Descending motor pathways carry motor signals from the brain down the spinal cord and to the target muscle or organ. They typically consist of an upper motor neuron and a lower motor neuron. The lateral corticospinal tract is a descending motor pathway that begins in the cerebral cortex, decussates in the pyramids of the lower medulla  (also known as the myelencephalon, the most posterior division of the brain ) and proceeds down the contralateral side of the spinal cord. It is the largest part of the corticospinal tract. It extends throughout the entire length of the medulla spinalis, and on transverse section appears as an oval area in front of the posterior column and medial to the posterior spinocerebellar tract.
Axons in the lateral corticospinal tract leave out of the tract and into the anterior horns of the spinal cord. It controls fine movement of ipsilateral limbs (albeit contralateral to the corresponding motor cortex) as it lies distal to the pyramidal decussation. Control of more central axial and girdle muscles comes from the anterior corticospinal tract.
Poliomyelitis and amyotrophic lateral sclerosis arise out of complications in the lateral corticospinal tract. Both of these diseases result in muscle atrophy. In poliomyelitis, the poliovirus destroys motor neurons found in the brainstem and anterior horn of the spinal cord. This inhibits the lower motor neurons of the lateral corticospinal tract from travelling to the target muscle or organ and delivering the signal from the brain. In amyotrophic lateral sclerosis, the lateral side of the spinal cord undergoes scarring. The astrocytes found in the spinal cord, which are vital to the healthy functioning of the central nervous system, fail to reabsorb a specific neurotransmitter. This causes the neurotransmitter to reach toxic levels and leads to scarring in the lateral regions of the spinal cord. This also disrupts neural signal transmission.