DESCENDING TRACTS OF SPINAL CORD

DESCENDING TRACTS OF SPINAL CORD

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

Pyramidal tracts were the first tracts to be found in man. Pyramidal tracts of spinal cord are the descending tracts concerned with voluntary motor activities of the body. These tracts are otherwise known as corticospinal tracts. There are two corticospinal tracts, the anterior corticospinal tract and lateral corticospinal tract. While running from cerebral cortex towards spinal cord, the fibers of these two tracts give the appearance of a pyramid on the upper part of anterior surface of medulla oblongata hence the name pyramidal tracts.

Nerve Fibers

All the fibers of the pyramidal tracts are present since birth. However, myelination of these fibers is

completed in about 2 years after birth. The pyramidal tracts on each side have more than a million fibers.

About 70% of the fibers are large myelinated fibers having a diameter of 4 to 22 micron. Large fibers of pyramidal tracts have the tendency to disappear at old age. Since these tracts are concerned with control of voluntary movements, the disappearance of the fibers of pyramidal tracts causes automatic shivering movements in old age. Fibers of pyramidal tracts are the axons of upper motor neurons.

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. Other parts of frontal lobe

4. Somatosensory areas of parietal lobe.

It is believed that 30% of pyramidal fibers arise from primary motor area (area 4) and supplementary

motor areas, another 30% from premotor area (area 6) and the remaining 40% of fibers arise from

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

Course

Corona radiata

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. is called corona radiata. Thus, corona radiata contains both ascending fibers from thalamus and descending fibers from cerebral cortex.

Internal capsule

While passing down towards the brainstem the corona radiata converges in the form of internal capsule. It is situated in between thalamus and caudate nucleus on the medial side and lenticular nucleus on the lateral side .

In pons

The fibers descend down through internal capsule, midbrain and pons. While descending through pons,

the fibers are divided into different bundles by the nuclei of pons. At lower border of pons, the fibers are

grouped once again into a compact bundle and then descend down into medulla oblongata.

In medulla

This compact bundle of corticospinal fibers gives the appearance of a pyramid in the anterior surface of

upper part of medulla. So, the corticospinal tracts are called the pyramidal tracts.

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.

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