Brachial Plexus Anatomy: Complete Guide to Nerves and Upper Limb Innervation

The brachial plexus is one of the most clinically significant nerve networks in the human body. It is a complex arrangement of nerve fibers that originates from the spinal cord and provides motor and sensory innervation to the entire upper limb, including the shoulder, arm, forearm, and hand. Understanding brachial plexus anatomy is essential for medical students, physical therapists, and any healthcare professional who treats injuries of the upper extremity.

The brachial plexus is formed by the ventral rami of spinal nerves C5, C6, C7, C8, and T1. These nerve roots emerge from the intervertebral foramina of the cervical and upper thoracic spine and converge in the posterior triangle of the neck before passing into the axilla. The network undergoes a series of merging and branching events that ultimately produce the named peripheral nerves responsible for upper limb innervation. This intricate organization allows the nervous system to distribute fibers from multiple spinal levels to individual muscles and skin regions, providing redundancy and precise motor control.

Brachial plexus anatomy is a cornerstone of gross anatomy courses and appears frequently on examinations such as the USMLE, COMLEX, and NBME shelf exams. Clinicians encounter brachial plexus injuries in trauma settings, obstetric complications, and surgical procedures involving the neck and shoulder. A firm grasp of the plexus structure enables accurate localization of nerve lesions based on a patient's motor and sensory deficits. For these reasons, dedicating focused study time to the brachial plexus pays dividends throughout a medical career.

In the sections that follow, we will walk through each organizational level of the brachial plexus, identify the major brachial plexus nerves that arise from it, discuss clinical correlations and common injuries, and share proven mnemonics and study strategies to help you retain this material efficiently.

The anatomy of the brachial plexus is organized into five structural levels: roots, trunks, divisions, cords, and branches. Each level represents a stage at which nerve fibers regroup before ultimately forming the terminal nerves of the upper limb. Learning these levels in order is the key to mastering brachial plexus anatomy.

The roots of the brachial plexus are the ventral rami of C5, C6, C7, C8, and T1. They emerge between the anterior and middle scalene muscles in the neck. At this level, several nerves branch off directly, including the dorsal scapular nerve (C5) and the long thoracic nerve (C5, C6, C7). These are sometimes called supraclavicular branches because they arise before the plexus crosses the clavicle.

The roots then merge to form three trunks. The superior trunk is formed by the union of C5 and C6. The middle trunk is a continuation of C7 alone. The inferior trunk is formed by C8 and T1. The trunks are located in the posterior triangle of the neck, superior to the clavicle. The suprascapular nerve and the nerve to the subclavius arise from the superior trunk at this level.

Each trunk divides into an anterior and a posterior division, producing six divisions in total. The anterior divisions generally supply flexor compartments of the upper limb, while the posterior divisions supply extensor compartments. This functional separation is a fundamental organizing principle of upper limb innervation.

The divisions regroup to form three cords, named by their relationship to the axillary artery. The lateral cord is formed by the anterior divisions of the superior and middle trunks (C5, C6, C7). The medial cord is a continuation of the anterior division of the inferior trunk (C8, T1). The posterior cord is formed by all three posterior divisions (C5-T1). The cords give off several important branches before terminating as the major brachial plexus nerves.

From the cords emerge the five terminal branches: the musculocutaneous nerve, the median nerve, the ulnar nerve, the radial nerve, and the axillary nerve. Additional branches from the cords include the medial pectoral nerve, lateral pectoral nerve, subscapular nerves, and thoracodorsal nerve. Understanding this hierarchical structure is essential for predicting which muscles and skin areas will be affected by injuries at different levels of the plexus.

The brachial plexus nerves that emerge as terminal branches are responsible for the motor and sensory functions of the entire upper limb. Each nerve follows a distinct anatomical course and innervates specific muscles and cutaneous regions. A systematic understanding of these nerves is critical for clinical diagnosis.

