One of the simplest forms of activity of the nervous system. (For the mechanism upon which it depends, see NERVOUS SYSTEM; NEURON(E).) Reflex acts are divided usually into three classes.
comprise the sudden movements which result when the skin is brushed or pricked, such as the movement of the toes that results from stroking the sole of the foot.
depend upon the state of mild contraction in which muscles are constantly maintained when at rest, and are obtained, as in the case of the knee-jerk (see below), by sharply tapping the tendon of the muscle in question. (see also BABINSKI REFLEX).
are those connected with various organs, such as the narrowing of the pupil when a bright light is directed upon the EYE, and the contraction of the URINARY BLADDER when distended by urine.
Faults in these reflexes give valuable evidence as to the presence and site of neurological disorders. Thus, absence of the knee-jerk, when the patellar tendon is tapped, means some interference with the sensory nerve, nerve-cells, or motor nerve upon which the act depends – as, for example, in POLIOMYELITIS, or peripheral NEURITIS; whilst an exaggerated jerk implies that the controlling influence exerted by the BRAIN upon this reflex mechanism has been cut off – as, for example, by a tumour high up in the SPINAL CORD, or in the disease known as MULTIPLE SCLEROSIS (MS).