The flexor pollicis brevis muscle is a hand muscle with two heads. He bends his thumb and participates in his adduction. The nervous signals to get the striated skeletal muscle from the Ramus profundis nervi ulinaris and from the median nerve. Damage to the muscle or nerve can result in motor impairment of the thumb, such as carpal tunnel syndrome or trauma.

What is the flexor pollicis brevis muscle?

The Latin name means translated as much as a short thumb flexor. In contrast, the "long thumb flexor" corresponds to the flexor pollicis longus muscle, which is located in the forearm and forms part of the deep muscles there. Like the flexor pollicis brevis muscle, the longer muscle flexes the thumb. In addition, the long flexor pollicis longus also supports the wrist when flexing.

The flexor pollicis brevis muscle is a skeletal muscle with striated fibers clustered into a muscle fiber. An envelope of connective tissue surrounds the fiber and stabilizes it. Several muscle fibers form a bundle - several muscle fiber bundles join together to form a muscle. This construction allows the flexor pollicis brevis muscle and other muscles to move dynamically and flexibly.

Anatomy & Construction

The flexor pollicis brevis muscle has two origins. At the carpal ligament (retinaculum flexorum) arises the superficial muscle head, which is also known as Caput superficiale. The Karpalband is located at the wrist and stretches over the flexor tendons that can be found there.

With its surface made of solid connective tissue, the wrist strap holds the tendons on the wrist and prevents the flexor tendons from sticking out during hand movements.

In addition to the superficial caput, the flexor pollicis brevis has a second head, the caput profundum. Its origin is distributed over the large polygon (Os trapezium), the small polygon (Os trapezoideum) and the head bone (Os capitatum). All three belong to the carpal bones. The caput superficiale and the caput profundum extend from the carpal to the bone, where they attach to the external sesamoid (os sesamoideum) and to the base of the thumb (the articulatio metacarpophalangealis pollicis).

Function & Tasks

The flexor pollicis brevis muscle participates in certain movements of the thumb. The control of the muscle flexor pollicis brevis occurs via two nerves. The middle arm nerve (median nerve) communicates with the caput superficiale. Its fibers originate from the brachial plexus. The median nerve also controls the movements of the flexor pollicis longus muscle.

The other nerve that innervates the flexor pollicis brevis muscle is the cervical nerve. The anatomy knows him as ulnar nerve. In its course, the ulnar nerve gives off five main branches, one of which is the Ramus volaris manu. From this branch branch off again two small nerves: the Ramus superficialis and the Ramus profundus. The latter pulls to the flexor pollicis brevis muscle and sends motor nerve signals to the caput profundum. The flexor pollicis brevis muscle belongs to the skeletal musculature of man and is subject to voluntary control: The command to contract comes from a motor center of the brain. An exception are reflexes, for example, the gripping reflex in infants.

The nerve fibers end up in a motor end plate, which releases biochemical messengers. When these irritate the membrane of the muscle cells, ion channels open and change the electrical balance of the cell. Biology also calls this change postsynaptic endplate potential. It stimulates a membrane system inside the muscle cell, the sarcoplasmic reticulum, to release calcium ions. These are deposited on special proteins, whereupon these telescope and shorten the muscle.

In the flexor pollicis brevis muscle contraction leads to flexion of the thumb or adduction. During adduction, the thumb moves towards the middle of the hand.


If the flexor pollicis brevis muscle is not functioning properly, there may be damage to the muscle or nerves innervating the short thumb flexor. Direct lesions can occur, for example, with hand injuries.

In a nerve paralysis that affects the median nerve, he is no longer able to flex the thumb, forefinger and middle finger. The medicine also refers to this sign of disease as a swear hand, as the finger position is reminiscent of the traditional gesture. The paralysis of the median does not spread to the other two fingers of the hand, as these are supplied by other nerve fibers. Only with additional damages also impairments of the ring finger as well as the small finger are possible.

The medial nerve contains not only motor nerve fibers, which control the activity of muscles, but also sensitive fibers. These transmit sensations such as heat, cold, pain and pressure to the central nervous system. As part of a paralysis of the medial nerve, this information transfer is disturbed and the affected person feels in these areas of the skin nothing more.

However, not every condition that affects the medial nerve causes a loss of sensation. Other sensory disorders such as paraesthesias can occur. These occur, for example, in carpal tunnel syndrome manifesting as tingling, "falling asleep", disturbances of temperature perception or feeling of numbness. In addition, the carpal tunnel syndrome often manifests itself in pain that varies in severity. The syndrome is often the result of overwork - but fractures, obesity, arthritis, diabetes, amyloidosis, bleeding, tumors, edema and other underlying diseases are also considered causes.

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