The spinal cord, a central nervous system pathway, connects the brain and body. A microscopic cross-section reveals its organized internal structure, allowing identification of distinct regions and cell populations. Understanding these components provides insight into the spinal cord’s role in transmitting sensory information and coordinating motor commands.
Understanding the Microscopic View
A spinal cord cross-section reveals two distinct regions: the central grey matter and the surrounding white matter. The grey matter often appears as an H-shaped or butterfly-shaped area in the center. This region is primarily composed of neuron cell bodies, unmyelinated axons, dendrites, and glial cells.
Encasing the grey matter is the white matter, which derives its lighter appearance from the abundance of myelinated axons. Myelin, a fatty substance, insulates these nerve fibers, allowing for rapid transmission of electrical signals. A small, fluid-filled channel known as the central canal is located at the very center of the grey matter, running the length of the spinal cord.
Identifying Grey Matter Structures
The H-shaped grey matter is organized into distinct projections known as horns. The dorsal horns, also called posterior horns, extend towards the back of the body. These horns primarily contain the cell bodies of interneurons and the endings of sensory neurons, which relay sensory information from the body to the brain.
The ventral horns, or anterior horns, project towards the front of the body and are larger than the dorsal horns, especially in regions controlling limb movement. These horns house the cell bodies of large motor neurons, which send signals directly to skeletal muscles. In the thoracic and upper lumbar regions, small lateral horns protrude to the sides. These lateral horns contain the cell bodies of preganglionic autonomic neurons, involved in regulating involuntary functions like heart rate and digestion.
Connecting the two halves of the H-shaped grey matter is the grey commissure, which surrounds the central canal. This bridge contains axons that cross from one side of the spinal cord to the other, allowing for communication and coordination between the two sides. The specific arrangement and size of these horns vary along the spinal cord’s length, reflecting the different demands of each body region.
Identifying White Matter Structures
The white matter, positioned around the grey matter, is organized into large columns or funiculi, each containing bundles of myelinated axons. These columns are responsible for transmitting information up and down the spinal cord. The posterior funiculus is located between the two dorsal horns, towards the back of the spinal cord.
The anterior funiculus lies between the two ventral horns, positioned towards the front. The lateral funiculus is situated on each side, between the dorsal and ventral horns. These funiculi are composed of numerous nerve tracts, which are collections of axons with similar origins, destinations, and functions.
Some tracts within the white matter are ascending, meaning they carry sensory information from the body towards the brain. Other tracts are descending, conveying motor commands from the brain down to the muscles and glands. The organization of these tracts allows for efficient and precise relay of sensory input and motor output throughout the nervous system.
Variations Along the Spinal Cord
The microscopic appearance of the spinal cord cross-section is not uniform throughout its length, displaying variations that reflect the different functions of each region. In the cervical region, particularly at the cervical enlargement (C5-T1), both the grey and white matter are significantly larger. This is due to the extensive number of neurons and nerve fibers required to innervate the upper limbs. The increased white matter volume in the cervical cord accommodates all ascending sensory tracts and descending motor tracts originating from or destined for the brain and lower body.
The thoracic region (T1-T12) shows a smaller and more circular grey matter, with the distinct presence of lateral horns, which are involved in sympathetic nervous system functions. This region also has a relatively high proportion of white matter, as it still contains many tracts passing through to other levels.
The lumbar region, especially at the lumbar enlargement (L1-S2), exhibits a large, rounded grey matter, similar to the cervical enlargement, to accommodate the extensive innervation of the lower limbs. The sacral region (S1-Co1) has the smallest overall diameter of the spinal cord. It features a very high grey matter-to-white matter ratio, meaning the grey matter occupies a larger proportion of the cross-sectional area. This is because most of the long ascending and descending tracts have already entered or exited the spinal cord at higher levels, leaving a relatively smaller volume of white matter.