The hand is a remarkable tool, capable of both immense strength and delicate, precise movements. This extraordinary function is entirely dependent on a complex and extensive network of nerves that carry signals between the brain and the muscles and skin of the arm, forearm, and hand. Understanding the distinct pathways of these nerves reveals why certain injuries or compressions lead to specific patterns of numbness or weakness. The vast majority of the hand’s function and sensation is governed by the three major nerves that travel from the neck, through the arm, and into the fingers.
The Three Major Nerves of the Hand and Forearm
The Median nerve travels down the center of the arm and forearm. From the armpit, it runs alongside the brachial artery before entering the forearm between two layers of muscle. In the wrist, the Median nerve is the only nerve that passes through the narrow carpal tunnel, positioned deep to the tough transverse carpal ligament.
The Ulnar nerve takes a path along the inner, or medial, side of the limb, following the bone of the little finger side. In the arm, it shifts to the back of the elbow, where it is highly exposed as it passes through the cubital tunnel, a shallow groove behind the bony prominence of the medial epicondyle. From the elbow, the nerve travels down the forearm between two deep muscles, the flexor carpi ulnaris and the flexor digitorum profundus, before entering the hand through a small channel at the wrist called Guyon’s canal.
The Radial nerve is primarily responsible for the muscles on the back of the arm and forearm. It starts by wrapping around the back of the humerus in a shallow groove. As it reaches the elbow, it divides into two main parts: a deep branch that supplies the muscles that straighten the wrist and fingers, and a superficial branch that continues into the hand, mostly providing sensation to the back of the thumb side.
Sensory and Motor Functions of Hand Nerves
The three main nerves divide their responsibilities between motor function (controlling muscle movement) and sensory function (providing feeling and touch). The Median nerve is the main motor nerve for the muscles that allow opposition, the movement that brings the thumb across the palm to touch the other fingers, which is essential for pinching and grasping. It also innervates most of the muscles that flex, or bend, the wrist and fingers on the palm side of the forearm, except for two specific muscles.
For sensation, the Median nerve covers the palmar surface of the thumb, the index finger, the middle finger, and the half of the ring finger nearest to the thumb. This sensory distribution also extends onto the dorsal, or back, side of the fingertips and nail beds for the same digits. A separate branch of the Median nerve provides sensation to the central portion of the palm, which is an area that does not pass through the narrow wrist tunnel.
The Ulnar nerve is the primary nerve for most of the small, intrinsic muscles within the hand, which are responsible for fine motor control and spreading the fingers. In the forearm, it supplies only two muscles: the flexor carpi ulnaris, which helps bend the wrist on the little finger side, and the portion of the flexor digitorum profundus that bends the ring and little fingers.
The sensory realm of the Ulnar nerve includes the entire little finger and the adjacent half of the ring finger on both the palm and back of the hand. The Radial nerve, in contrast to the other two, is predominantly a motor nerve in the forearm, supplying all the muscles that extend the wrist and fingers. Its sensory contribution is limited, providing feeling primarily to a small patch of skin over the back of the hand near the thumb and index finger.
Common Causes of Hand Nerves Compression
Nerves can become compressed when they pass through narrow anatomical tunnels or superficial areas. The most common condition is Carpal Tunnel Syndrome, which involves the Median nerve being compressed within the carpal tunnel at the wrist. This narrow passageway is already crowded with nine flexor tendons, and any swelling or inflammation of the surrounding tissues reduces the available space, putting pressure on the nerve.
The Ulnar nerve is most frequently compressed at the elbow, a condition known as Cubital Tunnel Syndrome. The nerve is highly vulnerable here because it runs directly behind the medial epicondyle, where it is only covered by skin and fat. Repeated or prolonged bending of the elbow can stretch the nerve, and leaning on the elbow for long periods can directly compress it.
Compression of the Radial nerve is less common in the forearm but can occur higher up in the arm. This typically happens when the nerve is pressed against the humerus bone due to prolonged, deep pressure, such as sleeping with the arm draped over a chair. The deep branch of the radial nerve can also be compressed in the forearm as it passes through a tunnel formed by the supinator muscle, which is known as Radial Tunnel Syndrome.