Brachial plexus
The brachial plexus is a complex network of nerves originating from the anterior rami of the spinal nerves C5 through T1, providing essential motor and sensory innervation to the upper extremity, including the shoulder, arm, forearm, and hand.[1] This intricate structure emerges from the lower neck, passes through the scalene muscles and supraclavicular region, and extends into the axilla, where it branches to control movement and sensation across the upper limb.[2] The brachial plexus is organized into a sequential arrangement remembered by the components roots, trunks, divisions, cords, and branches.[3] The roots consist of the ventral rami of C5–T1 (with occasional contributions from C4 or T2), which unite to form three trunks: the upper trunk (C5–C6), middle trunk (C7), and lower trunk (C8–T1).[1] Each trunk then splits into anterior and posterior divisions (yielding six divisions total), which rearrange in the axilla to form three cords named relative to the axillary artery: the lateral cord (from anterior divisions of upper and middle trunks), posterior cord (from posterior divisions of all trunks), and medial cord (from the anterior division of the lower trunk).[4] These cords give rise to the five major terminal branches—musculocutaneous, median, ulnar, axillary, and radial nerves—along with proximal branches such as the dorsal scapular, long thoracic, suprascapular, and nerve to subclavius, which innervate specific shoulder and scapular muscles.[3] Functionally, the brachial plexus transmits motor signals to innervate the muscles responsible for shoulder abduction and adduction, elbow flexion and extension, wrist and finger movements, and intrinsic hand functions, while providing sensory input from the skin and joints of the upper limb.[5] Its anatomical complexity and superficial course make it vulnerable to trauma, compression, or iatrogenic injury, leading to conditions such as brachial plexopathy, which can result in weakness, sensory loss, or chronic pain syndromes like those seen in birth injuries or sports-related trauma.[1] Understanding its structure is crucial for clinical diagnosis, surgical interventions, and rehabilitation in upper limb disorders.[4]Anatomy
Roots
The brachial plexus originates from the anterior rami of the spinal nerves C5 through T1, which form its five roots.[1] These roots represent the initial segments of the plexus, emerging directly from the ventral (anterior) divisions of the respective spinal nerves after they exit the intervertebral foramina of the vertebral column.[1] In typical anatomy, the C5 root arises from the fifth cervical vertebra, followed sequentially by C6, C7, C8, and T1 from the first thoracic vertebra, providing the foundational neural outflow for upper limb innervation.[1] Each root contributes specific fibers to the subsequent structure of the plexus: the C5 and C6 roots unite proximally to form the upper trunk, the C7 root independently forms the middle trunk, and the C8 and T1 roots combine to form the lower trunk.[1] Anatomically, the roots are located in the posterior triangle of the neck, positioned between the anterior scalene and middle scalene muscles, where they course inferiorly and laterally.[1] From this interscalene position, the roots pass anterior to the first rib, entering the scalene hiatus before transitioning toward the supraclavicular and infraclavicular regions.[6] The roots contain a mixture of motor and sensory fibers originating from the spinal cord, with motor axons destined for skeletal muscles of the shoulder, arm, and hand, and sensory axons conveying information from the skin and joints of the upper limb.[7] Variations in root contribution occur in approximately 10-20% of individuals, where the plexus may be prefixed (incorporating a contribution from C4) or postfixed (extending to include T2), altering the relative emphasis on cervical or thoracic inputs.[8]Trunks
The trunks of the brachial plexus are formed by the union of the anterior rami (roots) emerging from the spinal nerves. The upper trunk arises from the convergence of the C5 and C6 roots, the middle trunk from the C7 root alone, and the lower trunk from the C8 and T1 roots.[6][1] These three trunks course obliquely downward and laterally through the posterior triangle of the neck, positioned posterior to the clavicle and between the anterior and middle scalene muscles.[9][10] They lie in close proximity to the subclavian artery, which passes posterior to the anterior scalene muscle alongside the trunks, and the subclavian vein, which courses anterior to the anterior scalene muscle near the plexus in the thoracic outlet region.[11][12] The trunks represent short segments of the brachial plexus, typically measuring a few centimeters in length, before they reach the lateral border of the first rib where further divisions occur.[13][14]Divisions
