The Brachial Plexus Made Simple: Roots, Trunks, Divisions, Cords, and Branches Explained

The brachial plexus is a network of nerves that originates from spinal nerve roots C5, C6, C7, C8, and T1 and provides all motor and sensory innervation to the upper limb. It organizes into 5 levels: Roots → Trunks → Divisions → Cords → Branches. The mnemonic: Robert Taylor Drinks Cold Beer (or Real Texans Drink Cold Beer — pick the one that sticks). Roots (5): C5, C6, C7, C8, T1 — the ventral rami of these spinal nerves. These emerge from the intervertebral foramina and pass between the anterior and middle scalene muscles in the neck. Trunks (3): Upper (C5+C6), Middle (C7 alone), Lower (C8+T1). The roots combine into three trunks in the posterior triangle of the neck. Divisions (6): Each trunk splits into an anterior and posterior division — 3 anterior, 3 posterior. This is the least clinically tested level but it is structurally important because the anterior divisions supply flexors (front of the arm) and posterior divisions supply extensors (back of the arm). Cords (3): Named by their position relative to the axillary artery. Lateral cord (anterior divisions of upper and middle trunks), Posterior cord (all three posterior divisions), Medial cord (anterior division of lower trunk). Branches (5 terminal): Musculocutaneous (from lateral cord), Axillary (from posterior cord), Radial (from posterior cord), Median (from lateral + medial cords), Ulnar (from medial cord). This content is for educational purposes only and does not constitute medical advice.

Musculocutaneous nerve (C5-C7, from lateral cord): motor to the anterior compartment of the arm — biceps brachii, brachialis, and coracobrachialis. These are your elbow flexors. Sensory: becomes the lateral cutaneous nerve of the forearm after piercing the coracobrachialis. A musculocutaneous nerve injury produces weakness of elbow flexion and loss of the biceps reflex. Axillary nerve (C5-C6, from posterior cord): motor to the deltoid (shoulder abduction) and teres minor. Sensory: regimental badge area (lateral shoulder skin). The axillary nerve wraps around the surgical neck of the humerus — fractures at the surgical neck or anterior shoulder dislocations can damage it. Test: ask the patient to abduct the arm against resistance. Loss of the regimental badge sensation confirms the diagnosis. Radial nerve (C5-T1, from posterior cord — the largest branch): motor to all extensors of the elbow, wrist, and fingers. The triceps, brachioradialis, and all the extensor compartment muscles. Sensory: posterior arm, posterior forearm, dorsal hand (first 3.5 digits). The radial nerve spirals around the humerus in the radial groove — a midshaft humerus fracture can damage it, causing wrist drop (inability to extend the wrist and fingers). Wrist drop = think radial nerve = think humerus fracture. Median nerve (C5-T1, from lateral + medial cords): motor to most forearm flexors and the thenar muscles (the fleshy mound at the base of the thumb). Sensory: palmar side of the lateral 3.5 digits. Passes through the carpal tunnel — compression here causes carpal tunnel syndrome (numbness and tingling in the thumb, index, middle, and lateral ring finger, with thenar muscle weakness in advanced cases). The median nerve is the most commonly tested nerve in the upper limb. Ulnar nerve (C8-T1, from medial cord): motor to the intrinsic hand muscles (interossei, lumbricals 3-4, hypothenar muscles) and the flexor carpi ulnaris. Sensory: medial 1.5 digits. Passes behind the medial epicondyle (the funny bone) — hitting your funny bone is actually compressing the ulnar nerve. Ulnar nerve palsy produces a claw hand deformity (hyperextended MCP joints, flexed IP joints in digits 4-5). AnatomyIQ can trace each terminal branch from its cord origin to its motor and sensory territory when you snap a photo of a brachial plexus diagram — it highlights the pathway and explains the clinical significance of each nerve.

Erb-Duchenne palsy (upper trunk injury, C5-C6): caused by excessive lateral traction on the neck during delivery (birth trauma) or falling on the shoulder. Affects the suprascapular nerve, musculocutaneous nerve, and axillary nerve. The arm hangs in the waiter's tip position: shoulder adducted and internally rotated, elbow extended, forearm pronated. The patient cannot abduct the shoulder, flex the elbow, or supinate the forearm. This is the most commonly tested brachial plexus injury. Klumpke palsy (lower trunk injury, C8-T1): caused by excessive upward traction on the arm (grabbing onto something while falling) or during difficult breech delivery. Affects the intrinsic hand muscles (ulnar and median nerve territories). Produces a claw hand with loss of finger flexion and intrinsic hand muscle function. May also cause Horner syndrome (ptosis, miosis, anhidrosis) if the T1 sympathetic fibers are damaged — this combination (claw hand + Horner's) is a classic exam question. Long thoracic nerve injury (C5-C7, a branch that comes off the roots before the trunks form): innervates the serratus anterior muscle, which holds the scapula against the chest wall. Damage causes winged scapula — the medial border of the scapula protrudes when the patient pushes against a wall. Commonly injured during mastectomy (breast surgery) or axillary lymph node dissection because the nerve runs along the lateral chest wall. Saturday night palsy (radial nerve compression): falling asleep with the arm draped over a chair back (or after excessive drinking — hence the name) compresses the radial nerve in the radial groove of the humerus. Produces wrist drop and finger drop. Typically recovers in weeks because it is a neuropraxia (compression injury) rather than axonal damage.

The brachial plexus looks overwhelming because most diagrams show everything at once — 5 roots, 3 trunks, 6 divisions, 3 cords, 5 major branches, plus a dozen smaller branches, all tangled together. The key is to study it in layers, not all at once. Layer 1 (day 1): Learn the 5 levels and the mnemonic (Robert Taylor Drinks Cold Beer). Draw the basic structure from memory — just the shape, no branch details. Five roots combining into three trunks, splitting into six divisions, forming three cords, producing five terminal branches. This is the skeleton. Layer 2 (day 2-3): Add the terminal branches with their root values and cord origins. Musculocutaneous from lateral. Axillary and Radial from posterior. Median from lateral + medial. Ulnar from medial. Learn what each nerve does — motor function and sensory territory. Draw and label from memory. Layer 3 (day 4-5): Add the injuries. Erb = upper trunk (waiter's tip). Klumpke = lower trunk (claw hand ± Horner's). Long thoracic = winged scapula. Radial at humerus = wrist drop. Median at carpal tunnel = hand numbness. Ulnar at elbow = claw hand. Layer 4 (week 2): Do clinical vignette practice questions. A patient presents with X symptoms — where is the lesion? This is how exams test the brachial plexus. The question describes the deficit and you work backward to the nerve and the level of injury. Spacing these layers over 2 weeks produces durable long-term memory. Cramming the entire plexus the night before the exam produces confusion and the vague sense that all the nerves blur together. AnatomyIQ generates brachial plexus practice questions at each layer of difficulty — starting with basic anatomy and progressing to clinical localization scenarios.