Final answer:
The largest descending tract of the spinal cord is the corticospinal tract, which controls skeletal muscle movements and originates in the primary motor cortex located in the frontal lobe.
Step-by-step explanation:
The largest descending tract of the spinal cord is the corticospinal tract. This tract is primarily responsible for controlling skeletal muscle movements. It originates from the primary motor cortex in the frontal lobe of the brain. The upper motor neuron of the corticospinal tract starts in the primary motor cortex, and then synapses on the lower motor neuron, which is located in the ventral horn of the spinal cord, eventually projecting to the skeletal muscles in the periphery.
The corticospinal tract travels through various parts of the brain before reaching the spinal cord. It passes through the white matter of the cerebrum, the internal capsule, the cerebral peduncles, the pons, and then the large white matter tract in the medulla known as the pyramids. The pyramidal decussation occurs at the medullary-spinal border where most fibers cross to the opposite side. The tract then separates into two parts, including the anterior corticospinal tract, which controls the muscles of the body trunk without immediately decussating in the medulla. The axons in this tract decussate upon reaching the appropriate spinal level.