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The lumbricals are intrinsic
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...
s of the
hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each " ...
that flex the
metacarpophalangeal joint The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cav ...
s, and extend the interphalangeal joints. p. 97 The lumbrical muscles of the foot also have a similar action, though they are of less clinical concern.


Structure

The lumbricals are four, small, worm-like muscles on each hand. These muscles are unusual in that they do not attach to bone. Instead, they attach proximally to the tendons of
flexor digitorum profundus The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers (also known as digits). It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. Togethe ...
, and distally to the
extensor expansion An extensor expansion (extensor hood, dorsal expansion, dorsal hood, dorsal aponeurosis) is the special connective attachments by which the extensor tendons insert into the phalanges. These flattened tendons (aponeurosis) of extensor muscles spa ...
s. The first and second lumbricals are unipennate, while the third and fourth lumbricals are bipennate.


Nerve supply

The first and second lumbricals (the most radial two) are innervated by the
median nerve The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus. The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
. The third and fourth lumbricals (most ulnar two) are innervated by the deep branch of
ulnar nerve In human anatomy, the ulnar nerve is a nerve that runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is ...
. This is the usual innervation of the lumbricals (occurring in 60% of individuals). However 1:3 (median:ulnar - 20% of individuals) and 3:1 (median:ulnar - 20% of individuals) also exist. The lumbrical innervation always follows the innervation pattern of the associated muscle unit of
flexor digitorum profundus The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers (also known as digits). It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. Togethe ...
(i.e. if the muscle units supplying the
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its abilit ...
to the
middle finger The middle finger, long finger, second finger, third finger, toll finger or tall man is the third digit of the human hand, located between the index finger and the ring finger. It is typically the longest digit. In anatomy, it is also called ...
are innervated by the median nerve, the second lumbrical will also be innervated by the median nerve).


Blood supply

Four separate sources supply blood to these muscles: the superficial palmar arch, the common palmar digital artery, the
deep palmar arch The deep palmar arch (deep volar arch) is an arterial network found in the palm. It is usually primarily formed from the terminal part of the radial artery. The ulnar artery also contributes through an anastomosis. This is in contrast to the super ...
, and the dorsal digital artery.


Function

The lumbrical muscles, with the help of the interosseous muscles, simultaneously flex the
metacarpophalangeal joint The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cav ...
s while extending both interphalangeal joints of the digit on which it inserts. The lumbricals are used during an upstroke in writing.


Etymology

The term "lumbrical" comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
, meaning "
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wo ...
".


Additional images

File:Gray416.png, Tendons of forefinger and vincula tendina File:Slide2yyy.JPG, Lumbricals of the hand File:Slide5RRR.JPG, Lumbricals of the hand File:Slide14RRR.JPG, Lumbricals muscle File:Slide6PPP.JPG, Lumbricals muscle File:Slide4VVV.JPG, Lumbricals muscle File:Slide4AAAA.JPG, Lumbricals muscle File:Slide9AAAA.JPG, Lumbricals muscle File:Slide1MAI.JPG, Muscles of hand, cross section File:Slide1dsds.JPG, Wrist joint. Deep dissection.Anterior, palmar view File:Slide2dsds.JPG, Wrist joint. Deep dissection.Anterior, palmar view File:Slide3dsds.JPG, Wrist joint. Deep dissection.Anterior, palmar view


References

{{Authority control Muscles of the upper limb Hand