Three independent nerves supply the cutaneous innervation of the hand. From lateral to medial these nerves include the superficial branch of the radial nerve, the median nerve, and the ulnar nerve. These peripheral nerves originate from the C5 to T1 levels of the spinal column. The sensory distribution discussed here includes both the ventral and dorsal aspects of the hand ending proximally at the level of the wrist. Also briefly discussed here will be a set of muscles innervated by these nerves as they supply the cutaneous innervation of the hand.
The cutaneous innervation of the hand is formed primarily by the terminal branches of three nerves: the median nerve, the radial nerve, and the ulnar nerve. Each of these nerves is formed proximally by the brachial plexus, which arises from the cervical root levels C5 to T1. These nerves course through the axilla and various compartments of the arm and forearm until reaching the wrist where the sensory and motor branches of each nerve distribute throughout the hand. Both the radial and median nerve receive and send input via each brachial plexus level C5 to T1 while the ulnar nerve receives and sends inputs via just the C8 and T1 spinal levels. The anatomic course of these nerves is discussed in greater detail below in the 'neves' section of this article. These sensory branches include the superficial radial nerve, superficial and dorsal ulnar nerve, and the palmar branch of the median nerve. These nerves then give off digital nerve branches that feed into the individual digits providing cutaneous sensory information to the brain.
The function of these nerves summates the cutaneous innervation of the hand as well as the muscular innervation of a few muscles discussed in the muscles portion of this document. They provide the sensation of touch, pressure, pain, and temperature to the brain. The set of these nerves contain sympathetic and autonomic afferent fibers as well as sympathetic efferent fibers providing innervation to the cutaneous blood vessels, sweat glands, and arrector pili muscles of hair follicles. These autonomic afferent fibers primarily supply the cutaneous innervation.
As follows with all peripheral nerves; the cutaneous innervation of the hand arises from neural crest cells which derive from the embryonic ectoderm. These cells take the form of bilateral strips at the junction of the ectoderm and neural plate which detach and migrate with the merging of the two ectodermal regions during embryogenesis. These neural crest cells migrate in a rostral to caudal distribution ending in the formation of the peripheral nervous system, which includes the cutaneous innervation of the hand.
The blood supply of the hand takes the form of the superficial and deep palmar arches, which is an anastomotic landmark formed from the ulnar and radial artery rami at the level of the wrist. These arches give off palmar digital arteries that feed the individual digits, nerves, and muscles of the hand. The ulnar aspect of the arch follows with the course of the superficial ulnar nerve, discussed below, separate from it at the level of the bifurcation of this nerve. Branches of the median nerve follow the path of their respective palmar arteries.
Superficial Branch of Radial Nerve: This nerve yields sensory cutaneous innervation to a significant portion of the dorsolateral aspect of the hand. The anatomic course of this nerve follows the course of the radial nerve. Beginning lying slightly lateral to the radial artery, it runs hidden beneath the brachioradialis muscle. It continues this path until approximately 7 cm above the wrist where it then pierces the deep fascia and divides into two branches: lateral and medial. The lateral branch yields cutaneous innervation to the radial dorsal aspect of the thumb then joining the dorsal terminal branch of the lateral antebrachial cutaneous nerve. The course of the median branch is slightly more complicated, first communicating with the lateral antebrachial cutaneous just proximal to the level of the wrist and then communicates with the dorsal branch of the ulnar nerve on the dorsal aspect of the hand distal the level of the wrist. Along this course, distal to the wrist, the superficial radial nerve gives off fibers innervating the dorsum of the hand. Around the anatomical level just described it branches into terminal digital nerves with each innervating specific portions of the hand as follows:
Median Nerve: The first branch supplying cutaneous innervation is the palmar cutaneous branch of the median nerve. This branch begins just proximal to the wrist and travels superficial to the flexor retinaculum then giving off terminal fibers that supply cutaneous innervation to much of the lateral palm. After the median nerve passes through the carpal tunnel, it commonly divides into a smaller lateral branch and a thicker medial branch. The medial branch divides into the two common palmar digital nerves. The lateral branch divides into the recurrent median nerve as well as three proper digital nerves. The recurrent median nerve supplies motor innervation to the thenar muscle group which includes opponens pollicis, abductor pollicis brevis and the flexor pollicis brevis; it does not supply any of the cutaneous innervations of the hand and will receive any further discussion. Starting with the three proper digital nerves off the lateral branch, two of these supply the cutaneous sensory innervation for both ventral lateral aspects of the thumb while the remaining one supplies sensory innervation for the radial ventral aspect of the index finger. Next, the two common palmar digital nerves arising from the medial division of the median nerve yield their own proper digital nerves. These proper digital nerves innervate the radial ventral aspect of the index finger, the ventral aspects of the middle finger, and the radial ventral aspect of the ring finger. These terminal branches may also innervate the terminal dorsal end of the digits just discussed.
Ulnar Nerve: This nerve yields sensory cutaneous innervation to the dorsomedial and ventromedial aspect of the hand as well as motor innervation to the palmar brevis muscle; this is accomplished via two nerves: the dorsal cutaneous branch of the ulnar nerve and the palmar cutaneous branch of the ulnar nerve. The dorsal cutaneous ulnar branch originates on average 5 cm proximal to the ulnar styloid process which then travels superficially to the dorsum of the hand. Here it branches into common and proper digital nerves. The common digital nerve innervates the dorsum of the hand before splitting into more proper digital nerves. The summation of these proper digital nerves innervates the dorsum of the fifth and most commonly the ulnar dorsal aspect half of the fourth digit. The ulnar nerve bifurcates into the superficial and deep branches within the Guyon's canal. The superficial branch goes on to become the palmar cutaneous branch of the ulnar nerve within the palm. Here it gives off fibers that innervate the ulnar aspect of the palm. The nerve then branches into proper and common digital nerves that supply the cutaneous innervation to the ventral aspect of the fifth digit as well as most commonly to the ulnar ventral aspect of the fourth digit.
Anatomical variations primarily involve discrepancies between the anastomotic connections between the branches of the nerves discussed as well as variation in the origin of the common and proper digital nerves. Other variations usually involve the course or origin of the nerves as they enter the hand. Some of these variations are outlined here below.
Any surgery near the entire course of the radial, ulnar, or median nerve could have downstream cutaneous sensory deficits of the hand if trauma occurs. Also, any surgery involving the brachial plexus or the spinal cord levels C5 to T1 could present with similar or identical deficits distally. Of note, carpal tunnel release holds the risk for median nerve damage as it enters the hand through the wrist. Also, anatomic variations such as those discussed prior should always be taken into consideration when working within the space of the forearm, wrist, or hand.
There are significant clinical sequelae involving each the median, ulnar, and radial nerves. DIscussion of some follows, but it is important to note that there are relevant clinical consequences from almost every aspect of medicine.
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