Brachial Plexus Explained: Easy Access to Formation, Branches, and Relations

Following a high-speed collision, a 28-year-old motorcyclist complains of shoulder pain and an inability to raise his arm. Upon closer inspection, the arm is hanging limply with elbow extension and internal rotation—the traditional "waiter's tip" posture. But he still has a strong grip. Which particular brachial plexus trunk is damaged, and which nerve roots are affected? This is an upper trunk lesion at C5–C6 - Erb's palsy—according to the anatomy of the plexus.

FAST REPLY

The anterior rami of the C5-T1 spinal nerves form the brachial plexus, a network of nerves that provides the upper limb with motor and sensory innervation. Five roots, three trunks, six divisions, three cords, and five terminal branches make up this structure. The cervicoaxillary canal carries the plexus from the neck into the axilla.

NEET PG RELEVANCE

In NEET PG anatomy sections, the brachial plexus is frequently seen. Focus areas include winging of the scapula, muscles supplied by each nerve, Erb's vs. Klumpke's palsy presentations, and root values of terminal branches. Current research highlights the clinical association between nerve palsies and birth injuries.

What is the Brachial Plexus?

The master nerve network that governs your entire upper limb, including finger movements and shoulder shrugs, is the brachial plexus. It comes from the anterior rami (ventral divisions) of the spinal nerves C5 through T1, which leave the neck between the anterior and middle scalene muscles.

Imagine it as an electrical junction box, where five incoming wires (roots) combine, split, and reconnect in a predetermined pattern to become five primary outgoing cables (terminal branches). This intricate configuration prevents damage to a single root from totally paralyzing the entire limb by allowing fibres from several spinal levels to unite.

After leaving the posterior triangle of the neck, the plexus passes behind the clavicle and enters the axilla, where it encircles the axillary artery. After that, its terminal branches disperse throughout the hand, forearm, and arm.

Brachial Plexus Formation

There is a predictable five-part structure to the brachial plexus. "Robert Taylor Drinks Cold Beer" (or "Really Tired? Drink Coffee Black") is a helpful mnemonic for remembering: Cords → Branches → Roots → Trunks → Divisions.

Roots (5): The five roots are the anterior rami of C5, C6, C7, C8, and T1. These appear in the space between the muscles of the scalenus medius and anterior. Keep in mind that C5 (C3, C4, C5) contributes to the phrenic nerve as well.

Trunks (3): In the posterior triangle of the neck, roots unite to form three trunks:

• C5 + C6 = superior trunk
• C7 alone is the middle trunk.
• C8 + T1 = inferior trunk

Divisions (6): Behind the clavicle, each trunk divides into an anterior and posterior division. Anterior divisions generally supply flexor compartments, while posterior divisions supply extensor compartments.

Cords (3): Based on their connection to the second segment of the axillary artery, divisions recombine in the axilla.

• The anterior divisions of the superior and middle trunks make up the lateral cord.
• All three posterior divisions make up the posterior cord.
• The anterior division of the inferior trunk is called the medial cord.

Terminal Branches (5): The cords give rise to the five main nerves that supply the upper limb.

Also Read: 20 High Yield Anatomy Flashcards NEET PG 2026

The Brachial Plexus's branches

For NEET PG, it is crucial to comprehend branch origins. The branches are categorized as infraclavicular (derived from cords) and supraclavicular (derived from roots and trunks).

Branches of the supraclavicle:

From the ground up:

• The levator scapulae and rhomboids are supplied by the dorsal scapular nerve (C5).
• The serratus anterior (also known as the "nerve of Bell") is supplied by the long thoracic nerve (C5, C6, C7).
• Phrenic nerve contribution (C5)

From Superior Trunk:

• From the Superior Trunk: nerve (C5, C6) — supplies supraspinatus and infraspinatus
• Nerve to subclavius (C5, C6)

The supraspinatus and infraspinatus are supplied by the suprascapular nerve (C5, C6).

From the Lateral Cord:

• Pectoral nerve lateral
• C5, C6, and C7 musculocutaneous nerves
• The median nerve's lateral root

From the posterior cord:

• The nerve of the upper subscapular
• The nerve to the latissimus dorsi is the thoracodorsal nerve.
• Subscapular nerve at the bottom
• C5 and C6 axillary nerves
• (C5, C6, C7, C8, T1) Radial nerve

From the Medial Cord:

• The medial pectoral nerve
• The arm's medial cutaneous nerve
• Forearm's medial cutaneous nerve
• C7, C8, and T1 ulnar nerves
• The median nerve's medial root

The Purposes of Terminal Branches

"MARMU" can be used to recall the five terminal branches: musculocutaneous, axillary, radial, median, and ulnar.

Musculocutaneous Nerve (C5, C6, C7): Arises from the lateral cord, pierces the coracobrachialis, supplies all anterior arm muscles (BBC — Biceps, Brachialis, Coracobrachialis), and continues as the lateral cutaneous nerve of the forearm.

The axillary nerve (C5, C6) originates from the posterior cord, travels through a quadrangular space with the posterior circumflex humeral artery, supplies the deltoid and teres minor, and covers the deltoid with a sensory "regimental badge" area.

Lateral and medial roots form the median nerve (C5-T1), which has no branches in the arm. supplies the majority of the thenar and forearm flexors. Injury results in an ape thumb deformity and "hand of benediction" when making a fist.

The flexor carpi ulnaris and the medial half of the flexor digitorum profundus in the forearm are supplied by the ulnar nerve (C7, C8, T1) from the medial cord. Supplies most intrinsic hand muscles. Injury causes claw hand.

Brachial Plexus Relations

In the posterior triangle of the neck:

• lies between the middle and anterior scalene muscles.
• The supraclavicular region of the posterior triangle contains the trunks.
• The subclavian artery is located beneath and ahead of the inferior trunk.

