The median nerve is a major nerve in the upper limb that provides motor and sensory innervation to specific muscles of the forearm and hand, and sensory innervation to parts of the hand.
Course and Distribution of the Median Nerve
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Arm: The median nerve originates from the brachial plexus, specifically from the lateral and medial cords. It travels down the arm alongside the brachial artery, initially lateral to the artery, then crossing over to become medial to it around the mid-arm. Importantly, the median nerve typically does not provide any motor or sensory branches in the arm itself.
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Forearm: The nerve enters the forearm by passing between the two heads of the pronator teres muscle. In the forearm, the median nerve gives off motor branches to most of the flexor muscles, with one exception:
- Motor Innervation: It innervates the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. It also innervates the flexor pollicis longus, the radial half of the flexor digitorum profundus (responsible for flexing the distal interphalangeal joints of the index and middle fingers), and the pronator quadratus.
- Sensory Innervation (Palmar Cutaneous Branch): Before passing under the flexor retinaculum at the wrist, the median nerve gives off a palmar cutaneous branch that provides sensory innervation to the skin over the thenar eminence (the fleshy area at the base of the thumb).
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Hand: The median nerve enters the hand through the carpal tunnel. After passing through the tunnel, it divides into terminal branches:
- Motor Innervation (Recurrent Branch): A recurrent branch supplies the thenar muscles: the abductor pollicis brevis, flexor pollicis brevis (superficial head), and opponens pollicis, which are essential for thumb movements.
- Sensory Innervation (Digital Branches): Palmar digital branches provide sensory innervation to the palmar aspect of the thumb, index finger, middle finger, and the radial (thumb-side) half of the ring finger. It also provides sensory innervation to the dorsal (back) aspect of the distal phalanges of these same digits.
Key Functions
- Forearm Flexion and Pronation: The median nerve controls muscles involved in flexing the wrist and fingers, as well as pronating the forearm (turning the palm downwards).
- Thumb Opposition and Movement: It controls the thenar muscles, which are essential for thumb opposition (bringing the thumb across the palm to touch other fingers).
- Sensory Feedback: It provides sensory information from the skin of the palmar side of the thumb, index, middle, and radial half of the ring finger, allowing for fine motor control and tactile perception.
Clinical Significance
Compression or damage to the median nerve can lead to conditions like carpal tunnel syndrome, pronator teres syndrome, and high median nerve injuries. These conditions can result in weakness, numbness, tingling, and pain in the hand and forearm.
