Median nerve
Nervus medianus
Definition
The median nerve is a significant nerve of the upper extremity, essential for providing both motor and sensory innervation to the forearm and hand.
Origin in the axilla
The median nerve arises from two roots:
Lateral root: from the lateral cord of the brachial plexus.
Medial root: from the medial cord of the brachial plexus.
These roots unite in the axilla near the lower part of the axillary artery, either in front of or lateral to the vessel. Its fibers are derived from the C6, C7, C8, and T1 spinal nerves.
Median nerve in the arm
A. Course in the arm
The median nerve maintains a close relationship with the brachial artery throughout its course in the arm:
In the upper arm, the median nerve initially lies lateral to the brachial artery.
At the mid-arm level, near the insertion of the coracobrachialis muscle, the nerve crosses the brachial artery, transitioning from its lateral to medial side.
From this point to the elbow, the median nerve remains on the medial side of the brachial artery.
B. Branches in the Arm
Although the median nerve does not innervate arm muscles, it gives off the following branches near the elbow:
Muscular branches: To the pronator teres muscle, just proximal to the elbow joint.
Vascular branches: To the brachial artery, contributing to its vascular supply.
Articular branch: To the elbow joint, arising just above the elbow.
Median nerve at the elbow level
A. Course through the cubital fossa
At the elbow, the median nerve passes in front of the joint through the cubital fossa. Within the cubital fossa, the structures are arranged from lateral to medial in the order of TAN: Tendon (biceps brachii), Artery (brachial), and Nerve (median). The median nerve lies medial to the brachial artery and the biceps brachii tendon. It is covered by the bicipital aponeurosis and lies on the surface of the brachialis muscle, which separates it from the underlying bones. After passing through the cubital fossa, the nerve continues its descent by traveling between the two heads of the pronator teres muscle. Before doing so, it sends branches to innervate the pronator teres.
B. Branches near the elbow
These branches arise near the elbow and supply all the superficial flexor muscles of the forearm, except for the flexor carpi ulnaris. The muscles innervated include the flexor carpi radialis, palmaris longus, and flexor digitorum superficialis.
Median nerve in the forearm
The median nerve is the primary nerve serving the flexor compartment of the forearm, responsible for innervating most of its muscles.
A. Course in the forearm
Emerging as the most medial content of the cubital fossa, the median nerve enters the forearm by passing between the two heads of the pronator teres muscle. Here, it crosses over the ulnar artery, separated by the deep head of the pronator teres.
It continues its descent deep to the flexor digitorum superficialis, positioned between this muscle and the flexor digitorum profundus, before progressing toward the wrist.
Approaching the wrist, the nerve emerges from beneath the flexor digitorum superficialis and lies between the tendons of the flexor carpi radialis (laterally) and flexor digitorum superficialis (medially).
Finally, the median nerve passes under the flexor retinaculum within the carpal tunnel to enter the palm.
B. Branches in the Forearm
Anterior Interosseous Nerve:
1. Muscular branches supply the deep flexor muscles of the forearm, including the flexor pollicis longus, the lateral half of the flexor digitorum profundus, and the pronator quadratus.
2. Articular branches innervate the wrist and distal radioulnar joints.
Palmar Cutaneous Branch: Arising from the median nerve near the distal forearm, this branch travels superficial to the flexor retinaculum. It provides sensory innervation to the skin over the lateral two-thirds of the palm and the thenar eminence.
Articular Branches: These branches supply the elbow joint and the proximal radioulnar joint.
Vascular Branches: These branches contribute to the vascular supply of the forearm by innervating the radial and ulnar arteries.
Communicating Branch: A small branch that connects with nearby nerves for additional neural communication.
Transition in the palm
A. Course of the median nerve through the carpal tunnel
The median nerve traverses the carpal tunnel, passing beneath the flexor retinaculum, to enter the palm. Within the carpal tunnel, this nerve is particularly susceptible to compression, a condition known as carpal tunnel syndrome. This syndrome commonly affects individuals who engage in repetitive wrist movements, such as typing or other similar activities. It is worth noting that while the ulnar nerve also enters the palm from the flexor side of the forearm, it remains superficial to the flexor retinaculum and is therefore not impacted by carpal tunnel syndrome.
B. Branches of the median nerve in the palm
In the palm, the median nerve divides into two distinct portions—lateral and medial—giving rise to both motor and sensory branches.
1. Lateral Portion
a. Muscular Branches: Provides motor innervation to the thenar muscles, which include:
Superficial head of the flexor pollicis brevis
b. Proper Palmar Digital Nerves: Give rise to three nerves:
Two nerves supply sensory innervation to the skin on both sides of the thumb.
One nerve contributes two important functions:
i. Motor innervation to the first lumbrical muscle.
ii. Sensory innervation to the radial side of the index finger.
2. Medial Portion
The medial portion divides into two common palmar digital nerves, which further branch into proper palmar digital nerves:
a. First Common Palmar Digital Nerve:
Sends a motor branch to the second lumbrical muscle.
It splits into two proper palmar digital nerves that provide sensory innervation to the adjoining sides of the index finger.
b. Second Common Palmar Digital Nerve:
Splits into two proper palmar digital nerves that supply the adjoining sides of the middle finger.
Sometimes communicates with the ulnar nerve and may occasionally give motor branches to the third lumbrical muscle.
Note
The proper digital nerves also extend to the dorsum of the hand, where they provide sensory innervation to the skin of the dorsal aspect of the distal phalanges. Sensory supply to the remaining areas of the dorsal fingers is carried out by the radial and ulnar nerves.
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