Within the Axilla:

• The second segment of the axillary artery is surrounded by cords.
• Enclosed within the axillary sheath (extension of prevertebral fascia)
• Cords are located behind the pectoralis minor muscle. A helpful landmark during dissection is the "M" or "W" formed by the musculocutaneous nerve, lateral cord, median nerve, medial cord, and ulnar nerve surrounding the axillary artery.

The "M" or "W" shape formed by the musculocutaneous nerve, lateral cord, median nerve, medial cord, and ulnar nerve around the axillary artery is a useful landmark in dissection.

Clinical Associations: Injuries to the Brachial Plexus

Erb-Duchenne Palsy (Injury to the Upper Brachial Plexus – C5, C6):

• caused by the angle between the head and shoulder widening
• Frequent falls on the shoulder and birth trauma (shoulder dystocia)
• Presentation: "Waiter's tip" position: wrist flexed, elbow extended, forearm pronated, arm adducted, and internally rotated
• Deltoid, biceps, brachialis, brachioradialis, supraspinatus, and infraspinatus muscles are paralyzed.
• The grasp reflex is present, but the Moro reflex is absent.

Klumpke's Palsy (Injury to the Lower Brachial Plexus – C8, T1):

• caused by the arm's hyperabduction (clutching during a fall)
• Presentation: "Claw hand"—intrinsic hand muscle paralysis
• Loss of the grasp reflex

Winging of the scapula:

• Serratus anterior paralysis → medial winging → long thoracic nerve damage
• Seen when pushing against the wall or forward flexion
• Mastectomy, chest tube implantation, and carrying bulky backpacks are common

Also Read: Neural Crest Cells and Germ Layer Derivatives: The Complete NEET PG Guide

Erb's Palsy and Klumpke's Palsy Comparison Table

High-Yield Points for NEET PG

• Root value of radial nerve: C5, C6, C7, C8, T1 — the only terminal branch with contributions from all five roots.
  
• The most frequently injured location in the brachial plexus is Erb's point, which is where C5 and C6 converge to form the superior trunk.
• Axillary nerve injury: Common in surgical neck of humerus fractures and shoulder dislocations; results in loss of regimental badge sensation and deltoid paralysis.
• The only root nerve that supplies a single muscle (serratus anterior) is the long thoracic nerve (C5, C6, C7). Scapula winging is caused by injury.
• "Lucy Loves Me" is a mnemonic for the lateral pectoral, lateral root of the median, and musculocutaneous branches of the lateral cord.
• Saturday Night Palsy: Wrist drop with intact triceps function due to radial nerve compression in the spiral groove.
• The median nerve supplies the "LOAF" muscles. two lateral lumbricals. For questions about carpal tunnel syndrome, keep in mind the Opponens pollicis, Abductor pollicis brevis, and Flexor pollicis brevis.
• The cords are named after the second segment of the axillary artery, which is located behind the pectoralis minor.
• Only roots, trunks, and cords give rise to branches; divisions do not.
• Dermatome C6: Thumb; C7: Middle finger; C8: Little finger.

Also Read: 12 Cranial Nerves Made Easy: Names, Functions & Nuclei Explained

Frequently Asked Questions

What distinguishes Klumpke's palsy from Erb's palsy?

Erb's palsy affects the upper trunk (C5-C6), causing "waiter's tip" posture with paralysis of shoulder and elbow muscles, while Klumpke's palsy affects the lower trunk (C8-T1), causing "claw hand" with intrinsic hand muscle paralysis. Erb's preserves the grasp reflex; Klumpke's may have associated Horner's syndrome.

Which nerve injury results in scapular winging?

Due to serratus anterior paralysis, a long thoracic nerve injury results in medial winging of the scapula. The C5, C6, and C7 roots give rise to this nerve. When pushing against a wall or flexing the arm forward, winging becomes noticeable.

Which five terminal branches make up the brachial plexus?

The musculocutaneous nerve, axillary nerve, radial nerve, median nerve, and ulnar nerve are the five terminal branches. Use the acronym "MARMU" to help you remember them sequentially. The lateral, posterior, and medial cords give rise to them.

What is the structure of the brachial plexus?

The brachial plexus proceeds as follows: Five roots (C5-T1) → three trunks (superior, middle, inferior) → six divisions (three anterior, three posterior) → three cords (lateral, posterior, medial) → five terminal branches. Recall: "Robert Taylor Drinks Cold Beer."

The musculocutaneous nerve supplies which muscles?

The biceps brachii, brachialis, and coracobrachialis are the three muscles in the anterior compartment of the arm that are supplied by the musculocutaneous nerve (C5, C6, C7). Recall "BBC"; it continues as the forearm's lateral cutaneous nerve for sensory supply.

What is the ulnar nerve's root value?

Although C8 and T1 are the main contributors, the ulnar nerve has root values of C7, C8, and T1. The flexor carpi ulnaris, the medial half of the flexor digitorum profundus, and the majority of the hand's intrinsic muscles are supplied by it, which emerges from the medial cord.

PEARL CLINICAL

"Always look for Horner's syndrome when a patient is unable to abduct their arm, and you suspect brachial plexus pathology. Its existence suggests lower trunk involvement and transforms a straightforward Klumpke's palsy into a more concerning finding, potentially suggesting a Pancoast tumor compressing the plexus from above." Your differential diagnosis can be significantly altered by this one clinical observation.

Download the PrepLadder app now and unlock a 24-hour FREE trial of premium high-yield content. Access Video Lecturesalso in हिंglish, digital notes, QBank, and Mock Tests for FREE to ace your NEET PG preparation. Elevate your study experience and gear up for success. Start your journey with PrepLadder today!