The
thenar eminence (from the
Greek "θέναρ" -
thenar, "palm of the hand" and the
Latin word "eminentia", meaning projection,) refers to the group of
muscles on the palm of the human
hand at the base of the
thumb. The skin overlying this region is the area stimulated when trying to elicit a
palmomental reflex.
Muscles
The following three muscles are considered part of the thenar eminence:
Abductor pollicis brevis abducts the thumb. This muscle is the most proximal of the thenar group.
Flexor pollicis brevis, which lies next to the abductor, will flex the thumb, curling it up in the palm.
Opponens pollicis lies deep to abductor pollicis brevis. As its name suggests it opposes the thumb, bringing it against the fingers. This is a very important movement, as most of our dexterity comes from this action.
Another muscle that controls movement of the thumb is adductor pollicis. It lies deeper and more distal to flexor pollicis brevis. Despite its name, its main action is mainly rotation and opposition. It is not in the thenar group of muscles, so is supplied by the ulnar nerve.
Innervation
The Opponens pollicis and Abductor pollicis brevis are normally innervated by the
median nerve. The Flexor pollicis brevis can be innervated by the median or ulnar nerve. The adductor pollicis is typically innervated by the
ulnar nerve. There are normal variations. In a Cannieu-Riche anastomasis, fibers from the deep palmar branch of the ulnar nerve innervate the opponens pollicis and/or abductor pollicis brevis. Regardless of their final innervation, the nerves that reach the thenar muscles arise from the C8 and T1 roots, pass through the lower trunk of the plexus, and then through the lateral cord of the plexus.
The ulnar nerve is exclusively responsible for the innervations of the hypothenar eminence. Both nerves contribute to the innervations of the midpalmar group.
Flexor Pollicis Brevis (FPB) is typically an ulnar-innervated muscle. Due to a common interconnection between the Median and Ulnar nerves in the hand (Riche-Cannieu interconnection), the Median nerve may innervate the FPB in 35% of people. It is innervated by the Ulnar nerve in 50% of people and by both the Median and Ulnar nerves in 15%.
The muscles in this location are usually innervated by the recurrent branch of the median nerve. They all control movement of the thumb.
The innervation of these muscles by the median nerve is unusual, as most of the intrinsic muscles on the palm of the hand are supplied by the ulnar nerve. The lateral two lumbrical muscles are the other exception.
Mnemonics
The muscles of hand can be remembered using the
mnemonic,
"A OF A OF A" for,
Abductor pollicis brevis,
Opponens pollicis,
Flexor pollicis brevis,
Adductor pollicis (thenar muscles) and
Opponens digiti minimi,
Flexor digiti minimi,
Abductor digiti minimi (hypothenar muscles).
A good way to remember what muscles are in the thenar eminence is "OAF"; oafs have big thenar eminences. 'O' = opponens, 'A' = abductor, 'F' = flexor. There is only one opponens. The other two muscles each have a long partner and thus are called "brevis."
Meat-LOAF: the "M" in the word "Meat" helps you remember that the LOAF muscles of the hand (2 Lateral Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis) are innervated by the Median Nerve
ABOF (pronounced "above") the Law muscles. The law states muscles of the hand are supplied by ulnar nerve. However, ABOF the Law muscles are supplied by MEDIAN nerve. 'AB' = abductor, 'O' = Opponens, 'F' = Flexor, Law 'L' = Lateral Lumbricals (2).
MAINTAINER—Main(Median nerve) --Tainer(thenar muscles)
See also
Hypothenar eminence
References
External links
Category:Muscles of the upper limb
Category:Hand
