Knowledge of the locations and functions of the spinal tracts is essential in diagnosing and managing spinal cord injuries. Ascending tracts carry sensory information up the cord and descending tracts conduct motor impulses down. All nerve fibers in a given tract have a similar origin, destination, and function.
Several of these tracts undergo decussation10 (DEE-cuh-SAY-shun) as they pass up or down the brainstem and spinal cord—meaning that they cross over from the left side of the body to the right, or vice versa. As a result, the left side of the brain receives sensory information from the right side of the body and sends its motor commands to that side, while the right side of the brain senses and controls the left side of the body. A stroke that damages motor centers of the right side of the brain can thus cause paralysis of the left limbs and vice versa. When the origin and destination of a tract are on opposite sides of the body, we say they are contralateral11 to each other. When a tract does not decussate, so the origin and destination of its fibers are on the same side of the body, we say they are ipsilateral.12
The major spinal cord tracts are summarized in table 13.1 and figure 13.4. Bear in mind that each tract is repeated on the right and left sides of the spinal cord.
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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.