The spinal cord serves as the primary communication pathway between the brain and the rest of the body, transmitting sensory information from the body to the brain and motor commands from the brain to muscles. This bidirectional relay enables coordinated movement, reflex responses, and the integration of sensory inputs. Damage to the spinal cord can interrupt this signal transmission, leading to loss of motor and sensory function below the injury site. Thus, its main function is to relay signals, which is essential for both voluntary and involuntary nervous system activities.

Reference: Gray's Anatomy, Standring S., 42nd Edition.