Immediate Response To Stimulus in Living Organisms

A stimulus is a perceptible alteration in the physical or chemical composition of an organism's internal or external environment. The sensory receptors are reported to receive signals from outside the body, such as the light receptors in the eye or touch receptors.

The stimulus can be internal or external. An internal stimulus is the primary component of the homeostatic control system, whereas external stimuli can produce a systemic response throughout the body. The strength of the stimulus should be high enough to be detected with a high probability. The received signal is then transmitted to the CNS (central nervous system), where it is interpreted and generates a response.

Types of Stimuli

The stimulus definition states that anything that can cause a change in the behaviour, or physical state is a stimulus.

A stimulus can be broadly classified into external and internal stimuli.

• Internal stimuli can be defined as the alterations inside the body in response to viruses or the intake of poisonous food inside our stomach. For instance, hunger and thrust are some examples of internal stimuli that occur in response to a lack of food and water inside the body.

Another example is the mechanism of temperature control of the body. The stimulus here is the blood temperature, and the brain acts as the coordinator again. The receptor here is the hypothalamus of the brain. The effectors are multiple in this case, such as sweat glands and the skin's blood vessels.

• External stimuli can be defined as alterations in environmental factors, including temperature, any external danger, or pain. For instance, the light serves as a detectable change in a plant's environment in response to which the plant shows phototropism. Our five senses that are reported to perceive external stimuli are touch, vision, sound, smell, and taste.

Another example is the response of the iris of an eye to bright light. The light-sensitive cells of the retina act as receptors, and the brain acts as a coordinator. When the brain decides that the light is too bright for the eyes and it stimulates the muscles of the iris, which serve as the effectors. Eventually, the eyes contract as a response making the pupil narrow so that a reduced amount of light enters the eye.

• The other stimuli that are reported to induce a stress response are mentioned below.

• Chemical stimuli are generated when there is a decreased oxygen supply level and an acid-base concentration imbalance. Physiological stimuli are generated due to pain and hemorrhagic shock. Anxiety or fear generates psychological stimuli.

Any of these stimuli has the potential to immediately and noticeably raise the anterior pituitary hormone ACTH, which in turn raises cortisol production by the adrenal cortex.

Stimulus and Response Examples

All biological systems, from single-celled bacteria to multicellular humans, continually exchange matter and energy with their environment. All living systems must maintain homeostasis, and therefore they need to have the ability to regulate how they respond to changes in their internal or external environments.

• Response to Pain Stimuli: External stimuli that cause pain are connected to our sense of touch. Our skin's nociceptors alert us to potential dangers. Once the CNS determines whether the threat is significant enough, a substantial behavioural response is generated through a quick procedure known as a reflex arc. Before a person freely reacts to the threat, the reflex arc system triggers a reflex action—an unconscious response. For instance, touching something sharp.

Our hand's nociceptors identify the danger and send an electric impulse to our brain, which generates the sensation of pain and triggers a quick involuntary response to move away from the potential threat.

• Response to Temperature Stimuli: A steady temperature is essential for all chemical processes necessary to support life. Thermoregulation refers to all behavioural and physiological control processes necessary to maintain a consistent internal temperature despite changes in the outside temperature. All mammals, including humans, are capable of producing heat, and they all possess negative feedback systems that enable heat to be retained in cold settings while being lost in hot ones. The brain's hypothalamus controls thermoregulation in humans. This acts as our internal "thermostat" that maintains the temperature around 37 °C.

Examples of Stimulus and Response in Humans

Stimuli can be of two types external or internal. Some examples of stimulus and response in humans include the response of our body to any medicine. Alterations in the vital signs due to any change in the body are an example of internal stimuli.

Other examples are-

• When we feel cold, we wear a jacket, here feeling of cold is the stimulus. Shivering is a phenomenon reported to heat up our bodies when the temperature drops.

• In humans, the dilation and constriction of the pupils in order to adapt to light is one good example of stimulus and response.

• Sweating, which is an important phenomenon in adapting to heat, is also an example of stimulus and response.

• During workouts, we breathe deeper when we need more oxygen.

Humans can respond to a wide variety of stimuli. Some responses or reactions are innate, which means people are predisposed to know. For instance, a baby will cry when it is hungry to let its parents know that it needs to be fed.

Interesting Facts

• The body's reactions to external stimuli can take many forms, such as sweating, shivering, and tiredness.

• The body can react to internal stimuli by making us feel sick and hungry and raising our body temperature.

Important Questions

1. What are nociceptors?

Ans: Extremes in temperature, pressure, and chemicals associated with injury are detected by specialized peripheral sensory neurons known as nociceptors.

2. Name the organisms that can respond to stimuli.

Key Features

• Stimulus is the change that occurs in the internal or external environment.

• The response is the reaction to the external or internal stimulus.

• Stimulus is always received from the environment while the response is generated inside the organism.