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Brachial Plexus - Key Terms (Vocabulary)

The brachial plexus is a vital nerve network, originating from the ventral rami of spinal nerves covering the segments from C5 to T1. It provides all motor, sensory, and autonomic innervation to the upper extremity (UE), with all peripheral nerves of the UE stemming from it. This plexus is crucial for understanding UE anatomy, injury mechanisms, and clinical deficits.

Etymology and Overview

  • The term "plexus" derives from Latin "plectere," meaning to braid or weave, reflecting its complex interwoven structure.

  • It's a major nerve network supplying the upper extremity.

Origin and Anatomy of the Brachial Plexus

  • Originates from ventral rami of spinal nerves C5 to T1.

  • Composed of five main components in sequence:

    • Roots (Rami): C5 to T1.

    • Trunks: Superior (C5-C6), Middle (C7), and Inferior (C8-T*1).

    • Divisions: Each trunk forms an Anterior and a Posterior division, supplying flexor and extensor compartments, respectively.

    • Cords: Lateral, Posterior, and Medial, formed by the divisions.

    • Branches: Terminal nerves arising from the cords.

  • The plexus spans from the cervical region to the axilla, passing through the cervico-axillary canal.

Anatomical Pathways and Regions

  • Travels through the cervico-axillary canal (between the neck and armpit).

  • Thoracic Outlet: The region from the intervertebral foramina to the coracoid process, housing the brachial plexus and subclavian vessels.

  • Interscalene Triangle: Where the brachial plexus exits through the posterior cervical triangle, situated between the anterior and middle scalene muscles.

Classifications of the Brachial Plexus

  • By sections: Roots → Trunks → Divisions → Cords → Branches.

  • By bony regions:

    • Supraclavicular: Contains the Roots, Trunks, and Divisions.

    • Infraclavicular: Contains Divisions, Cords, and Branches.

  • By plexus regions:

    • Upper Plexus: C5 and C6 (shoulder and upper arm).

    • Lower Plexus: C7, C8, and T*1 (lower arm and hand).

Specific Components and Branching

  • Roots: Give rise to nerves like the Dorsal scapular (C5) and Long thoracic (C5-C*7) nerves.

  • Trunks: Superior trunk gives rise to the Suprascapular nerve (C5) and Nerve to subclavius (C5-C*6).

  • Divisions: Do not give off direct nerves but unite to form cords.

  • Cords: Give off specific branches (e.g., Pectoral nerves from Lateral/Medial Cords, Subscapular/Thoracodorsal from Posterior Cord) and lead to the terminal branches.

Branches (Peripheral Nerves) – Terminal Points

  • The five major terminal branches that innervate the UE are:

    • Musculocutaneous nerve: C5-C7

    • Ulnar nerve: C7-T1

    • Median nerve: C5-T1

    • Axillary nerve: C5-C6

    • Radial nerve: C5-T1

Motor and Sensory Distribution

  • Motor: Each terminal nerve supplies specific muscle groups (e.g., Musculocutaneous to forearm flexors, Radial to extensors, Axillary to deltoid and teres minor, Median to most forearm flexors and some hand muscles, Ulnar to most intrinsic hand muscles and some forearm flexors).

  • Sensory: Involves a wide array of cutaneous nerves, including supraclavicular nerves (C3-C4) and various medial, lateral, and posterior cutaneous nerves of the arm and forearm, and specific branches to the hand from radial, median, and ulnar nerves.

Vascular Anatomy Related to the Brachial Plexus

  • The axillary artery and vein are intimately associated with the brachial plexus cords, often enclosed within the axillary sheath.

  • The subclavian artery, a precursor to the axillary artery, also has important relationships with the plexus in the thoracic outlet, which is clinically significant.

Clinical Relevance and Injury Mechanisms

  • Highly susceptible to injury from stretch/traction (e.g., birth injuries, motor vehicle accidents), blunt force trauma, transection, or compression (e.g., muscle tightness, tumors).

  • Injuries can result in varied motor and sensory deficits depending on the affected plexus component.

Summary and Key Takeaways

  • The brachial plexus, originating from C5–T1, is the sole source of motor, sensory, and autonomic innervation to the entire upper limb.

  • It is classified by sections (roots, trunks, divisions, cords, branches), regional anatomy (supraclavicular/infraclavicular), and functional regions (upper/lower plexus).

  • Its anatomical course through the cervico-axillary canal into the axilla is defined by structures like the clavicle, first rib, and scalene muscles.

  • Understanding the specific contributions of each level (roots to branches) is vital for localizing lesions and predicting clinical deficits.

  • The five terminal nerves are the Musculocutaneous (C5-C7), Ulnar (C7-T1), Median (C5-T1), Axillary (C5-C6), and Radial (C5-T1).

  • Comprehensive knowledge of both motor and sensory distributions is essential for assessment, diagnosis, and treatment in clinical practice.