The three trunks of the brachial plexus—upper, middle, and lower—each bifurcate into an anterior and a posterior division, yielding a total of six divisions: three anterior and three posterior.[15] This bifurcation marks the point of transition from the supraclavicular to the infraclavicular portion of the plexus.[16] The divisions form behind the middle third of the clavicle, within the costoclavicular space, posterior to the subclavius muscle.[16] From this location, the divisions course anterior to the first rib, directing toward the axilla and the surrounding axillary structures.[16] Anatomically, the anterior divisions contribute to innervation of the flexor compartments of the upper limb, whereas the posterior divisions supply the extensor compartments, reflecting their positional orientation relative to limb musculature.[15]Cords
The cords of the brachial plexus represent the third level of organization in its structure, formed by the regrouping of the six divisions (three anterior and three posterior) that arise from the trunks as they pass beneath the clavicle. Specifically, the lateral cord is formed by the union of the anterior divisions of the upper and middle trunks, the posterior cord by the convergence of the posterior divisions from all three trunks (upper, middle, and lower), and the medial cord by the anterior division of the lower trunk alone. This reorganization occurs distal to the clavicle, transitioning the plexus from its supraclavicular to infraclavicular configuration.[9][10][1] The three cords are named according to their positional relationship to the second part of the axillary artery, which lies posterior to the pectoralis minor muscle: the lateral cord is positioned lateral to the artery, the medial cord medial to it, and the posterior cord posterior to it. This naming convention reflects their consistent spatial arrangement around the vessel, facilitating the neurovascular bundle's passage through the axilla.[9][10][17] Located within the axilla, the cords encircle the axillary artery, forming a protective neurovascular sheath enveloped by axillary fascia. They course distally from the level of the clavicle, where the divisions converge, extending through the axilla for approximately 5-6 cm until the axillary artery terminates as the brachial artery at the inferior border of the teres major muscle. Throughout this path, the cords maintain their relative positions to the artery, adapting to the conical shape of the axillary space.[9][10][1]Branches
The branches of the brachial plexus are categorized into supraclavicular and infraclavicular types based on their emergence relative to the clavicle. Supraclavicular branches arise from the roots and trunks in the neck, proximal to the clavicle, while infraclavicular branches originate from the cords in the axilla, distal to the clavicle.[1]Supraclavicular Branches
These branches primarily supply structures in the shoulder girdle and include the following major nerves.- Dorsal scapular nerve: Arising from the anterior ramus of C5 near the root, it pierces the middle scalene muscle and travels posteriorly to innervate the rhomboid major and minor muscles, as well as the levator scapulae muscle. It does not provide sensory innervation to dermatomes.[1][18]
- Long thoracic nerve: Formed by contributions from the anterior rami of C5, C6, and C7, it descends on the surface of the serratus anterior muscle, receiving additional fibers from C5-C7 roots, to supply the serratus anterior muscle along its medial border. It supplies no cutaneous dermatomes.[1][18]
- Suprascapular nerve: Originating from the upper trunk (C5-C6), it passes laterally through the suprascapular notch under the superior transverse scapular ligament to reach the posterior scapular region, innervating the supraspinatus and infraspinatus muscles. It provides sensory branches to the acromioclavicular and glenohumeral joints but no major dermatomes.[1][18]
- Nerve to subclavius: Emerging from the upper trunk (C5-C6), it descends anterior to the subclavian artery and vein to supply the subclavius muscle. It carries no significant sensory components to dermatomes.[1]
Infraclavicular Branches
These branches arise from the three cords (lateral, medial, and posterior) and form the primary nerves of the upper limb, supplying both muscular and cutaneous targets.Branches from the Lateral Cord
- Lateral pectoral nerve: Arising from the lateral cord (C5-C7), it pierces the clavipectoral fascia to innervate the pectoralis major muscle (clavicular head). It provides no direct dermatomal supply.[4][18]
- Musculocutaneous nerve: Originating from the lateral cord (C5-C7), it pierces the coracobrachialis muscle, then passes between the biceps brachii and brachialis muscles in the arm, continuing as the lateral cutaneous nerve of the forearm in the cubital fossa. It innervates the coracobrachialis, biceps brachii, and brachialis muscles, and supplies the lateral forearm dermatome (C5-C7).[1][4][18]
- Lateral root of the median nerve: A contribution from the lateral cord (C6-C7) that joins the medial root to form the median nerve distal to the axillary artery. (Details of the median nerve follow below.)[1]
Branches from the Medial Cord
