Spinal nerves arising from the cervical level of the cord are involved with sensory perception and motor function of the back of the head, neck, and arms. Nerves arising from the thoracic level innervate the upper trunk. Spinal nerves from the lumbar and sacral regions of the cord innervate the lower trunk, back, and legs. Lesions of the spinal cord interrupt sensation and motor function. The affected regions of the body are those innervated by spinal nerves below the level of the lesion. Interestingly, the phrenic nerves that innervate the diaphragm, the major muscle of inspiration, arise from spinal cord segments C3 through C5. Therefore, only lesions high in the cervical region will affect breathing.

The human spinal cord and the vertebral column initially grow at the same rate during embryonic development. In this way, spinal segments and the vertebral bones for which they are named are aligned. Therefore, the spinal nerves emerge from the vertebral column at the same level as the spinal cord segment from which they arise. However, after the third month of gestation, each vertebral bone becomes larger compared to the associated spinal segment; therefore, the vertebral column grows approximately 25 cm longer than the spinal cord. (This explains why the spinal cord extends only as far as the upper lumbar vertebrae.) As a result, the spinal cord segment from which each pair of spinal nerves arises is no longer aligned with its associated vertebral bone.

Because the vertebral column is now longer than the spinal cord, the intervertebral foramina have shifted downward relative to their corresponding spinal cord segment. Therefore, the spinal nerve roots arising from each segment must extend downward through the vertebral canal to reach their points of exit. This is the case especially for spinal nerves arising from the lumbar and sacral regions of the cord. As a result, only spinal nerve roots are found in the vertebral canal below the level of the first or second lumbar vertebrae. Because of its appearance, this bundle of nerve roots is collectively referred to as the cauda equina, or "horse's tail." A sample of cerebrospinal fluid may be obtained from this region by way of a lumbar puncture or "spinal tap." A needle may be safely inserted into the vertebral canal without the possibility of penetrating the spinal cord. The spinal nerve roots are easily pushed aside by the needle, significantly reducing the possibility of puncturing one of these nerves.

The spinal nerves associate with the spinal cord by way of two branches, or roots:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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