The Median Nerve is one of the five nerves that emerges from brachial plexus in axilla by two roots: (a) lateral and (b) medial. The lateral root (C5, C6, and C7) emerges from lateral cord of brachial plexus and the medial root (C8 and Tl) emerges from medial cord of the brachial plexus. The medial root crosses in front of the third part of axillary artery to connect with the lateral root in a Y-shaped manner possibly in front of or on the lateral side of the artery to compose the median nerve. So the root value of median nerve is C5, C6, C7, C8, and Tl.
In the axilla, the median nerve is located on the lateral side of the third part of the axillary artery. It enters the arm at the lower border of teres major.
In the arm, initially, median nerve is located lateral to brachial artery and then crosses in front of the artery from lateral to medial side at the level of midhumerus (i.e., level of insertion of coracobrachialis). After crossing, it runs downwards to enter cubital fossa.
In the cubital fossa, the median nerve lies medial to the brachial artery and tendon of biceps brachii. Here it is protected by bicipital aponeurosis, which divides it from the median cubital vein.
In the cubital fossa, it gives muscular branches from its medial side to supply all the superficial flexors of the forearm flexor carpi radialis, palmaris longus, and flexor digitorum superficialis) other than flexor carpi ulnaris.
Median nerve gets out of the cubital fossa by passing between the two heads of pronator teres. At this point, it gives off anterior interosseous nerve.
The anterior interosseous nerve is purely motor and supplies 2.5 muscles:
- Flexor pollicis longus.
- Lateral half of the flexor digitorum profundus (FDP).
In the forearm, the median nerve go on downwards behind the tendinous arch/bridge between the two heads of flexor digitorum superficialis and runs deep to the flexor digitorum superficialis. About 5 cm proximal to the flexor retinaculum, the median nerve arises from the lateral side of the FDS and turns into superficial, lying lateral to the tendons of FDS and posterior to the tendon of palmaris longus.
In the midarm, the median nerve gives muscular branch to the radial head of flexor digitorum superficialis, which gives rise to tendon for index finger.
Before entering the carpal tunnel, it gives off its palmar cutaneous branch,
which passes superficial to the flexor retinaculum to supply the skin over the thenar eminence and lateral part of the palm.
Median nerve gets in the palm by passing through carpal tunnel
where it is located deep to flexor retinaculum and superficial to the tendons of FDS, FDP, and FPL and their connected ulnar and radial bursae.
In the palm, the median nerve flattens at the distal border of the flexor retinaculum and splits into lateral and medial divisions. The lateral division gives a recurrent branch, which curls upwards to supply thenar muscles except the deep head of flexor pollicis brevis. It then splits into three palmar digital branches. The medial divisions give off two palmar digital nerves.
The five palmar digital nerves supply:
- sensory innervation to the skin of the palmar aspect of the lateral 3XA digits consisting of nail beds and skin on the dorsal aspect of distal phalanges, and
first and second lumbricals.
- Median nerve is also termed laborer’s nerve since the coarse movements of the hand required by laborers (e.g., digging the ground, lifting weight, etc.) are carried out by long flexors of the forearm which are mostly supplied by the median nerve.
- It is also called ‘eye of the hand’ or ‘peripheral eye’ because it offers sensory innervation to the pulp of the thumb and index finger which are used to see the thinness and texture of cloth and are also used for doing fine movements, e.g., buttoning a coat.
Injuries of the median nerve: The lesions of median nerve may take place at the following four sites: (a) at elbow, (b) at mid-forearm, (c) at wrist (distal forearm), and (d) in the carpal tunnel.
- Injury of the median nerve at the elbow: At elbow the median nerve can be injured due to:
- supracondylar fracture of humerus,
- application of tight tourniquet during venipuncture, and
entrapment of nerve between two heads of pronator teres or underneath the fibrous arch linking the two heads of flexor digitorum superficialis.
Characteristic clinical features in such cases will be as follows:
- Forearm kept in supine position (loss of pronation), due to paralysis of pronator teres.
- Wrist flexion is weak– due to paralysis of all the flexors of forearm other than medial half of FDP and flexor carpi ulnaris.
- Adduction of wrist– due to paralysis of FCR and unopposed action of FCU and medial half of FDP.
- No flexion is possible at the interphalangeal (IP) joints of index and middle fingers.
- Benediction deformity of the hand, i.e., when patient attempts to make fist, the index and middle fingers remain straight, due to paralysis of both superficial and deep flexors of these fingers leading to loss of flexion at PIP and DIP joints. The ring and the little finger can be kept in flexed position due to intact nerve supply of medial half of the FDP.
- Loss of flexion of terminal phalanx of thumb, due to paralysis of FPL.
- Ape-thumb deformity, in which thenar eminence is flattened and thumb is laterally rotated and adducted, due to paralysis of muscles of thenar eminence and normal adductor pollicis, respectively.
- Loss of sensation in lateral half of the palm and lateral 31/ 2 digits and also on the dorsal aspects of same digits.
- Injury of the median nerve at the mid-forearm: The injury of median nerve at mid-forearm effects in pointing index finger due to paralysis of radial head of FDS muscle that continues as tendon of index finger; other signs and symptoms will be same as those which happen in lesion at distal forearm and wrist.
- Injury of the median nerve at wrist (distal forearm): At wrist, median nerve and its palmar cutaneous branch may be injured just proximal to the flexor retinaculum by deep lacerated wounds (cut injury), e.g., suicidal cuts. Characteristic clinical features in such a case will be as follows:
- Ape-thumb deformity, because of paralysis of muscles of thenar eminence.
- Loss of sensation on the lateral part of the palm (including that over the thenar eminence) and lateral 31/ 2 digits involving loss of sensation on the dorsal aspect of these digits.
- Injury in the carpal tunnel: The median nerve is injured in the carpal tunnel due to its compression and creates a clinical condition called carpal tunnel syndrome. The carpal tunnel is formed by anterior concavity of carpus and flexor retinaculum. The tunnel is closely packed with nine long flexor tendons of fingers and thumb with their surrounding synovial sheaths and median nerve. The median nerve gets compressed in the tunnel due to its narrowing following a number of pathological conditions such as
- tenosynovitis of flexor tendons (idiopathic),
- myxedema (deficiency of thyroxine),
- retention of fluid in pregnancy,
- fracture dislocation of lunate bone, and
osteoarthritis of the wrist.
Characteristic clinical features of the carpal tunnel syndrome are as follows:
- Feeling of burning pain or ‘pins and needles’ along the sensory distribution of median nerve (i.e., lateral 31/ 2 digits) especially at night.
- There is no sensory loss over the thenar eminence because skin over thenar eminence is supplied by the palmar cutaneous branch of the median nerve, which passes superficial to flexor retinaculum.
- Weakness of thenar muscles.
- ‘Ape-thumb deformity’ may occur, if left untreated, due to paralysis of the thenar muscles.
- Positive Tinel’s sign and Phalen’s test.
Reduced conduction velocity in the median nerve
The signs and symptoms of the carpal tunnel syndrome are dramatically alleviated by decompressing the tunnel by giving a longitudinal incision through flexor retinaculum.