The median nerve is the only nerve that passes through the carpal tunnel.
After receiving inputs from both the lateral and medial cords of the brachial plexus, the median nerve enters the arm from the axilla at the inferior margin of the teres major muscle. It then passes vertically down and courses with the brachial artery on the medial side of the arm between biceps brachii and brachialis. At first it is lateral to the artery and lies anterior to the elbow joint; it then crosses anteriorly to run medial to the artery in the distal arm and into the cubital fossa.
Inside the cubital fossa the median nerve passes medial to the brachial artery, in front of the point of insertion of the brachialis muscle and deep to the biceps.
The median nerve gives off an articular branch in the upper arm as it passes the elbow joint. A branch to pronator teres may arise from the median nerve immediately proximal to the elbow joint.
The median nerve arises from the cubital fossa and passes between the two heads of pronator teres. It then travels between flexor digitorum superficialis and flexor digitorum profundus before emerging between flexor digitorum superficialis and flexor pollicis longus.
The unbranched portion of the median nerve (which arises from the cubital fossa) innervates muscles of superficial and intermediate groups of the anterior(flexor) compartment except flexor carpi ulnaris.
The median nerve does give off two branches as it courses through the forearm:
The anterior interosseous branch courses with the anterior interosseous artery and innervates all the muscles of the deep group of the anterior compartment of the forearm except the medial (ulnar half,which is supplied by ulnar nerve) half of flexor digitorum profundus and flexor carpi ulnaris muscle. It ends with its innervation of pronator quadratus muscle. The palmar cutaneous branch of the median nerve arises at the distal part of the forearm. It supplies sensory innervation to the lateral aspect of the skin of the palm (but not the digits).
The median nerve enters the hand through the carpal tunnel, deep to the flexor retinaculum along with the tendons of flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus.
From there it sends off several branches:
1. Recurrent branch to muscles of the thenar compartment (the recurrent branch is also called “the million dollar nerve”). Here it provides motor innervation to opponens pollicis, abductor pollicis brevis and superficial part of flexor pollicis brevis.
2. Digital cutaneous branches to common palmar digital branch and proper palmar digital branch of the median nerve which supply the:
a) lateral (radial) three and a half digits on the palmar side
b) dorsum of the tips of index, middle and thumb
3. The median nerve supplies motor innervation to the first and second Lumbricals of the hand.
There are multiple naturally occurring anomalies of the median nerve.
Bifurcation of the median nerve typically occurs after the nerve exits the carpal tunnel; however, in a small percentage (5%-10%) of individuals, the median nerve bifurcates more proximal in the carpal tunnel, wrist, or forearm.
During gestation, a median artery that serves the hand retracts. However, in some individuals the median artery does not retract and follows the course next to the median nerve into the hand. Martin-Gruber anastomoses can occur when branches of the median nerve cross-over in the forearm and merge with the ulnar nerve to innervate portions of the forehand.
Riche-Cannieu anastomoses can occur when there is connection between recurrent branch of the median nerve and deep branch of the ulnar nerve of the hand.
The median nerve has no voluntary motor or cutaneous function in the brachium. It gives vascular branches to the wall of the brachial artery. These vascular branches carry sympathetic fibers.
It innervates all of the flexors in the forearm except flexor carpi ulnaris and that part of flexor digitorum profundus that supplies the 4th and 5th digits. The latter two muscles are supplied by the ulnar nerve (specifically the Muscular branches of ulnar nerve).
The main portion of the median nerve supplies the following muscles:
Flexor carpi radialis
Flexor digitorum superficialis muscle
The anterior interosseus branch of the median nerve supplies the following muscles:
Flexor digitorum profundus (only the lateral half)
Flexor pollicis longus
The cutaneous innervation of the right hand. Areas supplied by the median nerve are colored green, the radial nerve red and the ulnar nerve blue. In the hand, the median nerve supplies motor innervation to the 1st and 2nd lumbrical muscles. It also supplies the muscles of the thenar eminence by a recurrent thenar branch. The rest of the intrinsic muscles of the hand are supplied by the ulnar nerve.
The median nerve innervates the skin of the palmar side of the thumb, the index and middle finger, half the ring finger, and the nail bed of these fingers. The lateral part of the palm is supplied by the palmar cutaneous branch of the median nerve, which leaves the nerve proximal to the wrist creases. This palmar cutaneous branch travels in a separate fascial groove adjacent to the flexor carpi radialis and then superficial to the flexor retinaculum. It is therefore spared in carpal tunnel syndrome.
The muscles of the hand supplied by the median nerve can be remembered using the mnemonic, “LOAF” for Lumbricals 1 & 2, Opponens pollicis, Abductor pollicis brevis and Flexor pollicis brevis. (NB: OAF are the thenar eminence)
In patients presenting with predominantly motor symptoms, some groups routinely biopsy mixed or pure motor nerves such as the the pronator branch of the median nerve 1).