17.15: Spinal Cord: Gross Anatomy
The spinal cord resides within the protective confines of the vertebral column. It is the main pathway for information traveling between the brain and the body. It plays a fundamental role in nearly all bodily functions, from simple reflexes to complex motor movements. The spinal cord begins at the medulla oblongata at the base of the brainstem and extends downward, terminating at the conus medullaris near the first and second lumbar vertebrae. The spinal cord's length in adults is approximately 45 cm (18 inches), significantly shorter than the vertebral column, which continues to extend beyond the termination of the spinal cord.
Two key enlargements along the spinal cord are where the nerve roots concentrate to innervate the limbs. These enlargements are crucial for limb movement and sensory processing. The cervical enlargement extends from the fourth cervical to the first thoracic vertebra and corresponds with the nerves controlling the upper limbs. The lumbosacral enlargement, located between the ninth thoracic and twelfth thoracic vertebrae, innervates the lower limbs.
The spinal cord concludes its downward trajectory in a cone-shaped taper known as the conus medullaris. From this point, the filum terminale, a thin strand of fibrous tissue, extends to anchor the spinal cord to the coccyx, stabilizing it within the vertebral column. Below the conus medullaris, a bundle of nerve roots known as the cauda equina ("horse's tail") fans out to reach various parts of the lower body. These nerve roots provide motor and sensory functions to the legs and lower parts of the body.
Enveloping the spinal cord are three layers of spinal meninges, which offer protection, physical stability, and shock absorption. The innermost layer, the pia mater, directly adheres to the surface of the spinal cord. The arachnoid mater, the middle layer, provides a protective barrier. It is separated from the pia mater by the subarachnoid space, which contains cerebrospinal fluid. The outermost layer, the dura mater, is a tough protective sheath that forms the outermost covering of the spinal cord.
