3.4. Deeper Dive: The Brachial Plexus
What is a Deeper Dive?
Roots and Trunks
The brachial plexus supplies almost all of the nerves to the upper limb. The only exception to this is T2, which supplies part of the axilla.
Note that in anatomy, the upper arm where the biceps brachii muscle is located is referred to as ‘arm’, while the lower arm is referred to as ‘the forearm’.
The brachial plexus lies in the lower neck and axilla and can be divided up into roots, trunks, divisions, cords and branches, in this order from proximal to distal. The roots have already been mentioned; these are C5 – T1. The neurons from these nerves combine to form trunks, namely the superior/upper, middle and inferior/lower trunks. Each trunk gives off an anterior division and a posterior division; there are three trunks so there are three anterior and three posterior divisions. These divisions mix and match their contents in a specific way to give the cords:
- The three posterior divisions combine to give the posterior cord – easy enough to remember!
- The anterior divisions of the upper trunk and the anterior cord of the middle trunk combine to give the lateral cord.
- The anterior division of the lower trunk continues as the medial cord.
Divisions, Cords and Branches
From these cords, the main nerves of the upper limb arise. These are as follows the:
- Musculocutaneous nerve (from the lateral cord, which goes on to supply the elbow flexors (biceps brachii, coracobrachialis and brachialis). Once it supplies these muscles, it goes on to become the lateral cutaneous nerve of the forearm.
- Axillary nerve (from the posterior cord, which goes on to supply deltoid and teres minor and the regimental badge skin area.
- Radial nerve (from the posterior cord too, which goes on to supply the extensors of the wrist and digits and the posterior skin of the forearm, the lateral palmar aspect of the palm and the lateral three and a half digits.
- Ulnar nerve (from the medial cord, which goes on to supply some of the medially-lying wrist and digit flexors (namely flexor carpi ulnaris and the ulnar half of flexor digitorum profundus, as well as the hypothenar eminence, the medial two lumbricals, the skin of the medial one and a half digits on the palmar and dorsal aspects and the proximal palmar area.
- Median nerve (from both the medial and lateral cords, which is a big brute of a nerve that goes on to supply most of the wrist and digit flexors (apart from those supplied by the ulnar nerve, as well as the thenar eminence and the lateral two lumbricals. It also supplies the skin of the medial dorsum of the hand, the medial three and a half digits and the fingertips on both the palmar and dorsal aspects.
Because the nerves mix in the plexus, we can create an approximate dermatome map, i.e. see which spinal nerve roots innervate which areas of skin. We can also see which specific nerve branches innervate which areas of skin. When I first learnt this, I found it confusing, but it all hinges on the concept that the neurons from each spinal nerve redistribute in the brachial plexus. For example, the spinal nerve root C6 supplies the palmar skin of the thumb, but this can also be said to be supplied by the radial nerve (which contains fibres from C5 – T1).
Note that there are varying dermatome maps produced by different researchers but generally the thumb is in the C6 dermatome and the baby finger (digit 5) is in the C8 dermatome.
