The median nerve

The median nerve is a sensory and motor nerve of the arm (or upper limb). The median nerve is derived from the medial and lateral cords of the brachial plexus. It contains fibres from roots C6-T1 and can contain fibres from C5 in some individuals.

Motor functions of the median nerve are mainly flexor. The median nerve also provides sensory innervation to the dorsal aspect (nail bed) of the distal first two digits of the hand, the palmar aspect of the thumb, index, middle, and half of the ring finger, the palm, as well as the medial aspect of the forearm.

In the axilla, the median nerve lies lateral to the axillary artery. It enters the arm from axilla at the inferior margin of the teres major muscle and descends medially between biceps brachii and triceps brachii. In the arm, the median nerve courses laterally to the brachial artery and then crosses it, usually anteriorly, from lateral to medial. It lies initially on coracobrachialis, then brachialis. In the cubital fossa, the median nerve lies medial to the brachial artery and the biceps brachii tendon. The median nerve leaves the bicipital aponeurosis and enters the forearm between the two heads of pronator teres muscle and gives off the anterior interosseous nerve. It courses towards the wrist deep to flexor digitorum superficialis and superficial to flexor digitorum profundus muscle. It emerges lateral to the flexor digitorum superficialis about 5 cm proximal to the wrist (giving off the palmar cutaneous branch) and enters the hand through the carpal tunnel (the only nerve to traverse the carpal tunnel), passing deep to the flexor retinaculum at the wrist. On entering the palm, it gives off motor and cutaneous branches.

The median nerve innervates some of the major muscles of the forearm and hand, which allows for a two-way communication between the brain and spinal cord, and the muscles and overlying skin. The brain and spinal cord can send signals through the medial nerve, to the muscles that it innervates, with instructions on when to contract and complete specific actions. Similarly, the muscles and overlying skin can transmit sensations and sensory information, such as heat and pain, through the median nerve, and back to the brain and spinal cord for processing.

The muscles innervated by the median nerve can be divided into four layers of the anterior compartment of the forearm. The first layer arises from the medial epicondyle and contains three muscles: the pronator teres, flexor radialis longus, and palmaris longus. The second layer contains the flexor digitorum superficialis, whose tendons insert into the middle phalanx of the four fingers. Thus it is a flexor of the fingers at the proximal interphalangeal joint. The first and second layers are innervated by the median nerve proper.

The muscles of the third layer are the lateral half of the flexor digitorum profundus that innervates the index and ring fingers, the flexor pollicis longus, and the pronator quadratus. These are innervated by the anterior interosseous nerve, which has no sensory branches.

The median nerve is usually damaged at either the elbow, due to a fracture of the humerus bone of the upper arm, or the wrist, due to either carpal tunnel syndrome or a wrist laceration or gashing. If the median nerve is damaged at the elbow region, it is known as a proximal injury to the median nerve. Proximal injury to the median nerve often presents with the hand of benediction, a sign that occurs when an individual is unable to make a complete fist. This occurs because flexion of the finger joints, specifically the first and third finger joints (the metacarpophalangeal and distal interphalangeal joints, respectively) of the 1st and 2nd fingers, are lost with median nerve damage. Consequently, when an individual with such nerve damage attempts to make a fist, the thumb and first two fingers remain partially distended, resembling the hand gesture of a Pope as he gives blessings, hence, the name “hand of benediction”.

Carpal Tunnel Syndrome, a common condition caused by compression of the median nerve in the carpal tunnel. In general, patients with this condition will experience symptoms of pain and paresthesia in the distribution of the median nerve, which includes the thumb, index finger, middle finger, and half of the ring finger. Normal pressure in the carpal tunnel ranges from 2 to 10 mm Hg. There are several things that can increase the pressure in the carpal tunnel, known as wrist extension increases the pressure by 10 times, and wrist flexion can increase the pressure of the carpal tunnel by 8 times. Increased carpal tunnel pressure is thought to be the cause of the compression on the median nerve.

Published : Nov 20 2023