - every part of your body from your head to your toes.

How the Nervous System Works

The nerve impulses for muscle movement begin in the brain and travel down through the spinal cord to the spinal nerves and out the spinal nerves to the muscles (the motor pathway). When that pathway is interrupted by spinal cord injury, the result is paralysis - the inability to move a muscle. Sensation or feeling occurs when impulses travel from the skin or organs through the spinal nerves and travel up through the spinal cord to the brain (the sensory pathway). Spinal cord injury also interrupts these messages, resulting in a lack of sensation.

The Spinal Column

The spinal cord is protected by bony structures called vertebrae - more commonly known as the "backbone". These vertebrae form a curve held together by muscles and tough, fibrous tissue. The spinal canal runs through the center of the vertebrae and the spinal cord is in this canal.

There are four major divisions of the spinal column:

  1. The cervical region, or neck;
  2. The thoracic region, or chest;
  3. The lumbar region, or lower back; and,
  4. The sacral region, or tailbone.

The cervical region contains the first seven vertebrae and the first eight spinal nerves. The thoracic region is composed of the next 12 vertebrae and the next 12 spinal nerves. The next five vertebrae and spinal nerves make up the lumbar region. The last section of the spinal column is the sacrum and coccyx. In this section, nine vertebrae fuse into two separate bone structures before adulthood. This area contains six spinal nerves. The brain and spinal cord are pictured below. The bony parts of the spinal column, vertebrae, are shown in black. The nervous tissue is shown in shades of gray.

Spinal nerves exit the spinal canal between the vertebrae. Because the spinal cord is shorter than the bony spine, spinal nerves exit below their spinal cord segments. The spinal cord ends at approximately L-1. The spinal nerves that exit below I 1 are called the caude equine.

If the spinal cord injury occurs in the cervical region, the diagnosis is tetraplegia, or weakness of all four extremities. If the injury occurs in the other regions, the diagnosis is paraplegia, or weakness of the two lower extremities.

Your SCI diagnosis is based on the level of the neurologic injury and refers to spinal nerves that still have function. You also have a diagnosis on the level of the spinal fracture. The numbered level of your fracture may be slightly different from the numbered level of your neurologic injury depending on the extent of the spinal cord injury.

ASIA Impairment Scale (AIS)

The American Spinal Injury Association impairment scale is part of the ASIA spinal cord injury classification. It divides spinal cord injuries into 5 categories, with optional clinical syndromes. The following definitions are used in grading the degree of impairment resulting from a spinal cord injury:

  • A = Complete. No sensory or motor function is preserved in the sacral segments S4-S5.
  • B = Incomplete. Sensory but not motor function is preserved below the neurological level and includes the sacral segments S4-S5.
  • C = Incomplete. Motor function is preserved below the neurological level and more than half of key muscles below the neurological level have a muscle grade less than 3 (Grades 0-2).
  • D = Incomplete. Motor function is preserved below the neurological level, and at least half of key muscles below the neurological level have a muscle grade greater than or equal to 3.
  • E = Normal. Sensory and motor functions are normal.

Note: To receive the grade of C or D, the individual must be incomplete, that is have sensory or motor function in the sacral segments S4-S5. In addition, the individual must have either (1) voluntary anal sphincter contraction or (2) sparing of motor function more than three levels below the motor level.

Incomplete/Incomplete Injuries and the Zone of Partial Preservation

A complete injury is defined as the absence of sensory and motor function in the lowest sacral segment. An incomplete injury is defined as a partial preservation of sensory and/or motor function in the lowest sacral segment (S4-5). Sacral sensation includes sensation at the anus as well as deep anal sensation. The test of motor function is the presence of voluntary contraction of the external anal sphincter upon digital examination. The zone of partial preservation (ZPP) is used only with complete injuries, and refers to those dermatomes and myotomes caudal to the neurological level that remain partially innervated.

The Autonomic Nervous System

Besides the parts of the nervous system that control voluntary movement, the body has another system of nervous tissue that controls the automatic functions of internal organs and glands. This is known as the autonomic nervous system. This system operates day and night without any awareness or conscious control on your part.

There are two divisions of the autonomic nervous system: the sympathetic and the parasympathetic. The sympathetic and parasympathetic nervous systems regulate involuntary functions and, while they generally have opposing effects, they normally balance each other. One system predominates in certain situations and the other system predominates in other situations, depending on the body's needs.

The sympathetic nervous system supplies energy for sudden responses, such as the reactions of fight, flight or fear. When you confront situations warranting a response, your sympathetic nervous system prepares you for immediate action: your blood pressure goes up; your heart beats faster; your pupils get larger. In this way the sympathetic system alerts and readies your body for upcoming reactions.

The parasympathetic nervous system supplies a slowing down action. It keeps your body on an "even keel" preventing it from becoming over excited and enabling it to function properly at all times. Through the parasympathetic system the blood pressure lowers, the heart beats more slowly, and the size of the pupils decreases.

What Happens After Spinal Cord Injury

The spinal cord can be injured at any point along its entire length (usually by a broken bone of the vertebrae or dislocation of the vertebrae). Generally, the higher the point of injury, the greater the loss of function. The parts and functions of the body located above the point of injury will continue to function unimpaired. The parts and functions that are below the point of injury, however, cannot function in their normal way. Messages from below the level of injury are blocked by the damage to the spinal cord and can no longer reach the brain for an appropriate response.

Spinal cord injury also interrupts the autonomic nervous system, and the brain can no longer regulate the functions of this system. This can result in changes in blood pressure, the body's temperature control, the functioning of the digestive tract, sexual organs, the bladder and bowel system function.

As we mentioned earlier, the parasympathetic and sympathetic nervous systems are normally in complete balance. However, when a spinal cord injury occurs, the brain can no longer maintain the balance because of the interruption of the lower parasympathetic system and some portion of the sympathetic system or in quadriplegia, the entire sympathetic system. This interruption of the autonomic nervous system has more serious implications for the tetraplegic and can cause episodes of autonomic dysreflexia. (AD).


Some messages do not need to go to the brain to initiate an action. This is known as a reflex. The spinal cord has natural reflex functions that are influenced by the brain but do not require brain control.

Most spinal cord injury patients have a healthy, functioning spinal cord below the level of injury. Before the injury, the brain regulated these automatic spinal cord functions, but an effect of spinal cord injury is the loss of brain influence over these functions. The brain can no longer modify and regulate these reflex actions because they are exaggerated causing spasms. In other words, these previously normal reflex actions increase and are exaggerated.


In spinal cord injury, the loss of function that occurs below the level of injury may result in any or all of the following:

  • Paralysis of the skeletal muscles
  • Loss of sensation, i.e., sense of touch, pain, temperature, position, vibration and deep pressure
  • Changes in breathing patterns and capacity
  • Abnormal function of the autonomic nervous system that may effect pulse, blood pressure, body temperature, the ability to sweat, bladder and bowel function, glandular secretions and sexual function.
  • Each SCI is unique and depends on the extent of damage to the spinal nerves. Ongoing testing and monitoring of your function will determine level of injury and whether the injury is complete or incomplete.


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