The spinal cord serves as a fundamental component of the central nervous system, acting as the primary communication link between the brain and the broader body. Examining a cross-section of the spinal cord provides a clear view of its intricate internal organization. This internal structure is fundamental to understanding how the body processes sensory information and generates motor commands.
Visualizing the Spinal Cord Cross Section
When observed in cross-section, the spinal cord typically appears as a roughly oval or circular structure, though its exact shape can vary slightly depending on the vertebral level. A striking feature within this cross-section is the distinct butterfly or H-shaped region of grey matter centrally located. This grey matter is surrounded by lighter-colored white matter. The cross-section also reveals bilateral symmetry.
The H-Shaped Core: Grey Matter
The H-shaped grey matter forms the processing and integration center within the spinal cord. It is primarily composed of neuron cell bodies, their dendrites, unmyelinated axons, and various glial cells that support neuronal function. The “H” shape is formed by two dorsal (posterior) horns and two ventral (anterior) horns, connected across the midline by the grey commissure.
The dorsal horns receive and process sensory information entering the spinal cord from the body, containing interneurons and the cell bodies of sensory neurons. The ventral horns, conversely, house the cell bodies of motor neurons that send signals out to skeletal muscles, controlling voluntary movement. In the thoracic and upper lumbar regions of the spinal cord, small lateral horns protrude outwards from the grey matter, containing the cell bodies of neurons involved in the autonomic nervous system, which regulates involuntary bodily functions. A narrow central canal, filled with cerebrospinal fluid, runs through the center of the grey commissure.
The Surrounding Pathways: White Matter
Encasing the central grey matter is the white matter, which functions as the primary conduit for long-distance neural communication. This region is predominantly made up of myelinated axons, which are nerve fibers coated in a fatty substance called myelin that enhances the speed of electrical signal transmission. These myelinated axons are organized into distinct bundles known as tracts or fasciculi. The white matter is broadly divided into three main regions or columns on each side: the dorsal columns, lateral columns, and ventral columns.
The dorsal columns, located between the dorsal horns, primarily contain ascending sensory tracts that carry precise information about touch, pressure, and proprioception (body position) up to the brain. The lateral columns, positioned on either side of the grey matter, contain a mix of both ascending sensory tracts and descending motor tracts. These descending motor tracts convey commands from the brain down to the spinal cord, controlling skilled movements. Similarly, the ventral columns, located anterior to the ventral horns, also contain both ascending and descending tracts, contributing to various sensory and motor functions.
Bridging to the Body: Spinal Nerves
Spinal nerves serve as the connection points, extending from the spinal cord to link with the rest of the body. Each spinal nerve forms from the merger of two distinct roots: a dorsal root and a ventral root. The dorsal root carries sensory information, such as touch or pain signals, from the body towards the spinal cord. A swelling known as the dorsal root ganglion, containing the cell bodies of these sensory neurons, is located along the dorsal root just outside the spinal cord.
The ventral root, in contrast, carries motor commands away from the spinal cord to muscles and glands. These commands originate from motor neurons within the ventral horns of the grey matter. As the dorsal and ventral roots merge, they form a mixed spinal nerve, which then branches out to innervate specific regions of the body.