17.16: Spinal Cord: Cross-sectional Anatomy
The cross-sectional anatomy of the spinal cord offers a detailed view of its complex structure and function within the central nervous system. At the core of the spinal cord lies the gray matter, characterized by its butterfly or "H"-shaped appearance in cross-section. This central region is enveloped by white matter, with the overall structure divided into symmetrical halves by the dorsal median sulcus and the ventral median fissure.
Gray Matter and its Components
Central to the gray matter is the gray commissure, encircling the central canal filled with cerebrospinal fluid (CSF). This commissure acts as a bridge connecting the left and right sides of the spinal cord. Projections from the gray commissure extend outward, forming the dorsal and ventral horns.
The dorsal horns contain multipolar interneurons and the axons of sensory neurons, which organize into dorsal rootlets and dorsal roots. These structures receive input from descending pathways and are pivotal in processing incoming sensory information. Dysfunction of the dorsal horns can lead to sensory abnormalities and contribute to conditions such as chronic pain syndromes.
The ventral horns comprise multipolar somatic motor neurons and interneurons essential for initiating motor responses. Their axons bundle into ventral rootlets and ventral roots, forming spinal nerves that transmit motor signals from the spinal cord to muscles, controlling voluntary movements and motor functions. The dorsal and ventral roots converge on each side of the spinal cord to form the spinal nerves. This union is a critical juncture where sensory inputs are integrated with motor outputs, facilitating the spinal cord's primary role in relaying information.
The gray matter exhibits lateral horns in certain regions of the spinal cord, notably the thoracic and upper lumbar areas. These contain autonomic motor neurons that give rise to preganglionic sympathetic neurons. These neurons are then projected to sympathetic ganglia located outside the spinal cord. As a result, these neurons play a crucial role in controlling various involuntary functions, such as heart rate, blood pressure, and respiration.
The quantity of gray matter within the spinal cord varies along its length, reflecting the density of neural connections necessary to innervate different body parts. Regions controlling the limbs, for example, contain a higher concentration of gray matter due to the complexity of motor and sensory functions required.
White Matter Organization
The white matter surrounds the gray matter and is segmented into dorsal, ventral, and lateral columns, known as funiculi. These columns house ascending and descending nerve fiber tracts dedicated to sensory and motor functions.
The ascending pathways carry sensory information from the peripheral nerves to the brain. These tracts process sensory data such as touch, temperature, pain, and proprioception. Notable ascending tracts include the dorsal columns fasciculus gracilis and fasciculus cuneatus.
Conversely, the descending tracts are involved in transmitting motor commands from the brain to the spinal cord. These tracts influence muscle movements and coordinate activities such as walking, grasping, and posture. Key descending tracts include the corticospinal tract.
In addition to containing portions of ascending and descending tracts, the lateral funiculus also includes important pathways like the lateral corticospinal tract and the rubrospinal tract, which play roles in voluntary movement control and modulation of limb movements.
