Le système pyramidal

A voluntary movement ^[5], regardless of its nature, must go through several stages and numerous neural circuits before being executed ^{[3, 130]}.

The gathering of sensory information is important before, during, and even after the movement is performed ^[39]. Constant feedback is essential for correcting our movements while they are happening ^[75]; This is why the primary motor cortex sits directly in front of the primary somatosensory cortex, with numerous nerve fibers physically linking the two regions ^[75].

Cortical areas :

The planning of voluntary movement takes place largely in the prefrontal cortex (area 8) ^[39]. Movement programming occurs in the premotor cortex (area 6) ^[32], which comprises two distinct regions: the supplementary motor area ^[38] and the premotor area ^{[74, 75]}.

The execution of movements is performed by the primary motor cortex (area 4) ^{[4, 39]}, which occupies the entire precentral gyrus. Like its somatosensory counterpart, it is characterized by a somatotopic distribution of the various body parts. This distribution is disproportionate; regions where the musculature is responsible for fine movements are overrepresented in the cortex compared to other regions. This representation is illustrated by the Penfield homunculus ^{[4, 41]}, which has hands and a face that are large relative to the rest of the body.

Deep in the brain, structures like the basal ganglia act as a quality control center. They use feedback loops to fine-tune our actions, making sure every movement is smooth, accurate, and coordinated ^[5].

The pyramidal tract :

The fibers originating from the primary motor cortex form what is known as the pyramidal tract. It is so called because the neurons forming it have pyramidal cell bodies in the cerebral cortex (layer 5) ^[157]. Another plausible explanation is that the main path of the pyramidal tract (the corticospinal tract) forms the two pyramids in the medulla oblongata ^[32]; this definition excludes the corticonuclear tract.

The pyramidal tract ^[179] is the main pathway for voluntary motor activity ^{[130, 227]}. It should be noted that this pathway also includes fibers from the premotor areas and the somatosensory and associative cortices ^[31]. Indeed, only 40% of the fibers of the pyramidal tract originate from the primary motor cortex ^[3].

The pyramidal tract consists of two bundles: the corticospinal and the geniculate (also called corticonuclear or corticobulbar) tracts.

The corticospinal tract :

The corticospinal tract ^[39] travels from the cortex, passing through the corona radiata (the white matter of the cerebral hemispheres), then through the posterior limb of the internal capsule ^[4], the middle part of the cerebral peduncle, and the pons.

In the medulla oblongata, the corticospinal tract forms the two medullary pyramids. At the lower limit of the medulla (pyramidal decussation), 80% ^{[44, 57, 75]} of the fibers of this tract cross the midline to form the lateral corticospinal tract ^[5] tract descends along the lateral column of the spinal cord ^[38]. The remaining fibers form the anterior corticospinal tract ^[5].

In the ventral horn of the spinal cord, the fibers of the lateral corticospinal tract connect either to interneurons or directly to motor neurons for the muscles involved in fine movements ^[39].

The fibers of the anterior corticospinal tract continue directly down the ventral column of the spinal cord. They cross the midline at each segment in the anterior commissure to connect to the corresponding interneurons located in the anterior horns of the cord. These fibers provide bilateral innervation of the axial musculature ^[41].

Together, these two corticospinal tracts - the anterior and the lateral - provide what is known as contralateral control. This simply means that each side of your brain manages the movements of the opposite side of your body.

The corticonuclear tract :

The corticonuclear tract ^{[31, 157]} also originates from the cortex. It passes through the genu of the internal capsule (hence its name), then provides fibers to the various cranial nerve nuclei in the brainstem. From there, the fibers may or may not cross the midline, depending on the target musculature.