PYRAMIDAL TRACT
Pyramidal tract (major pathway) The pyramidal tract is ensemble motor neurons regulating the precise voluntary movements Those motor neurons originate in the motor cortex the encephalon (mainly at the level the precentral gyrus that is to say the primary motor cortex but also the postcentral gyrus pre-motor cortex) Each skeletal muscle the body is represented by precise area 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 three pairs motor neurons : the lateral corticospinal tract (in red) the ventral corticospinal tract (in yellow) the corticobulbar tract (in green) The lateral corticospinal tract (or crossed pyramidal tract in red) decussates (see definition bellow) at the level the medulla oblongata transits through the lateral cordons the spinal cord passes through the spinal nerves heading towards the contralateral skeletal muscles the members The ventral corticospinal tract (or anterior cerebrospinal tract in yellow) crosses vertically the spinal cord (at the level the ventral cordons) It exits the spinal cord via the spinal nerves at different levels Those neurons decussate at the exit the lumbar spinal cord They innervate the contralateral skeletal muscles part the neck the trunk The corticobulbar tract (in green) decussates at different levels the brain stem Those neurones end in the nuclei nine pairs cranial nerves so as to innervate the skeletal muscles the head the neck The decussation is the crossing the motor neurons inside the central nervous system In fact the neurons cerebral hemisphere innervate the muscles the other side the body (contralateral muscles) : the nerve fibers issue the left motor cortex innervate the left half the body 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 the brain stem (medulla
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