The brachial plexus is usually formed by the ventral nerve roots of the spinal nerves C5 to T1. It supplies motor and sensory function to the upper arm and shoulder regions. The plexus is divided into roots, trunks, divisions, cords and peripheral branches.
The roots of the brachial plexus are the ventral branches of the spinal nerves C5 to T1. The roots lie between the anterior and middle scalene muscles, where they form the trunks.
Branches from the roots
Unfortunately there are several branches which arise from the roots of the brachial plexus.
- The dorsal scapular nerve which pierces the middle scalene (posterior to the plexus) and descends to supply the rhomboid muscles; it arises from the C5 root
- The long thoracic nerve which passes through the cervico-axillary canal (between the neck and the axilla) posterior to the plexus; it supplies serratus anterior and arises from the C5 - C7 roots.
The trunks of the brachial plexus are formed by:
- The union of the C5 and C6 root forms the superior trunk.
- The C7 root continues as the middle trunk
- The C8 and T1 roots form the inferior trunk.
The plexus continues as trunks from the lateral border of the anterior scalene. The trunks lie posterior and superior to the subclavian vessels.
Branches from the trunks
The superior trunk gives a small branch to the subclavius muscle. The distal part of the superior trunk also gives off the suprascapular nerve, which passes across the posterior triangle of the neck and over the clavicle, supplying the supraspinatus and infraspinatus.
Each trunk divides into anterior and posterior divisions as they pass inferior to the clavicle. These divisions enter the axilla and reform to create the cords.
The cords lie in relation to the axillary artery in its second part (beneath pectoralis minor), giving them their names.
- The lateral cord lies on the lateral surface of the artery, and is formed by the anterior divisions of the superior and middle trunks.
- The medial cord is formed by the anterior division of the inferior trunk
- The posterior cord is formed by the union of all three posterior divisions
Each cord gives off terminal branches which become the peripheral nerves.
Branches from the cords
Each cord also gives off some smaller and less important nerves.
- The lateral cord only gives off a single branch; the lateral pectoral nerve which supplies the pectoral muscles.
- The posterior cord has three branches. The upper and lower subscapular nerves supply the subscapularis muscle. The thoracodorsal nerve passes inferiorly to the latissimus dorsi muscle.
- The medial cord also has three branches, all named 'medial' something. The medial pectoral nerve supplies pectoralis minor. The medial cutaneous nerve of the arm is a bit longer, supplying (strangely) the medial side of the arm. The medial cutaneous nerve of the forearm is longer still, accompanying the basilic vein to the forearm.
The major peripheral nerves are formed by the final branches of the cords.
- The medial cord divides into the ulnar nerve and the medial root of the median nerve.
- The lateral cord divides into the musculocutaneous nerve and the lateral root of the median nerve
- The posterior cord divides into the axillary and radial nerves
The ulnar nerve receives input from the C8 and T1 nerve roots only. It runs beneath the deep fascia of the medial arm and passes posterior to the medial epicondyle of the humerus (the 'funny bone'). It then courses more anteriorly and accompanies the ulnar artery into the wrist and the hand. It supplies the skin of the hand medially from a line midway through the 4th digit. It also supplies the muscles controlling the interossei muscles and the flexors of the 4th and 5th digit.
The median nerve receives input from C6 to T1. It accompanies the brachial artery through the arm, but leaves it in the cubital fossa, instead running deeply within the anterior compartment, supplying the flexors of the forearm and digits (except those supplied by the ulnar nerve, above). It passes through the carpal tunnel of the wrist with the flexor tendons, and supplies the skin of the palmar surface of the hand, lateral to the midpoint of the 4th digit.
The musculocutaneous nerve passes through the anterior compartment of the forearm, between biceps brachii and brachialis. It supplies these muscles and continues into the forearm as the lateral cutaneous nerve, supplying the skin of the lateral forearm.
The axillary nerve exits the axilla with the posterior circumflex humeral artery (a branch of the third part of the axillary artery). It curves around the proximal humerus to supply the deltoid muscle. It also gives off the lateral cutaneous branch of the arm, which supplies the skin over the inferior deltoid.
The radial nerve is the largest peripheral nerve of the plexus, and receives input from all five roots. It initially lies posterior to the axillary artery, and departs with the deep brachial artery. It supplies the posterior compartment of the arm, and enters the cubital fossa from the lateral side (as opposed to the median nerve which enters medially). It divides into superficial and deep radial nerves; the superficial nerve supplies the skin of the lateral wrist and dorsum of the hand (lateral to the midpoint of the 4th digit). The deep nerve accompanies the radial artery and supplies the posterior compartment of the forearm.