Muscles in the Thenar Eminence

What is the Thenar Eminence

The thenar eminence is the fleshy mound at the base of the thumb on the radial side of the palm of the hand. It is formed by the bulge of the bellies of three intrinsic hand muscles. Together these are called the thenar muscles.

Names of the Thenar Muscles With Anatomy and Functions

1. Opponens Pollicis: The deepest and largest of the three thenar muscles, it originates from the trapezium’s tubercle and its attached flexor retinaculum. It then inserts along the length of the 1st metacarpal’s radial or lateral surface. This muscle works on the 1st carpometacarpal joint to flex, adduct, and rotate the thumb — movements that lead to the opposition of the thumb.
2. Flexor Pollicis Brevis (FPB): Lying superficial to the opponens pollicis, the FPB forms the medial side of the thenar eminence. The muscle has two heads; the superior head originating from the trapezium’s tubercle and the deep head arizing from the trapezoid and capitate bones. Both heads course radially together to insert into the 1st proximal phalangeal base. This muscle works on the metacarpophalangeal joint to flex the thumb.
3. Abductor Pollicis Brevis (APB): It is the most superficial and laterally located thenar muscle with multiple points of origin. It arises from the tubercles of the trapezium and scaphoid, as well as the attaching flexor retinaculum. It shares its point of insertion with FPB, and inserts into the base of the thumb’s proximal phalanx. As evident from its name, it is primarily responsible for abducting the thumb.

The adductor pollicis is another muscle in the thenar area, sometimes counted as the fourth thenar muscle. It originates via two heads, from the bases of the 2nd and 3rd metacarpal and the capitate bone. Like the FPB and APB, this muscle also inserts into the base of the 1st proximal phalanx. The actions of this muscle are antagonistic to those of the APB as it works to adduct the thumb.

Functions of the Muscles of the Thenar Eminence

Together, the three thenar muscles function to move and rotate the thumb. They are responsible for the opposition of the thumb that allows you to grab, hold, and pinch objects with your thumb and the other fingers.

So, without the thenar muscles, your thumb would be useless. And a “hand without a thumb is at worst nothing but an animated spatula and at best a pair of forceps whose points don’t meet properly” — as said by the British primatologist and physician John Russell Napier, in his book ‘Hands’

Innervation

All the thenar muscles receive innervation from the recurrent branch of the median nerve arising from nerve roots C8 and T1. Additionally, the deep head of the flexor pollicis brevis is innervated by the deep branch of the ulnar nerve (C8 and T1), which also innervates the adductor pollicis.

Blood Supply

The primary blood supply to the thenar eminence comes from the superficial palmar branch of radial artery. This artery courses over the flexor retinaculum to reach the thenar eminence and then anastomoses with the ulnar artery’s superficial palmar arch.