Corpus: Pyramidal tract

The efferent tract of the motor cortex (pre-central gyrus) is referred to as the pyramidal tract'. It is the largest descending pathway and innervates the alpha motoneurones.

The pyramidal tract functionally includes two motor fibre tracts:

• the corticospinal tract and
• the corticonuclear tract

However, the corticonuclear tract is only part of the pyramidal tract in a broader sense, as it does not run through the pyramid in the medulla oblongata, but sends fibres to the motor cranial nerve nuclei.

The origin of the pyramidal tract lies in the primary motor cortex of the precentral gyrus (area 4 according to Brodmann). From there, the fibres run through the capsula interna and reach the crura cerebri in the mesencephalon. The fibres then continue through the pons into the medulla oblongata.

In the ventral medulla oblongata, around 70 to 90 % of the fibres cross over to the contralateral side in the pyramidal crossing (decussatio pyramidum). These fibres continue as the lateral corticospinal tract, while the remaining fibres that do not cross descend as the anterior corticospinal tract.

The pyramidal tract serves as the pyramidal motor system for voluntary motor function and is assigned to fine motor function - in contrast to the extrapyramidal motor system, which is the functional correlate for gross motor function or mass movements of the trunk and proximal limb muscles. The pyramidal motor system and the extrapyramidal motor system both belong to the somatomotor system. The earlier assessment that the extrapyramidal motor system is part of the involuntary motor system is now considered obsolete.

Damage to the pyramidal tract within the brain, e.g. due to a stroke, initially results in flaccid paresis with impairment of fine motor skills. After a short period, the flaccid paralysis turns into spastic paralysis. All of the neurological changes that occur are summarised under the term pyramidal tract signs.

Babinski-Reflex[Bearbeiten]

In the case of pyramidal tract damage, the otherwise suppressed propiospinal circuits are reactivated. This makes the Babinski reflex positive. This primitive reflex is normally suppressed in adults by the activity of the pyramidal tract. In infants, it can still be triggered physiologically.

Memory aid[Bearbeiten]

In lesions of the pyramidal tract, a distinction is made between a defect in the area of the first and the second neuron. Assume that the first neuron is a rider on the 2nd neuron, the horse:

• If the horse is injured, the rider can no longer ride: Flaccid paralysis
• If the rider has fallen off the horse, the horse is no longer led: Spastic paralysis