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Main Model

Terminal filament

Spinal Cord

The spinal cord is the major reflex center and conduction pathway between the body and brain. This cylindrical structure, slightly flattened anteriorly and posteriorly, is protected by the vertebrae, their associated ligaments and muscles, the spinal meninges, and the cerebrospinal fluid (CSF).

The spinal cord begins as a continuation of the medulla oblongata (often called the medulla), the caudal part of the brainstem. In adults, the spinal cord is 42-45 cm long and extends from the foramen magnum in the occipital bone to the level of the L₁ or L₂ vertebra. However, its tapering inferior end, the conus medullaris, may terminate as high as T₁₂ vertebra or as low as L₃ vertebra. Thus the spinal cord occupies only the superior two thirds of the vertebral canal.

The spinal cord is enlarged in two regions in relationship to innervation of the limbs. The cervical enlargement extends from C₄ through T₁ segments of the spinal cord, and most of the anterior rami of the spinal nerves arising from it form the brachial plexus of nerves that innervates the upper limbs. The lumbosacral enlargement extends from T₁₁ through S₁ segments of the spinal cord, inferior to which the cord continues to diminish as the conus medullaris. The anterior rami of the spinal nerves arising from this enlargement make up the lumbar and sacral plexuses of nerves that innervate the lower limbs.

Spinal Nerves and Nerve Roots

The portion of the spinal cord giving rise to the rootlets and roots that ultimately form one bilateral pair of spinal nerves is designated a spinal cord segment, the identity of which is the same as the spinal nerves arising from it.

Cervical spinal nerves (except C₈) bear the same alphanumeric designation as the vertebrae forming the inferior margin of the IV foramina through which the nerve exits the vertebral canal. The more inferior spinal (T₁ through Co₁) nerves bear the same alphanumeric designation as the vertebrae forming the superior margin of their exit. First cervical nerves lack posterior roots in 50% of people, and the coccygeal nerve may be absent.

In embryos, the spinal cord occupies the whole length of the vertebral canal; thus cord segments lie approximately at the vertebral level of the same number, the spinal nerves passing laterally to exit the corresponding IV foramen. By the end of the embryonic period (8^th week), the tail-like caudal eminence has disappeared, and the number of coccygeal vertebrae is reduced from six to four segments. The spinal cord in the vertebral canal of the coccyx atrophies.

During the fetal period, the vertebral column grows faster than the spinal cord; as a result, the cord "ascends" relative to the vertebral canal. At birth, the tip of the conus medullaris is at the L₄-L₅ level. Thus, in postnatal life, the spinal cord is shorter than the vertebral column; consequently, there is a progressive obliquity of the spinal nerve roots. Because the distance between the origin of a nerve's roots from the spinal cord and the nerve's exit from the vertebral canal increases as the inferior end of the vertebral column is approached, the length of the nerve roots also increases progressively.

The lumbar and sacral nerve roots are therefore the longest, extending far beyond the termination of the adult spinal cord at approximately the L₂ level to reach the remaining lumbar, sacral, and coccygeal IV foramina. This loose bundle of spinal nerve roots, arising from the lumbosacral enlargement and the conus medullaris and coursing within the lumbar cistern of CSF caudal to the termination of the spinal cord, resembles a horse's tail, hence its name - the cauda equina (Latin horse tail).

Arising from the tip of the conus medullaris, the filum terminale descends among the spinal nerve roots in the cauda equina. The filum terminale is the vestigial remnant of the caudal part of the spinal cord that was in the tail-like caudal eminence of the embryo. Its proximal end (the filum terminale internum, or pial part of the terminal filum) consists of vestiges of neural tissue, connective tissue, and neuroglial tissue covered by pia mater. The filum terminale perforates the inferior end of the dural sac, gaining a layer of dura and continuing through the sacral hiatus as the filum terminale externum (or dural part of the terminal filum, also known as the coccygeal ligament) to attach to the dorsum of the coccyx. The filum terminale is an anchor for the inferior end of the spinal cord and spinal meninges.