The spinal cord is a communication pathway linking the brain with the peripheral nervous system. In neuroscience research, the rat is a common model organism for studies related to spinal cord injuries. This preference is due to the anatomical and physiological parallels between the rat and human spinal cords. Understanding the rat’s spinal cord structure provides a framework for investigating nerve damage and recovery mechanisms.
Macroscopic Organization
The rat spinal cord is a long, cylindrical structure of nervous tissue, housed and protected within the vertebral column. It extends from the medulla oblongata at the base of the brainstem down towards the tail. The cord itself is shorter than the vertebral column that encases it. This discrepancy in length means that the spinal segments do not always align with their corresponding vertebrae.
Along its length, the spinal cord is organized into five distinct regions: cervical, thoracic, lumbar, sacral, and caudal. Two areas, the cervical and lumbar enlargements, are visibly thicker than the rest of the cord. This increased diameter is a direct result of the larger number of nerve cells required to control the complex movements of the forelimbs and hindlimbs.
The spinal cord tapers to a point in its lower region, forming a structure known as the conus medullaris. Beyond this point, the spinal nerves continue downward, fanning out in a formation that resembles a horse’s tail, hence the name cauda equina. This bundle of nerves extends into the lower vertebral canal, innervating the pelvic organs and hindlimbs.
Microscopic Structure
A cross-section of the spinal cord reveals an organized internal structure composed of gray and white matter. The gray matter is centrally located and has a characteristic butterfly or “H” shape. This area contains the cell bodies of neurons, interneurons, and glial cells, as well as unmyelinated axons. It is functionally segregated into distinct regions or “horns.”
The dorsal horn, located at the back of the “H,” is the primary receiving area for sensory information from the body. Sensory data from skin receptors, for instance, terminates almost exclusively within this region in rats. The ventral horn, at the front, contains the cell bodies of motor neurons, which send commands to the skeletal muscles. The intermediate zone, between the dorsal and ventral horns, contains interneurons and neurons that connect to the autonomic nervous system.
Surrounding the gray matter is the white matter, which consists of myelinated nerve fibers. The myelin sheath provides electrical insulation, allowing for the rapid transmission of nerve impulses. This white matter is organized into three large columns on each side, known as funiculi: the dorsal, lateral, and ventral funiculi. These columns contain the ascending and descending tracts that carry information to and from the brain.
Neural Pathways and Connectivity
Spinal nerves emerge from each segment of the cord, acting as communication lines. These nerves are formed by the joining of two distinct roots: a dorsal root and a ventral root. This arrangement ensures a directional flow of information.
Sensory (afferent) information travels from the body’s periphery toward the spinal cord. These signals are carried by sensory neurons whose cell bodies are in the dorsal root ganglion, located just outside the spinal cord. The axons of these neurons then enter the spinal cord through the dorsal root to connect with neurons in the dorsal horn.
Motor (efferent) commands originate within the central nervous system. Motor neurons in the ventral horn send their axons out of the spinal cord through the ventral root. These axons travel to skeletal muscles, initiating movement. The white matter funiculi contain the ascending tracts carrying sensory data to the brain and descending tracts delivering motor commands from the brain.
Supporting and Protective Systems
The nervous tissue of the spinal cord is protected by bone and specialized membranes called meninges. There are three distinct meningeal layers that encase the cord. The outermost layer is the dura mater. Beneath it lies the arachnoid mater, a web-like membrane. The innermost layer, the pia mater, adheres closely to the surface of the spinal cord.
Between the arachnoid mater and the pia mater is the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This fluid acts as a shock absorber, cushioning the spinal cord from physical impact. It also circulates nutrients and removes waste products, contributing to the health of the neural tissue.
A network of blood vessels ensures the spinal cord receives a constant supply of oxygen and nutrients. The vascular system includes major vessels like the vertebral arteries and a series of segmental arteries that branch off to supply different regions of the cord. This blood supply is fundamental for maintaining the high metabolic activity of the neurons and glial cells.