Stimulus- Definition, Types (External, Internal), Mechanism

A stimulus is any detectable change in the physical or chemical structure of an organism’s internal and external environment, leading to a change in environmental or physiological factors. These factors may be any conditions that are required for the cell’s optimal function and survival, like the temperature or pH of the cell.

  • Receptors play a vital role in the detection of stimuli.
  • Receptors are a type of stimulus or proteins complementary to specific proteins found intracellularly and extracellularly, which can initiate downstream reactions when stimulated.
  • These receptors accept the information from the external environment in the form of light receptors found in the eye and the touch receptors found in the skin.
  • Sensory receptors are also found inside the body.
  • These sensory receptors detect any kind of stimulus and can respond to that stimulus in the form of reflex through stimulus transductions.

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Coordination and Response

The receptors are stimulated when any changes and variations from an optimal level exist. Effector organs trigger the response and then corrects the imbalance. In the case of multicellular organisms like animals and plants, the response is quite complicated where the stimuli are received by the receptors in one part of the body, and the effector generates the response in another part of the body.

Effector organs include skeletal muscles and different glands. For proper coordination of the generated responses, it is necessary to have a proper coordination system so as to connect the receptors to the effectors through different signals and control centers. In animals, the responses are conducted through the nervous and endocrine systems with the help of electrical, hormonal, or chemical signaling. But in the context of plants, these reactions are conducted through chemical systems like plant hormones.

Types of stimulus

There are two main types of stimulus: external and internal.

External stimulus

External sensory stimuli are generally activated by the external changes and include pain, vision, smell, taste, sound, and balance.

  • Pain and touch: Pain is the stimulus that can change the behavior of the organism and cause a major response from the body. When there is a pain response, then the brain decides to give a response to where the signal is sent to muscles, and it starts to behave accordingly. Nociceptors are a kind of pain receptors that sense any kind of pain stimuli. Another kind of external stimuli is touch, which causes an organism’s behavioral change. For example, some plants respond to the touch stimuli and close their leaves, also known as the touch-me-not plant.
  • Vision: photoreceptor cells are a special type of neuron that plays a vital role in sensing vision stimuli. Vision helps in the movement and visualizing of the surroundings for all of the organisms. With the help of visual stimuli, an animal is able to move toward its prey. Similarly, the same visual stimuli help prey to run for its survival. In humans, visual stimuli help perform almost everything related to vision, like crossing a road or driving a vehicle.
  • Taste is also a kind of external stimuli as it comes from touching the food into our mouth with the help of the tongue. Gustatory cells are the cells that are present on tongue taste buds. Different enzymes are produced due to the stimulation of the mouth by these taste stimuli. It also stimulates the production of saliva.
  • Smell: many of the organisms move towards the food or away from the rotten food because of the smell, and all of this action is performed by the smell stimuli. In most animals, it is most important to identify good food. In human smell, stimuli help to stimulate and generate saliva from the salivary glands. Similarly, it helps to smell the poison like ammonia and helps us to move back or away from it. An olfactory organ is present inside the nose that helps to smell things.
  • Sound: For many animals, the sound is also a kind of stimulus that helps to recognize the presence of animals and objects. One of the best examples is bats that use high-frequency sound waves for navigating objects during their flight. Similarly, different animals like goats, sheep, or mice use sound to stay near the herd and be alert when the predators are near. In humans, some sounds are pleasant (sound of a guitar), whereas some sounds are noisy (sound of a bullet fire).
  • Balance: All the animals require balance to move from one place to another and remain stable. The orientation of animals is an external factor and also acts as a stimulus where four-legged animals have to maintain themselves lesser than two-legged animals. information to maintain the balance is sensed by the brain and provides the information to muscles for keeping the balance.

Internal stimulus

Internal stimulus is the type of stimulus that comes from inside of the body. One example is a pang of hunger which makes our body get food as there is low energy in the body.

  • Blood pressure: Blood pressure is measured by the receptors of the arteries and is a type of internal stimulus in mammals. When there is a rise in the blood pressure, then the arteries shrink, and the nerve receptor sends the message or signal to the brain. The brain commands the heart to slow down, and if there are no more signals sent by the receptors, then this means the blood pressure is low. All this happens without us knowing, but when the blood pressure is too high, humans feel pain in the back of the head.
  • Homeostasis: Homeostasis is the internal condition of the body that is maintained by most mammals for its survival that is maintaining physical and chemical balance. Blood levels, temperature, nutrient levels, etc., are some examples of homeostasis. A variety of receptors works in combination to maintain the body’s internal conditions.

Mechanism of Nervous system and stimuli conduction

  • The organization of the receptors, adjustors, and effector units is the basic pattern of stimulus-response coordination in all animals.
  • The receptors cells receive stimuli from the external environment; in most cases, these receptors are neurons.
  • The external or the internal stimulus thus received is modified or transduced into electrical impulses in the receptors of neurons.
  • The impulses then pass through the axon receptor to the adjustor, also known as interneurons.
  • The impulses are transmitted from the axon of one neuron to another without losing the signal reaction or information.
  • The inputs from the receptors are modified, selected, and interpreted by the interneuron adjustors.
  • After this, these impulses are sent to the efferent response, such as a motor neuron.
  • The response is produced after the efferent neuron makes contact with the effectors like muscle or glands.
  • Finally, the reflex arc is formed by the simple arrangement of the receptor-effector-adjustor unit.
  • The sensory cells carry afferent impulses to a central interneuron and make contact with the motor neurons.
  • Then the motor neurons carry the efferent impulses to the effectors, which produce the responses.
  • Some examples of simple reflexes are blinking of the eye by the touch of the cornea, contraction of the muscle in response to stretching, and release of the saliva in response to food.
  • In the context of the higher nervous system brain and spinal cord mediates the impulse between the afferent and efferent neurons.
  • Induced afferent impulses travel through a specific neuron pathway received from a particular receptor to the central nervous system.
  • The information is processed and passed through the efferent motor neurons to produce a response towards any of the effectors.
  • Because of this, all organisms can react accordingly and respond to the stimulus, which may be external or internal.



About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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