Pronator quadratus is a square-shaped muscle on the distal

forearm The forearm is the region of the upper limb between the elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, a word which is most often used to describe the entire appendage of the upper limb, but which in ...

that acts to

pronate Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, Limb (anatomy), limbs, and specific sections of the body. The terminology used de ...

(turn so the palm faces downwards) the hand.

Structure

Its fibres run perpendicular to the direction of the arm, running from the most distal quarter of the anterior

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

to the distal quarter of the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

. It has two heads: the superficial head originates from the anterior distal aspect of the

diaphysis The diaphysis is the main or midsection (shaft) of a long bone. It is made up of cortical bone and usually contains bone marrow and adipose tissue (fat). It is a middle tubular part composed of compact bone which surrounds a central marrow cavit ...

(shaft) of the ulna and inserts into the anterior distal diaphysis of the radius, as well as its anterior

metaphysis The metaphysis is the neck portion of a long bone between the epiphysis and the diaphysis. It contains the growth plate, the part of the bone that grows during childhood, and as it grows it ossifies near the diaphysis and the epiphyses. The metap ...

. The deep head has the same origin, but inserts proximal to the ulnar notch. It is the only muscle that attaches only to the ulna at one end and the radius at the other end. Arterial blood comes via the anterior interosseous artery.

Innervation

Pronator quadratus muscle is innervated by the

anterior interosseous nerve The anterior interosseous nerve (volar interosseous nerve) is a branch of the median nerve that supplies the deep muscles on the anterior of the forearm, except the ulnar (medial) half of the flexor digitorum profundus. Its nerve roots come from C8 ...

, a branch of 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 cont ...

Function

When pronator quadratus contracts, it pulls the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

side of the radius towards the ulna, thus pronating the hand. Its deep fibers serve to keep the two bones in the forearm bound together. Moreover, this muscle can be absent in some humans, however, that does not affect the action of pronation very notably, as the pronator teres does the major role in that action.

Spinal tracts

The lateral corticospinal tract is responsible for the motor pathway of the pronator quadratus. This tract begins in the precentral gyrus of the motor cortex where a signal is transmitted from the upper motor nerve through the progression tracts of the internal capsule and through the cerebral peduncles of the midbrain. It decussates in the medulla and travels down the lateral corticospinal tract in the lateral column of the spinal cord. It then decussates in the spinal cord and synapses at the anterior horn to the lower motor neurons of the skeletal muscles. The cuneate fasciculus tract is responsible for the sensation of the pronator quadratus position and movement, deep touch, visceral pain, and vibration. This tract begins in the dorsal nerve root where the signal is transmitted through the dorsal horn and up the posterior column of the spinal cord. It synapses with an interneuron in the gracile nucleus. It then decussates in the medial lemniscus of the medulla, travels through the cuneate nucleus and through the medial lemniscus of the midbrain to synapse in the thalamus. It synapses with a third order neuron and transmits the signal to the postcentral gyrus of the somesthetic cortex. This could apply to any muscle in the upper limb and not specific to this muscle.

Additional images

File:Slide10MMMM.JPG, Pronator quadratus muscle File:Slide27OOOO.JPG, Pronator quadratus muscle File:Slide2VVVV.JPG, Pronator quadratus muscle File:Slide5VVVV.JPG, Pronator quadratus muscle

References

External links

* {{Authority control Muscles of the upper limb Forearm pronators