A Spinal Nerve

The body is supplied by peripheral nerves in a segmental fashion. These nerves arise from the spinal cord in a standard pattern. Within each peripheral nerve may be motor fibres to voluntary striated muscle, somatic sensory fibres from the skin or musculoskeletal system, sympathetic fibres to blood vessels and glands, parasympathetic fibres to pelvic viscera and visceral afferent fibres. Each of these fibre types has a precise source or destination within the spinal cord.

Spinal roots

The ventral root arises from the ventral aspect of the spinal cord segment. All motor fibres (somatic and autonomic) leave the cord via the ventral roots. The ventral roots from T1 - L2 carry sympathetic fibres. The dorsal roots enter the spinal cord on its dorsal aspect. Only sensory fibres run in the dorsal root. The cell bodies of sensory neurons form ganglia on the dorsal roots. Cells of the dorsal root ganglia have a peripheral process in the tissues, and a central process which terminates in the spinal cord.

Primary rami

The dorsal and ventral roots join to form a peripheral nerve. Immediately after formation the nerve divides into a small dorsal (posterior) primary ramus and a much larger ventral (anterior) primary ramus. The posterior primary rami serve a column of muscles on either side of the vertebral coulmn, and a narrow strip of overlying skin. All of the other muscle and skin is supplied by the anterior primary rami, which form the cervical, brachial, lumbar and sacral plexuses and the intercostal nerves.

Clinical significance

Compression or cutting a nerve results in the loss of all modalities beyond the lesion site.

Loss of motor but not sensory modalities suggests torn ventral roots.

Loss of sensation in the territory supplied by anterior but not posterior primary rami suggests a peripheral nerve lesion.

Loss of sympathetic innervation without motor or sensory loss suggests a sympathetic chain lesion.