Median Nerve: Structure, Function, Clinical Significance and Associated Lesions

It is a nerve in humans and other animals in the upper limb. It is one of the five main nerves that originate from the brachial plexus.

The median nerve originates from the lateral and medial cords of the brachial plexus and has contributions from the ventral roots of C5-C6 (lateral cord) and C8 and T1 (medial cord).

The median nerve is the only nerve that passes through the carpal tunnel . The syndrome carpal tunnel is disability resulting from pressing the median nerve in the carpal tunnel.

Structure

The median nerve arises from the branches of the lateral and medial cords of the brachial plexus, runs through the front of the arm, forearm, and hand, and ends up supplying the muscles of the hand.

Arm

After receiving inputs from the lateral and medial cords of the brachial plexus, the median nerve enters the arm from the axilla at the lower margin of the teres major muscle.

It then passes vertically downward and passes laterally to the brachial artery between the biceps brachii (above) and the brachialis (below).

At first it is lateral to the artery and is anterior to the elbow joint; it is then crossed anteriorly to run medial to the artery in the distal arm and in the ulnar fossa. Within the ulnar fossa, the median nerve passes medial to the brachial artery.

The median nerve emits an articular branch to the elbow joint. A branch of the pronator teres muscle arises from the median nerve just above the elbow joint.

Forearm

The median nerve continues in the ulnar fossa medial to the brachial artery and passes between the two heads of the pronator teres, deep to the bicipital aponeurosis (biceps aponeurosis) and to the superficial brachialis muscle.

It crosses the ulnar artery (branch of the brachial artery) while it is separated by the deep head of the pronator teres.

It then travels between flexor digitorum superficialis / superficial common flexor of the fingers (top) and deep flexor of the fingers (bottom). The median nerve is accompanied by a median artery (a branch of the anterior interosseous artery) during this course.

Then, about 5 cm above the flexor retinaculum (wrist), it emerges between the superficial flexor of the fingers (medially) and the flexor carpi radialis (laterally) in the hand.

The main trunk of the median nerve supplies the superficial and deep groups of muscles in the anterior compartment of the forearm, with the exception of the flexor carpi ulnaris. The median nerve does this by emitting two branches as it passes through the forearm:

The muscle branches branch off in the ulnar fossa to supply the flexor carpi radialis, palmar longus, and flexor digitorum superficialis.

The anterior interosseous branch is detached in the upper part of the forearm, courses with the anterior interosseous artery and innervates the long flexor of the thumb and the lateral half of the deep flexor of the fingers (the ulnar half is supplied by the ulnar nerve, as the flexor ulnar carpal muscle).

It ends with its pronator square innervation. In addition to its supply to the muscles, this nerve also supplies the distal radioulnar joint and the wrist joint.

The median nerve also emits sensory and other branches in the forearm. The palmar cutaneous branch of the median nerve originates from the distal part of the forearm. It supplies sensory innervation to the thenar emineration of the palm and the central palm.

The articular branches are delivered to the elbow joint and the proximal radioulnar joint. Vascular branches supply the radial and ulnar arteries. Meanwhile, a communicating branch is given to the ulnar nerve.

Mano

The median nerve enters the hand through the carpal tunnel, deep in the flexor retinaculum along with the tendons of the superficial flexor of the fingers, the deep flexor of the fingers, and the long flexor of the thumb.

From there it is divided into a recurrent muscular branch and a digital cutaneous branch:

The muscular branch (also known as the recurrent branch) supplies the thenar muscles (opponens pollicis / opposing muscle of the thumb, abductor pollicis brevis / abductor pollicis brevis muscle and superficial part of the flexor pollicis brevis / flexor pollicis longus muscle)

Digital cutaneous branches to the appropriate palmar digital branch and common palmar digital branch. The appropriate palmar digital branch gives three digital branches to the lateral one and a half digits (two digital branches to the thumb, one digital branch to the lateral side of the index finger).

The digital branch to the index finger also supplies the first lumbrical. The common palmar digital branch is further divided into two branches.

Both the medial and lateral branches provide the second and third interdigital clefts with the adjacent index, central half, and lateral half of the ring finger. The lateral branch also supplies the second lumbrical.

Variation

There are multiple natural abnormalities of the median nerve.

