The arm, from the shoulder to the fingertips, is connected to the central nervous system through peripheral nerves. These nerves transmit signals that allow the brain to control muscle movement and receive sensory information (touch, temperature, and pain). Understanding the location of these nerves helps explain how we move and why certain injuries can lead to numbness or weakness. These communication lines begin high in the neck before branching out into the arm.
The Origin: From the Neck to the Shoulder
The nervous supply for the arm starts with the brachial plexus, a network of nerve fibers. This plexus is formed by nerve roots exiting the spinal cord in the neck, specifically from the fifth cervical vertebra (C5) down to the first thoracic vertebra (T1). These roots interlace behind the collarbone and travel into the armpit area (the axilla).
The intermingling of fibers ensures that the main nerves continuing into the arm receive input from multiple spinal levels. This formation acts as a central distribution point for all motor and sensory signals intended for the shoulder, arm, forearm, and hand. From this network, the major individual nerves emerge to take their distinct paths down the limb.
Mapping the Major Nerve Pathways
Three main terminal nerves exit the brachial plexus and traverse the length of the arm. Each nerve occupies a specific anatomical location, controlling distinct muscle groups and skin sensation. Their pathways are defined by the bones and muscles of the upper limb, which dictates where they are most exposed and what functions they govern.
Radial Nerve
The radial nerve is the largest of the three and primarily supplies the muscles responsible for extension. It begins in the armpit and travels down the back of the upper arm, spiraling around the shaft of the humerus bone in a shallow groove. This positioning allows it to innervate the triceps muscle, which straightens the elbow.
Continuing into the forearm, the radial nerve then controls the muscles that extend the wrist and fingers. It also provides sensory feedback to a large area of skin on the back of the hand, near the thumb and index finger. The path of the radial nerve along the humerus makes it susceptible to injury from fractures of that bone.
Median Nerve
The median nerve generally follows a central path down the arm, staying close to the main artery of the limb. It runs along the front of the upper arm, passing through the elbow joint on the inner side. This nerve is responsible for the majority of the flexing and gripping actions of the hand and forearm.
In the forearm, the median nerve powers most of the muscles that bend the wrist and fingers toward the palm. It then enters the hand through the narrow carpal tunnel at the wrist. Once in the hand, it gives sensation to the thumb, index finger, middle finger, and half of the ring finger on the palm side.
Ulnar Nerve
The ulnar nerve travels along the inner side of the arm until it reaches the elbow. At the elbow, it passes through the cubital tunnel, where it is very close to the skin and bone. This location is commonly associated with the sensation felt when the “funny bone” is struck.
The nerve continues down the forearm, controlling specific wrist flexors and providing sensation to the little finger and the adjacent half of the ring finger. Most significantly, the ulnar nerve governs nearly all the small muscles within the hand itself, which are necessary for fine motor control.
Understanding Nerve Compression and Injury
Nerves are vulnerable where they pass through tight anatomical spaces or lie close to the bone, making them susceptible to pressure (nerve compression or entrapment). This pressure disrupts the nerve’s ability to transmit signals effectively. The resulting symptoms depend on the nerve’s function and the location of the pressure.
Common symptoms of nerve disruption include paresthesia (tingling or “pins and needles”) and numbness in the area of skin the nerve supplies. Motor function can also be impaired, resulting in muscle weakness or difficulty performing fine motor tasks. For example, compression of the ulnar nerve at the elbow, known as cubital tunnel syndrome, causes numbness in the little and ring fingers.
Carpal tunnel syndrome is a frequent example, where the median nerve is compressed as it passes through the wrist. This leads to numbness and tingling in the thumb and first two fingers. Recognizing the specific pathway of each arm nerve allows for accurate identification of the source of pain, weakness, or altered sensation.