The functions of reflexes are numerous and varied. As previously mentioned, reflexes adjust many important biological functions rapidly and efficiently without conscious effort, while other reflexes are largely protective. For example, the eye and the ear, the most delicate and sensitive sensory systems, can be damaged or destroyed by overstimulation or by accident. The amount of light admitted to the retina is controlled by the pupillary reflex, which in humans can effect a change in pupillary diameter from approximately eight millimeters in darkness to two millimeters in bright light. A sudden flash of intense light, for example, can evoke the reflex closing of the eyelids, further protecting the delicate retina. The eyeball itself is protected from drying by the blink reflex and from mechanical injury by the eyelid closure reflex. The latter reflex is triggered when an object approaches the eye or when the lashes or cornea are touched. The ear is also protected from potentially damaging sounds through the reflex contraction of middle-ear muscles in response to loud noise. This reflex functions to lower the efficiency with which sound vibrations are transmitted through the bones of the middle ear and thus reduces the possibility of damage to the delicate hair cell receptor of the inner ear.
Another category of protective reflexes exhibited by many animals is stereotyped escape responses. For example, a startled squid takes evasive action by contracting its mantle muscles (the membranous flap or folds of the body wall), forcing a jet of water through its siphon. Fish respond to vibrations carried through water by contracting the muscles on one side of their bodies; the reflex contraction occurs on the side opposite the source of vibration, and the result is a sudden move away from potential danger.
Equally essential for survival are the numerous feeding reflexes exhibited by animals. For example, flies as well as many other insects possess chemoreceptors located on their feet, mouth parts, and antennae. Thus, when a hungry fly walks on a surface moistened with nutrients, a set of reflexes is triggered. A reflex extension of the proboscis occurs. If the proboscis receptors are favorably stimulated, then the animal begins to drink. Drinking continues until the crop is sufficiently distended to stimulate its stretch receptors. Finally, this stimulus initiates the reflex termination of feeding.
Instinctive or innate behaviors, such as courtship rituals, nest-building, aggression, and territorial behaviors, demonstrate many similarities to reflexes. Although generally more complex, they are, like reflexes, unlearned, species-specific, genetically determined, and stereotypic in nature. Importantly, fixed action patterns such as these are similar to reflexes in that they are initiated by a specific stimulus, called a sign stimulus or a re-leaser. Generally, both forms of behavior are also comparable in that they are thought to be controlled by specific sets of neurons that underlie each behavior. Like the more complex learned behaviors, however, the neural basis of instinctive behaviors remains largely unknown.
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