Motor skills are the association of several mechanisms that are voluntary, automatic, or reflexes.
The combination of these three elements results in a precise and measured gesture.
Voluntary motor skills are very fast. It is made up of a neuron that leaves the cortex. The fibers will concentrate to form a bundle which will gather at the level of the oval center, then pass through the internal capsule and descend through the brainstem. There, part of the fibers will cross the midline, decuss, and reach the nuclei of the cranial nerves.
These elements will constitute the cortico-nuclear bundle or geniculated bundle. The characteristic of this bundle is that all the fibers cross the midline, it is a bundle which is a pure motor. It runs out and disappears at the bottom of the bulb.
The second contingent is the cortico-spinal bundle, the fibers of which also arise at the level of the motor cortex or ascending frontal convolution, and are concentrated at the level of the oval center, then push through the internal capsule and pass through the elements of the brainstem or the midbrain , the protuberance and the bulb. All these fibers reach the spinal cord, but the vast majority, around 80%; goes to the lower part of the bulb, cross the midline to form the crossed pyramidal bundle. The other fibers, 20%, remain homolateral and form the direct pyramidal bundle which will cross the midline at each metameric level.
The pyramid path is that of voluntary motor skills. It discutes, so a hemisphere controls the other side. Any lesion will therefore affect the opposite side.
2. Starting point: the Cortex
The Pyramidal Beam is formed from large cells:
Betz cells, in a specific place:
The area immediately behind the central fissure is the ascending frontal convolution limited behind by the Rolando fissure and forwards by the precentral groove.
This zone starts above the lateral fissure, covers the entire lateral surface, the upper and medial parts of the brain and stops at the frontocallous groove. A muscle or a group of muscles corresponds to each point of this convolution: there is a precise somatotopy between each point and each muscle or group of muscles.
The fibers will describe a radiant crown and will gather in the oval center. At this point, a rotational movement takes place, the most lateral fibers are arranged inside, forward then they reach the internal capsule. This is located between, inside the thalamus, and outside the middle part of the lenticular nucleus called pallidum. All the fibers will go to the knee and the posterior arm.
Those who go to the knee arrange themselves inside and forward and occupy the knee.
The entire cortico-nuclear bundle occupies the knee of the internal capsule, hence the name of geniculated bundle.
The cortico-spinal fibers will be placed at the level of the posterior arm of the internal capsule.
Passed the internal capsule and the diencephalon, the fibers descend and arrive at the level of the midbrain, the peduncles.
2. In the midbrain
The pyramidal bundle is arranged in the cerebral peduncle, in its 3/5 means, bounded behind by the niger locus. At the level of the cerebral peduncle, the cortico-spinal fibers will be lateral and the cortico-nuclear contingent is medial. At the level of the brainstem, the nuclear beam leaves on each floor a part of its fibers which will decuss and gain the motor nuclei on the opposite side: the third cranial nerve is thus formed at the level of the peduncle. Part of the fibers unstuck and joined, further back the motor nucleus of the IV cranial nerve.
3. Annular protuberance
The pyramidal fibers will be arranged in front but they are dissociated by the transverse fibers and the nuclei of the bridge which go from one cerebellar peduncle to the other.
The fibers of the cortico nuclear bundle remain medial, those of the spinal remain lateral.
Part of the fibers unscrew and join the motor nuclei of the cranial nerves at the level of the protuberance:
• 5th cranial nerve, masseter contingent;
• 6th cranial nerve, born lower at the level of the bulging protuberant groove;
• 7th cranial nerve, in the same way, more laterally to the VI, above the bulbar olive.
4. At Bulb level
The fibers of the pyramidal bundle remain anterior.
There, 80% of the fibers of the cortical spinal bundle decussate at the lower part of the bulb to constitute the crossed pyramidal bundle.
The remaining 20% form the direct pyramidal bundle, the fibers remain homolateral.
The cortico-nuclear bundle will run out at the bottom of the bulb. The last fibers will reach the motor nuclei of the cranial nerves of the bulb: in the lower part of the bulb, there are no longer fibers of the cortico nuclear bundle.
5. With the Spinal Cord
In the cord, the horns are made up of gray matter, in the axis of the anterior collateral emissure for the anterior horn, posterior for the posterior horn.
The gray central-medullary substance presents in its center an often virtual orifice, it is the channel of the ependyma, which contains cerebrospinal fluid.
In front of the canal, it is the anterior or ventral gray commissure, and behind, the posterior or dorsal.
At each metameric level there is an anterior root facing the anterior collateral furrow and a posterior facing the posterior collateral furrow.
The anterior root is pure motor. The posterior root is pure sensitive and has a spinal ganglion.
The direct pyramidal bundle is placed in front opposite the anterior medial groove. The crossed pyramidal bundle is arranged in front of the posterior horn of the cord. The white area is myelinated, it is not the cell bodies of neurons.
At each metameric level, the crossed pyramidal bundle abandons fibers which will reach the anterior horn of the cord and there, act as a relay with the second neuron, the deutoneurone, which will lead to the constitution of the anterior root of the cord, the root which ends directly on the muscle. This second neuron is peripheral.
All the fibers of the crossed pyramidal bundle come from the contralateral motor cortex.
The direct pyramidal bundle will, at each metameric level, abandon fibers which will decuss, reach the head of the anterior horn, and there will act as a relay with the effector neuron named motoneuron, or deutoneurone, which will exit through the anterior root. The pyramidal bundle which has not crossed the midline at the level of the bulb will do so in the spinal cord. Thus, all the pyramidal fibers will have crossed the midline.