Response to Stimuli

All biological systems, from single-cell bacteria to multicellular animals, constantly exchange matter and energy with the outside environment. When animals eat or plants take in sunlight, they interact with their environment, exchanging matter and energy. These interactions ensure the survival of the organism but also cause changes within its system. To survive, all living systems must maintain homeostasis, meaning a constant internal environment for optimal functioning despite these interactions. Therefore, all living beings must be able to regulate their responses to stimuli, which are changes in the internal or external environment.

Responding to stimuli

Both our body and our individual cells are constantly reacting to stimuli that can come from the outside (like the prick of a needle) or from the body itself (like the release of a particular hormone).

Responding to stimuli is essential for survival. It's the process that allows a rabbit to run when it sees a fox, and also allows wounds to seal through coagulation so that we don't bleed out when we cut ourselves. However, the body needs to go back to its resting state to survive too. The rabbit would die of exhaustion if it never stopped running once it started, and we run the risks of blood clots if the coagulation process is not controlled. Maintaining a constant internal environment is known as homeostasis.

Homeostasis mechanisms

There are two ways in which we regulate this internal environment when met with changes:

• Negative feedback loops
• Positive feedback loops

Negative feedback loop

The homeostatic balance is maintained through control mechanisms that regulate all organisms. Most of these mechanisms are called negative feedback control loops because they counter any change and restore the previous balance through corrective actions.

A change is detected when any factor relevant to homeostatic balance rises above or falls below a certain optimal value. Once the change is detected and assessed, a response is triggered to decrease or increase the factor following the change. These corrections maintain stability in various factors such as the internal temperature (thermoregulation) of biological systems even when the external temperature varies uncontrollably. Figure 1 exemplifies a generic negative loop mechanism.

Positive feedback loop

There are also less common homeostatic control mechanisms required for organism survival called positive feedback loops.

Positive loops enhance the change once detected by stimuli instead of correcting it. These systems are far less common because they lead to cascades of repeating events that enhance the stimuli. This type of positive loop is unstable by nature but can also be very important for homeostasis.

From stimuli to a response

A stimulus is any detectable change in an environmental or physiological factor. These factors can include any condition necessary to the organism and its cell’s optimal function and survival, like temperature or cell pH. Receptors detect stimuli.

Coordination and response

Any variation from an optimal level stimulates the receptors. This triggers an adaptive response by effector organs that corrects the imbalance. These responses can be complicated, especially in multicellular organisms like animals and plants, where receptors receive stimuli in one part of the body and effectors generate a response in another part of the body.

This requires coordination systems to connect the receptors to the effectors through signals and control centres. This happens through the nervous and endocrine systems via electric or chemical/hormonal signalling in animals. In plants, these reactions happen through chemical systems like plant hormones.

External stimuli

External stimuli are changes in environmental factors, meaning external conditions to the organism that can affect its function. This includes environmental temperature or survival threats like environmental dangers.

Our five senses perceive external stimuli, including touch, vision, sound, smell, and taste. These include various types of receptors that can detect environmental changes.

Internal stimuli

Internal stimuli result from variations in physiological factors detected by internal receptors. These factors include systemic blood pressure and blood water content.

Blood pressure needs to be maintained at a relatively constant level to ensure proper blood flow and oxygen supply reaches the organ systems and their cells. Changes in arterial blood pressure are detected by baroreceptors, a type of pressure sensor present in blood vessels like the aorta. When blood pressure is too high or too low, baroreceptors receive stimuli that trigger a necessary corrective response by effector organs.

Stimuli and response examples

Let’s review two classic examples of control mechanisms and responses to different stimuli: pain and temperature.

Response to pain stimuli

Pain stimuli are external stimuli most often associated with our sense of touch. This stimulus is received by nociceptors (“pain receptors”) on our skin that signal towards possible threats. This process can elicit a significant behavioural response once the central nervous system determines if the threat presents a true danger to survival or physical integrity.

If significant enough, the control centre may even elicit an automatic response to mitigate the danger through a fast process called a reflex arc. A reflex arc mechanism causes a reflex action, an unconscious effectuated response that happens even before responding voluntarily to the threat. One common example is touching something sharp. Nociceptors on your hand detect the threat and transmit the signal through electric impulses to our brain, which creates the sensation of pain and triggers a fast, involuntary response to move away from the threat, thus stopping its harm.

Response to temperature

Temperature is perhaps one of the most important factors to be maintained in any organism in homeostatic balance. A stable temperature is vital to allow all chemical reactions that sustain life.

Organisms have different strategies to respond to heat change. All mammals, including humans, can generate heat and have negative feedback loops that allow heat retention in cold environments and heat loss in hot environments. This heat balance allows us to maintain a constant temperature independently.

Most other animals rely on the environment for heat sources and are therefore more vulnerable to its variation. They must rely on behavioural responses to maintain the internal temperature constant in these cases. This includes seeking shelter at night when it’s cold or the shade during the day when it’s hot to balance heat exchange.

In humans, thermoregulation is coordinated by the hypothalamus in the brain. This is our internal 'thermostat' and measures our core temperature, which needs to be around 37°C. Receptors spread across the skin and within the body monitor internal and external temperature variation and relay the information to the hypothalamus, which acts upon any change with corrective actions once a stimulus is created.

If the core temperature decreases as a result, for example, of a cold environment, the hypothalamus will try to increase heat production and retention in our body through:

• Vasoconstriction – reduction of blood vessels’ lumen diameter near the skin, decreasing blood flow and heat loss
• Shivering – involuntary muscle contractions that generate extra heat
• Raising body hairs – muscle contraction near the skin to trap air and heat

Meanwhile, if the core temperature increases as a result, for example, of a hot environment, the hypothalamus will try to increase heat loss in our body through:

• Vasodilation – an increase of blood vessels’ lumen diameter near the skin increasing blood flow and heat loss
• Sweat production – increased evaporation from the skin in the form of sweat helps to increase heat loss.