Reflex Arc

In biology, a type of action consisting of comparatively simple segments of behaviour that usually occur as direct and immediate responses to particular stimuli uniquely correlated with them. Reflex actions have a widespread occurrence among complex animals.

Many reflexes of placental mammals appear to be innate. They are transmitted by heredity and are the common property of the species, and often of the genus. They include not only such simple acts as mastication, swallowing, the blink reflex, the knee jerk, and the scratch reflex, but also stepping, standing, the cat's righting reflex, basic sexual acts, etc. Built up into complex patterns of many coordinated muscular actions, reflexes form the basis of much instinctive behaviour in animals.

Humans also exhibit a variety of innate reflexes, which are variously concerned with adjusting the musculature for optimum performance of the body's distance receptors (the eye and the ear), with orienting parts of the body in spatial relation to the head, and with managing the complicated acts involved in ingesting food. Among the innate reflexes concerning just the eyes, for example, are:

(1) paired shifting of the eyeballs, often combined with turning of the head, to perceive some interesting object in the field of vision;

(2) contraction of the intraocular muscles to adjust the focus of the retina for the viewing of near or far objects;

(3) constriction of the pupil of the eye to reduce excessive illumination of the retina;

(4) blinking due to intense light or touching of the cornea.

In its simplest and most elementary form, a reflex is now viewed as a function of an idealized mechanism called a reflex arc. The primary components of the reflex arc have been identified as the sensory-nerve cell (or receptor) that receives the stimulation, in turn connecting to another nerve cell that activates the muscle cell (or effector), which thus performs the reflex action. In most cases, however, the basic physiological mechanism is more complicated than this simple arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are invariably present in reflex circuits. As a result of the integrative action of the nervous system in higher animals, the behaviour of such organisms is more than the simple sum of their reflexes; it is a unitary whole that exhibits coordination between many individual reflexes and is characterized not by inherited, stereotyped responses but by flexibility and adaptability to circumstances.