Chapter 27: Spinal Cord Anatomy

The cord is a segmented structure, giving rise to 31 bilateral pairs of spinal nerves via dorsal (afferent) and ventral (efferent) rootlets, and features two noticeable enlargements in the cervical and lumbosacral regions corresponding to upper and lower limb innervation. Internally, the organization is defined by an inner core of grey matter surrounded by white matter. The grey matter is structurally organized into Rexed’s laminae (I through IX and Area X), which delineate functional cell groupings. For instance, the dorsal horn (Laminae I–IV) is the primary termination site for cutaneous afferent fibers, including nociceptive input, and contains the substantia gelatinosa (Lamina II), which is essential to local pain modulation circuits like the Gate Control Theory. The ventral horns (Laminae VII, VIII, IX) house the somatic motor neurons, including large alpha motor neurons supplying skeletal muscle and smaller gamma motor neurons for muscle spindles, which are organized somatotopically with medial groups controlling axial muscles and lateral groups controlling limb muscles. The surrounding white matter is divided into dorsal, lateral, and ventral funiculi, containing specific ascending, descending, and propriospinal fiber tracts. Key ascending tracts include the Dorsal Columns (fasciculus gracilis and cuneatus) which transmit conscious proprioception and fine touch, and cross over in the medulla. The Spinothalamic Tracts (lateral and ventral) transmit pain, temperature, and coarse touch, crossing promptly at the spinal segment level. Major descending tracts, primarily involved in motor control, posture, and reflex modulation, originate from the cortex and brainstem. The vital Lateral Corticospinal Tract descends mostly contralaterally after crossing in the motor decussation of the medulla, exerting control over fine, distal movements. Other influential pathways include the Vestibulospinal and Reticulospinal Tracts, which are key to regulating muscle tone, particularly in anti-gravity muscles. The spinal cord supports fundamental functions through intrinsic connections, such as the monosynaptic muscle stretch reflex and the polysynaptic flexor and crossed extensor reflexes. Clinical syndromes, such as Brown-Séquard syndrome (hemisection), Anterior Cord syndrome, or Central Cord syndrome (often due to damage to crossing spinothalamic fibers), demonstrate how specific lesion locations affect different segmental functions and longitudinal tract transmissions, resulting in distinct patterns of motor weakness, hyperreflexia, and sensory loss.