Spinal Nerves

Spinal nerves are mixed peripheral nerves formed by the union of sensory and motor nerve roots arising from the spinal cord. They provide communication between the central nervous system and the body.

• 8 Cervical pairs (C1-C8): C1 emerges superior to the atlas (C1 vertebra); C1-C7 exit superior to their pedicles, whereas C8 exits inferior to the C7 pedicle.
• 12 Thoracic pairs (T1-T12): Exit inferior to corresponding vertebral pedicles.
• 5 Lumbar pairs (L1-L5): Exit inferior to corresponding vertebral pedicles.
• 5 Sacral pairs (S1-S5): Exit via anterior and posterior sacral foramina.
• 1 Coccygeal pair (Co1): Exits via the sacral hiatus.

Root Architecture and Nerve Assembly

Each spinal nerve forms within or adjacent to the intervertebral foramen through the fusion of morphologically and functionally distinct dorsal and ventral roots:

• Dorsal Root (Posterior Root): Carries primary afferent (sensory) fibers into the dorsolateral sulcus. It features an expansion, the dorsal root ganglion (DRG), housing the cell bodies of pseudounipolar sensory neurons.
• Ventral Root (Anterior Root): Carries efferent fibers (somatic motor axons from anterior horn alpha/gamma motor neurons, plus preganglionic autonomic fibers from the lateral horn) exiting via the ventrolateral sulcus.
• Spinal Nerve Trunk: Formed distal to the dorsal root ganglion by the convergence of the dorsal and ventral roots. Because it fuses sensory and motor components, every spinal nerve trunk is an anatomically mixed nerve.

Ramification and Target Innervation

Immediately upon passing through or exiting the intervertebral foramen, each spinal nerve trunk divides into four main branches:

Branch	Functional Fiber Modality	Anatomical Distribution and Targets
Dorsal Ramus	Mixed (somatic motor, somatic sensory, sympathetic)	True deep muscles of the back (erector spinae, transversospinales); overlying skin of the posterior trunk.
Ventral Ramus	Mixed (somatic motor, somatic sensory, sympathetic)	Ventrolateral trunk wall, intercostal regions, and limbs via major nerve plexuses.
Meningeal (Recurrent) Branch	Mixed sensory and autonomic	Re-enters intervertebral foramen; supplies posterior longitudinal ligament, outer anulus fibrosus, dura mater, and epidural blood vessels.
Rami Communicantes	Autonomic (visceral motor and visceral sensory)	Connects spinal nerve to sympathetic chain. White rami (myelinated preganglionic, T1-L2 only); Gray rami (unmyelinated postganglionic, all spinal levels).

Major Somatic Plexuses

Except for thoracic levels T2 through T12 (which retain segmental arrangement as intercostal and subcostal nerves), ventral rami intertwine to form somatic networks:

• Cervical Plexus (C1-C4): Innervates infrahyoid muscles, neck cutis, and diaphragmatic motor/sensory supply via the phrenic nerve (C3-C5).
• Brachial Plexus (C5-T1): Organizes through roots, trunks, divisions, cords, and terminal branches (musculocutaneous, axillary, median, radial, and ulnar nerves) to provide complete motor and sensory innervation to the upper limb and shoulder girdle.
• Lumbar Plexus (L1-L4): Embedded within the psoas major; gives rise to the femoral nerve and obturator nerve, supplying anterior and medial thigh compartments.
• Sacral Plexus (L4-S4): Situated on the piriformis muscle; consolidates into the sciatic nerve–composed of tibial and common fibular divisions–innervating the posterior thigh, entire leg, and foot.
• Coccygeal Plexus (S4-Co1): Minor somatic contribution innervating pelvic floor musculature and skin overlying the coccyx.

Functional Fiber Classifications

Axons within peripheral spinal nerves fall into four classical physiological fiber modalities:

1. General Somatic Afferent (GSA): Transmits exteroceptive (pain, temperature, light touch) and proprioceptive impulses from the body wall, joints, and skeletal muscles to the CNS.
2. General Somatic Efferent (GSE): Carries alpha and gamma motor signals from anterior horn cells directly to striated skeletal muscle fibers.
3. General Visceral Afferent (GVA): Transmits visceral sensory signals (ischemia, distension, physiological status) from internal organs and blood vessels back to the spinal cord.
4. General Visceral Efferent (GVE): Carries autonomic motor outflow (sympathetic from T1-L2 intermediolateral cell columns; parasympathetic from S2-S4 sacral autonomic nuclei) targeting smooth muscle, cardiac tissue, and glands.