Spinal cord

Spinal cord

The spinal cord is situated in the vertebral canal. It starts at the foramen magnum as a continuation of the medulla and ends at the level of L1 or L2 lumbar vertebrae in adults and at L3 in the newborn. It gradually grows so that by the age of 10 years, its location in children is the same as adults. The spinal cord ends as the tapered “conus medullaris”. Denticulate ligaments are extensions of the pia mater that anchor the spinal cord to the dura mater, which is in close proximity to the vertebrae. Filum terminale continues as the coccygeal ligament. It is an extension of the pia mater that extends from the conus medullaris, penetrates the dura mater and ultimately attaches to the coccyx bone.

It is covered by three layers of meninges - outermost tough dura mater, inner arachnoid mater and innermost pia mater. Spinal dura mater and arachnoid mater end at the level of S2 vertebra. Epidural space is the space just outside the dura mater, filled with nerve roots, blood vessels and adipose tissue. It is the site for epidural anesthesia. Subdural space is in between the dura mater and arachnoid mater. Spinal subdural space is not clinically relevant. Between the arachnoid and pia mater is the subarachnoid space. It contains CSF. Spinal taps and spinal anesthesia are done in this space. The pia mater is tightly associated with the spinal cord.

Spinal cord segments and spinal nerves

The spinal cord is divided into the following 4 regions:

• cervical
• thoracic
• lumbar
• sacral

The spinal cord is enlarged in two places:

• cervical enlargement from C3 to T1
• lumbar enlargement from L1 to S2

There are 31 segments in the spinal cord:

• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal

Each segment corresponds to the respective spinal nerve, meaning 31 pairs of spinal nerves emerge from the spinal cord. Each nerve has dorsal sensory nerve roots. The intervertebral foramen transmits the spinal nerve, parts of the dorsal and ventral nerve roots, and part of the dorsal root ganglion.

The spinal nerves which leave the spinal cord are numbered according to the vertebra at which they exit the spinal column. So, the spinal nerve T4, exits the spinal column through the foramen in the 4th thoracic vertebra. During intrauterine development of the fetus, the vertebra grows at a faster rate than the spinal cord. As a result, spinal nerves do not always exit at the same level as the corresponding vertebra. All cervical spinal nerves, except the C8 nerve, exit above their vertebrae at the same level, while the C8 nerve exits between C7 and T1 vertebrae. In the thoracic and upper lumbar regions, the difference between cord level and vertebral level is 3 segments.

A dermatome is an area of skin in which sensory nerves derive from a single spinal nerve root. Dermatomes can help to localize spinal cord level of lesions like radiculopathies and also reveal infected spinal cord segments in Herpes Zoster.

Important dermatomes and associated nerve roots

Viruses that infect spinal nerves, such as herpes zoster infections (shingles), can reveal their origin by showing up as a painful dermatomic area. Herpes zoster, a virus that can be dormant in the dorsal root ganglion, migrates along the spinal nerve to affect only the area of skin served by that nerve.

Cross section of spinal cord

Cross section of spinal cord shows butterfly shaped grey matter composed of neurons and white matter composed of nerve fibers and a central canal containing CSF. The grey matter is divided into 4 horns/columns - dorsal horn, ventral horn, intermediate horn and lateral horn. The dorsal horn consists of sensory nuclei. The ventral horn has alpha, beta and gamma motor nuclei. The intermediate and lateral horns are composed of autonomic neurons innervating the viscera and pelvic organs.

The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM × 40.

Spinal motor neurons are also called lower motor neurons. They are located in the ventral horn of the spinal cord and control effector muscles in the periphery. Somatic motor neurons are located in lamina IX of the spinal cord and innervate skeletal muscles. They are divided into alpha, beta and gamma according to the muscle fibre type that they innervate. Alpha innervate extrafusal muscle fibbers, gamma innervate intrafusal muscle fibers (muscle spindles) while beta innervate both intra and extrafusal muscle fibers. Topographically, in the spinal cord, motor nuclei supplying extensor muscles are located anteriorly while motor nuclei supplying flexor muscles are located posteriorly. From lateral to medial side the distribution is muscles of hand- forearm- arm- shoulder- trunk.

Spinal cord nuclei are organized into 10 lamina I to X. Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn, whereas laminae V and VI are concerned primarily with proprioceptive sensations. Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-IX comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia. Intermediolateral columns located from C8 to L3 form the sympathetic outflow supplying preganglionic sympathetic fibers. The parasympathetic outflow is located in the intermediolateral horns extending from S2 to S4 supplying preganglionic parasympathetic fibers.

The white matter contains myelinated and unmyelinated nerve fibers. The white matter is divided into the dorsal (or posterior) column (or funiculus), lateral column and ventral (or anterior) column. Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and anterior columns.

All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root has an enlargement called the dorsal root ganglion which consists of cell bodies of sensory neurons also called first-order neurons. After exiting from the spinal cord, the spinal nerves form plexuses which give rise to peripheral nerves. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs.