Brachial Plexus

Introduction

Introduction to the Brachial Plexus

The brachial plexus is a complex network of nerves that provides somatic motor and sensory innervation to the upper limb and parts of the pectoral girdle. Originating in the neck, it extends into the axilla, serving as the anatomical highway through which peripheral nerves reach their target tissues. Understanding its structure is fundamental to diagnosing upper extremity neurological deficits, regional anesthetic blocks, and surgical exposures.

Study

Anatomical Structure: The Five Structural Components

The brachial plexus is organized sequentially from proximal to distal into five distinct parts, traditionally remembered by the mnemonic 'Robert Taylor Drinks Cold Beer': Roots, Trunks, Divisions, Cords, and Branches. 1. Roots (Ventral Rami): Formed by the anterior rami of spinal nerves C5, C6, C7, C8, and T1. These rami emerge between the anterior and middle scalene muscles in the neck. They receive contributions from pre-ganglionic sympathetic fibers via gray rami communicantes. 2. Trunks: As the roots emerge, they merge to form three distinct trunks in the posterior triangle of the neck: - Superior Trunk: Fusion of C5 and C6 roots. - Middle Trunk: Continuation of the C7 root. - Inferior Trunk: Fusion of C8 and T1 roots. 3. Divisions: Posterior to the clavicle (at the apex of the axilla), each trunk splits into an anterior division and a posterior division, yielding six divisions in total. Anterior divisions supply muscles of the anterior (flexor) compartments, while posterior divisions supply muscles of the posterior (extensor) compartments. 4. Cords: The divisions reorganize within the axilla around the second part of the axillary artery to form three cords, named according to their relationship to the artery: - Lateral Cord: Formed by the anterior divisions of the superior and middle trunks (carrying fibers from C5, C6, C7). - Posterior Cord: Formed by the posterior divisions of all three trunks (carrying fibers from C5 to T1). - Medial Cord: Continuation of the anterior division of the inferior trunk (carrying fibers from C8 and T1).

Terminal Branches and Non-Terminal Branches

The cords give rise to five prominent terminal branches that run down the upper limb: - Musculocutaneous Nerve (C5-C7): Arises from the lateral cord; innervates the anterior compartment of the arm (biceps brachii, coracobrachialis, brachialis) and terminates as the lateral cutaneous nerve of the forearm. - Axillary Nerve (C5-C6): Arises from the posterior cord; exits via the quadrangular space to innervate the deltoid and teres minor, providing sensation over the lateral shoulder. - Radial Nerve (C5-T1): Arises from the posterior cord; innervates all muscles of the posterior compartments of the arm and forearm, and supplies posterior skin. - Median Nerve (C5-T1): Formed by a contribution from both the lateral and medial cords; travels down the arm to innervate most flexors in the forearm and the thenar muscles. - Ulnar Nerve (C7-T1): Arises from the medial cord; innervates the intrinsic muscles of the hand (except thenar and lateral two lumbricals) and one and a half muscles in the forearm. Crucial pre-terminal branches also arise along the plexus: - Long Thoracic Nerve (C5-C7): Arises directly from the roots to innervate the serratus anterior muscle. - Dorsal Scapular Nerve (C5): Arises from the C5 root to supply the rhomboids and levator scapulae. - Suprascapular Nerve (C5-C6): Arises from the superior trunk to supply supraspinatus and infraspinatus muscles. - Lateral and Medial Pectoral Nerves: Arise from their respective cords to innervate the pectoralis major and minor muscles.

Clinical Correlations and Neurological Injuries

Injuries to the brachial plexus can be classified based on the location of the lesion: 1. Upper Brachial Plexus Injury (Erb-Duchenne Palsy): Caused by an excessive increase in the angle between the neck and the shoulder (e.g., fall from a horse or traction during a difficult birth), tearing the upper roots or superior trunk (C5-C6). This results in loss of function in the deltoid, biceps, and brachialis. The clinical presentation is the classic 'waiter's tip position': the limb hangs by the side, medially rotated, and the forearm is extended and pronated. 2. Lower Brachial Plexus Injury (Klumpke Palsy): Occurs when the upper limb is suddenly pulled upward (e.g., clutching an object when falling from a tree or excessive breech delivery traction), damaging the C8-T1 roots or inferior trunk. This compromises the intrinsic muscles of the hand, leading to a full 'claw hand' (ape hand and clawing of digits 2-5) due to the unopposed action of forearm extensors and long flexors. It may be accompanied by Horner syndrome if the T1 sympathetic rami are avulsed. 3. Thoracic Outlet Syndrome (TOS): Compression of the cords or roots of the brachial plexus and/or subclavian vessels as they pass through the interscalene triangle, costoclavicular space, or retro-pectoralis minor space. It can be precipitated by a cervical rib or muscular hypertrophy, presenting with paresthesia down the medial aspect of the forearm and hand.

Summary

Summary of Key Concepts

The brachial plexus (C5-T1) is responsible for the motor and sensory control of the upper extremity. Its structure progresses linearly through roots, trunks, divisions, and cords before ending in terminal nerves. Anterior divisions control flexor compartments via the lateral and medial cords, while posterior divisions control extensor compartments via the posterior cord. Pathologies range from upper trunk injuries (Erb's palsy, C5-C6, affecting shoulder abduction and forearm flexion/supination) to lower trunk lesions (Klumpke's palsy, C8-T1, causing profound intrinsic hand weakness and a claw deformity).