Flexor Pollicis Brevis

Superficial Head Median Nerve, Medial Cord, Anterior Division, Lower Trunk, C8, Tl. 

Superficial Head Ridge of the trapezium and the flexor retinaculum Deep Head Ulnar side of first metacarpal. 

Superficial Head Radial side of the base of the proximal phalanx thumb. 

Deep Head Ulnar side of base of proximal phalanx of the thumb. 

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The flexor pollicis longus tendon traverses the thenar eminence between the superficial and deep bellies of the flexor pollicis brevis muscle. It then enters the osteofibrous canal of the thumb to insert at the volar aspect of the base of the distal phalanx. At the metacarpal head level, this tendon courses between two sesamoid bones, one (lateral) included in the combined tendon of the flexor pollicis brevis and abductor pollicis longus, the other (medial) in the adductor pollicis tendon. 

The thenar muscles are innervated by the recurrent branch of the median nerve and are basically responsible for the complex movement of the thumb. They include the superficial abductor pollicis brevis, the flexor pollicis brevis and the deep opponens pollicis muscles. The abductor pollicis brevis is a broad flat muscle located on the more lateral side of the thenar eminence. It arises from the radial side of the transverse carpal ligament and the tubercle of the trapezium and inserts into the base of the proximal phalanx. The flexor pollicis brevis is located medial to the abductor pollicis and has two muscular bellies the superficial head arises from the transverse carpal ligament and the tubercle of the trapezium, the deep head (first palmar interosseous muscle) from the trapezoid and the capitate. Both heads insert through the radial sesamoid over the metacar-... 

If the electrode is inserted too deeply it will be in the flexor pollicis brevis. 

This muscle inserts into the lateral half of the palmar surface of the 1st Metacarpal. It lies deep to the Abductor Pollicis Brevis and lateral to the Flexor Pollicis Brevis. 

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