Flexor pollicis brevis muscle

Origin and insertion

Flexor pollicis brevis has two heads that originate from the two separate areas; 

• Superficial head: arises from the distal border of the flexor retinaculum, as well as the distal part of the tubercle of trapezium bone. 
• Deep head: arises from the trapezoid and capitate bones, as well as the palmar ligaments of the distal row of carpal bones.

The deep and superficial heads course obliquely and meet at the radial side of the base of the proximal phalanx of the thumb. Here they insert by a common short tendon which contains an embedded sesamoid bone.

Relations

Flexor pollicis brevis is the most medial of the thenar muscles. It lies medial to the abductor pollicis brevis and opponens pollicis muscles, while it is lateral to adductor pollicis muscle. 

Along its course, the superficial head of the muscle passes along the radial side of the tendon of flexor pollicis longus, whereas the deep head passes deep to the same tendon. Additionally, the superficial head is commonly blended with the opponens pollicis muscle. The superficial surface of the muscle is crossed by the motor branch of the median nerve.

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Innervation

The two heads of the flexor pollicis brevis usually differ in their innervation. The superficial head of flexor pollicis muscle receives nervous supply from the recurrent branch of the median nerve, whereas the deep head receives innervation from the deep branch of the ulnar nerve, derived from spinal roots C8 and T1.

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Blood supply

Flexor pollicis brevis receives arterial supply from branches of the radial artery; superficial palmar artery, branches of the princeps pollicis artery and radialis indicis artery.

Function

As a part of the thenar muscles, flexor pollicis brevis acts on the thumb and produces flexion at the metacarpophalangeal and carpometacarpal joints. This action aids in opposition of the thumb and, if continued, it produces the medial rotation of thumb.

Flexor pollicis brevis can be tested and palpated on the thenar eminence when the thumb is flexed against resistance.