INNERVATION OF THE HAND
Nerve branches from the ulnar, median, and radial nerve supply motor, sensory, and autonomic vasomotor function in the hand.
The ulnar nerve (C, 8; T1) is the main continuation of the medial cord of the brachial plexus. In the forearm and hand, the ulnar nerve gives off articular, muscular, palmar, dorsal, superficial and deep terminal, and vascular branches. It divides into branches for the areas of skin on the medial side of the back of the hand and fingers (see Plate 4-12). The ulnar nerve enters the hand to the radial side of the pisiform between the palmar carpal ligament and the flexor retinaculum. Just distal to the pisiform, the ulnar nerve divides into superficial and deep branches.
The superficial terminal branch supplies the palmaris brevis muscle, innervates the skin on the medial side of the palm, and gives off two palmar digital nerves. The first is the proper palmar digital nerve for the medial side of the small finger; the second, the common palmar digital nerve, communicates with the adjoining common palmar digital branch of the median nerve before dividing into the two proper palmar digital nerves for the adjacent sides of the small and ring fingers. Rarely, the ulnar nerve supplies two and one-half rather than one and one-half digits, and the areas supplied by the median and radial nerves are reciprocally reduced. The deep terminal branch of the ulnar nerve, with the deep branch of the ulnar artery, sinks between the origins of the abductor digiti minimi and the flexor digiti minimi brevis muscles and perforates the origin of the opponens digiti minimi muscle. It supplies these muscles and then curves around the hamulus of the hamate into the central part of the palm of the hand in conjunction with the deep palmar arterial arch. As it crosses the hand deep to the flexor tendons to the digits, the nerve gives twigs to the ulnar two lumbrical muscles and to all the interosseous muscles, both dorsal and palmar. It then supplies the adductor pollicis muscle and gives articular twigs to the wrist joint, and it may send a terminal branch into the deep head of the flexor pollicis brevis muscle.
Variations in the nerve supplies of the palmar muscles are as common as the variations in the cutaneous distribution; they are due to the variety of interconnections between the ulnar and median nerves.
The dorsal branch of the ulnar nerve completes the cutaneous supply of the dorsum of the hand and digits. It arises about 5 cm above the wrist, passes dorsalward from beneath the flexor carpi ulnaris tendon, and then pierces the forearm fascia. At the ulnar border of the wrist, the nerve divides into three dorsal digital branches. There are usually two or three dorsal digital nerves, one supplying the medial side of the small finger, the second splitting into proper dorsal digital nerves to supply adjacent sides of the small and ring fingers, and the third (when present) supplying contiguous sides of the ring and long fingers.
The first branch courses along the ulnar side of the dorsum of the hand and supplies the ulnar side of the small finger as far as the root of the nail. The second branch divides at the cleft between the ring and small fingers and supplies their adjacent sides. The third branch may divide similarly; it may supply the adjacent sides of the long finger and ring finger, or it may simply anastomose with the fourth dorsal digital branch of the superficial branch of the radial nerve. The dorsal branches to the ring finger usually extend only as far as the base of the second phalanx, with the more distal parts of the ring and small finger supplied by palmar digital branches of the ulnar nerve.
The palmar branch of the ulnar nerve arises about the middle of the forearm, descending under the ante brachial fascia in front of the ulnar artery. It perforates the fascia just above the wrist and supplies the skin of the hypothenar eminence and the medial part of the palm.
The median nerve (C, 6, 7, 8; T1) is formed by the union of medial and lateral roots arising from the corresponding cords of the brachial plexus (see (Plate 1-18).
The palmar branch of the median nerve arises just above the wrist (see Plate 4-13). It perforates the palmar carpal ligament between the tendons of the palmaris longus and flexor carpi radialis muscles and distributes to the skin of the central depressed area of the palm and the medial part of the thenar eminence.
The digital branches of the median nerve, the proper palmar digital nerves, lie subcutaneously along the margins of each of the digits distal to the webs of the fingers (see Plates 4-12 and 4-13). They arise from common palmar digital nerves, which lie under the dense palmar aponeurosis of the central palm. The first common palmar digital nerve gives rise to the muscular branch to the short muscles of the thumb and then divides into three proper palmar digital nerves. Just distal to the flexor retinaculum, its motor, or recurrent, branch curves sharply into the thenar eminence and supplies the abductor pollicis brevis, flexor pollicis brevis (sometimes only its superficial head), and opponens pollicis muscles. This branch frequently arises from the median nerve together with its first common digital branch. The first common digital branch then runs to the radial and ulnar sides of the thumb, giving numerous branches to the pad and small, dorsally running branches to the nail bed of the thumb. The third proper digital branch supplies the radial side of the index finger. The second common palmar digital branch provides two proper palmar digital nerves, which reach the adjacent sides of the index and long fingers. The third common palmar digital nerve communicates with a digital branch of the ulnar nerve in the palm and divides into two proper palmar digital nerves supplying adjacent sides of the long finger and ring fingers.
Proper palmar digital nerves are large because of the density of nerve endings in the fingers. They lie superficial to the corresponding proper palmar digital arteries and veins. As each nerve passes toward its termination in the pad of the finger, it gives off branches for the innervation of the skin of the dorsum of the digits and the matrices of the fingernails. These dorsal branches innervate the dorsal skin of the distal segment of the index finger, the two terminal segments of the long finger, and the radial side of the ring finger. The common and proper palmar digital nerves vary in their origins and distributions, but the usual arrangement innervates the skin (including the nail beds) over the distal and dorsal aspects of the lateral three and one-half digits. Occasionally, they supply only two and one-half digits. The proper palmar digital branches to the radial side of the index finger and to the contiguous sides of the index and long fingers also carry motor fibers to supply the first and second lumbrical muscles, respectively. Therefore, the digital nerves are not concerned solely with cutaneous sensibility. They contain an admixture of efferent and afferent somatic and autonomic fibers, which transmit impulses to and from sensory endings, vessels, sweat glands, and arrectores pilorum muscles and between fascial, tendinous, osseous, and articular structures in their areas of distribution.
Dorsally the superficial branch pierces the deep fascia and commonly subdivides into two branches, which usually split into four or five dorsal digital nerves. The cutaneous area of supply is shown in Plate 4-14. The smaller lateral branch supplies the skin of the radial side and eminence of the thumb and communicates with the lateral antebrachial cutaneous nerve. The larger medial branch divides into four dorsal digital nerves. The first dorsal digital nerve supplies the ulnar side of the thumb; the second supplies the radial side of the index finger; the third distributes to the adjoining sides of the index and long fingers; and the fourth supplies the adjacent sides of the long and ring fingers.
There is usually an anastomosis on the back of the hand between the superficial branch of the radial nerve and the dorsal branch of the ulnar nerve, and there is some variability in the apparent source of the last (more median) branch of either nerve. In some such cases, the adjacent sides of the long and ring fingers are in the territory of the ulnar nerve. Dorsal digital nerves fail to reach the extremities of the digits. They reach to the base of the nail of the thumb, to the distal interphalangeal joint of the index finger, and not quite as far as the proximal interphalangeal joints of the long and ring fingers. The distal areas of the dorsum of the digits not supplied by the radial nerve receive branches from the stout palmar digital branches of the median nerve.
The dorsal digital nerves also supply filaments to the adjacent vessels, joints, and bones. (Note that the dorsal digital nerves extend only to the levels of the distal interphalangeal joints and that the first dorsal digital nerve gives off a twig that curves around the radial side of the thumb to supply the skin over the lateral part of the thenar eminence.)