Stimulus-response coordination

The simplest type of response is a direct one-to-one stimulus-response reaction. A change in the environment is thestimulus; the reaction of the organism to it is the response. In single-celled organisms, the response is the result of a property of the cell fluid calledirritability. In simple organisms, such asalgae,protozoans, andfungi, a response in which the organism moves toward or away from the stimulus is calledtaxis. In larger and more complicated organisms—those in which response involves the synchronization andintegration of events in different parts of the body—a control mechanism, or controller, is located between the stimulus and the response. In multicellular organisms, this controller consists of two basic mechanisms by which integration is achieved—chemical regulation and nervous regulation.

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Human Organs

Inchemical regulation, substances calledhormones are produced by well-defined groups of cells and are either diffused or carried by theblood to other areas of the body where they act ontarget cells and influencemetabolism or induce synthesis of other substances. The changes resulting from hormonal action are expressed in the organism as influences on, or alterations in, form,growth,reproduction, and behaviour.

Plants respond to a variety of externalstimuli by utilizing hormones ascontrollers in a stimulus-response system. Directional responses of movement are known astropisms and are positive when the movement is toward the stimulus and negative when it is away from the stimulus. When a seed germinates, the growingstem turns upward toward the light, and theroots turn downward away from the light. Thus, the stem shows positive phototropism and negative geotropism, while the roots show negative phototropism and positive geotropism. In this example, light and gravity are the stimuli, and directional growth is the response. The controllers are certain hormonessynthesized by cells in the tips of the plant stems. These hormones, known asauxins, diffuse through the tissues beneath the stem tip and concentrate toward the shaded side, causing elongation of these cells and, thus, a bending of the tip toward the light. The end result is the maintenance of the plant in an optimal condition with respect to light.

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Inanimals, in addition to chemical regulation via theendocrine system, there is another integrative system called thenervous system. A nervous system can be defined as an organized group of cells, called neurons, specialized for the conduction of an impulse—an excited state—from asensory receptor through a nerve network to an effector, the site at which the response occurs.

Organisms that possess a nervous system are capable of much more complex behaviour than are organisms that do not. The nervous system, specialized for the conduction of impulses, allows rapid responses to environmental stimuli. Many responses mediated by the nervous system are directed toward preserving the status quo, orhomeostasis, of the animal. Stimuli that tend to displace or disrupt some part of the organism call forth a response that results in reduction of the adverse effects and a return to a more normal condition. Organisms with a nervous system are also capable of a second group of functions that initiate a variety of behaviour patterns. Animals may go through periods of exploratory or appetitive behaviour, nest building, andmigration. Although these activities arebeneficial to the survival of the species, they are not always performed by the individual in response to an individual need or stimulus. Finally, learned behaviour can be superimposed on both the homeostatic and initiating functions of the nervous system.