The carpal tunnel refers to a narrow osseofibrous passageway on the palmar side of your wrist, and ten precise structures pass through it: nine flexor tendons that bend the fingers and thumb, plus the median nerve, the single cable delivering sensation and fine motor control to most of the hand. Slide your thumb across the heel of your hand and you’ll feel the firm band that forms the tunnel’s roof.
Press your thumb pad against a desk and the median nerve fires sensation back to your brain. Inside that postage-stamp-sized channel, every structure must stay in its lane.
The guide below walks through every layer, from the bony floor to the ligamentous roof, so you can picture what occupies the space and understand why compression produces such distinctive symptoms in the hand.
The Carpal Tunnel Is a Narrow Passageway, Not a Diagnosis
The carpal tunnel is an osseofibrous canal on the palmar side of your wrist, built partly of bone and partly of fibrous tissue. In cross-section, it measures roughly an inch long and a bit narrower across, about the size of a United States postage stamp. Inside that small footprint sits a remarkably crowded neighborhood.
Three structural facts make this anatomy clinically important. The tunnel is a real anatomical channel with a defined floor, walls, and roof, not an abstract region of vague discomfort. Its dimensions are fixed once adulthood is reached; the bones do not spread apart, and the ligamentous roof does not stretch to accommodate swelling. Anything that increases the volume of its contents, including inflammation, fluid retention, or a thickened tendon sheath, reduces the available space for everything inside.
A small pressure change inside a non-expanding tunnel affects every structure it contains.
What the Tunnel Does and Doesn’t Do
The tunnel’s function is purely protective and organizational. It keeps the flexor tendons from bowstringing away from the wrist during gripping, and it gives the median nerve a predictable route from the forearm into the hand. The structure does not move on its own, does not expand, and does not heal by widening. This rigidity is the entire reason a swollen tendon inside can produce numbness in your fingertips.
The Floor and Walls Formed by Eight Small Carpal Bones
Eight small carpal bones are arranged in a U-shape to form the tunnel’s floor and side walls. Eight carpal bones exist in total, though only six contribute directly to the tunnel’s groove.
The Radial Side: Scaphoid and Trapezium
Two bones on the thumb side,the scaphoid and trapezium,form the lateral wall and part of the floor. The scaphoid is boat-shaped and sits just past the radius. The trapezium sits beneath the base of the thumb and gives the thumb its wide range of motion.
The Central Floor: Lunate and Capitate
In the middle, the lunate and capitate complete the deepest part of the bony groove. The lunate is crescent-shaped and articulates with the radius above. The capitate, the largest carpal bone, sits centrally and anchors the middle of the tunnel floor.
The Ulnar Side: Triquetrum and Hook of Hamate
On the pinky side, the triquetrum and the hook of the hamate together form the medial wall of the tunnel. The triquetrum is pyramidal and lies just past the ulna. The hook of the hamate is a small bony projection you can sometimes feel as a tender bump in the heel of your palm, and it serves as an important attachment point for the roof ligament.
| Carpal Bone | Position | Contribution to Tunnel |
|---|---|---|
| Scaphoid | Radial side, proximal | Lateral wall and proximal floor |
| Trapezium | Radial side, distal | Lateral wall and distal floor |
| Lunate | Central, proximal | Central floor |
| Capitate | Central, distal | Central floor |
| Triquetrum | Ulnar side, proximal | Medial wall |
| Hamate (hook) | Ulnar side, distal | Medial wall and attachment point |
| Pisiform | Superficial to triquetrum | Sits over the tunnel, not part of the floor |
| Trapezoid | Between trapezium and capitate | Adjacent to but not forming the tunnel |
The pisiform is worth singling out. Although it sits near the tunnel, it lies superficially over the triquetrum rather than forming the floor directly. Most quick explanations gloss over this distinction, but it matters because the pisiform becomes an attachment point for the roof ligament rather than a load-bearing wall.
Because the pisiform only forms part of the floor, the true roof has to come from somewhere else entirely.
The Roof Created by the Flexor Retinaculum and Its Many Names
The bony groove alone is just an open channel. To become a true tunnel, it needs a roof, and that roof is supplied by a thick fibrous band called the flexor retinaculum.
