The median nerve is one of the major peripheral nerves of the upper limb, providing both sensory and motor innervation to various parts of the forearm and hand. It is formed from branches of both the medial and lateral cords of the brachial plexus.
Structure and Appearance
- Type: The median nerve is a mixed nerve, containing both motor and sensory fibers.
- Origin: It is formed by the union of two roots: one from the lateral cord of the brachial plexus (usually containing fibers from C5, C6, and C7) and one from the medial cord (usually containing fibers from C8 and T1).
- Branches: It gives off several branches along its course, including the palmar cutaneous branch, anterior interosseous nerve, and several digital nerves in the hand.
Location
- Origin and Course: The median nerve originates in the axillary region, near the shoulder, where it forms from the lateral and medial cords of the brachial plexus.
- Pathway: It travels down the arm, lying initially lateral to the brachial artery. As it descends, it crosses over to the medial side of this artery. It then enters the forearm between the two heads of the pronator teres muscle and continues down the forearm, passing under the flexor retinaculum in the carpal tunnel to enter the hand.
- Innervation Points:
- Motor: Supplies the majority of the flexor muscles in the forearm and several muscles in the hand, including the thenar muscles responsible for thumb movement.
- Sensory: Provides sensation to the palmar aspect of the thumb, index, middle finger, and the lateral half of the ring finger, as well as the back of the tips of these fingers.
Functions
The median nerve has both motor and sensory functions, making it one of the most versatile nerves in the upper limb.
Motor Functions
- Forearm Flexors: The median nerve supplies most of the muscles in the anterior compartment of the forearm, including the pronator teres, flexor carpi radialis, palmaris longus, and the flexor digitorum superficialis. These muscles are primarily responsible for flexing the wrist and fingers and pronating the forearm.
- Thenar Muscles: In the hand, it supplies the thenar muscles (opponens pollicis, abductor pollicis brevis, and the superficial head of the flexor pollicis brevis), which are crucial for thumb movements like opposition, abduction, and flexion.
- Lumbricals: The median nerve also supplies the first and second lumbricals, which flex the metacarpophalangeal joints and extend the interphalangeal joints of the index and middle fingers.
- Anterior Interosseous Nerve: A branch of the median nerve, the anterior interosseous nerve supplies deeper muscles in the forearm like the flexor pollicis longus and the radial half of the flexor digitorum profundus, which are involved in flexing the thumb and fingers, respectively.
Sensory Functions
- Palmar Aspect: Provides sensation to the palmar aspect of the thumb, index finger, middle finger, and the lateral half of the ring finger.
- Dorsal Aspect: Also gives sensory innervation to the skin over the distal phalanges on the dorsal aspect (back) of the same fingers.
- Palmar Cutaneous Branch: This branch arises before the median nerve enters the carpal tunnel and provides sensation to the skin over the thenar eminence.
Integrated Functions
- Fine Motor Skills: The median nerve plays a critical role in activities requiring fine motor skills like writing, gripping, and manipulating objects with the fingers and thumb.
- Sensory Feedback: Its sensory functions are critical for tasks that require a tactile sense, such as picking up small objects or feeling texture.
- Coordination and Dexterity: By innervating muscles responsible for a wide range of motions in the thumb and fingers, the median nerve is key for tasks requiring hand-eye coordination and dexterity.
Clinical Significance
The median nerve holds significant clinical importance due to its extensive motor and sensory functions in the forearm and hand.
Diagnostic Importance
- Carpal Tunnel Syndrome: One of the most common clinical conditions involving the median nerve is carpal tunnel syndrome, where compression of the nerve within the carpal tunnel leads to symptoms like numbness, tingling, and weakness in the areas it innervates.
- Motor Dysfunction: Damage to the median nerve can result in weakness of the flexor and pronator muscles in the forearm and loss of thumb opposition, which are often diagnostic clues.
- Sensory Deficits: Altered or diminished sensation in the lateral palmar aspect of the hand (thumb, index, and middle fingers) can also indicate median nerve dysfunction.
Clinical Conditions
- Nerve Injury: The median nerve can be injured in forearm or wrist fractures, leading to sensory and motor deficits that can severely impact hand function.
- Compression Neuropathies: Beyond carpal tunnel syndrome, the median nerve can be compressed at other points along its course, leading to varying degrees of dysfunction.
- Anterior Interosseous Syndrome: A less common condition affecting a branch of the median nerve that leads to specific motor deficits, notably the inability to make the “OK” sign with the hand.
Therapeutic Implications
- Surgical Decompression: Carpal tunnel release is a common surgical procedure to alleviate median nerve compression in cases of carpal tunnel syndrome.
- Rehabilitation: Physical therapy and occupational therapy are often recommended for patients with median nerve injuries to regain function.
- Pain Management: For chronic pain or neuropathic conditions affecting the median nerve, treatments may include medication, nerve blocks, or other interventions.
