Peripheral Nerves: Spinal Nerves, Plexuses, and Dermatomes

Spinal Nerves: Rami and Plexuses

Overview of Spinal Nerves

Spinal nerves are mixed nerves that emerge from the spinal cord and branch to supply various regions of the body. They are organized into rami and plexuses, which facilitate the distribution of motor and sensory fibers.

• Ventral Rami: All ventral rami except those from T1–T12 form interlacing nerve networks called plexuses.

• Dorsal Rami: Innervate the muscles and skin of the back via several branches.

• Intercostal Nerves: The ventral rami of T1–T12 do not form plexuses; instead, they become intercostal nerves that supply the muscles of the ribs, anterolateral thorax, and abdominal wall.

• Spinal Roots: Become longer as they descend the spinal cord, with lumbar and sacral roots extending as the cauda equina.

• Rami Communicantes: Autonomic pathways that join the ventral rami.

Cervical Plexus and the Neck

Structure and Function

The cervical plexus is formed by the ventral rami of spinal nerves C1–C4. It provides sensory and motor innervation to the neck and parts of the head, shoulders, and diaphragm.

• Cutaneous Nerves: Most branches are cutaneous, supplying the skin of the neck, ear, back of the head, and shoulder.

• Muscular Branches: Other branches innervate muscles of the neck.

• Phrenic Nerve: The major motor and sensory nerve of the diaphragm, essential for breathing. Irritation of this nerve can cause hiccups.

Brachial Plexus and Upper Limb

Structure and Major Nerves

The brachial plexus is formed by the ventral rami of C5–C8 and T1 (sometimes with contributions from C4 and/or T2). It gives rise to nerves that innervate the upper limb.

• Axillary Nerve: Innervates the deltoid, teres minor, and skin and joint capsule of the shoulder.

• Median Nerve: Innervates most flexors of the forearm, forearm pronators, wrist and finger flexors, and thumb opposition muscles.

• Ulnar Nerve: Supplies the flexor carpi ulnaris, part of the flexor digitorum profundus, most intrinsic hand muscles, and the skin of the medial aspect of the hand; involved in wrist and finger flexion.

• Radial Nerve: Innervates essentially all extensor muscles, supinators, and the posterior skin of the limb.

Lumbar Plexus

Structure and Major Nerves

The lumbar plexus arises from the ventral rami of L1–L4. It innervates the thigh, abdominal wall, and psoas muscle.

• Femoral Nerve: Innervates the quadriceps and skin of the anterior thigh and medial surface of the leg.

• Obturator Nerve: Passes through the obturator foramen to innervate the adductor muscles of the thigh.

Sacral Plexus

Structure and Major Nerves

The sacral plexus arises from the ventral rami of L4–S4. It serves the buttock, lower limb, pelvic structures, and perineum.

• Sciatic Nerve: The longest and thickest nerve in the body. It innervates the hamstring muscles, adductor magnus, and most muscles in the leg and foot.

Innervation of Skin: Dermatomes

Definition and Clinical Importance

A dermatome is an area of skin innervated by the cutaneous branches of a single spinal nerve. Dermatomes are important in clinical diagnosis of nerve injuries and diseases.

• All spinal nerves except C1 participate in dermatomes.

• Dermatomal maps are used to determine the level of spinal injuries.

Dermatome Table (Main Purpose: Classification of Skin Innervation by Spinal Nerves)

Spinal Nerve	Dermatome Region
C2–C4	Neck and upper shoulders
C5–T1	Upper limbs
T2–T12	Thorax and abdomen
L1–L4	Anterior and inner surfaces of lower limbs
L5–S2	Posterior and outer surfaces of lower limbs
S3–S5	Perineum

Example: Loss of sensation in the lateral forearm may indicate injury to the C6 spinal nerve.

Additional info: Dermatomes overlap, so destruction of a single spinal nerve will not completely numb a region.