- Medial pectoral nerve: Emerging from the medial cord (C8-T1), it passes anterior to the axillary artery, pierces the pectoralis minor, and supplies the pectoralis minor and pectoralis major muscles (sternocostal head). It has no major dermatomal distribution.[4][18]
- Medial cutaneous nerve of the arm: Arising from the medial cord (C8-T1), it travels with the basilic vein to supply the medial skin of the arm. It corresponds to the medial brachial cutaneous dermatome (C8-T1).[1][18]
- Medial cutaneous nerve of the forearm: Originating from the medial cord (C8-T1), it descends medial to the axillary artery and accompanies the basilic vein to the elbow, innervating the medial forearm skin. It supplies the medial antebrachial cutaneous dermatome (C8-T1).[1][18]
- Medial root of the median nerve: A branch from the medial cord (C8-T1) that unites with the lateral root to form the median nerve. (Details of the median nerve follow below.)[1]
- Ulnar nerve: Arising directly from the medial cord (C8-T1), it descends posterior to the medial epicondyle of the humerus in the arm, then along the medial forearm to the hand. It innervates the flexor carpi ulnaris and medial half of the flexor digitorum profundus in the forearm, hypothenar muscles, interossei, and medial two lumbricals in the hand, and supplies the medial hand dermatome including the little finger and medial half of the ring finger (C8-T1).[1][4][18]
Branches from the Posterior Cord
- Upper subscapular nerve: Originating from the posterior cord (C5-C6), it supplies the subscapularis muscle (upper part). It has no cutaneous supply.[1]
- Thoracodorsal nerve: Arising from the posterior cord (C6-C8), it descends to pierce the latissimus dorsi muscle, innervating it entirely. It provides no dermatomal innervation.[1][18]
- Lower subscapular nerve: Emerging from the posterior cord (C5-C6), it divides into branches supplying the lower subscapularis and teres major muscles. It lacks sensory dermatomes.[1]
- Axillary nerve: Branching from the posterior cord (C5-C6), it winds around the surgical neck of the humerus through the quadrangular space to reach the deltoid and teres minor muscles, which it innervates, and provides sensory branches to the lateral shoulder skin (upper lateral cutaneous nerve of the arm, C5 dermatome).[1][4][18]
- Radial nerve: The largest terminal branch from the posterior cord (C5-T1), it descends in the arm posterior to the humerus in the spiral groove, then pierces the lateral intermuscular septum to the cubital fossa, continuing into the forearm. It innervates the triceps brachii, anconeus, brachioradialis, extensor carpi radialis longus, and extensors of the wrist and fingers, and supplies the posterior arm, forearm, and dorsal hand dermatomes (C5-T1).[1][4][18]
Terminal Branch Formed by Lateral and Medial Cords
- Median nerve: Formed by the union of lateral (C6-C7) and medial (C8-T1) roots around the axillary artery, it travels down the arm medial to the brachial artery, crosses the cubital fossa, and enters the forearm between the heads of pronator teres. It innervates the pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, lateral half of flexor digitorum profundus, pronator quadratus, thenar muscles, and lateral two lumbricals in the hand, and supplies the palmar surface of the lateral three-and-a-half digits and corresponding palm dermatome (C6-T1).[1][4][18]
Variations
The brachial plexus exhibits anatomical variations in approximately 10-25% of individuals, depending on the specific component assessed, with meta-analyses indicating an overall variability in trunk formation of about 16%.[19] These deviations from the standard configuration, formed by the ventral rami of spinal nerves C5-T1, can affect the roots, trunks, divisions, cords, or branches, and are documented across cadaveric and imaging studies. Common variations include prefixed and postfixed types, as well as alterations in trunk composition such as the absence of the middle trunk. A prefixed brachial plexus, characterized by the inclusion of a substantial contribution from the C4 ventral ramus alongside the typical C5-T1 roots, occurs in roughly 11% of cases (95% CI 6-17%).[19] This rostral shift results in an upper plexus that extends more cephalad, potentially altering the positions of subsequent trunks and cords. In contrast, a postfixed plexus incorporates the ventral ramus of T2 and may exclude part or all of C5, with a lower prevalence of about 1% (95% CI 0-1%).[19] The absence of the middle trunk, typically formed by C7, is a rarer anomaly reported in isolated case studies and small series, often leading to compensatory fusion of C6 and C8 contributions into upper and lower trunks, with an estimated prevalence under 5% based on aggregated cadaveric data.[20] Variations in the median nerve formation are also frequent, with the standard dual origin from the lateral and medial cords present in approximately 90% of cases (95% CI 84-95%), implying variable contributions—such as additional roots from C4 or T1—in about 10%.