Extensor muscles of the carpus and digits are located cranially on the antebrachial region of the forelimb and innervated by the radial nerve. These muscles (except for abductor pollicus longus) all originate from the lateral epicondyle of the humerus. Extensor carpi radialis inserts onto the second and third metacarpal bone. It sits in the groove for extensor carpi radialis on the radius. Extensor carpi radialis is found medially to the other extensor muscles. On the forelimb dissected it was measured to be approximately three centimetres wide and 29 centimetres long (including the tendon). When contracting, the muscle extends the carpus, effectively increasing the angle the joint makes from the vertical position.…show more content… It divides into three tendons of insertion, where they insert on the distal phalanges of digits three to five. The lateral digital extensor is a similar length to the common digital extensor, although it is lacking the shorter tendon that inserts on the smaller first digit. It is significantly narrower than the common digital extensor, measuring at its widest point just over one centimetre (on the forelimb shown in the photograph) whereas the common digital extensor measured to be just over three centimetres. Because the lateral digital extensor runs over the carpus and distal phalanges, it extends both joints, though it only extends the lateral digits unlike the common digital…show more content… When a painful stimulus, such as pin prick, is applied to the palmar surface of the third digit, it is received by a sensory neuron (branched from the median or ulnar nerves). This impulse travels to the dorsal horn of the spinal cord, where it is then relayed to interneurons. As well as being transmitted to the brain the pin prick is transmitted through motor neurons to inhibit the digital extensors and stimulate the digital extensors. Ultimately, this reflex action allows the digit to be retracted away from the harmful impetus.
Both the deep and superficial digital flexor (innervated by the ulnar and median nerves respectively) contract when stimulated as a result of the withdrawal reflex. This occurs via a neural pathway that transmits the sensation of the pin pick from the sensory neurons that detected it through to stimulation of the motor neurons that innervate the superficial and deep digital flexor. When these flexor muscles contract, the third digit is flexed, which allows it to recoil from the source of pain thus taking it away from potential danger. Often this reflex occurs before the animal has felt any