Spinal Cord: Cross-sectional Anatomy

The H-shaped structure is gray matter, the central region of the spinal cord composed of multipolar interneurons and neuron cell bodies. It is surrounded by white matter and divided into symmetrical halves by the dorsal median sulcus and ventral median fissure. At its core lies the gray commissure, which encircles the cerebrospinal fluid-filled central canal and connects the left and right sides of the spinal cord.

Dorsal roots contain sensory neuron axons organized into dorsal rootlets, while ventral roots contain motor neuron axons organized into ventral rootlets. These dorsal and ventral roots fuse laterally on each side of the spinal cord to form spinal nerves. This union creates a critical junction where sensory inputs integrate with motor outputs, enabling the spinal cord to relay information between the brain and peripheral nervous system ganglia and nerves.

The dorsal horns contain multipolar interneurons and sensory neuron axons that process incoming sensory information from descending pathways. The ventral horns contain multipolar somatic motor neurons and interneurons that initiate motor responses and control voluntary movements. Together, these horn structures enable the spinal cord to receive sensory input and generate appropriate motor commands to muscles.

Lateral horns are additional gray matter projections found in the thoracic and upper lumbar regions of the spinal cord. They contain autonomic motor neurons that give rise to preganglionic sympathetic neurons projecting to sympathetic ganglia outside the spinal cord. These neurons control involuntary functions including heart rate, blood pressure, and respiration.

White matter surrounds the gray matter and is segmented into three columns called funiculi: dorsal, ventral, and lateral. Each funiculus contains ascending and descending nerve fiber tracts dedicated to sensory and motor functions. Ascending tracts carry sensory information from peripheral nerves to the brain, while descending tracts transmit motor commands from the brain to control muscle movements and coordinate activities like walking and grasping.

The gray commissure is the middle part of gray matter that surrounds the cerebrospinal fluid-filled central canal. It acts as a bridge connecting gray matter projections, or horns, on the left and right halves of the spinal cord. This structure enables communication and integration between sensory and motor pathways across both sides of the spinal cord.

Gray matter proportion varies because different spinal cord regions require different densities of neural connections to innervate specific body parts. Regions controlling the limbs contain higher concentrations of gray matter due to the complexity of motor and sensory functions required for precise limb control. This variation reflects the spinal cord's specialized role in processing information for different anatomical regions.