A stimulus is any detectable change in an organism’s environment that causes it to react. A change can arise from within the organism itself, which is what biologists refer to as an internal stimulus. Responding to these internal signals is a continuous, automatic process that dictates the body’s moment-to-moment function. The physiological reaction to an internal stimulus is not a conscious choice but a hardwired adjustment necessary for survival.
Defining Internal Signals and Reactions
An internal stimulus is a deviation from an ideal set point for a measurable biological variable inside the body. These signals are constantly generated by fluctuations in the body’s internal environment. Examples of these measurable variables include the concentration of oxygen in the blood, the level of acidity (pH), or the stretching of arterial walls due to blood pressure.
When the body’s internal conditions stray from their optimal range, this change acts as the stimulus. A reaction, or response, is the immediate biological adjustment triggered by that stimulus to counteract the change. For instance, if carbon dioxide levels in the blood rise, that rise is the internal stimulus, and the reaction is an automatic increase in the rate of breathing to expel the excess gas.
These internal signals are detected by specialized cells and tissues, such as chemoreceptors (sensitive to chemical changes) or mechanoreceptors (sensitive to physical changes). The resulting reaction is a physiological adjustment, such as the release of a hormone or the contraction of a muscle, designed to restore the variable to its ideal state.
The Physiological Control Loop
The body processes and responds to internal stimuli through a precise, three-part system known as a feedback loop. This mechanism ensures that any internal variable that drifts too far from its set point is quickly brought back into line.
The three components of the control loop are:
- Sensor: Also called a receptor, the sensor constantly monitors the specific variable, such as blood glucose concentration or core body temperature. When it detects a change away from the normal range, it generates a signal, typically an electrical nerve impulse or a chemical message.
- Control Center: Often residing in the brain (like the hypothalamus) or an endocrine gland, the control center receives the signal and compares the current value to the established set point. If a significant deviation is noted, it initiates a corrective action.
- Effector: This is any cell, tissue, organ, or gland that executes the command from the control center. Effectors carry out the final physiological response, such as causing a muscle to contract, a blood vessel to dilate, or a gland to secrete a hormone.
Maintaining Stability Through Homeostasis
The purpose of responding to internal stimuli through the control loop is to achieve and maintain a stable internal environment, a state known as homeostasis. This internal balance is necessary because the body’s chemical reactions, which sustain life, operate effectively only within narrow ranges of temperature and chemical concentrations. Without constant adjustment, cellular function would quickly fail.
Negative feedback is the primary mechanism for maintaining stability, as it reverses the direction of the initial internal stimulus. If a variable increases, the loop triggers a response that causes it to decrease; conversely, if it decreases, the response causes it to increase. For example, if body temperature rises, the loop activates cooling mechanisms like sweating, which are actions opposite to the initial change.
While negative feedback accounts for the vast majority of internal responses, a less common mechanism called positive feedback exists. Positive feedback intensifies or amplifies the change in the same direction as the stimulus until a specific endpoint is reached. An example is the release of oxytocin during childbirth, which causes stronger uterine contractions, accelerating the process until the baby is delivered. Once the endpoint is achieved, the positive feedback loop is broken.
Applying the Concept to Human Biology
The regulation of blood glucose provides a clear example of the control loop. After a meal, the internal stimulus is a rise in the concentration of glucose in the bloodstream. Specialized beta cells in the pancreas act as the sensors and control center, detecting the elevated sugar level.
In response, these cells secrete the hormone insulin, which acts as the effector by signaling liver, muscle, and fat cells to absorb and store the excess glucose. As the glucose concentration returns to its normal range, the negative feedback reduces the stimulus, and insulin secretion slows. This precise response prevents blood sugar levels from remaining dangerously high.
Osmoregulation is the body’s management of water balance. If the body becomes dehydrated, the internal stimulus is an increase in the concentration of solutes in the blood. Receptors in the brain detect this shift and trigger the release of antidiuretic hormone (ADH) as the effector. ADH signals the kidneys to reabsorb more water, conserving fluid and diluting the blood back toward its optimal concentration.