The carpal tunnel is a narrow passageway in the wrist, about an inch wide. The floor and sides of the tunnel are formed by small wrist bones called carpal bones.

The carpal tunnel contains the median nerve and flexor tendons that bend the fingers and thumb.

Reproduced and adapted from Rodner C, Raissis A, Akelman E: Carpal tunnel syndrome. Orthopaedic Knowledge Online Journal. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2009; 7(5). Accessed March 2016.

The roof of the tunnel is a strong band of connective tissue called the transverse carpal ligament. Because these boundaries are very rigid, the carpal tunnel has little capacity to stretch or increase in size.

The median nerve is one of the main nerves in the hand. It originates as a group of nerve roots in the neck; these roots then come together to form a single nerve in the arm.

The median nerve travels down the upper arm, across the elbow, and into the forearm, then passes through the carpal tunnel at the wrist on its way to the hand and fingers. It separates into several smaller nerves along the way, particularly once it reaches the palm. These nerves allow for feeling in the thumb, index finger, middle finger, and half of the ring finger (the thumb side).

The median nerve also controls the muscles around the base of the thumb.

The nine tendons that bend the fingers and thumb also travel through the carpal tunnel with the nerve. These tendons are called flexor tendons because they flex the fingers and thumb.

Description

Carpal tunnel syndrome occurs when the tunnel becomes narrowed or when the tissue surrounding the flexor tendons (known as synovium) swells, putting pressure on the median nerve and reducing its blood supply.  

This abnormal pressure on the nerve can result in numbness, tingling, pain, and weakness in the hand.
Carpal Tunnel Syndrome. Ortho Info (n.d.) https://orthoinfo.aaos.org/en/diseases--conditions/carpal-tunnel-syndrome/