Stimuli: Definition & Examples

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  • 0:05 A Look at Stimuli
  • 0:47 Detection of Stimuli
  • 1:37 Tropisms
  • 2:29 Why Do Humans Detect Stimuli?
  • 3:46 Lesson Summary
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Lesson Transcript
Instructor: John Williams
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.

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