The flexor pollicis brevis muscle is a hand muscle with two heads. It flexes the thumb and participates in its adduction. The striated skeletal muscle receives the nervous signals for this from the ramus profundis nervi ulinaris and from the median nerve. Damage to the muscle or nerve can result in motor restrictions of the thumb, for example, in carpal tunnel syndrome or as a result of trauma.
What is the flexor pollicis brevis muscle?
The Latin name translates as short thumb flexor. The “long thumb flexor,” on the other hand, 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 flexor pollicis longus muscle also assists the wrist in bending. The flexor pollicis brevis muscle is a skeletal muscle and has striated fibers that are grouped together to form a muscle fiber. A sheath of connective tissue surrounds the fiber and stabilizes it. Several muscle fibers each form a bundle – several muscle fiber bundles join together to form the muscle. This structure allows the flexor pollicis brevis muscle and other muscles to move dynamically and flexibly.
Anatomy and structure
The flexor pollicis brevis muscle has two origins. At the carpal ligament (retinaculum flexorum) originates the superficial muscle head, also known as the caput superficiale. The carpal ligament is located at the carpus and spans the flexor tendons found there. With its surface made of strong connective tissue, the carpal ligament holds the tendons to the wrist and prevents the flexor tendons from sticking out during hand movements. In addition to the caput superficiale, the flexor pollicis brevis muscle has a second head, the caput profundum. Its origin is distributed among the large polygonal bone (Os trapezium), the small polygonal bone (Os trapezoideum) and the capitate bone (Os capitatum). All three belong to the carpal bones. The caput superficiale and the caput profundum extend from the carpus to the bone, where they attach to the external sesamoid bone (Os sesamoideum) and the proximal phalanx of the thumb (to the articulatio metacarpophalangealis pollicis).
Function and Tasks
The flexor pollicis brevis muscle participates in certain movements of the thumb. The flexor pollicis brevis muscle is controlled by two nerves. The median arm 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 ulnar nerve. It is known to anatomists as the ulnar nerve. In its course, the ulnar nerve gives off five main branches, one of which embodies the ramus volaris manu. From this branch, in turn, two small nerves branch off: the ramus superficialis and the ramus profundus. The latter draws to the flexor pollicis brevis muscle and sends motor nerve signals to the caput profundum. The flexor pollicis brevis muscle is part of the skeletal musculature of humans and is subject to voluntary control: the command to contract originates from a motor center of the brain. Reflexes, for example the grasping reflex in infants, are an exception. The nerve fibers end 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 the postsynaptic endplate potential. It stimulates a membrane system inside the muscle cell, the sarcoplasmic reticulum, to release calcium ions. These attach themselves to special proteins, whereupon these push into each other and shorten the muscle. At the flexor pollicis brevis muscle, contraction leads to flexion of the thumb or adduction. In adduction, the thumb moves toward the center of the hand.
Diseases
If the flexor pollicis brevis muscle is not working properly, there may be damage to the muscle or to one of the nerves that innervate the short thumb flexor. Direct lesions can occur, for example, with hand injuries. In the case of nerve palsy affecting the median nerve, the affected person is no longer able to flex the thumb, index finger and middle finger.Medicine also calls this disease sign the swearing hand, since the finger position is reminiscent of the traditional gesture. The medianus paralysis does not extend to the other two fingers of the hand, as these are supplied by other nerve fibers. Only with additional damage, impairment of the ring finger as well as the little finger is 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. In the context of a paralysis of the medial nerve, this transmission of information is also disturbed and the affected person no longer feels anything in these areas of the skin. However, loss of sensation does not occur in every clinical picture affecting the medial nerve. Other sensory disturbances such as paresthesias can also occur. These occur, for example, in carpal tunnel syndrome manifest as tingling, “falling asleep,” disturbances in temperature perception or feelings of numbness. In addition, carpal tunnel syndrome often manifests as pain, which varies in severity. The syndrome often results from overuse – but fractures, obesity (adiposity), arthritis, diabetes, amyloidosis, hemorrhage, tumors, edema and other underlying diseases are also possible causes.
