The Anatomy of the Spinal Cord: The Nerves and the Meninges
The spinal cord serves as an information pathway between your brain and the peripheral nerves that serve the rest of your body. It’s quite delicate and requires a lot of protection. Your spinal cord has three coverings and is sheltered in the vertebral column.
The nerves of your spinal cord
The vertebral canal is home to the spinal cord, which extends from the brainstem down into the lumbar portion of the vertebral column. It begins as an extension of the medulla oblongata and runs inferiorly to around the 1st or 2nd lumbar vertebra, where it terminates as the conus medullaris.
The spinal cord has two enlarged areas, the cervical enlargement and the lumbosacral enlargement:
The cervical enlargement gives rise to the spinal nerves that exit this portion of the cord and form the brachial plexus, which innervates the upper extremities.
The lumbosacral enlargement includes spinal nerves that form the lumbar and sacral plexuses, which innervate the lower extremities.
Speaking of spinal nerves: You have 31 pairs of spinal nerves that leave the spinal cord to innervate various structures throughout the body. They’re categorized by regions: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
Each spinal nerve is formed from the convergence of posterior and anterior nerve roots. The cell bodies of the anterior nerve roots are located in the anterior horns of gray matter in the spinal cord, and the cell bodies of posterior nerve roots are located as a mass of cell bodies called the spinal ganglia (posterior root ganglia) outside of the cord. The anterior nerve roots contain motor fibers, and the posterior nerve roots contain sensory fibers. The spinal nerve roots merge to form spinal nerves (spinal nerves contain both sensory and motor fibers) where they leave the vertebral canal.
Just past the point where the nerve roots merge, each spinal nerve divides into a posterior ramus and an anterior ramus. The posterior ramus innervates the skin and deep back muscles, and the anterior ramus innervates the rest of the trunk and the extremities. The rami (like the spinal nerves) are mixed, containing both sensory and motor fibers. The recurrent meningeal branch of the spinal nerves innervates most of the vertebral column; however, the zygapophysial joints are innervated by the medial branches of the posterior rami.
The spinal cord tapers into a conical-shaped conus medullaris and actually ends around the level of the 2nd lumbar vertebra, so the nerve roots that emerge past that point become quite long because they have to extend down to exit the intervertebral foramens in the remaining lumbar and sacral levels. The collection of those spinal roots resembles a horse’s tail, so it’s referred to as the cauda equina.
The meninges and cerebrospinal fluid
The spinal cord is surrounded by three meninges (membranes) and cerebrospinal fluid. The meninges also cover the brain. Together the meninges and cerebrospinal fluid help to protect the spinal cord and the spinal nerve roots. Here are the three meninges:
Dura mater: This fibrous, outermost layer of the meninges forms a tough dural sac that’s separated from the vertebrae by an extradural space. The dural sac ends at the 2nd sacral vertebra. In fact spinal blocks (type of anesthetic) are given between the 2nd lumbar and the 2nd sacral vertebrae because only the cauda equina is located there, not the spinal cord. Dura mater also covers the nerve roots with dural root sheaths.
Arachnoid mater: This more-delicate meninx forms the middle layer. It’s separated from the dura mater by a thin layer of cells called the dura-arachnoid interface.
Pia mater: The pia mater is a vascular membrane that covers the spinal cord and is separated from the arachnoid mater by the subarachnoid space, which is filled with cerebrospinal fluid. It also covers the nerve roots and spinal blood vessels.
The spinal cord is held in place by the filum terminale, a slender extension of dural linings that run from the conus medullaris (end of the spinal cord) to the coccyx. In addition, 21 pairs of denticulate ligaments attach the spinal cord to the arachnoid mater and dura mater. These ligaments are formed from pia mater.
Cerebrospinal fluid is formed mainly by the choroid plexuses of the brain. It circulates over the brain and down around the spinal cord, where it cushions the spinal cord and removes waste products before it passes into the blood stream.