2.13 Stimuli: Definition & Examples

Jan 18, 2020 | ch2 Biological Bases of Behaviour, Courses, Intro to Psychology

Stimuli are constantly affecting us and our daily lives. Therefore, it is important to understand how stimuli can cause changes in our environment and how we are able to adapt to them.

A Look at Stimuli

Our environment is constantly changing, and in order for us and other living things to survive those changes, we must be able to respond to them. Any detectable change in the environment is called a stimulus. Usually, a stimulus is a product of change in an energy source or force, such as light, or sound, or heat. As humans, we detect and respond to stimulus in order to survive.

For example, if you walk outside on a very sunny day, your pupils will constrict to protect your eye from taking in too much light and being damaged. Your body reacts to the stimulus (the light) to protect you. Let’s take a closer look at how a stimulus is detected.

Detection of Stimuli

In order for an organism (such as a plant or an animal) to adjust to a change in the environment, it must first be able to detect (or physically recognize) the change. This detection of a stimulus is called sensitivity.

In humans, sensitivity is due to portions of the nervous system called receptors. Receptors are typically neurons or cells that are able to sense changes in the environment. For example:

  • Photoreceptors in the eyes detect changes in light
  • Thermoreceptors detect changes in heat
  • Baroreceptors detect changes in pressure

And there are many other examples that can be used to illustrate the point. What’s important to remember is that in order for us to be sensitive to a particular stimulus, we must have receptors for that stimulus. If not, we won’t be able to recognize the change.


After detecting a stimulus, organisms must provide a response in order to account for the change. A tropism is a response that an organism makes to a stimulus. An example of a common tropism in plants is phototropism (or light response). Plants grow towards light sources, and if the direction of light is changed, the plant will also change its direction of growth to accommodate for survival.

Another example would be a geotropism (or response to gravity). Jellyfish, which are marine organisms, swim based on gravity, and when the force of gravity is changed in their environment, they also change their swimming activity to accommodate. Likewise, humans have gravity receptors in our balance, or vestibular systems, and we use this to adjust to gravity during our daily activities.

Why Do Humans Detect Stimuli

Detection of stimuli is important for adaptation, or adjusting to changes in the environment. The human body is equipped with response mechanisms that allow us to adjust to changes within the environment in order for survival. As mentioned earlier, if you are flashed with a bright light, your photoreceptors will be sensitive to this change in light energy. As a response, your pupils will constrict in order to prevent damage to the eye. In this case, the stimulus is light, the receptors are found in the eyes, and the response is a constriction of the pupils.

Stimuli and responses are not only associated with the external environment, but are also found in the internal environment of organisms as well. Any stimulus within the body of the organism must be detected as well in order for the organism to survive.

A good example of this would be chemical changes that take place during exercise. As humans exercise, the amount of oxygen in the body decreases. Chemoreceptors (or chemical detectors) are responsible for detecting this stimulus, and as a response, we begin to breathe harder in order to bring in more oxygen. In this case, the stimulus is low oxygen, the receptors are chemoreceptors, and the response is an increase in breathing.

Lesson Summary

These examples are only a small sample of the types of stimuli that affect an organism’s daily existence. Stimuli are changes in the environment, and organisms must be able to adapt, or adjust, to these changes in order to survive these conditions. Stimuli come in many forms, but are usually the result of an energy source, such as light or sound, or a natural force, such as gravity. Finally, all organisms are affected by stimuli, whether we are talking about humans, plants, or microbes, and they will have methods for responding to stimuli in order to live.

Stimuli Vocabulary & Definitions

  • Stimuli: changes in the environment
  • Sensitivity: the detection of a stimulus
  • Receptors: neurons or cells that are able to sense changes in the environment
  • Photoreceptors: receptors in the eyes that detect changes in light *Thermoreceptors: detect changes in temperature
  • Baroreceptors: detect changes in pressure
  • Tropism: a response that an organism makes to a stimulus
  • Phototropism: light response in plants
  • Geotropism:a response to gravity
  • Adaptation,: adjusting to changes in the environment
  • Chemoreceptors: chemical detectors of stimulus

Learning Outcomes

When this lesson ends, students should be able to:

  • Define stimuli
  • Identify the receptors in humans that are involved in detecting stimuli
  • Describe examples of how living things react to stimuli
  • Explain what adaptation is
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