Median nerve bifurcation typically occurs after the nerve exits the carpal tunnel; however, in a small percentage (5-10%) of individuals, the median nerve bifurcates more proximally at the carpal tunnel, wrist, or forearm.

During gestation, a median artery serving the hand is retracted. However, in some individuals, the median artery does not retract and follows the course alongside the median nerve in the hand.

Martin-Gruber anastomoses can occur when branches of the median nerve cross in the forearm and fuse with the ulnar nerve to innervate portions of the right.

Riche-Cannieu anastomoses can occur when there is a connection between the recurrent branch of the median nerve and the deep branch of the ulnar nerve in the hand.

Function

The median nerve is the main nerve in the front of the forearm. It supplies the muscles of the front of the forearm and the muscles of the thenar eminence, thus controlling the gross movements of the hand. Therefore, it is also called “worker’s nerve.”

Arm

The median nerve does not have a 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.

Forearm

It innervates all the flexors of the forearm except the flexor carpi ulnaris and the part of the deep flexor of the fingers that supplies the 4th and 5th digits. The last two muscles are supplied by the ulnar nerve (specifically the muscular branches of the ulnar nerve).

The main portion of the median nerve supplies the following muscles:

Surface group:

  • Pronator teres / round pronator muscle.
  • Flexor carpi radialis / flexor carpi radialis muscle or palmar major.
  • Palmaris longus / long palmar.

Intermediate group:

  • The superficial flexor muscle.

The anterior interosseous branch of the median nerve supplies the following muscles:

Deep group:

  • Deep flexor digitorum (lateral half only).
  • Flexor pollicis longus / flexor pollicis longus muscle.
  • Pronator quadratus / pronator quadratus muscle.

Mano

In the hand, the median nerve supplies motor innervation to the first and second lumbrical muscles. It also supplies the muscles of the thenar eminence by means of a recurrent thenar branch. The rest of the intrinsic muscles of the hand are supplied by the ulnar nerve.

The median nerve supplies the skin on the palmar (volar) side of the index, thumb and middle finger, the middle of the ring finger, and the nail bed.

The radial aspect of the palm is supplied by the palmar cutaneous branch of the median nerve, which leaves the nerve proximal to the folds of the wrist.

This palmar cutaneous branch travels in a separate fascial groove adjacent to the flexor carpi radialis and then superficial to the flexor retinaculum. Hence, it is spared in carpal tunnel syndrome.

Clinical significance

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

  • Carpal tunnel syndrome.
  • Anterior interosseous syndrome.
  • Pronator Teres Syndrome.

Injury to the median nerve at different levels causes different syndromes with various motor and sensory deficits.

Carpal tunnel syndrome

It is a common condition that causes a tingling sensation, numbness, and sometimes pain in the hand and fingers. These sensations usually develop gradually and begin to worsen at night. They tend to affect the thumb, index, and middle fingers.

Other symptoms of carpal tunnel syndrome include:

Anterior intraosseous syndrome

The anterior intraosseous syndrome is a pure motor neuropathy, since the anterior interosseous nerve does not contain sensory fibers; dull forearm pain is sometimes mentioned by patients.

Patients generally fail to make the “OK” sign as flexion of the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger are affected.

Another sensitive test is the pinch test, a patient with anterior intraosseous syndrome will also not be able to pinch a sheet of paper between their thumb and forefinger, instead of holding the sheet between their extended thumb and forefinger, similar to a forceps than a forceps. .

Weakness of the pronator quadratus muscle manifests itself in weak pronation with a flexed elbow.

The anterior intraosseous syndrome can be confused with the Martin-Gruber anastomosis, present in up to 25% of the population, in these cases, the anterior interosseous nerve emits branches to the ulnar nerve, creating patterns of atypical motor innervation of the forearm and hand and thus erasing the typical clinical symptoms.

Pronator Teres Syndrome

Due to the position of this muscle on your median nerve, pressure on the nerve can cause pain and hinder movement of your forearm. It occurs when the 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.

Symptoms of carpal tunnel syndrome are often mistaken for post-traumatic stress disorder, however in this case the symptoms are made worse by movements of the elbow.

Median nerve entrapment

The median nerve, colloquially known as the “eye of the hand,” is one of the three main nerves of the forearm and hand. It flows from the brachial plexus in the armpit to innervate the intrinsic muscles of the hand.

