The document summarizes the descending corticospinal tract, which controls voluntary movement. It has three neurons: 1) upper motor neurons with cell bodies in the cortex, 2) internuncial neurons in the spinal cord, and 3) lower motor neurons in the spinal cord that innervate muscles. The first neuron projects from the motor cortex to the spinal cord. Its fibers decussate at the pyramids and continue in the lateral corticospinal tract to terminate in the spinal cord's anterior gray column, controlling movement.

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2. 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
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3. Cortico-spinal tract : Rapid, skilled & voluntary
movements
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4. Cortico-spinal tract : Rapid, skilled & voluntary
movements
1st order neuron
1)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
1.1/3 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 & 3)
2)Descending fibers converge in the corona radiata & 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
3)The tract then passes through the middle 3/5 of the basis pedunculi of the
midbrain; Organization of fibers in the midbrain:
1.Medially : Cervical parts of the body
2.Laterally : Lower limbs
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5. Cortico-spinal tract : Rapid, skilled & voluntary
movements
4. When the tract enters the pons, it’s broken into many bundles by the
transverse pontocerebellar fibers
5.In the medulla oblongata, the bundles group together to form the pyramids
6.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)
7.LCST descends length of the spinal cord; terminate in the anterior grey
column of all the spinal segments
8.The fibers which didn’t cross, descend in the anterior white column of the
spinal cord as the anterior corticospinal tract (ACST)
9.Fibers of the ACST eventually cross & terminate in the anterior grey column
of the spinal cord segments in the cervical & upper thoracic regions
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6. Cortico-spinal tract : Rapid, skilled & voluntary
movements
2nd order neuron : it’s an internuncial neuron
3rd order neuron : it’s a alpha or gamma motor neuron
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