The brachial plexus is a particular arrangement of nerve fibers that run from the spine, and are formed by the ventral rami of the lower four cervical and the first thoracic nerve roots. It proceeds through the neck, the axilla (armpit) region, and into the arm.
It is a very important structure that is responsible for cutaneous and muscular intervention of the entire upper limb, except for the trapezius muscle (which is neurally innervated by the spinal accessory nerve), and the skin near the axilla, which is innervated by the intercostobrachial nerve.
The rami (divisions) of the spinal nerves from C5, C6, C7, C8, and T1, are referred to as the five roots of brachial plexus. They merge together in a certain order to form three trunks: C5 and C6 roots merge to form the superior or the upper trunk. C7 travels alone, and continues as the middle trunk. C8 and T1 roots join to form the inferior or lower trunk.
Each of these trunks splits into two, to form a total of six divisions. These are the anterior divisions of the upper, middle, and lower trunks, and the posterior divisions of the upper, middle, and lower trunks. These divisions merge to form three cords. These cords are named in accordance with their relative position with respect to the axillary artery.
The posterior cord is formed by the three posterior divisions of the trunks C5 and T1. The posterior cord gives rise to the upper subscapular nerve, lower subscapular nerve, thoracodorsal nerve, axillary nerve and the radial nerve. The axillary nerve gives sensation to the shoulder region, while the radial nerve innervates the upper and lower arm.
The lateral cord is the anterior division from the upper and middle trunk, that is, from C5 and C7. This cord gives rise to the lateral pectoral nerve, which innervates the pectoralis major muscle.
The mucocutaneous nerve is the second nerve, which innervates the biceps. The median nerve is the last on that arises from the lateral cord, and also, it partly arises from the medial cord.
The medial cord is the last cord that gives rise to various nerves. It is a continuation of the anterior division of the lower trunk (C8 - T1). The median pectoral nerve innervates the pectoralis muscle. The medial brachial cutaneous nerve and the medial antebrachial cutaneous nerve arise from T1 and C8 respectively.
The median nerve partly arises from the medial cord, and partly originates from the lateral cord. The ulnar nerve is the last one that originates in the nerve roots C7, C8, and T1. It provides sensation to the ring finger and the pinky finger.
Injuries or lesions can occur in the brachial plexus region, as a result of shoulder trauma, inflammations, and even due to tumors. By studying the anatomy on MRI, one can locate the exact point of trauma. Furthermore, the signs and symptoms will also help to indicate the injured section of the plexus.