The Brachial Plexus

Overview

• Definition: A plexus is a branching network of nerves or blood vessels.
• Brachial Plexus: Network of nerves extending from anterior rami of cervical nerves (C5 to C8) and the first thoracic root (T1).
• Pathway: Passes through the cervical axillary canal (between the scapula and clavicle, bordered medially by the first rib) from the neck over the first rib into the axilla (armpit).
• Function: Supplies skin and muscles of the upper limb, including shoulder, arm, and hand.

Structure of the Brachial Plexus

1. Roots
   • Start from the central nervous system.
   • Includes C5, C6, C7, C8, and T1 roots.
   • Note: 8 cervical nerves exist despite 7 cervical vertebrae.
2. Trunks
   • C5 and C6 roots form the upper trunk.
   • C7 root forms the middle trunk.
   • C8 and T1 roots form the lower trunk.
   • Muscle Relationship: Scalene muscles are lateral to the cervical tract.
3. Divisions
   • Each trunk splits into an anterior and posterior division.
   • Functionality:
     • Anterior divisions: Supply anterior part of the upper limb.
     • Posterior divisions: Supply posterior part of the upper limb.
4. Cords
   • Formation:
     • Three posterior divisions form the posterior cord.
     • Anterior divisions of upper and middle trunks form the lateral cord.
     • Anterior division of lower trunk continues as the medial cord.
   • Named based on their position relative to the axillary artery.
5. Branches
   • Posterior Cord:
     • Splits into the axillary and radial nerves.
     • Axillary nerve: Passes through the quadrangular space; innervates deltoid and teres minor.
     • Radial nerve: Supplies the posterior upper limb, triceps, posterior forearm, associated joints, and overlying skin.
   • Lateral Cord:
     • Splits into musculocutaneous nerve and lateral root of the median nerve.
     • Musculocutaneous nerve: Supplies muscles in front of the arm (biceps brachii, brachialis, coracobrachialis); provides sensation to lateral forearm.
   • Medial Cord:
     • Splits into medial root of the median nerve and ulnar nerve.
     • Median Nerve: Supplies muscles of the front of the forearm and thenar eminence for hand movements.
     • Ulnar Nerve: Innervates part of the forearm and majority of the hand; gives sensation to medial forearm, wrist, and 1.5 fingers.

Mnemonic

• Mnemonic for Sections: "The Bear Came Down The River"
  • Roots, Trunks, Divisions, Cords, and Terminal Branches.

Summary

• The brachial plexus consists of five roots, three trunks, six divisions (three anterior and three posterior), three cords, and five branches, serving various motor and sensory functions in the upper limb.