Descending tracts have three neurons:
1. 1st order neurons (UMN): cell bodies are in the cerebral cortex and other supra spinal areas
2. 2nd order neurons: short and situated in the anterior grey column of the spinal cord
3. 3rd order neuron (LMN): situated in the anterior grey column and innervate the skeletal muscles through anterior roots of the spinal nerves
Corticospinal tract: rapid, skilled and voluntary movements
1st order neuron
Axons arise from the pyramidal cells of the cerebral cortex (situated in the 5th layer), 2/3 from the pre central gyrus and 1/3 from the post central gyrus:
1. 1/3 of fibers arise from the 1stry motor cortex (Area 4)
2. 1/3 of fibers arise from the 2ndry motor cortex (Area 6)
3. 1/3 of fibers arise from the parietal lobe
(Area 1, 2 and 3).
Descending fibers converge in the corona radiata and pass though the posterior limb of the internal capsule; organization of fibers within the internal capsule:
1. close to genu (medial): concerned with the cervical parts of the body
2. away from the genu (lateral): concerned with the lower extremity.
The tract then passes through the middle 3/5 of the basis pedunculi of the midbrain; organization of fibers in the midbrain:
- medially: cervical parts of the body
- laterally: lower limbs.
When the tract enters the pons, it’s broken into many bundles by the transverse pontocerebellar fibers. In the medulla oblongata, the bundles group together to form the pyramids. At the junction of the MO and the spinal cord, most fibers cross the midline at the decussation of the pyramids and enter the lateral white column of the spinal cord to form the lateral corticospinal tract (LCST). LCST descends length of the spinal cord and terminates in the anterior grey column of all the spinal segments.
The fibers which didn’t cross, descend in the anterior white column of the spinal cord as the anterior corticospinal tract (ACST). Fibers of the ACST eventually cross and terminate in the anterior grey column of the spinal cord segments in the cervical and upper thoracic regions.
2nd order neuron:
It’s an internuncial neuron.
3rd order neuron:
It’s a alpha or gamma motor neuron.
To read more click on this link to the full article: Descending Tracts