Flexor Retinaculum and Transverse Carpal Ligament: Same Structure
A common source of confusion is terminology. The flexor retinaculum and the transverse carpal ligament refer to the same structure. Older anatomy texts often use “transverse carpal ligament,” while current clinical literature tends to use “flexor retinaculum.” When a surgeon or physical therapist uses either name, they’re pointing at the same band of tissue that closes the tunnel.
Where the Roof Attaches
On the radial side, the retinaculum attaches to the scaphoid tubercle and the crest of the trapezium. On the ulnar side, it attaches to the pisiform and the hook of the hamate. These four bony attachment points convert the open groove into a closed tunnel with no give.
The Palmaris Longus and the Palmar Carpal Ligament
The palmaris longus tendon, when present (it’s absent in roughly 10–15% of people), runs superficial to this roof rather than through the tunnel itself. The palmar carpal ligament is a separate, more superficial band that lies just beneath the skin and does not form the tunnel’s true roof. Mixing these up is one of the most common errors in casual descriptions of wrist anatomy.
When reading anatomy references, treat “flexor retinaculum” and “transverse carpal ligament” as interchangeable. The “palmar carpal ligament” is a different, more superficial structure.
Nine Flexor Tendons and One Nerve Occupy the Tunnel
Exactly ten structures occupy the tunnel: nine flexor tendons wrapped in synovial sheaths and one nerve. Counting them individually removes most of the mystery behind carpal tunnel symptoms.
The Four Flexor Digitorum Superficialis Tendons
Four tendons from the flexor digitorum superficialis pass through the tunnel in two stacked rows. These tendons flex the middle phalanges of the index, middle, ring, and little fingers, the motion that curls your fingers at the middle joint while the fingertip stays extended.
The Four Flexor Digitorum Profundus Tendons
The flexor digitorum profundus tendons lie in the deepest layer of the tunnel. These tendons flex the distal phalanges, the fingertip bones, and are the muscles responsible for making a tight fist. The profundus tendons share a common synovial sheath, a thin fluid-filled covering that lets them glide without friction.
The Single Flexor Pollicis Longus Tendon
Sitting nearest the radial wall, the flexor pollicis longus tendon is enclosed in its own synovial sheath. This tendon flexes the thumb and makes pinch grip possible. Its dedicated sheath explains why thumb tendon problems can sometimes mimic or contribute to carpal tunnel symptoms.
The Median Nerve
Only one nerve runs through the tunnel, and it is the median nerve. It runs alongside the flexor tendons, typically just beneath the flexor retinaculum and superficial to the tendon bundle. After exiting the tunnel, it branches to provide sensation to the thumb, index, middle, and the radial half of the ring finger, and it controls the thenar muscles that move the thumb.
Mapping what those contents actually do clarifies why the median nerve, specifically, ends up vulnerable.
| Structure | Number | Function |
|---|---|---|
| Flexor digitorum superficialis tendons | 4 | Flex middle phalanges of fingers 2–5 |
| Flexor digitorum profundus tendons | 4 | Flex distal phalanges; make a fist |
| Flexor pollicis longus tendon | 1 | Flex the thumb; enable pinch |
| Median nerve | 1 | Sensation and motor control to thumb and first 3.5 fingers |
The Median Nerve and the Ulnar Nerve Take Different Pathways
That passes through the carpal tunnel, but the ulnar nerve deliberately avoids it. That separation is the anatomical basis for distinguishing two common wrist compression syndromes.
Guyon’s Canal and the Ulnar Nerve
Just proximal to the wrist, the ulnar nerve diverts into a separate channel called Guyon’s canal, which lies near the pisiform and hook of hamate. The ulnar artery travels with it. Because the ulnar nerve never enters the carpal tunnel, compression there cannot affect it.
Why This Split Matters Clinically
This anatomical separation is why symptoms localize so precisely. Median nerve compression in the carpal tunnel affects the thumb, index, middle, and half of the ring finger. Ulnar nerve compression in Guyon’s canal affects the ring and pinky fingers. When your pinky goes numb, the problem is not in your carpal tunnel.
