Spinal Nerves

Spinal Nerves

The 31 pairs of axon bundles associated with neuroglial cells are part of the PNS, which connects to the CNS with sensory receptors, muscles, and glands throughout the body.

Spinal Nerves: Naming

Spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge.

Spinal Nerves: C1

The first cervical spinal nerve (C1) exits between the occipital bone and the atlas (C1 vertebra).

Spinal Nerves: C1–C7

Spinal nerves C1–C7 exit the vertebral canal above their corresponding vertebrae.

Spinal Nerves: C8

Spinal nerve C8 exits the vertebral canal between the C7 and T1 vertebrae.

Spinal Nerves: T1–L5

Spinal nerves T1–L5 exit the vertebral canal below their corresponding vertebrae.

Spinal Nerves: Sacral and Coccygeal Roots

The roots of S1–S5 and Co1 enter the sacral canal before exiting the sacrum.

Sacral Nerves: S1–S4

Spinal nerves S1–S4 exit the sacral canal through the four pairs of anterior and posterior sacral foramina.

Sacral and Coccygeal Nerves: S5 and Co1

Spinal nerves S5 and Co1 exit the sacral canal through the sacral hiatus.

Spinal Nerves: Two Roots

Each spinal nerve connects to the spinal cord through a posterior root and an anterior root.

Posterior Root

The posterior root contains sensory axons that carry sensory information into the CNS.

Anterior Root

The anterior root contains motor axons that carry motor information away from the CNS.

Spinal Nerve: Formation

The posterior and anterior roots unite at the intervertebral foramen to form a spinal nerve.

Spinal Nerve: Mixed Nerve

A spinal nerve is a mixed nerve because it contains both sensory axons from the posterior root and motor axons from the anterior root.

Spinal Ganglion

A spinal ganglion is a swelling on the posterior root that contains the cell bodies of sensory neurons.

Nerve Coverings

Spinal and cranial nerves contain many axons protected by three layers of connective tissue: endoneurium, perineurium, and epineurium.

Nerve Coverings: Endoneurium

The innermost connective tissue covering each axon, whether myelinated or unmyelinated, consists of a mesh of collagen fibers, fibroblasts, and macrophages.

Nerve Coverings: Nerve Fascicles

Bundles of axons, each surrounded by its own endoneurium.

Nerve Coverings: Perineurium

The middle connective tissue layer that surrounds each nerve fascicle, consisting of thick connective tissue layer with up to 15 layers of fibroblasts within a network of collagen fibers.

Nerve Coverings: Epineurium

The outermost connective tissue covering that surrounds the entire nerve, consisting of fibroblasts and thick collagen fibers.

Epineurium: Fascicle Support

Extensions of the epineurium fill the spaces between individual nerve fascicles.

Epineurium: Fascicle Support: Dura Mater

The dura mater fuses with the epineurium as a spinal nerve passes through an intervertebral foramen.

Rami

After the spinal nerve exits the spinal column through an intervertebral foramen, it splits into distinct mixed branches that carry both sensory and motor signals: posterior and anterior ramus

Posterior Ramus

A nerve branch that carries both motor and sensory signals that serves the deep muscles and skin of the posterior surface of the trunk

Anterior Ramus

A nerve branch that carries both motor and sensory signals that serves the muscles, upper and lower limbs, and the skin of the lateral and anterior surfaces of the trunk

Meningeal Branch

A branch of a spinal nerve that reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, spinal cord blood vessels, and meninges.

Plexus

A network of axons forms when anterior rami from adjacent spinal nerves join and redistribute nerve fibers from different spinal nerves so that the nerves leaving the plexus can supply specific regions of the body.

Plexus: Principal Types

The principal plexuses are the cervical, brachial, lumbar, sacral plexuses, and coccygeal

Plexus: Nerve Formation

Nerves emerging from a plexus contain axons from multiple spinal nerves and are often named according to the region they supply or the course they take.

Intercostal Nerves

The anterior rami of spinal nerves T2-T12 do not form plexuses, as these nerves directly connect to structures they supply in the intercostal spaces.

Thoracic Nerve T2

The anterior ramus of T2 supplies the intercostal muscles of the second intercostal space and the skin of the axilla and posteromedial arm.

Thoracic Nerves T3–T6

Nerves T3–T6 travel along the costal grooves of the ribs that supply the intercostal muscles and skin of the anterior and lateral chest wall.

Thoracic Nerves T7–T12

Nerves T7–T12 supply the intercostal muscles, abdominal muscles, and overlying skin.

Thoracic Nerves: T2–T12 Pattern

T2 supplies the upper thorax and axilla/arm region; T3–T6 supply the chest wall; T7–T12 supply the lower thorax and abdominal wall.

Dermatome

An area of skin that provides sensory input to the CNS through one pair of spinal nerves or the trigeminal (V) nerve.

Dermatome: Sensory Function

Dermatomes contain somatic sensory neurons that carry sensory information from the skin into the spinal cord and brain, with each spinal nerve supplying sensation to a specific, predictable region of the skin.

Dermatome: Face and Scalp

The trigeminal (V) cranial nerve supplies most of the skin of the face and scalp.

Dermatome: Overlap

Adjacent dermatomes overlap somewhat, meaning more than one spinal nerve may supply sensation to parts of the same skin region.

Dermatome: Nerve Damage

If a skin region is stimulated but the sensation is not perceived, the spinal nerves supplying that dermatome may be damaged.

Dermatome: Effect of Overlap

Because adjacent dermatomes overlap, damage to only one spinal nerve may cause little or no noticeable loss of sensation.

Dermatome: Posterior Roots

Cutting posterior roots can permanently block sensory signals, including pain, from a specific dermatome.

Dermatome: Local Anesthetic

Local anesthetics can temporarily block sensory signals from a dermatome.

Dermatome: Complete Anesthesia

Because dermatomes overlap, completely eliminating sensation from a region may require blocking at least three adjacent spinal nerves.