Nine tendons and one nerve pass through the carpal tunnel. Specifically, the tunnel contains the median nerve, four tendons of the flexor digitorum superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis longus. These ten structures are tightly packed into a space roughly the size of your index finger in diameter, which is why even small changes in swelling or pressure can cause problems.
The Tunnel Itself
The carpal tunnel is a narrow passageway on the palm side of your wrist. Its floor and walls are formed by a U-shaped arrangement of small wrist bones called carpal bones. The roof is a thick band of connective tissue called the flexor retinaculum (sometimes called the transverse carpal ligament), which stretches across the top like a bridge. This ligament is about 3 cm long and 2.5 cm wide, and it thickens from about 1.5 mm at one end to 6 mm at the other. It anchors to bony landmarks on each side of the wrist: the scaphoid and trapezium bones on the thumb side, and the pisiform and hamate bones on the pinky side.
The retinaculum does more than just form a roof. It acts as a pulley, keeping all those tendons from bowstringing outward when you flex your wrist and fingers. It also provides an attachment point for some of the small muscles in your palm.
The Median Nerve
The median nerve is the only nerve that travels through the carpal tunnel, and it’s the structure that causes trouble when things get crowded. It provides sensation to your thumb, index finger, middle finger, and the thumb side of your ring finger. It also controls several small muscles at the base of the thumb that help with gripping and pinching.
The nerve sits just beneath the retinaculum, toward the top of the tunnel. That position makes it especially vulnerable to compression because it’s pressed between the ligament above and the tendons below.
The Nine Flexor Tendons
The remaining nine structures are all tendons, the tough cords that connect forearm muscles to finger bones. They break down into three groups.
Flexor digitorum superficialis (4 tendons): These run to each of your four fingers (index through pinky) and allow you to curl the middle joints of your fingers. They sit closer to the surface within the tunnel.
Flexor digitorum profundus (4 tendons): These also run to the same four fingers but insert deeper, at the fingertips. They let you bend the last joint of each finger. Inside the tunnel, these tendons sit behind the superficialis tendons, closer to the wrist bones.
Flexor pollicis longus (1 tendon): This is the lone thumb tendon in the tunnel. It runs along the thumb side and lets you bend the tip of your thumb. It sits on the outermost (radial) edge of the tunnel, which actually makes it the most commonly irritated tendon when problems develop.
Protective Sheaths Inside the Tunnel
The tendons don’t slide through the tunnel bare. They’re wrapped in fluid-filled sleeves called synovial sheaths, organized into two compartments known as bursae. The radial bursa wraps only the flexor pollicis longus tendon. The ulnar bursa envelops all eight of the other flexor tendons together. These sheaths produce a small amount of lubricating fluid that lets the tendons glide smoothly against each other and the surrounding walls during the thousands of finger movements you make each day.
Between and around the sheaths is a web of loose connective tissue called the subsynovial connective tissue. This tissue helps distribute forces and allows the tendons to slide independently. Changes in this connective tissue, such as thickening or fibrosis, are now recognized as a key part of what goes wrong in carpal tunnel syndrome.
Anatomical Variations
Not everyone’s carpal tunnel contains exactly the same structures. About 10 to 14 percent of people have a persistent median artery, a blood vessel that normally disappears during fetal development but sometimes remains into adulthood and runs alongside the median nerve through the tunnel. Roughly 11 to 14 percent of people have a bifid median nerve, meaning the nerve splits into two branches before entering the tunnel rather than after. Some people have both variations at once.
Despite early suspicion that these extra structures might crowd the tunnel and raise the risk of carpal tunnel syndrome, recent ultrasound studies comparing patients with healthy controls found no significant difference in the prevalence of these variations between groups. They appear to be normal anatomical quirks rather than independent risk factors.
Why Crowding Causes Problems
In a healthy wrist, pressure inside the carpal tunnel is remarkably low: about 2 to 10 mmHg in a neutral position, similar to the gentle pressure inside relaxed muscle tissue. In people with carpal tunnel syndrome, that baseline pressure rises to 32 mmHg or higher. Flexing the wrist can push it to 94 mmHg, and extending the wrist sends it to 110 mmHg.
Those pressure spikes squeeze the tiny blood vessels that feed the median nerve. The nerve begins to swell, and the swelling itself takes up more space, creating a cycle of increasing pressure. Early on, the result is intermittent numbness and tingling, especially at night when people tend to sleep with flexed wrists. If compression persists, the insulating layer around the nerve fibers starts to break down, slowing electrical signals. Over time, the nerve fibers themselves can be damaged, leading to constant numbness and weakness in the thumb muscles.
This is why the contents of the tunnel matter so much. Ten structures sharing a small, rigid space with no room to expand means any swelling of the tendons, thickening of the sheaths, or fluid retention (from pregnancy, thyroid conditions, or repetitive strain) puts direct pressure on the one structure that can’t tolerate it: the nerve.