Do not be daunted by the brachial plexus! It is simply the nerve plexus of the upper limb and is one of the easiest to learn. The five major branches are the musculocutaneous, radial, axillary, ulnar, and median nerves. As long as you focus on these five main branches, you will not go wrong. The plexus runs in the axilla and innervates the muscles and skin of the upper limb.
The brachial plexus is formed by the ventral rami of the C5-T1 spinal nerves. It emerges between the anterior and middle scalene muscles, and runs in the axilla, where the cords of the plexus surround the axillary artery, which is reflected in their names. It is divided into roots (C5-T1), trunks (upper, middle and lower), divisions, cords (medial, lateral and posterior), and branches (5 main branches are the radial, axillary, musculocutaneous, ulnar and median).
The mixed spinal nerves emerge from the intervertebral foramen. The C4 and C5 roots give rise to the dorsal scapular nerve (supplies infraspinatus and supraspinatus), C5, 6 and 7 give rise to the long thoracic nerve (supplies serratus anterior), and the C5 nerve root contributes to the phrenic nerve (supplies the diaphragm).
C5 and C6 merge to form the upper trunk, C7 continues alone, C8 and T1 merge to form the lower trunk. Each of these trunks gives a branch which all merge to form the posterior cord (lies posterior to the axillary artery). Once these posterior cord-forming branches have been given off, the upper trunk continues as the lateral cord, and the lower trunk continues as the medial cord.
The posterior cord gives rise to the radial nerve (supplies the extensor compartment of the arm and forearm, as well as sensation over the posterior surface of the arm and hand), the axillary nerve (supplies teres minor and deltoid muscles, also supplies sensation to the regimental patch C5 dermatome), the thoracodorsal nerve (motor supply to the latissimus dorsi muscle), and the upper and lower subscapular nerves (both supply subscapularis, and the lower also supplies teres major).
The medial cord gives rise to the ulnar nerve (supplies flexor carpi ulnaris and the ulnar head of flexor digitorum profundus in the forearm, and in the hand supplies the interossei, adductor pollicis brevis and medial two lumbricals). The medial cord also gives rise to the medial cutaneous nerves of the arm and forearm as well as the medial pectoral nerve (which supplies the pectoral muscles, mainly pectoralis minor). It also gives a branch to form the median nerve.
The lateral cord gives rise to the musculocutaneous nerve (supplies the flexor compartment of the arm, as well as the sensation to the lateral forearm). The lateral pectoral nerve also arises from the cord, and supplies the pectoral muscles (mainly pectoralis major). It also gives a branch to form the median nerve.
Developmental precursor- Alar and basal plate of C5-T1 spinal nerves
From the roots- Long thoracic nerve, suprascapular nerve, dorsal scapular nerve
Medial cord- Ulnar nerve, medial cutaneous nerve of the arm, medial cutaneous nerve of the forearm, medial branch to form the median nerve, medial pectoral nerve
Lateral cord- Musculocutaneous nerve, lateral pectoral nerve, lateral branch to form the median nerve
Posterior cord- Radial nerve, axillary nerve, upper subscapular nerve, lower subscapular nerve, thoracodorsal nerve
Muscles supplied- All muscles of the upper limb, scapular region, pectoral region, and the serratus anterior muscle of the superolateral thorax.
Dermatome- All of the upper limb.
Place your hand against your axilla with your fingers pointing towards your hand, the position of your five fingers reflects the position of the five main branches of the plexus.