Brachial Plexus : Overview, Branches & Brachial Plexus Injury

Brachial Plexus: A brachial plexus is a group of nerves that formed by the anterior rami of the lower four cervical nerves (C5-C8) and first thoracic nerve ( T1) and travel down the arm. It supplies afferent and efferent nerve fibers to the chest, shoulder, arm, and hand. These nerves control the muscles of the shoulder, elbow, wrist, and hand, as well as provide feeling in the arm, with the exception of the trapezius and levator scapula.

The brachial plexus is subdivided into roots, trunks, divisions, cords, and branches. The brachial plexus is typically composed of 5 roots, 3 trunks, 6 divisions, 3 cords.

Roots
The ventral rami of the spinal nerves C5 to T1 are referred to as the “roots” of the brachial plexus. The typical spinal nerve originating in the anterior horn cells of the spinal cord and the dorsal nerve rootlets which join the spinal ganglion in the region of the intervertebral foramen.

The roots emerge from the transverse processes of the cervical vertebrae immediately posterior to the vertebral artery, which travels through the transverse foramina. Each transverse process consists of a posterior and anterior tubercle, that meet laterally to form a costotransverse bar. The transverse foramen lies medial to the costotransverse bar and between the anterior and posterior tubercles. The spinal nerves that form the brachial plexus run in inferior and anterior direction within the sulci created by these structures.

Trunks

Superior trunk: Combination of C5 and C6 roots.
Middle trunk: Continuation of C7.
Inferior trunk: Combination of C8 and T1 roots.

After emerging from the intervertebral foramina, the 5 roots (C5-T1) meet to form 3 trunks. The trunks of brachial plexus pass between the middle and anterior scalene muscles.

The ventral rami of the C5 and C6 unit to form the upper trunk. The suprascapular nerve and the subclavius nerve arise from the upper trunk. The suprascapular nerve contributes sensory fibers to shoulder joint and motor innervation to the infraspinatus and supraspinatus muscles.

The ventral ramus of the C7 stays in the middle trunk. The ventral rami of the C8 and T1 unit to form the lower trunk.

Divisions
Each trunk splits into two divisions- an anterior and a posterior division. These separate the innervation of the dorsal and ventral aspect of the upper limb. The posterior divisions usually supply extensor muscles. The anterior divisions usually supply flexor muscles.

Cords
After entered the axilla, the cords are referred to as lateral, posterior, and the medial cord, according to the relationship with the axillary artery. The cords pass over the first rib close to the dome of the lung and continue under the clavicle immediately posterior to the subclavian artery.

Lateral Cords: The anterior divisions of the upper and middle trunks unite to form the lateral cord.
Medial Cords: The anterior division of the lower trunk forms the medial cord.

Posterior Cords: The posterior division of the superior, middle and inferior trunk formed the posterior cords.

Major Branches of the Brachial Plexus

Musculocutaneous Nerve

Roots: C5, C6, C7.
Motor Functions: Biceps brachii, brachialis and coracobrachialis muscles.

Sensory Functions: Lateral cutaneous branch of the forearm, that innervates the lateral half of anterior forearm, and a small lateral portion of the posterior forearm.

Axillary Nerve

Roots: C5 and C6.
Motor Functions: Teres minor and deltoid muscles.
Sensory Functions: The Superior lateral cutaneous nerve of the arm, that innervates the inferior region of the deltoid.

Median Nerve

Roots: C5 – T1.
Motor Functions: Flexor muscles in the forearm, the thenar muscles, and two lateral lumbricals that move the middle and index fingers.

Sensory Functions: Palmar cutaneous branch, that innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior surface of the hand.

Radial Nerve

Roots: C5-C8 and T1.
Motor Functions: Triceps brachii, and the extensor muscles in the posterior compartment of the forearm.

Sensory Functions: Posterior aspect of the arm and forearm, and the posterior, lateral aspect of the hand.

Ulnar Nerve
Roots: C8 and T1.
Motor Functions: Innervates the muscles the thenar muscles and two lateral lumbricals, flexor carpi ulnaris and medial half of the flexor digitorum profundus.

Sensory Functions: Anterior and posterior surfaces of the medial one and half fingers, and associated palm area.