Median nerve entrapment syndrome is a mononeuropathy that affects movement or sensation in the hand. It is caused by compression of the median nerve at the elbow or distally in the forearm or wrist, with symptoms in the distribution of the median nerve.

Forms of median nerve entrapment include the following:

  • Carpal tunnel syndrome.
  • Anterior interosseous syndrome.
  • Pronator Teres Syndrome.

Since the first launch of the carpal tunnel described by Learmonth, the surgical technique for carpal tunnel syndrome has remained constant, with more than 95% of cases performed through a small longitudinal incision oriented distal to the palmar crease of the wrist.

Although an endoscopic approach has been used for carpal tunnel release, the open procedure remains the most popular operation. Ultrasound-guided percutaneous carpal tunnel release may be a viable alternative.

Specific air injuries

The median nerve is particularly vulnerable to damage to the elbow and wrist.

Over elbow

Common injury mechanism : supracondylar humerus fracture.

Engine deficit:

Loss of pronation of the forearm, weakness in flexion of the hand at the wrist, loss of flexion of the radial half of the fingers and thumb, loss of abduction and opposition of the thumb.

It produces paralysis of the flexors and pronators in the forearm, with the exception of the flexor carpi ulnaris and the medial half of the deep flexor of the fingers.

The forearm is constantly supinated and flexion is weak (often accompanied by adduction, due to the pull of the flexor carpi ulnaris). Flexion in the thumb is also prevented, as the long and short muscles are paralyzed.

Presence of an ape hand deformity when the hand is at rest, due to hyperextension of the index finger and thumb, and a thumb adducted.

Presence of a sign of blessing when trying to form a fist, due to the loss of flexion of the radial half of the digits.

Sensory deficit : loss of sensitivity in 3 1/2 lateral digits including nail beds and thenar area.

It gives rise to the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.

Both lateral lumbrical muscles are paralyzed, and the patient will not be able to flex at the metacarpophalangeal joints or extend to the interphalangeal joints of the index and middle fingers.

Therefore, when trying to flex the fingers and thumb to form a fist, the first 3 digits remain in extension, and the posture is known as the “blessing hand.”

In ulnar nerve injuries, the same posture occurs with the hand at rest, due to medial lumbrical paralysis. In these cases, it is known as the “ulnar claw.”

On the elbow

Impingement at the elbow or proximal forearm could be due to pronator teres syndrome.

Within the proximal forearm: anterior interosseous syndrome

Injury to the anterior interosseous branch in the forearm causes the anterior interosseous syndrome.

Common mechanisms : tight cast, forearm bone fracture.

Motor deficit : loss of pronation of the forearm, loss of flexion of the radial half of the fingers and thumb.

Sensory deficit : none.

On the wrist

Common mechanism : laceration of the wrist.

Engine deficit:

Weakness in flexion of the radial half of the fingers and thumb, loss of abduction and opposition of the thumb.

The presence of an ape hand deformity when the hand is at rest may be likely, due to hyperextension of the index finger and thumb, and an adducted thumb.

However, an ape hand deformity is not a requirement for the diagnosis of carpal tunnel syndrome.

Presence of the sign of blessing when trying to form a fist, due to the weakness in the flexion of the radial half of the digits.

Sensory deficit : loss of sensitivity in 3 1/2 lateral digits including nail beds and thenar area.

Inside the wrist : carpal tunnel syndrome.

Common Mechanism : Carpal Tunnel Syndrome, a compression injury to the carpal tunnel, without transection of the median nerve, due to overuse of activities such as typing and cooking.

Engine deficit:

Weakness in flexion of the radial half of the fingers and thumb, weakness in abduction and opposition of the thumb.

Presence of a monkey hand deformity or when attempting to form a fist, the sign of blessing, due to compression of the median nerve, as opposed to complete paralysis of the median nerve.

Sensory deficit : numbness and tingling in 3 1/2 lateral digits, including the nail beds, but excluding the thenar eminence which is supplied by the palmar cutaneous branch of the median nerve.

Unlike a wrist laceration, there is still sensation in the central palm area. Sensation is not lost because the palmar cutaneous branch runs above the flexor retinaculum and is not affected by compression in carpal tunnel syndrome.