Question

In most hominins (especially humans) some upper motoneurons make direct, rapid, excitatory synaptic connections with the lower motoneurons at the level of the spinal segment, with no intervening inhibitory interneuron(s). What is this pathway called and what is its adaptive significance?

Answer 1

Answer -

The motor pathway, or the pyramidal tract or the corticospinal tract, play important role as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. There are upper and lower motor neurons in the corticospinal tract.

The motor impulses originate in the giant pyramidal cells of the motor area, i.e., the precentral gyrus of the cerebral cortex. These are the upper motor neurons of the corticospinal tract. The axons of these cells pass from the cerebral cortex to the midbrain and the medulla oblongata. Peripheral motor nerves carry the motor impulses from the anterior horn to the voluntary muscles.

Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6, then descend into the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract.

These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. And the remaining, axons descend on the lateral side as the ventral corticospinal tract. These axons also synapse with lower motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord just before synapsing.