Median Nerve

 

 Median Nerve

1.  Origin:

The Median Nerve extends along the arm and forearm to the wrist. It arises by two roots, one from the lateral and one from the medial cord of the brachial plexus; these embrace the lower part of the axillary artery, uniting either in front of or lateral to that vessel. Its fibers are derived from the sixth, seventh, and eighth cervical and first thoracic nerves.

2.  Course:

· Axilla: Starts in axilla (lateral or anterior to axillary artery)

· Arm: Runs along with brachial artery – initially lateral to it (in arm) and later medial to it (cubital fossa)

· Forearm: Enters the forearm between 2 heads of pronator teres (anterior interosseous nerve) and then travels between Flexor digitorum superficialis and Flexor digitorum profundus

· Wrist: Just above wrist, the nerve lies between Flexor digitorum superficialis and Flexor carpi ulnaris

· Hand: Enters the hand via carpal tunnel (Palmar cutaneous branch runs superficial to flexor retinaculum to supply thenar skin)

3.  Branches:

With the exception of the nerve to the Pronator teres, which sometimes arises above the elbow-joint, the median nerve gives off no branches in the arm. As it passes in front of the elbow, it supplies one or two twigs to the joint.

In the forearm its branches are:

·         muscular

·         volar interosseous

·         palmar

 

·         muscular branch

All the superficial muscles on the front of the forearm except the Flexor Carpi Ulnaris. Deep muscles on the front of the forearm, except the ulnar half of the Flexor digitorum profundus

                           I.            Pronator teres

                          II.            Flexor carpi radialis

                        III.            Flexor carpi sublimis

                        IV.            Anterior interosseous (motor)

                         V.            Flexor pollicis longus

                        VI.            Flexor digitorum profundus to 2nd & 3rd fingers

                      VII.            Pronator quadratus

 

·         Volar interosseous branch

Deep muscles on the front of the forearm, except the ulnar half of the Flexor digitorum profundus

                        I.            Abductor pollicis brevis

                       II.            Opponens pollicis

                     III.            Lumbricals: 1st & 2nd

                     IV.            ± Flexor pollicis brevis (also innervated by ulnar nerve)

 

·         Palmar branch

It pierces the volar carpal ligament and divides into:

·         lateral branch (branch arises proximal to carpal tunnel)

·         skin over thenar theeminence

·         communicates with the volar branch of the lateral antibrachial cutaneous nerve.

4.  Clinical relevance:

There are three entrapment syndromes involving the medial nerve or its branches:

·         Carpal Tunnel Syndrome

Is a common condition that causes a tingling sensation, numbness and sometimes pain in the hand and fingers. These sensations usually develop gradually and start off being worse during the night. They tend to affect the thumb, index finger, and middle finger.

Other symptoms of carpal tunnel syndrome include:

·         pins and needles (paraesthesia)

·         thumb weakness

·         a dull ache in the hand or arm ^[3]

·         Anterior Anterosseus

·         AINS is a pure motor neuropathy, as the anterior interosseous nerve contains no sensory fibers; dull forearm pain is however sometimes mentioned by patients.

·         Typically, patients fail to make an “O.K.”-sign, as flexion of the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger, is impaired.

·         Another sensitive test is the pinch test: a patient with AINS will also not be able to pinch a sheet of paper between his thumb and index finger, instead of clamping the sheet between his extended thumb and index fingers, akin to a tong rather than a clamp. The weakness of the pronator quadratus muscle manifests itself in pronation weakness with a flexed elbow.

·         AINS can be confounded by the Martin-Gruber anastomosis, present in up to 25% of the population: in these cases, the anterior interosseous nerve gives off branches to the ulnar nerve, creating atypical motor innervation patterns of the forearm and hand and thus effacing the typical clinical symptoms. ^[4]

 

·         Pronator Teres Syndrome

Due to the position of this muscle that sits over your median nerve, the pressure on the nerve can elicit pain while hindering your forearm movement. It occurs when your pronator teres muscle becomes tight or overworked, compressing the median nerve. Hammering, repeatedly using a screwdriver, cleaning fish can lead to overuse of the pronator teres. Symptoms include pain and reduced mobility. Carpal Tunnel Syndrome symptoms are often confused with PTS, however in this case symptoms worsen with elbow movements.