The spinal cord lies within the vertebral canal. It begins at the foramen magnum as a continuation of the medulla and ends at the level of the L1 or L2 lumbar vertebrae in adults, and at L3 in the newborn. As a child grows, the vertebral column lengthens faster than the spinal cord, so the cord’s ending level appears to “ascend.” By about age 10, the spinal cord level matches the adult position.
The spinal cord tapers at its inferior end to form the conus medullaris. Denticulate ligaments are extensions of the pia mater that anchor the spinal cord to the dura mater (which lies close to the vertebrae). The filum terminale is another pia mater extension: it continues from the conus medullaris, pierces the dura mater, and ultimately attaches to the coccyx as the coccygeal ligament.
The spinal cord is covered by three layers of meninges:
Spinal dura mater and arachnoid mater end at the level of the S2 vertebra.
The pia mater is tightly associated with the spinal cord.
The spinal cord is divided into the following 4 regions:
The spinal cord is enlarged in two places:
There are 31 segments in the spinal cord:
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.
Spinal nerves are numbered according to the vertebra at which they exit the spinal column. For example, the T4 spinal nerve exits through the foramen at the 4th thoracic vertebra. During intrauterine development, the vertebral column grows faster than the spinal cord. As a result, spinal nerves do not always exit at the same level as the corresponding spinal cord segment.
A dermatome is an area of skin whose sensory innervation comes from a single spinal nerve root. Dermatomes help localize the spinal cord level of lesions such as radiculopathies, and they can also indicate which spinal nerve roots are involved in herpes zoster.
| Nerve root | Dermatome area |
|---|---|
| C2 and C3 | posterior head and neck |
| C6 | thumb |
| C7 | middle finger |
| C8 | little finger |
| T1 | inner forearm |
| T2 | upper inner arm |
| T4 | nipple |
| T6 | bottom of sternum |
| T10 | umbilicus |
| T12 | pubic bone |
| L3 | medial aspect of knee, anterior aspect of each thigh, medial aspect of the upper posterior lower leg |
| L4 | anterior aspect of knee, medial malleolus, anteromedial half of leg, anterior thigh (below L3 area), medial most ventral and dorsal surfaces of great toe |
| L5 | anterolateral leg, most of great toe, dorsum of foot, including the toes |
| S1 | lateral malleolus and little toe, heel, posterolateral thigh |
| S4 and S5 | Skin over perineum, perianal region, genitals |
Viruses that infect spinal nerves, such as herpes zoster (shingles), often present in a painful dermatomal distribution. Herpes zoster can remain dormant in the dorsal root ganglion and then migrate along a spinal nerve to affect only the area of skin supplied by that nerve.
A cross section of the spinal cord shows butterfly-shaped gray matter (composed of neurons) and surrounding white matter (composed of nerve fibers), with a central canal containing CSF. The gray matter is divided into 4 horns/columns:
The dorsal horn contains sensory nuclei. The ventral horn contains alpha, beta, and gamma motor nuclei. The intermediate and lateral horns contain autonomic neurons that innervate 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 muscle. They are divided into alpha, beta, and gamma types based on the muscle fiber type they innervate:
Within the spinal cord, motor nuclei are arranged topographically:
Spinal cord nuclei are organized into 10 laminae (I to X).
The intermediolateral columns from C8 to L3 form the sympathetic outflow and supply preganglionic sympathetic fibers. The parasympathetic outflow is located in the intermediolateral horns from S2 to S4 and supplies preganglionic parasympathetic fibers.
The white matter contains myelinated and unmyelinated nerve fibers. It is divided into:
Ascending tracts are found in all columns, whereas descending tracts are found only in the lateral and anterior columns.
All spinal nerves contain both sensory and motor axons, which separate into two nerve roots.
The dorsal root has an enlargement called the dorsal root ganglion, which contains the cell bodies of sensory neurons (first-order neurons). After leaving the spinal cord, spinal nerves form plexuses that give rise to peripheral nerves. Spinal nerves of the thoracic region (T2 through T11) do not form plexuses; instead, they give rise to the intercostal nerves found between the ribs.
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