Describe the formation, relations and the distribution of the brachial plexus

Definition: the brachial plexus is a plexus of nerves from which the upper limb is innervated.
Parts: it is made up of five parts: roots, trunks, division, cords and branches.
Relations:
1. The roots lie between the scalenus anterior and medius
2. The trunks cross the lower part of the posterior triangle of the neck
3. The divisions lie posterior to the clavicle
4. The cord lie in the axilla
A. Roots: formed from the anterior rami of the fifth cervical to the first thoracic vertebra (C5, 6, 7, 8 and T1)
B. Trunks: the roots join together to form the trunks as follows:
– the fifth and the sixth cervical roots join to form upper trunk
– the seventh cervical root continues as the middle trunk
– the eighth cervical and the first thoracic roots merge to form the lower trunk.
C. Divisions: the three trunks divided into anterior and posterior divisions respectively.
D. Cords: these are formed as follows
– the anterior division of the upper and middle trunks unite to form the lateral cord.
– the anterior division of the lower trunk runs as the medial cord and
– all posterior divisions unite to form the posterior cord.
Branches:
1. Roots – nerve to subclavian
               – dorsal scapular nerve
               – long thoracic nerve
2. Upper trunk – suprasapular nerve
3. Lateral cord – musculocutaneous  nerve
                           – lateral root of median nerve
                           – lateral pectoral nerve
4. Medial cord – medial root of median nerve
                          – medial cutaneous nerve of arm
                          – medial cutaneous nerve of forearm
                          – medial pectoral nerve
                          – ulnar nerve
5. Posterior cord – upper subscapular  nerve
                               – lower subscapular nerve
                               – thoracodorsal nerve
                               – radial nerve
                               – axillary nerve.
Clinical correlates:
1. Damage to the upper trunk (C5, 6) leads to a condition called Erb-duchenne paralysis. There is loss of abduction, supination and flexion. The arm is adducted and medially rotated and the forearm is extended and probated.
2. Injury to the lower trunk (C8, T1) results in Klumpke’s paralysis. There is paralysis of the intrinsic muscles of the hand and some flexors of the wrist and fingers.
3. Damage of the brachial plexus in the axilla by pressure from a crutch causes crutch paralysis. The radial nerve is most frequently involved.