produce energy and to make proteins. The cell bodies of neurons cluster together in the gray matter of the spinal cord and are not located in the white matter. Two regions of the gray matter are of special interest: the ventral horn and the dorsal horn. The ventral horn contains the cell bodies of motor neurons, which induce muscles to contract. The dorsal horn contains the primary sensory pathways that transmit information from the skin and muscles into the spinal cord and up to the brain. The cell bodies of sensory neurons lie outside the spinal cord in a discrete cluster known as the dorsal root ganglion.
The term “spinal column” refers to the vertebral bones and discs that collectively encase and protect the soft tissue of the spinal cord. The spinal cord is made up of nerve tracts carrying signals back and forth between the brain and the rest of the body. The spinal cord is traditionally divided into four levels, beginning with the highest (most rostral) portion and ending with the lowest (most caudal) portion: cervical, thoracic, lumbar, and sacral (Figure 2-2). Each of the four levels of the spinal cord controls the functions of a particular region of the body through a defined set of spinal nerves that enter and exit the spinal nerve roots (Figure 2-1) through particular openings in the vertebrae. Injury at one level can often lead to the loss of sensory and motor functions below that level because the injury disrupts nerve conduction to and from the brain (Table 2-1).
On the basis of pathology, there are at least three general types of spinal cord injuries: contusion, laceration, and solid cord injuries (Table 2-2). Contusion injuries, often of the cervical spine, are the most frequent and can be simulated in the most widely used animal models (see Chapter 3). The type of spinal cord injury, as well as its level and severity, dictates its functional impact and prognosis.
In an effort to systematize the classification of spinal cord injuries, the American Spinal Injury Association (ASIA) developed in 1992 a uniform and comprehensive way of assessing the level and extent of injury severity. The ASIA International Standards for Neurological Classification are based on the systematic examination of neurological function to assess any deterioration or improvement in neurological function throughout the course of the injury. The classification, which has prognostic, therapeutic, and research value, has four components: (1) sensory and motor levels, (2) the completeness of the injury, (3) the ASIA Impairment Scale, and (4) the zone of partial preservation for complete injuries (ASIA, 2000).
The sensory and motor levels refer to the spinal location of the injury and indicate the lowest (most caudal) segment with normal function. The sensory level is identified after extensive testing of skin areas via light touch