Examples of internal stimuli include: hunger, hormone levels, sensations of anxiety and changes in vital signs such as blood pressure. A stimulus is defined as anything that can cause a behavioral or physical change. Stimuli are both internal and external.
Internal stimuli, such as blood pressure rates, occur due to receptors in the body that cause blood vessels to open or constrict. External stimuli include the senses such as sight, sound, touch and smell. A body's response to ingesting medicine is one example of external stimuli.
The reactions to stimuli are known as responses. Responses can include contractions of muscles, perspiration and brain wave changes.