The musculocutaneous nerve (C5, C6, C7) arises from the lateral cord and pierces the coracobrachialis muscle. It innervates the flexors of the arm, including the biceps brachii and brachialis. After passing through these muscles, it continues as the lateral cutaneous nerve of the forearm, providing sensation to the lateral forearm. Injury to this nerve results in weakness of elbow flexion and loss of the biceps reflex.

The median nerve (C5-T1) is formed by contributions from both the lateral and medial cords. It descends along the medial aspect of the arm, passes through the cubital fossa, and enters the forearm between the two heads of the pronator teres. The median nerve innervates most of the forearm flexors and the thenar muscles of the hand. It also provides sensory innervation to the palmar surface of the lateral three and a half digits. Carpal tunnel syndrome, one of the most common compression neuropathies, involves the median nerve at the wrist.

The ulnar nerve (C8, T1) arises from the medial cord, passes posterior to the medial epicondyle of the humerus at the elbow, and enters the hand through Guyon's canal. It innervates the intrinsic muscles of the hand, including the interossei and the medial two lumbricals, as well as the hypothenar muscles. Sensory distribution covers the medial one and a half digits. The ulnar nerve is vulnerable at the elbow, where it can be compressed or stretched, producing numbness and tingling in the ring and little fingers.

The radial nerve (C5-T1) is the largest terminal branch of the posterior cord. It spirals around the humerus in the radial groove, supplying the triceps and the extensors of the forearm and wrist. Sensory branches cover the posterior arm, forearm, and the dorsal aspect of the lateral three and a half digits. Radial nerve injury in the spiral groove, often associated with humeral shaft fractures, results in wrist drop due to paralysis of the wrist extensors.

The axillary nerve (C5, C6) branches from the posterior cord and passes through the quadrangular space. It innervates the deltoid and teres minor muscles and provides sensation to the regimental badge area of the lateral shoulder. Anterior shoulder dislocations frequently injure the axillary nerve, leading to weakness in arm abduction beyond fifteen degrees.

Brachial plexus injuries are encountered across multiple medical specialties, including orthopedics, neurology, neurosurgery, and obstetrics. These injuries range from mild traction neuropraxia to complete avulsion of nerve roots from the spinal cord. Understanding the anatomy of the brachial plexus allows clinicians to localize the injury and predict functional deficits.

Erb-Duchenne palsy, also known as upper brachial plexus injury, involves the C5 and C6 nerve roots or the superior trunk. It is the most common form of obstetric brachial plexus injury and occurs when excessive lateral flexion of the neck during delivery stretches the upper roots. The affected infant presents with the arm adducted, internally rotated, and extended at the elbow, a posture known as the waiter's tip position. The deltoid, biceps, brachialis, and supraspinatus are primarily affected, resulting in loss of arm abduction, external rotation, and elbow flexion.

Klumpke palsy is a lower brachial plexus injury involving C8 and T1. It may occur during a difficult delivery when the arm is pulled overhead or in adults who grab an object while falling. The intrinsic muscles of the hand are affected, producing a claw hand deformity. If the T1 root is involved proximal to the sympathetic chain, an ipsilateral Horner syndrome (ptosis, miosis, anhidrosis) may also be present.

Traumatic brachial plexus injuries in adults often result from motorcycle accidents, falls, or contact sports. High-velocity traction injuries can avulse nerve roots directly from the spinal cord, a devastating injury that is not amenable to direct nerve repair. Findings such as a positive Tinel sign, electromyographic changes, and MRI evidence of pseudomeningocele formation help determine the level and severity of injury.

Thoracic outlet syndrome is a clinical condition in which the brachial plexus or subclavian vessels are compressed as they pass through the thoracic outlet between the scalene muscles and the first rib. Patients may experience upper limb pain, paresthesias, and weakness, particularly with overhead activities. Treatment ranges from physical therapy and postural correction to surgical decompression in refractory cases. Accurate knowledge of brachial plexus anatomy is indispensable for diagnosing and managing all of these conditions.

The complexity of brachial plexus anatomy makes it one of the most mnemonic-dependent topics in medical education. Well-crafted mnemonics can dramatically accelerate memorization and improve recall during high-stakes examinations. Below are some of the most popular and effective memory aids used by students worldwide.

