SCHMITT / SCIENCE PHOTO LIBRARY SCHMITT / SCIENCE PHOTO LIBRARY
Pyramidal tract (major pathway). The pyramidal tract is a ensemble of motor neurons regulating the precise voluntary movements. Those motor neurons originate in the motor cortex of the encephalon (mainly at the level of the precentral gyrus, that is to say from the primary motor cortex, but also the postcentral gyrus and pre-motor cortex). Each skeletal muscle of the body is represented by a precise area of the motor cortex. Thoses neurons descending vertically into the brain stem (mesencephalon then pons then medulla oblongata), forming the corticospinal tract. The corticospinal tract is composed of three pairs of motor neurons : the lateral corticospinal tract (in red), the ventral corticospinal tract (in yellow) and the corticobulbar tract (in green). The lateral corticospinal tract (or crossed pyramidal tract, in red) decussates (see definition bellow) at the level of the medulla oblongata, transits through the lateral cordons of the spinal cord, passes through the spinal nerves heading towards the contralateral skeletal muscles of the members. The ventral corticospinal tract (or anterior cerebrospinal tract, in yellow) crosses vertically the spinal cord (at the level of the ventral cordons). It exits from the spinal cord, via the spinal nerves, at different levels. Those neurons decussate at the exit of the lumbar spinal cord. They innervate the contralateral skeletal muscles of a part of the neck and the trunk. The corticobulbar tract (in green) decussates at different levels of the brain stem. Those neurones end in the nuclei of nine pairs of cranial nerves, so as to innervate the skeletal muscles of the head and the neck. The decussation is the crossing of the motor neurons inside the central nervous system. In fact, the neurons of a cerebral hemisphere innervate the muscles from the other side of the body (contralateral muscles) : the nerve fibers issue from the left motor cortex innervate the left half of the body, and vice-versa. Then, the neurons must cross the median plane to join their target. This crossing, named decussation, realize mainly (at 90%) at the exit of the brain stem (medulla oblongata).
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