Pyramidal tracts

A voluntary movement [5], regardless of its nature, must pass through several stages and numerous neural circuits before being implemented [3, 130]. Gathering sensory information is crucial before, during, and even after the movement's execution [39]. We constantly need feedback from our gestures to correct them throughout their action [75]; this is why the primary motor cortex is located directly in front of the primary somatosensory cortex; furthermore, several fibers link the neurons of these two regions [75].

1. Cortical Areas:

The planning of voluntary movement occurs largely in the prefrontal cortex (Brodmann area 8) [39]. Movement programming takes place in the pre-motor cortex (area 6) [32]; the latter includes two distinct regions: the supplementary motor area [38] and the pre-motor area [74, 75].

The execution of movements is handled 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 different body parts. This distribution is disproportionate. Indeed, regions where the musculature is responsible for fine movements are overrepresented on the cortex compared to other regions. This representation is illustrated by the Penfield homunculus [4, 41], which has much larger hands and a face compared to the rest of the body.

Up to this stage, there are control loops and feedback loops involving other subcortical structures (particularly the basal ganglia) to ensure proper movement execution [5].

2. The Pyramidal Tract:

Fibers originating from the primary motor cortex form what is known as the pyramidal tract. It is so named because the neurons forming it have a pyramidal cell body in the cerebral cortex (layer 5) [157]. Another equally 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 function [130, 227]. It should be noted that this tract also includes fibers from the premotor areas and the somatosensory and associative cortex [31]. In fact, only 40% of the fibers in the pyramidal tract originate from the primary motor cortex [3]!

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

2.1. The Corticospinal Tract:

The corticospinal tract [39] travels from the cortex, passes through the corona radiata (white matter in the cerebral hemispheres), then the internal capsule at its posterior limb [4], followed by 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 in this tract cross the midline to form the lateral corticospinal tract [5]. This 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 bind either to interneurons or directly to motor neurons for the musculature involved in fine movements [39].

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

These two tracts (anterior and lateral) thus ensure contralateral control of the body's motor function.

2.2. The Corticonuclear Tract:

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