A discussion of the anatomy may be helpful.
A large number of descending and ascending tracts have been identified in the spinal cord. The three most important of these in terms of neuroanatomic localization of cord lesions are the corticospinal tracts, spinothalamic tracts, and dorsal (posterior) columns.
The corticospinal tract is a descending motor pathway. Its fibers descend from the cerebral cortex through the internal capsule and the middle of the crus cerebri. The tract then breaks up into bundles in the pons and finally collects into a discrete bundle, forming the pyramid of the medulla. In the lower medulla, approximately 90% of the fibers cross (decussate) to the side opposite that of their origin and descend through the spinal cord as the lateral corticospinal tract. These fibers synapse on lower motor neurons in the spinal cord. The 10 percent of corticospinal fibers that do not decussate in the medulla descend in the anterior funiculus of the cervical and upper thoracic cord levels as the ventral corticospinal tract. Damage to the corticospinal tract neurons (upper motor neurons) in the spinal cord results in ipsilateral clinical findings such as muscle weakness, spasticity, increased deep tendon reflexes, and a Babinski sign.
The two major ascending pathways that transmit sensory information are the spinothalamic tracts and the dorsal columns. The first neurons of both of these afferent systems begin as sensory receptors situated in the skin and stretch receptors of muscles. Their cell bodies are located in the dorsal root ganglia of the spinal nerves. The spinothalamic tract transmits pain and temperature sensation. As the axons of the first neurons enter the spinal cord, most rise one or two levels before entering the dorsal gray of the spinal cord, where they synapse with the second neuron of the spinothalamic tract. The second neuron immediately crosses the midline in the anterior commissure of the spinal cord and ascends in the anterolateral funiculus as the lateral spinothalamic tract. When the spinothalamic tract is damaged in the spinal cord, the patient experiences loss of pain and temperature sensation in the contralateral half of the body. The sensory loss begins one or two segments below the level of the lesion. The dorsal columns transmit vibration and proprioceptive information. Neurons enter the spinal cord proximal to pain and temperature neurons. They differ from pain and temperature neurons in that they do not immediately synapse. Instead, these axons enter the ipsilateral dorsal column and do not synapse until they reach the gracile or cuneate nuclei of the medulla. From these nuclei, fibers cross the midline and ascend in the medial lemniscus to the thalamus. Injury to one side of the dorsal columns will result in ipsilateral loss of vibration and position sense. The sensory loss begins at the level of the lesion. Light touch is transmitted through both the spinothalamic tracts and the dorsal columns. Therefore, light touch is not completely lost unless there is damage to both the spinothalamic tracts and the dorsal columns.
Concerning the spinal nerves and their relationship to the vertebrae, each spinal nerve is named for its adjacent vertebral body. Because there is an additional pair of spinal nerve roots compared to the number of vertebral bodies, the first seven spinal nerves are named for the first seven cervical vertebrae, each exiting through the intervertebral foramen above its corresponding vertebral body. The spinal nerve exiting below C7, however, is referred to as the C8 spinal nerve, though no eighth cervical vertebra exists. All subsequent nerve roots, beginning with T1, exit below the vertebral body for which they are named.
During fetal development, the downward growth of the vertebral column is greater than that of the spinal cord. Because the adult spinal cord ends as the conus medullaris at the level of the lower border of the first lumbar vertebra, the lumbar and sacral nerve roots must continue inferiorly below the termination of the spinal cord to exit from their respective intervertebral foramina. These nerve roots form the cauda equina. A potential consequence of this arrangement is that injury to a single lower vertebra can involve multiple nerve roots in the cauda equina. For example, an injury at the L3 vertebra can involve the L3 nerve root as well as the lower nerve roots that are progressing to a level caudal to the L3 vertebra.
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