Surgeons rely on this split to selectively decompress one tunnel without disturbing the other. The two surgeries are performed through different incisions and target entirely different anatomical spaces, even though they may be discussed together under the umbrella term “wrist nerve compression.”
| Feature | Carpal Tunnel | Guyon’s Canal |
|---|---|---|
| Nerve inside | Median nerve | Ulnar nerve |
| Fingers affected if compressed | Thumb, index, middle, half of ring | Half of ring, pinky |
| Other structures | 9 flexor tendons | Ulnar artery |
| Roof structure | Flexor retinaculum | Volar carpal ligament |
Why the Median Nerve Suffers When the Tunnel Becomes Crowded
Tendons and nerves share the same narrow space, yet compression symptoms almost always involve the nerve, not the tendons. The reason lies in tissue biology.
Why Nerves Are Vulnerable and Tendons Are Not
Nerve tissue is soft, electrically active, and exquisitely sensitive to ischemia, or reduced blood flow. Even modest pressure disrupts the nerve’s ability to transmit signals, producing tingling, numbness, and eventually weakness. Tendons, by contrast, are dense bundles of collagen designed to withstand enormous mechanical load. They tolerate pressure that would cripple a nerve, which is why most carpal tunnel syndrome cases present with sensory symptoms long before any loss of grip strength.
Common Causes of Tunnel Crowding
Several conditions can reduce the tunnel’s internal volume. Tenosynovitis, or inflammation of the tendon sheaths, is one of the most frequent contributors. Fluid retention during pregnancy or hormonal shifts can temporarily swell the tendons. Trauma such as a wrist fracture can change the bony architecture. Anatomical variations, including an extra muscle belly or a low-lying median nerve branch, occasionally crowd the space from birth.
The Symptom Cascade
When pressure rises inside a non-expanding tunnel, the median nerve responds in a predictable sequence. Intermittent tingling or “pins and needles” appears first, often at night or during sustained gripping. Persistent numbness follows as the nerve’s sensory fibers lose function. Thenar muscle weakness, the fleshy pad at the base of your thumb, develops as motor fibers fail. Without intervention, prolonged compression can produce permanent nerve damage.
Recognizing the exact contents of the tunnel clarifies which symptoms warrant evaluation. Numbness in the thumb, index, middle, or half the ring finger points toward the median nerve specifically, while symptoms in the pinky suggest a different pathway entirely.
This anatomy also explains why a clinician’s physical examination focuses on specific spots. Tapping over the tunnel (Tinel’s test) or holding the wrist flexed (Phalen’s test) reproduces symptoms by briefly raising pressure on the nerve. Understanding the contents makes those tests make sense.
Bottom Line
Ten structures share a space the size of a postage stamp: nine flexor tendons that bend your fingers and thumb, plus the median nerve that powers sensation and fine motor control in most of your hand. When that space becomes crowded, the nerve suffers first because nerve tissue is far more sensitive to pressure than the dense collagen of tendons.
Knowing exactly what occupies the tunnel clarifies why symptoms appear where they do, and why two nerves producing two different patterns of numbness travel through two different canals on either side of your wrist.
FAQ
What goes through the carpal tunnel?
Ten structures traverse the tunnel: four superficialis tendons, four profundus tendons, one pollicis longus tendon, and the median nerve. Together they occupy a space roughly the size of a postage stamp.
Where is the carpal tunnel located in the wrist?
On the palmar surface of the wrist, just distal to the wrist crease, lies the entrance of the carpal tunnel. It is bounded by the carpal bones below and the flexor retinaculum above, forming a fixed osseofibrous canal.
How many tendons pass through the carpal tunnel?
Nine flexor tendons cross the tunnel: four from the superficialis, four from the profundus, and one from the pollicis longus.
What is the role of the median nerve in the carpal tunnel?
The median nerve provides sensation to the thumb, index, middle, and half of the ring finger, and it controls the thenar muscles that move the thumb. It is the only nerve traveling through the carpal tunnel.
What bones form the carpal tunnel?
Six carpal bones border the tunnel: the scaphoid and trapezium on the radial side, the lunate and capitate centrally, and the triquetrum and hook of hamate on the ulnar side. The pisiform and trapezoid sit adjacent to it without forming the tunnel.
What is the flexor retinaculum and what does it do?
The flexor retinaculum is a thick fibrous band that forms the roof of the carpal tunnel. It attaches to the scaphoid tubercle, trapezium, pisiform, and hook of the hamate, converting the open bony groove into a closed passageway. It is the same structure as the transverse carpal ligament.
