Figure 1 - The brachial plexus with its main branches.
In the posterior triangle of the neck, C5 and C6 join to form the upper, or superior, trunk, and C8 and T1 join to form the lower, or inferior, trunk. C7, the largest of the rami, is renamed the middle trunk. Within the posterior triangle the trunks, along with the subclavian artery, acquire the axillary sheath, formed from deep fascia derived from the prevertebral layer of deep fascia in the neck. Each trunk splits into an anterior and a posterior division in the supraclavicular triangle, and within the axilla these six trunks become three cords, the lateral, from C5 to C7, the medial from C8 and T1, and the posterior, from all of them.
The lateral cord then proceeds to split into the musculocutaneous nerve and part of the median nerve, while the medial cord splits to form the remaining part of the median nerve and the ulnar nerve. The posterior cord splits into the radial and axillary nerves. It is recognised that this is not a complete picture, other nerve branches such as the scapular, thoracic and medial pectoral nerves branch off at different point in the brachial plexus.
The arrangement of splitting and joining acts to supply each of the main nerves by a number of the origin rami. This has the function of providing a 'back-up' for the end nerves. If damage occurs to one of the anterior rami or roots, then while the affected area will cease to operate, there will still be some nerve supply to areas around the affected area supplied by the other rami, which can, to a small extent, replace the function of the damaged rami. In order to know which areas of the arm will be affected by a nerve lesion it is necessary to know what is supplied by each of the main branches. It is accepted, following Hilton's law, that any nerve that passes near to a joint will supply that joint, and that nerve will also supply the muscles operating across the joint.
The axillary nerve, originating from C5 and C6, has branches to the shoulder joint in the axilla, and then splits into the anterior and posterior branch, the former provides the nerve supply to the skin over the lower half of the deltoid and to the deltoid itself, while the latter branch provides the nerve supply to the teres minor and the skin. Loss of the axillary nerve results in paralysis of the deltoid and a loss of skin sensation of the lower half of the deltoid.
The radial nerve provides a nerve supply to the triceps in the upper arm, as well as to the elbow joint, brachialis and extensor carpi radialis longus, and in the forearm it provides the motor nerve supply for the long extensors of the wrist among other and the sensory nerve supply for the skin over the lateral part of the dorsum of the hand. Any major damage to the radial nerve results in a slight loss of sensation down the back of the forearm and in the lateral part of the dorsum in the hand. However the damage also results in the triceps, anconeus and long extensors being paralysed, resulting in a permanent flexion of the wrist.
The median nerve provides a nerve supply for the elbow joint and for all the muscles of the front of the upper third of the forearm except for the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. It also provides the sensory nerve supply for the skin on the lateral half of the palm and a motor nerve supply to the muscles allowing flexion of the interphalangeal joints of the index and middle fingers. Damage to the median nerve results in paralysis of most of the pronator muscles in the forearm and in the some of the hand, and can be diagnosed by loss of sensation in the lateral half of the palm.
The ulnar nerve gives off no branches in the upper arm, apart from a small one supplying the elbow joint. In the forearm it provides the nerve supply for the remainder of the muscles ignored by the median nerve. In the hand a sensory nerve supply is provided for the skin over the medial third of the dorsum and the medial one and a half, sometimes two and a half, fingers. In addition to this it supplies all the small muscles of the hand, once again excluding the muscles of the thenar eminence and the first two lumbricals, these are supplied by the median nerve. Injuries to the ulnar nerve results in paralysis of most of the small muscles of the hand, resulting in the removal of the ability to adduct and abduct the fingers, and to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus muscles. An injury can be diagnosed through loss of sensation over the anterior and posterior surfaces of the medial third of the hand and the medial one and a half fingers.