17.22: Spinal Nerves: Anatomy
Spinal nerves are pivotal conduits in the nervous system, bridging the central nervous system (CNS) with the peripheral nervous system (PNS). These nerves enable a complex communication network between the brain, spinal cord, and the rest of the body, facilitating sensory input, motor output, and autonomic functions.
There are 31 bilateral pairs of spinal nerves, each emerging from the spinal cord through the intervertebral foramina—openings between adjacent vertebrae. These nerves are symmetrically organized and are categorized based on the region of the spine from which they emerge — 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal (Co1).
- • Cervical nerves (C1-C8): These eight nerve pairs transmit signals to and from the head, neck, and upper extremities. The cervical nerves are:
- C1: Suboccipital nerve
- C2: Greater occipital nerve
- C3: Lesser occipital nerve
- C4: Supraclavicular nerve
- C5: Dorsal scapular nerve
- C6: Superior plexus root
- C7: Middle plexus root
- C8: Inferior plexus root
- • Thoracic nerves (T1-T12): The 12 pairs of thoracic nerves correspond to the 12 thoracic vertebrae and control the sensation and motor function of the chest, upper back, and abdominal muscles. The T1 to T11 nerves are the intercostal nerves, while T12 is the subcostal nerve.
- • Lumbar nerves (L1-L5): Five pairs of lumbar nerves control the lower back, hips, and parts of the legs. The five lumbar nerves are:
- L1: Iliohypogastric nerve
- L2: Ilioinguinal nerve
- L3: Genitofemoral nerve
- L4: Lateral femoral cutaneous nerve
- L5: Lumbar plexus root
- • Sacral nerves (S1-S5): The sacral nerves consist of five pairs, and they are responsible for the function of the lower extremities, pelvic organs, and some muscles in the hips and legs. The sacral nerves are:
- S1: Superior gluteal nerve
- S2: Inferior gluteal nerve
- S3: Sciatic nerve
- S4: Pudendal nerve
- S5: Sacral plexus root
- • Coccygeal nerve (Co1): The single coccygeal nerve pair, called anococcygeal nerve, provides sensory innervation to the skin over the coccyx and surrounding area, as well as to some of the muscles in the pelvic floor. Its motor functions are minimal and not as well defined as those of the other spinal nerves.
Structure of Spinal Nerve
Each spinal nerve is formed by the union of two distinct roots — the dorsal (posterior) root and the ventral (anterior) root.
- • Dorsal Root: The dorsal root carries sensory information from the peripheral receptors to the spinal cord. It contains sensory neurons whose cell bodies are located in the dorsal root ganglion, a bulge in the root just before it merges with the ventral root.
- • Ventral Root: In contrast, the ventral root conveys motor information from the spinal cord to the muscles and glands. This root consists of axons from motor neurons whose cell bodies are housed within the gray matter of the spinal cord.
These two roots combine to form a mixed spinal nerve carrying both sensory and motor fibers. Shortly after emerging from the spinal column, each spinal nerve splits into two primary branches — the dorsal ramus and the ventral ramus.
- • Dorsal Ramus: This branch innervates the muscles and skin of the posterior trunk, providing sensory and motor functions to these areas.
- • Ventral Ramus: The larger of the two branches, the ventral ramus, supplies the anterolateral parts of the trunk and the limbs. It carries fibers that form the nerve plexuses — networks of interconnecting nerves that give rise to peripheral nerves innervating the limbs.
Additionally, each spinal nerve has a small meningeal branch that reenters the vertebral column to innervate the vertebrae, ligaments, blood vessels, and meninges of the spinal cord. Spinal nerves also feature communicating rami that connect with the sympathetic trunk of the autonomic nervous system (ANS). These rami consist of:
- • Gray Rami Communicantes: These carry postganglionic sympathetic fibers back to the spinal nerves for distribution to the target organs and tissues.
- • White Rami Communicantes: These are present only in the thoracic and upper lumbar regions. They carry preganglionic sympathetic fibers from the spinal nerve to the sympathetic trunk.