The most widely used mnemonic for the organizational levels of the brachial plexus is "Robert Taylor Drinks Cold Beer." Each word corresponds to a structural level: Roots, Trunks, Divisions, Cords, and Branches. This simple phrase provides an immediate framework for recalling the five levels in order, which is the foundation of understanding the entire plexus.

To remember the nerve roots that form the brachial plexus, students often use "C5, C6, C7, C8, T1 — five roots make the plexus fun." While simple, this rhyme reinforces the specific spinal levels involved and helps prevent confusion with other nerve plexuses such as the cervical or lumbar plexus.

For the terminal branches, the mnemonic "My Aunt Roberta Urinates Mustard" corresponds to Musculocutaneous, Axillary, Radial, Ulnar, and Median nerves. Some students prefer the variant "MARMU" as an acronym. The key is choosing a mnemonic that creates a vivid mental image, as research on memory encoding consistently shows that emotional or absurd associations are retained far better than abstract lists.

The relationships between cords and terminal branches can be recalled with the following associations. The lateral cord gives rise to the lateral pectoral nerve, the musculocutaneous nerve, and the lateral contribution to the median nerve. Think "LML" — Lateral pectoral, Musculocutaneous, Lateral root of median. The medial cord yields the medial pectoral nerve, medial cutaneous nerves of the arm and forearm, the ulnar nerve, and the medial contribution to the median nerve. The posterior cord gives rise to the subscapular nerves, the thoracodorsal nerve, the axillary nerve, and the radial nerve. The mnemonic "STAR" helps: Subscapular, Thoracodorsal, Axillary, Radial.

For remembering brachial plexus nerves and their root values, use "C5-C6 pick up sticks" for the musculocutaneous nerve and "C5-C6-C7-C8-T1" for the median and radial nerves, which draw from the broadest range of roots. Combining multiple mnemonics into a study sheet and reviewing it daily using spaced repetition will lock this material into long-term memory.

Successfully learning brachial plexus anatomy requires a multi-modal study approach that goes beyond passive reading. The plexus is a three-dimensional structure, and students who engage with it through drawing, visualization, and active recall consistently outperform those who rely solely on textbook diagrams.

One of the most effective strategies is to draw the brachial plexus from memory repeatedly. Start by sketching the five roots (C5-T1) on the left side of a blank page. Then draw lines merging into three trunks, splitting into six divisions, reforming into three cords, and finally branching into the five terminal nerves. Label every structure as you go. Doing this exercise daily for a week will build a strong mental map of the entire plexus. Many students use color-coding, assigning different colors to each cord and its branches, which adds a visual dimension that enhances encoding.

Clinical case-based learning is another powerful approach. Rather than memorizing isolated facts, work through clinical vignettes that require you to identify the injured nerve based on motor and sensory findings. For example, if a patient presents with wrist drop after a humeral shaft fracture, you should be able to trace the deficit to the radial nerve in the spiral groove. These kinds of applied questions are exactly what you will encounter on board examinations and in clinical rotations.

Cadaveric dissection or prosection review, when available, offers unmatched spatial understanding of upper limb innervation. If you do not have access to a cadaver lab, high-quality anatomy atlases and 3D anatomy apps can serve as effective substitutes. Rotating a digital model of the brachial plexus and examining it from different angles helps solidify the spatial relationships between nerves, vessels, and muscles.

Group study sessions focused on the brachial plexus can be highly productive. Quiz each other on nerve root values, motor functions, sensory distributions, and clinical scenarios. Teaching the material to a peer is one of the most effective ways to identify gaps in your own understanding. Finally, integrate your brachial plexus study with related topics such as the cranial nerves and the action potential in neurons. Seeing the connections between nervous system topics builds a cohesive framework that supports long-term retention and clinical reasoning.

LectureScribe's AI-generated study materials, including slides, infographics, and practice questions, can supplement your brachial plexus study by providing spaced repetition prompts and visual summaries that reinforce the core concepts covered in this guide.