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1. In the midbrain - they occupy the central + lateral parts of the central crus of the midbrain peduncle. The arm fibers are medial to the leg fibers.
2. In the pons - the fibers become broken up into smaller bundles to lie amongst the pontine nuclei.
3. In the medulla - the fiber bundles collect into large pyramidal masses on each side of the midline.
4. Each pyramid contains approximately 1,000,000 fibers, of which 700,000 are small + myelinated (1-4um in diameter).
5. In the lower medulla - 85% of the fibers decussate to the contralateral side (motor decussation) to form the lateral corticospinal tract.
6. The few uncrossed fibers form the anterior corticospinal tract, but these eventually decussate lower down in the spinal cord.


1. Mcminn. (2003). Last's Anatomy. Elsevier Australia. ISBN:0729537528. Read it at Google Books - Find it at Amazon
2. Netter, F. H. (2019). Atlas of human anatomy. Philadelphia, PA: Elsevier.
3. Image: no reference available.

Ⓒ A. Manickam 2018

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