Stimulus and response

Organisms act according to the changes happening in their environment. That change is a stimulus; the action taken towards it is the response.

Stimulus

A change arising in the external or internal environment that is capable of producing a response is called a stimulus.

Figure to be added

Figure 8.10 — responding to a stimulus: a trumpet blown close to the ear, and the person covering both ears.

In that figure the stimulus is the sound and the response is covering both ears. Acting on the changes happening in the environment is what is called responding.

Where does a stimulus come in?

Stimuli are received by the sense organs — the eye, ear, nose, tongue and skin. Light, sound, and chemical or mechanical vibrations can all act as a stimulus.

Irritability

The ability to respond to stimuli arriving from the external and internal environments is irritability.

Not every change is a stimulus

A change is a stimulus only when it has enough strength to produce a response. If the room temperature changes by 0.01 °C you do nothing — it is a change, but it is not a stimulus to you.