[21] Cadaveric studies report higher rates of multi-root origins for the median nerve, up to 25% in males and 21% in females, often involving three or more rami that fuse distal to the typical site.[22] These variations arise embryologically from irregularities in the migration and fusion of ventral rootlets during the fourth to sixth weeks of gestation, when spinal nerve precursors extend axons toward peripheral targets in a segmental manner; incomplete fusion or differential caudal-cranial shifts can lead to atypical plexus configurations.[23] Such developmental anomalies reflect the plasticity of neural crest-derived cells in establishing limb innervation patterns. Clinically, these variations heighten risks during surgical interventions like axillary lymph node dissection or brachial plexus blocks, where unrecognized prefixed or postfixed patterns may result in inadvertent nerve injury or incomplete anesthesia, necessitating preoperative imaging for confirmation.[24]Function
Motor Innervation
The brachial plexus consists of mixed nerves that transmit efferent motor fibers to the skeletal muscles of the upper limb, enabling voluntary movements from shoulder girdle stabilization to fine finger manipulations. These fibers originate from the ventral rami of spinal nerves C5 through T1, with the network organized to support a hierarchical pattern of innervation progressing from proximal structures (shoulder and arm) to distal ones (forearm and hand). This proximal-to-distal gradient reflects the functional division of the plexus, where upper roots (C5-C6) dominate control of shoulder and elbow actions, middle root (C7) contributes to elbow and wrist extension, and lower roots (C8-T1) govern hand intrinsics and grip precision.[25][4] Key branches from the plexus provide targeted motor supply to specific muscle groups. For instance, the suprascapular nerve (C5-C6), emerging from the superior trunk, innervates the supraspinatus and infraspinatus muscles, which initiate shoulder abduction and external rotation, respectively. The axillary nerve (C5-C6), from the posterior cord, supplies the deltoid and teres minor for shoulder abduction and external rotation. These proximal innervations underscore the role of C5-C6 roots in scapulohumeral stability and mobility.[9][1] Moving distally, the musculocutaneous nerve (C5-C7), arising from the lateral cord, innervates the coracobrachialis, biceps brachii, and brachialis muscles to facilitate elbow flexion and forearm supination. The radial nerve (C5-T1), the largest branch from the posterior cord, provides motor innervation to the triceps brachii for elbow extension, as well as the brachioradialis and posterior forearm extensors (e.g., extensor carpi radialis longus, extensor digitorum) for wrist and finger extension. C7 fibers within the radial nerve are particularly crucial for these extensor functions at the elbow and wrist.[9][26][25] The median nerve (C6-T1), formed by contributions from the lateral and medial cords, innervates anterior forearm flexors such as the pronator teres, flexor carpi radialis, and flexor digitorum superficialis for wrist and finger flexion, along with the thenar eminence muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) and the first two lumbricals for thumb opposition and index/middle finger flexion. In contrast, the ulnar nerve (C8-T1), from the medial cord, supplies the flexor carpi ulnaris and medial half of the flexor digitorum profundus for wrist and finger flexion, as well as most hand intrinsics (hypothenar muscles, interossei, and medial two lumbricals) for finger adduction/abduction and fine dexterity. The dominance of C8-T1 roots in these distal branches enables precise hand function.[9][10][25] Additional terminal and collateral branches contribute to proximal motor control, including the dorsal scapular nerve (C5), which innervates the rhomboids and levator scapulae for scapular retraction and elevation, and the long thoracic nerve (C5-C7), supplying the serratus anterior for scapular protraction and upward rotation. These elements ensure coordinated upper limb positioning before distal actions.[1][26]| Nerve | Root Levels | Primary Muscles Innervated | Key Movements |
|---|---|---|---|
| Suprascapular | C5-C6 | Supraspinatus, infraspinatus | Shoulder abduction, external rotation |
| Axillary | C5-C6 | Deltoid, teres minor | Shoulder abduction, external rotation |
| Musculocutaneous | C5-C7 | Coracobrachialis, biceps brachii, brachialis | Elbow flexion, forearm supination |
| Radial | C5-T1 | Triceps brachii, brachioradialis, forearm extensors | Elbow extension, wrist/finger extension |
| Median | C6-T1 | Forearm flexors (e.g., pronator teres, flexor digitorum superficialis), thenar muscles, lateral lumbricals | Wrist/finger flexion, thumb opposition |
| Ulnar | C8-T1 | Flexor carpi ulnaris, medial flexor digitorum profundus, hand intrinsics (e.g., interossei) | Wrist/finger flexion, finger adduction/abduction |
| Dorsal scapular | C5 | Rhomboids, levator scapulae | Scapular retraction, elevation |
| Long thoracic | C5-C7 | Serratus anterior | Scapular protraction, upward rotation |