The median nerve is a peripheral nerve and one of the five nerves originating from the brachial plexus. It is found in the upper extremities of humans and other animals. This nerve allows sensitivity and movement of the hand; specifically, it allows for fast fine movements, including the palm of the hand and all fingers except the little finger.
The brachial plexus is a nerve structure found in the neck, armpits, and arms. The lateral and medial fascicles give rise to the median nerve (C5, C6, C7, C8 and T1). Its beginning has a V shape between which the axillary artery passes.
The median nerve is the only one that crosses the carpal tunnel. When this tunnel presses on the nerve, what is known as carpal tunnel syndrome arises, which results in weakness, numbness and tingling in the hand and fingers.
The median nerve starts from the armpit and then runs vertically through the arm, passing alongside the brachial artery.
This nerve is related to the pectorals by crossing the brachial artery at the inner root of the arm. It also crosses the ulnar artery, lying under the superficial common flexor muscle of the fingers.
The median nerve runs down the inner face of the arm next to this artery. Although, when it reaches the front of the arm it goes next to the brachial artery. It enters the anterior compartment of the forearm through the ulnar fossa.
In the forearm the median nerve is located between the muscles of the deep flexor of the fingers and the superficial flexor. This nerve has two main branches in the forearm:
– Anterior interosseous nerve, which supplies the deep muscles of the anterior aspect of the forearm.
– Cutaneous palmar nerve, which supplies the skin of the lateral palm of the hand.
After passing the carpal tunnel, the median nerve divides into two branches:
– Recurrent branch, which innervates the thenar muscles.
– Palmar digital branch, which innervates the surface of the palm of the hand and the thumb, index, middle and middle of the ring finger.
Many people are born with natural abnormalities of the median nerve. The most common variants are the Martin Gruber and Riche Cannieu anastomoses. The first occurs on the forearm in 17% of people, with five types.
While the Riche Cannieu anastomosis occurs in the hand when there is no connection between the recurrent branch of the median nerve and the deep branch of the ulnar nerve. It is observed that the median nerve innervates the adductor pollicis and the first dorsal interosseus in 1% of cases.
The median nerve supplies a large part of the muscles of the anterior aspect of the forearm and some intrinsic muscles of the hand.
The median nerve does not have voluntary motor functions or cutaneous functions in the arm, however, it provides vascular branches to the walls of the brachial artery, which carry sympathetic fibers ( sympathetic nervous system ).
On the other hand, it innervates all the flexor muscles of the forearm, except the ulnar carpus and the deep common flexor of the fingers of the hand. These last two flexor muscles are innervated by the ulnar nerve.
The median nerve gives sensitivity and movement, mainly, to the following muscles:
– Round pronator
– Flexor carpi radialis
– Palmar major
– Superficial flexor muscle of the fingers of the hand
– The lateral half of the deep flexor muscle of the fingers
– Long flexor of the thumb
– Square pronator
These muscles mainly allow pronation of the forearm, flexion of the wrist and flexion of the fingers of the hand.
The median nerve gives mobility to the first and second lumbrical muscles. These muscles allow flexion of the metacarpophalangeal joint of the index and middle fingers.
They also move the muscles of the thenar eminence. The latter are related to the movements of the thumb.
It also innervates the skin on the palmar side of the thumb, index and middle fingers, as well as the middle of the ring finger and the nail bed.
The median nerve also allows skin innervation to certain areas of the hand. This is possible through two branches: the cutaneous palmar branch and the digital cutaneous palmar branch. The first provides sensitivity to a large part of the palmar surface. The second innervates some parts of the fingers.
Clinical significance of the median nerve
Carpal tunnel syndrome
The best known pathology associated with the median nerve is carpal tunnel syndrome. This disease is caused by extreme pressure on this nerve in the area of the wrist where it enters the hand.
It can have multiple causes, although these are often unknown. It can arise from any inflammation, damage, or compression of the nerve. There are people who present it because they were born with a smaller carpal tunnel.
It can also appear after performing the same hand movement repeatedly, as with using vibrating tools.
This syndrome is facilitated by alcoholism, bone fractures, arthritis, tumors in the wrist, infections, obesity, etc.
The most common symptoms are weakness, tingling, and numbness. In addition, the patient may notice clumsiness when trying to grasp objects, pain in the hand or wrist that can extend to the elbow. Other symptoms are difficulty carrying bags and trouble making fine, precise movements with the fingers.
Treatment usually consists of the use of a splint. Corticosteroid injections into the carpal tunnel are also used for severe pain. In more severe cases, it may be necessary to resort to surgical intervention to decompress the nerve.
Another pathology that affects the median nerve is the humerus fracture. This is characterized by lack of sensation in certain areas of the hand, paralysis of the flexor and pronator muscles of the forearm, inability to flex the thumb, and the metacarpophalangeal joints.
If the patient tries to put the hand into a fist, he can only fully flex the little and ring fingers. This results in a characteristic shape of the hand called a “blessing sign.”
If the damage is to the wrist, the symptoms are the same as described above. The thenar muscles are paralyzed, as well as the two lateral lumbricals. The thumb and flexion of the index and middle fingers are affected.
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