From cortex to cord
A command to move a finger starts in the precentral gyrus. The fibres of the pyramidal tract run down through the internal capsule, the cerebral peduncles of the midbrain and the pons, and gather in the pyramids on the front of the medulla. There most cross to the other side and run down the cord in the corticospinal tracts.
The motor tract: fibres from the motor cortex run through the internal capsule and cross at the decussation of the pyramids into the spinal cord, Gray's Anatomy, 20th edition, 1918, figure 764Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray764.png)
Each pyramid carries about a million fibres, and the thickest conduct at about 70 metres a second. In the cord the fibres end on motor neurons, whose axons run out to the muscles. The axon of a motor neuron at the lower end of the cord reaches the foot, about a metre away.
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapters on the internal organization of the spinal cord and on the brainstem.
- Arthur Guyton and John Hall, Textbook of Medical Physiology, 13th ed. (2016), ch. 56.
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