**2.4 Brachial plexus and upper extremity innervation**

The brachial plexus is formed by the ventral primary rami of C5, C6, C7 and C8, and nearly allof the ventral primary ramus from T1. The rami passes between the scalenus anterior muscle and the scalenus medius muscle and reach the posterior triangle of the neck. The rami from C7 and C8 are larger than those from C5 and T1. Before the main nerves of the upper limb are formed, a complex branch exchange occurs between the ventral branches C5-T1. Trunks, divisions and cords of brachial plexus are formed with complex branch exchange. The upper trunk is formed by C5 and C6. The middle trunk is the continuation of C7. The lower trunk is formed by C8 and T1. Trunks are divided into anterior and posterior branches after a short course. These are called anterior and posterior divisions. The anterior and posterior divisions of the trunks form cords by performing a number of combinations among themselves. The posterior divisions of the three trunks unite and form the posterior cord behind the axillary artery. The anterior divisions of the superior and medium trunks unite and form the lateral cord. The anterior division of the inferior trunk form the medial cord. The cords divide into terminal branches. Terminal branches of lateral cord are musculocutaneus nerve and lateral root of median nerve. Terminal branches of posterior cord are axillary nerve and radial nerve. Terminal branches of medial cord are ulnar nerve and medial root of median nerve (**Figure 1**) [16].
