User blog comment:Auslly~Bobbie~Music/Talk Like Crazy!/@comment-24099722-20140619165829/@comment-5384369-20140619180848

This is what I found: Source

Over one million axons travel through the spinal cord, including the longest axons in the central nervous system.

Neurons in the motor cortex, the region of the brain that controls voluntary movement, send their axons through the corticospinal tract to connect with motor neurons in the spinal cord. The spinal motor neurons project out of the cord to the correct muscles via the ventral root. These connections control conscious movements, such as writing and running.

Information also flows in the opposite direction resulting in involuntary movement. Sensory neurons provide feedback to the brain via the dorsal root. Some of this sensory information is conveyed directly to lower motor neurons before it reaches the brain, resulting in involuntary, or reflex movements. The remaining sensory information travels back to the cortex.