Spinal nerves connect the central nervous system (CNS) to the rest of the body as part of the peripheral nervous system. These mixed nerves carry sensory information to the CNS and motor commands from the CNS to muscles and glands, enabling sensation, movement, and the regulation of involuntary bodily functions. A systematic naming and numbering convention helps identify their precise location and function.
Spinal Nerve Regions
The human spine is divided into five distinct regions, and spinal nerves are named based on the region of the vertebral column from which they emerge. These regions are the cervical (neck), thoracic (upper back), lumbar (lower back), sacral (pelvis), and coccygeal (tailbone).
The cervical region supports the head and allows for extensive movement. The thoracic region forms the upper back and attaches to the rib cage, protecting vital organs. The lumbar region, in the lower back, bears the majority of the body’s weight. The sacrum connects the spine to the hip bones, while the coccyx forms the tailbone.
Numbering the Nerves
Within each spinal region, the nerves are further distinguished by a numerical system. There are 31 pairs of spinal nerves: eight cervical (C1-C8), twelve thoracic (T1-T12), five lumbar (L1-L5), five sacral (S1-S5), and one coccygeal (Co1) pair.
A unique aspect of this numbering is observed in the cervical region: there are eight cervical spinal nerves (C1-C8) despite only seven cervical vertebrae (C1-C7). The numerical sequence for cervical nerves extends beyond the number of vertebrae in that segment. The remaining regions, from thoracic to coccygeal, have an equal number of nerve pairs as their corresponding vertebrae.
Connection to Vertebrae
The anatomical relationship between spinal nerves and vertebrae determines their specific naming convention. In the cervical spine, the first seven spinal nerves (C1-C7) emerge above their corresponding vertebrae. For instance, the C1 nerve exits between the skull and the first cervical vertebra, and C7 exits above the C7 vertebra.
The C8 spinal nerve presents an exception to this pattern, as it exits below the C7 vertebra and above the first thoracic vertebra (T1). This placement accounts for the presence of eight cervical nerves despite only seven cervical vertebrae. From the thoracic region downwards, all subsequent spinal nerves exit below their corresponding vertebrae. For example, the T1 nerve exits below the T1 vertebra, and this pattern continues through the lumbar, sacral, and coccygeal regions.