Pyramidal Tracts

Pyramidal TractsThe pyramidal tracts are the axons of nerve cells situated in the surface or cortex of the cerebral hemispheres. They extend down through the brain, one on either side, till they reach the medulla where they undergo partial decussation.

One portion of each tract crosses to the opposite side and runs down in the lateral part of the cord between the anterior and posterior horns as the crossed pyramidal tract. The remainder of each tract passes down on its own side on the mesial aspect of the anterior horn as the direct pyramidal tract (Fig. 38).

A number of the fibres of this tract cross to the opposite side in each segment, so that by the time the end of the tract is reached all the fibres have crossed to the other side of the cord.

The fibres of the pyramidal tracts terminate by arborising around cells of the posterior horn, the axons of which in turn arborise around motor cells in the anterior horn. By this arrangement an impulse passing down a pyramidal fibre will set in action not simply a single muscle but a co-ordinated group of muscles which customarily act together.