Chapters 5 - Motor systems 02. Pyramidal tracts

02. Pyramidal tracts

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Voluntary movement [ 5 ], whatever its nature, must go through several steps and many neural circuits before being implemented [ 3 , 130 ]. The collection of sensory information is important before, during and even after the completion of the movement [ 39 ]. It needs constant feedback gestures we do in order to correct all along in their development action [ 75 ], that is why the primary motor cortex seat directly in front of the somatosensory cortex primary, also several fibers bind neurons in these two regions [ 75 ].

1. Cortical areas:

Planning of voluntary movement happens largely in the prefrontal cortex (area 8) [ 39 ]. Programming the movement takes place at the premotor cortex (area 6) [ 32 ], it contains two distinct regions: The supplementary motor area [ 38 ] and the pre-motor area [ 74 , 75 ].

The execution of movements is provided by the primary motor cortex (area 4) [ 4 , 39 ] which occupies the precentral gyrus. It is characterized in the image of its counterpart by a somatotopic somatosensory distribution of the different body parts. This distribution is disproportionate. In fact, the areas where muscles are responsible for movements are overrepresented in the cortex compared to other regions. This representation is represented by (the Penfield homunculus) [ 4 , 41 ] which has hands and a bigger face than the rest.

So far there will be control loops and feedback involving other subcortical structures (basal ganglia in particular) to ensure successful completion of the movement [ 5 ].

2. The pyramidal tract:

The fibers from the primary motor cortex are called the pyramidal tract. So called because the neurons that form have a pyramidal cell bodies in the cerebral cortex (layer 5) [ 157 ]. Another explanation is also plausible that the main course of the pyramidal tract (the corticospinal tract) form two pyramids in the medulla [ 32 ], this definition excludes corticonuclear beam.

The pyramidal tract [ 179 ] is the main route of voluntary movements [ 130 , 227 ]. You should know that this path also includes fibers from the premotor areas and the somatosensory cortex and associations [ 31 ]. In fact, only 40% of the fibers of the pyramidal tract are from the primary motor cortex [ 3 ]!

The pyramidal tract consists of two beams: corticospinal and geniculate (also called cortico-nuclear or cortical bulbar).

2.1. The corticospinal tract:

The corticospinal tract [ 39 ] leads from the cortex through the corona radiata (white matter in the cerebral hemispheres) and the internal capsule at its posterior arm [ 4 ], then the middle part of the cerebral peduncle and pons.

At the level of the medulla oblongata, the corticospinal tract shape both medullary pyramids. A lower limit of the bulb (decussation pyramidal), 80% [ 44 , 57 , 75 ] this channel fibers will intersect the center line for forming the lateral corticospinal tract [ 5 ]. It goes down along the side of the spinal cord [ 38 ]. The rest of the fibers form prior corticospinal tract [ 5 ].

At the ventral horn of the spinal cord, the fibers of the lateral corticospinal tract will bind either interneurons or directly to motoneurons to the muscles for movement [ 39 ].

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

These two beams (anterior and lateral) thus provide a control contralateral body movements ..

2.2. The way corticonuclear:

Geniculate beam [ 31 , 157 ] as part of the cortex. It crosses the internal capsule at the knee (hence its name), then book different fibers at different cranial nerve nuclei in the brainstem. From there, fibers or will not cross the midline to the target muscles.