Question

What is a descending tract? Identify each descending pathway discussed in class. Identify the function, where they are located within the spinal cord, where they terminate, the structures involved, and whether they decussate or not. If they do decussate, identify where that process occurs and why it is important for that pathway to decussate.

Answer 1

Descending tracts of the spinal cord are formed by motor nerve fibers arising from brain and descend into the spinal cord. These tracts carry motor impulses from brain to spinal cord.
Descending tracts of spinal cord are of two types:

A. Pyramidal tracts
B. Extrapyramidal tracts

Pyramidal Tracts -

These tracts are also called as corticospinal tracts.

There are two corticospinal tracts, the anterior corticospinal tract and lateral corticospinal tract.

Origin - Fibers of pyramidal tracts arise from following cells or areas of cerebral cortex:

1. Giant cells or Betz cells or pyramidal cells in precentral gyrus of the motor cortex. These cells are situated in area 4 (primary motor area) of frontal lobe.

2. Other areas of motor cortex namely, premotor area (area 6) and supplementary motor areas

3. parts of frontal lobe

4. Somatosensory areas of parietal lobe.

All the above fibers form fibers of upper motor neurons of motor pathway.

Course After taking origin, the nerve fibers run downwards in a diffused manner through white matter of cerebral hemisphere and converge in the form of a fan-like structure along with ascending fibers, which project from thalamus to cerebral cortex. This fan-like structure s called corona radiata.  While passing down towards the brainstem the corona radiata converges in the form of internal capsule. The fibers descend down through internal capsule, midbrain and pons.

In medulla This compact bundle of corticospinal fibers gives the appearance of a pyramid in the anterior surface of upper part of medulla. At the lower border of medulla, pyramidal tract on each side is divided into two bundles of unequal sizes. About 80% of fibers from each side cross to the opposite side. While crossing the midline, the fibers of both sides form the pyramidal decussation.

In spinal cord -

Fibers which cross the midline and form pyramidal decussation descend through posterior part of lateral white column of spinal cord. This bundle of crossed fibers is called as lateral corticospinal tract

Remaining 20% of fibers do not cross to the opposite side but descend down through the anterior white column of the spinal cord. This bundle of uncrossed f ibers is called as anterior corticospinal tract .  

Function of pyramidal tract -

Pyramidal tracts are concerned with voluntary movements of the body. Fibers of the pyramidal tracts transmit motor impulses from motor area of cerebral cortex to the anterior motor neurons of the spinal cord. These two tracts are responsible for fine, skilled movements.

Extrapyramidal tract - Descending tract of spinal cord other than pyramidal tracts are known as Extrapyramidal tracts

1. Medial longitudinal fasciculus

2. Anterior vestibulospinal tract

3. Lateral vestibulospinal tract

4. Reticulospinal tract

5. Tectospinal tract

6. Rubrospinal tract

7. Olivospinal tract