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Descending tract


Descending tracts have 3 neurons 1st order neurons (UMN) : Cell bodies are in the cerebral cortex & other supra spinal areas 2nd order neurons : Short & situated in the anterior grey column of the spinal cord 3rd order neuron (LMN) : Situated in the anterior grey column & innervate the skeletal muscles through anterior roots of the spinal nerves Cortico-spinal tract : Rapid, skilled & voluntary movements image 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 & 1/3 from the post central gyrus 1/3 of fibers arise from the 1stry motor cortex (Area 4) 1/3 of fibers arise from the 2ndry motor cortex (Area 6) 1/3 of fibers arise from the parietal lobe (Area 1, 2 & 3) Descending fibers converge in the corona radiata & pass though the posterior limb of the internal capsule; Organization of fibers within the internal capsule: Close to genu (Medial) : Concerned with the cervical parts of the body 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 & the spinal cord, most fibers cross the midline at the decussation of the pyramids & enter the lateral white column of the spinal cord to form the lateral corticospinal tract (LCST) LCST descends length of the spinal cord; terminate 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 & terminate in the anterior grey column of the spinal cord segments in the cervical & upper thoracic regions 2nd order neuron : it’s an internuncial neuron 3rd order neuron : it’s a alpha or gamma motor neuron
Related lecturesDescending tract 1
Descending tract 2

Descending tract Descending tract Reviewed by Ucfm on March 03, 2012 Rating: 5
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