Brachial Plexus Made Simple: Roots, Trunks, Divisions, Cords, Branches

The brachial plexus is one of the most clinically tested and most feared topics in anatomy. It's the network of nerves (C5-T1) that supplies the entire upper limb — every muscle you use to lift, grip, write, and gesture is innervated by branches of this plexus. Understanding the brachial plexus is essential for clinical reasoning: injuries at different levels produce different patterns of motor and sensory loss, and being able to localize the lesion requires knowing the anatomy. The good news is that the brachial plexus follows a logical organization: Roots → Trunks → Divisions → Cords → Branches. Once you understand this framework, the individual components fall into place.

The brachial plexus begins as five ventral rami emerging from spinal cord segments C5 through T1. These are the roots of the plexus. Each root carries both motor and sensory fibers destined for specific regions of the upper limb. Important branches arise directly from the roots before they combine into trunks: the dorsal scapular nerve (C5) innervates the rhomboids and levator scapulae, and the long thoracic nerve (C5, C6, C7) innervates the serratus anterior. The long thoracic nerve is clinically significant because damage produces a "winged scapula" — the medial border of the scapula protrudes posteriorly when the patient pushes against a wall. This can occur during axillary lymph node dissection or from carrying heavy backpacks.

The five roots converge into three trunks. C5 and C6 join to form the upper trunk, C7 continues alone as the middle trunk, and C8 and T1 join to form the lower trunk. Two important nerves branch from the upper trunk: the suprascapular nerve (innervates supraspinatus and infraspinatus — critical for shoulder abduction and external rotation) and the nerve to subclavius. The upper trunk is clinically important because of Erb-Duchenne palsy (Erb's palsy), which results from forceful downward traction on the shoulder during birth or trauma. This stretches C5-C6, producing the classic "waiter's tip" position: the arm hangs at the side, medially rotated, with the forearm pronated and wrist flexed. The patient cannot abduct the shoulder, flex the elbow, or supinate the forearm.

Each trunk splits into an anterior and posterior division, creating six divisions total. This is the simplest level conceptually: the anterior divisions supply the flexor (front) compartments of the upper limb, while the posterior divisions supply the extensor (back) compartments. No named branches arise at the division level, but this split is the key organizational principle of the plexus. Think of it as the plexus sorting fibers by function — flexors versus extensors — before recombining them into cords. The three anterior divisions will form the lateral and medial cords (which give rise to nerves supplying flexors), while all three posterior divisions converge to form the posterior cord (which gives rise to nerves supplying extensors).

The divisions recombine into three cords, named for their relationship to the axillary artery. The lateral cord forms from the anterior divisions of the upper and middle trunks (C5, C6, C7). The posterior cord forms from all three posterior divisions (C5-T1). The medial cord forms from the anterior division of the lower trunk (C8, T1). Each cord gives off important named branches. The lateral cord gives off the lateral pectoral nerve and contributes to the musculocutaneous nerve and the lateral root of the median nerve. The posterior cord gives off the upper and lower subscapular nerves, the thoracodorsal nerve (latissimus dorsi), and terminates as the axillary nerve and radial nerve. The medial cord gives off the medial pectoral nerve, medial cutaneous nerve of the arm, medial cutaneous nerve of the forearm, and terminates as the ulnar nerve and the medial root of the median nerve.

Five major terminal branches emerge from the cords and supply the upper limb. The musculocutaneous nerve (C5-C7, from lateral cord) innervates the anterior arm compartment — biceps brachii, brachialis, and coracobrachialis — and provides sensation to the lateral forearm. The axillary nerve (C5-C6, from posterior cord) innervates the deltoid and teres minor and provides sensation to the regimental badge area. The radial nerve (C5-T1, from posterior cord) is the largest branch, innervating all extensors of the arm and forearm. The median nerve (C5-T1, from lateral and medial cords) innervates most forearm flexors and the thenar muscles. The ulnar nerve (C8-T1, from medial cord) innervates the hypothenar muscles, interossei, and medial lumbricals, and provides sensation to the medial 1.5 fingers.

The classic mnemonic for the five levels is: "Robert Taylor Drinks Cold Beer" (Roots, Trunks, Divisions, Cords, Branches). For the terminal branches, remember "MARMU" — Musculocutaneous, Axillary, Radial, Median, Ulnar. For the cord branches, a helpful approach is to associate each cord with its most important nerve: lateral cord → musculocutaneous, posterior cord → radial (the largest), medial cord → ulnar. The median nerve is unique because it receives contributions from both the lateral and medial cords. When studying, draw the plexus from memory at least five times — the act of drawing reinforces the spatial relationships better than passive reading. Snap a photo of your drawn diagram with AnatomyIQ to check your labeling and get detailed feedback on any structures you may have missed.