The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and enters the upper arm.
The anterior rami of cervical spinal nerves C5, C6, C7 and C8 along with the first thoracic spinal nerve T1 forms this plexus.
The Brachial plexus is divided into Roots, Trunks, Divisions, Cords and Branches for easier understanding. This division is of no functional and practical significance. Roots and trunks lie in the neck in relation with the subclavian artery, divisions lie behind the clavicle, and cords and branches lie in the axilla around the axillary artery.
Roots of the Brachial Plexus are the spinal nerve rami from where it originates. As we already saw the roots are C5, C6, C7, C8 and T1. At each vertebral level, a pair of spinal nerves arise on either side through intervertebral foramina and divides into anterior and posterior nerve fibres. The posterior rami innervate the skin and muscles of the body. The anterior rami form the plexus.
At the base of the neck, the roots join together to form the trunks. They are Superior, Middle and Inferior trunks. While C5 and C6 forms the superior trunk and C8 and T1 forms the inferior trunk, the middle trunk is a continuation of C7. These trunks move laterally and cross the posterior triangle of the neck.
In the posterior triangle of the neck, the trunks divide into two branches one of which moves anteriorly and the other moves posteriorly, thus forming anterior and posterior divisions. The three anterior and three posterior nerve fibres thus formed moves to the axilla.
These divisions join together to form cords in the axilla. The cords are named based on its relative position to the axillary artery. Anterior divisions of superior and middle trunks merge together to form lateral cord and all the posterior divisions merge together to form posterior cord. The anterior division of inferior trunk continues as the medial cord.
The cords give out five major branches in and around axilla which are the main innervations of the upper limb. They are as follows.
Musculocutaneous Nerve arising from Roots C5, C6 and C7. Its motor Functions is to Innervate the brachialis, biceps brachii and coracobrachialis muscles. It gives off the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral portion of the posterior forearm.
|Branches of the Brachial Plexus
Branches from the roots
• Nerve to serratus anterior (C5, C6, C7)
• Dorsal scapular nerve (C5)
• Muscular branches to the 3 scalene muscles
Branches from the trunks
• Suprascapular nerve (C5, C6)
• Subclavius nerve (C5, C6)
Branches from the cords
o Medial cord
• Medial head of median nerve (C8, T1)
• Medial pectoral (C8, T1)
• Ulnar nerve (C8, T1)
• Median cutaneous nerve of forearm (C8, T1)
• Medial cutaneous nerve of arm (T1)
o Lateral cord
• Lateral pectoral (C5, C6, C7)
• Lateral head of median (C5, C6, C7)
• Musculocutaneous (C5, C6, C7)
o Posterior cord
• Radial (C5,C6,C7,C8,T1)
• Axillary (C5, C6)
• Nerve to latissimus dorsi (C6, C7, C8)
• Subscapular (C5, C6)
Axillary Nerve originates from C5 and C6and innervates the teres minor and deltoid muscles. It gives off the superior lateral cutaneous nerve of the arm, which innervates the inferior region of the deltoid (“regimental badge area”). Median Nerve originates from C6 – T1. (Also contains fibres from C5 in some individuals) and innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbricals that move the index and middle fingers. It gives off the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand. Radial Nerve arises from C5-C8 and T1 and innervates the triceps brachii and the extensor muscles in the posterior compartment of the forearm. It also innervates the posterior aspect of the arm and forearm and the posterior, lateral aspect of the hand.Ulnar Nerve is formed from C8 and T1. It innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus. It senses the anterior and posterior surfaces of the medial one and half fingers and associated palm area.
There are two major types of injuries that can affect the brachial plexus. An upper brachial plexus injury(Erb’s Palsy) affects the superior roots, and a lower brachial plexus injury(Klumpke Palsy) affects the inferior roots.
Erbs Palsy results in abduction at the shoulder, lateral rotation of the arm, supination of the forearm, and flexion at the shoulder. Loss of sensation down the lateral side of the arm, which covers the sensory innervation of the axillary and musculocutaneous nerves.
The affected limb hangs limply, medially rotated by the unopposed action of pectoralis major. The forearm is pronated due to the loss of biceps brachii. This is position is known as ‘waiter’s tip’, and is characteristic of Erb’s palsy.
Nerves originating from the roots C5 and C6 are affected, including Musculocutaneous and Axillary.
In Klumpke Palsy, Ulnar and median nerves originating from T1 are affected. All the small muscles of the hand (the flexors muscles in the forearm are supplied by the ulnar and median nerves, but are innervated by different roots). Loss of sensation along the medial side of the arm. The metacarpophalangeal joints are hyperextended, and the interphalangeal joints are flexed. This gives the hand a clawed appearance.
Innervation to remember
|Terminal Branch||Sensory Innervation||Muscular Innervation|
|musculocutaneous nerve||Skin of the anterolateral forearm||Brachialis, biceps brachii, coracobrachialis|
|axillary nerve||Skin of lateral portion of the shoulder and upper arm||Deltoid and teres minor|
|radial nerve||Posterior aspect of the lateral forearm and wrist; posterior arm||Triceps brachii, brachioradialis, anconeus, extensor muscles of the posterior arm and forearm|
|median nerve||Skin of lateral 2/3rd of hand and the tips of digits 1-4||Forearm flexors, thenar eminence, lumbricals of the hand 1-2|
|ulnar nerve||Skin of palm and medial side of hand and digits 3-5||Hypothenar eminence, some forearm flexors, thumb adductor, lumbricals 3-4, interosseous muscles|