The nerves in your arm are the communication cables that transmit signals between your brain and the muscles and skin of your upper limb. This intricate wiring allows for the complex movements of your hand and fingers while relaying sensory information like touch, temperature, and pain back to the central nervous system. Understanding the location and path of these nerves is key to grasping how the arm operates.
The Nerve Highway Origin: The Brachial Plexus
The nerves that travel through the arm emerge from a complex junction box located in the neck and shoulder area called the brachial plexus. This network is formed by nerve fibers originating from the spinal nerves C5 through T1. These nerve roots converge, divide, and recombine to form trunks, divisions, and cords, acting as a rerouting station before traveling from the neck, under the collarbone, and into the armpit (axilla) to supply the upper limb.
The Three Major Nerves and Their Routes
Three major nerves—the median, ulnar, and radial—emerge from the brachial plexus to provide motor function and sensation to the arm, forearm, and hand. These nerves follow distinct anatomical paths, allowing them to innervate specific muscle groups and skin regions.
The Median Nerve
The median nerve is formed from contributions from both the lateral and medial cords of the brachial plexus. It runs down the center of the arm, initially alongside the brachial artery, before passing through the forearm between layers of flexor muscles. This nerve supplies most of the muscles that flex the wrist and fingers. It also provides sensation to the palm side of the thumb, index, middle, and half of the ring finger. It is the only nerve that passes through the carpal tunnel at the wrist to reach the hand.
The Ulnar Nerve
The ulnar nerve is formed from the medial cord of the brachial plexus and is often called the “funny bone” nerve. It travels along the inner side of the arm and passes directly behind the bony knob on the inside of the elbow, known as the medial epicondyle. In the forearm, it runs alongside the ulna bone, supplying the flexor carpi ulnaris muscle and the medial half of the deep finger flexors. The ulnar nerve is the primary nerve for almost all of the small, intrinsic muscles of the hand, and it provides sensation to the little finger and the adjacent half of the ring finger.
The Radial Nerve
The radial nerve is the continuation of the posterior cord of the brachial plexus. This nerve takes a winding path, wrapping around the back of the humerus bone in a shallow depression called the radial groove. It then divides into two main branches near the elbow, which continue into the forearm. The radial nerve supplies the muscles responsible for extending the elbow, wrist, and fingers, and its sensory branch supplies the skin on the back of the hand and the thumb side.
Vulnerable Points and Common Nerve Issues
Knowing the precise routes of these nerves helps explain why certain common injuries and compression syndromes occur at specific points in the arm. The median nerve’s path through the carpal tunnel at the wrist makes it susceptible to compression. Increased pressure within this tunnel leads to Carpal Tunnel Syndrome, causing numbness and tingling in the thumb, index, and middle fingers.
The ulnar nerve’s superficial location behind the medial epicondyle is its most exposed point, commonly known as the cubital tunnel. This is where the nerve is easily bumped, creating the sensation of hitting the “funny bone.” Chronic compression or prolonged elbow flexion at this site can lead to Cubital Tunnel Syndrome, resulting in pain and tingling in the ring and little fingers.
The radial nerve is most vulnerable as it spirals around the middle of the humerus bone in the upper arm. Sustained pressure on the back of the arm, such as from sleeping in an awkward position, can compress the nerve against the bone, sometimes referred to as “Saturday Night Palsy.” This compression results in a characteristic weakness in straightening the wrist and fingers, known as wrist drop.