Britannica Sources used in this answer

How does the corticospinal decussation cause each brain hemisphere to control the opposite side of the body?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a human figure showing the full nervous system, including the brain, spinal cord, and branching peripheral nerves throughout the body, with labels for each part. — human nervous system The conduction of electrochemical stimuli from sensory receptors in the human nervous system occurs via organized groups of specialized cells, consisting largely of neurons, various neural support cells, and tracts of nerve fibers.

The corticospinal tract is responsible for controlling voluntary motor activity throughout the body. The mechanism that allows each brain hemisphere to control the opposite side of the body is known as the corticospinal decussation (or decussation of the pyramids).123

As the fibers of the corticospinal tract descend from the cerebral cortex through the brainstem, they form compact bundles on the ventral surface of the medulla oblongata called the medullary pyramids. In the lower portion of the medulla, approximately 90 percent of these fibers cross over to the opposite side of the nervous system.142

Following this crossing, the fibers descend into the dorsolateral funiculus of the spinal cord. Because the majority of these motor fibers decussate (cross) before reaching the spinal cord, signals originating in the left cerebral hemisphere are transmitted to the right side of the body, and signals from the right hemisphere are transmitted to the left side of the body.125