THE BRACHIAL PLEXUS AND UPPER LIMB INNERVATION

A plexus is a branching network of nerves or blood vessels. The brachial plexus is a network of nerves which extend from the anterior rami of cervical nerves C5-C8, as well as the first thoracic root, T1. The network passes through the cervicoaxillary canal, a passageway between the scapula and clavicle, bordered medially by the first rib. So the brachial plexus makes its way from the neck, over the first rib, into the axilla, or armpit. The brachial plexus supplies the skin and muscles of your upper limb, or upper extremity, which includes your shoulder, arm, and hand.
The brachial plexus can be divided into several sections – Roots, Trunks, Divisions, Cords, and Terminal Branches. A mnemonic for that is The Bear Came Down the River. A nerve root is the first section of a nerve leaving the central nervous system. We call these roots the C5 root to the C8 root and the T1 root. Note that although there are seven cervical vertebrae, there are 8 cervical nerves. The nerves emerge through foramina in the vertebral column.
The C5 and C6 roots join to form the upper trunk. The C7 root continues on to form the middle trunk and the C8 and T1 roots join to form the lower trunk. The scalene muscles are on either side of the brachial plexus, lateral to the cervical tract.
All three trunks split into an anterior and posterior division, with the anterior divisions being more superficial. These have different functionality – the anterior divisions supply the anterior part of the upper limb while the posterior divisions supply the posterior part of the upper limb.
The three posterior divisions fuse to form the posterior cord. The anterior divisions of the upper and middle trunks fuse to form the lateral cord. The medial cord is a continuation of the anterior division of the lower trunk. The cords surround the axillary artery and are named based on their position relative to it. The posterior cord is posterior to the axillary artery. The lateral and medial ones are on either side.
The posterior cord splits into the axillary and radial nerve. The axillary nerve passes through the quadrangular space – between teres minor and major, the long head of triceps brachii, the humerus, and the subscapularis - to innervate the deltoid and the teres minor. It also gives feeling to the shoulder joint and the skin overtop of the inferior part of the deltoid.
The radial nerve supplies the posterior upper limb, innervating the triceps, muscles in the posterior forearm, the associated joints, and the overlying skin. The lateral cord splits into the musculocutaneous nerve and the lateral root of the median nerve. The musculocutaneous nerve supplies muscles in the front of the arm - the biceps brachii, the brachialis, and the coracobrachialis muscles. It provides sensation to the lateral part of the forearm.
The median nerve proper supplies the muscles of the front of the forearm and muscles of the thenar eminence to control hand movements.
The medial cord splits into the medial root of the median nerve and the ulnar nerve. The lateral and medial root of the median nerve, the median nerve proper, and the musculocutaneous nerve and ulnar nerve on either side of it together form an “M-shape” which serves as an important anatomical reference point. The ulnar nerve innervates part of the forearm and the majority of the hand. It gives sensation to the medial forearm, medial wrist, and medial one and one-half fingers. In summary, there are five roots, three trunks, six divisions (three anterior and three posterior), three cords, and five branches.

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