Stimulus: Definition, Types, Cell Response, Systematic Response, Methods and Techniques

In physiology, it is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external impulses is called sensitivity.

When a stimulus is applied to a sensory receptor, it usually causes or influences a reflex through the transduction of stimuli.

These sensory receptors can receive information from outside the body, such as tactile receptors that are on the skin or light receptors in the eye, as well as from inside the body, such as chemoreceptors and mechanoreceptors.

An internal stimulus is often the first component of a homeostatic control system . External stimuli are capable of producing systemic responses throughout the body, such as in the fight or flight response.

For a stimulus to be detected with high probability, its level must exceed the absolute threshold; If a signal reaches the threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision is made on how to react.

Although stimuli commonly cause the body to respond, it is the central nervous system that ultimately determines whether a signal causes a reaction or not.



Homeostatic imbalances

Homeostatic imbalances are the main driving force of body changes. These stimuli are monitored closely by receivers and sensors in different parts of the body.

These sensors are mechanoreceptors, chemoreceptors and thermoreceptors that, respectively, respond to pressure or stretching, chemical changes or changes in temperature.

Examples of mechanoreceptors include baroreceptors that detect changes in blood pressure, Merkel discs that detect sustained pressure and contact, and hair cells that detect sound stimuli.

Homeostatic imbalances that can serve as internal stimuli include levels of nutrients and ions in the blood, oxygen levels and water levels.

Deviations from the homeostatic ideal can generate a homeostatic emotion, such as pain, thirst or fatigue, which motivates the behavior that will restore the body to stasis (such as abstinence, drink or rest).

Blood pressure

Blood pressure, heart rate and cardiac output are measured by the stretch receptors found in the carotid arteries.

The nerves are embedded within these receptors and when they detect the stretching, they stimulate and trigger action potentials to the central nervous system.

These impulses inhibit the constriction of the blood vessels and decrease the heart rate.

If these nerves do not detect the stretching, the body determines that the low blood pressure is a dangerous stimulus and signals are not sent, which prevents the inhibition of the action of the central nervous system.

The blood vessels contract and the heart rate increases, causing an increase in blood pressure in the body.


Touch and pain

Sensory feelings, especially pain, are stimuli that can cause a great response and cause neurological changes in the body. Pain also causes a change in behavior in the body, which is proportional to the intensity of the pain.

Sensation is recorded by sensory receptors in the skin and travels to the central nervous system, where it is integrated and a decision is made on how to respond; If it is decided that a response must be made, a signal is sent to a muscle, which acts according to the stimulus.

The postcentral gyrus is the location of the primary somatosensory area, the main sensory receptive area for the sense of touch.

Pain receptors are known as nociceptors. There are two main types of nociceptors, fiber A nociceptors and fiber C nociceptors. Fiber A receptors are myelinated and conduct currents rapidly.

They are mainly used to drive pain types (fast and sharp). In contrast, C-fiber receptors are unmyelinated and transmit slowly. These receptors produce slow, strong and diffuse pain.

The absolute threshold for touch is the minimum amount of sensation needed to obtain a response from touch receptors.

This amount of sensation has a definable value and is often considered to be the force exerted by dropping the wing of a bee on its cheek from a distance of one centimeter. This value will change depending on the part of the body that is touched.


Vision provides the opportunity for the brain to perceive and respond to changes that occur around the body. The information, or stimuli, in the form of light enters the retina, where it excites a special type of neuron called the photoreceptor cell .

A local graduated potential begins in the photoreceptor, where it excites the cell enough for the impulse to pass through a track of neurons into the central nervous system.

As the signal travels from the photoreceptors to the larger neurons, action potentials must be created so that the signal has sufficient strength to reach the central nervous system.

The absolute threshold for vision is the minimum amount of sensation necessary to obtain a response of the photoreceptors in the eye.

This amount of sensation has a definable value and is often considered as the amount of light present from someone holding a single candle 30 miles away, if the eyes adjusted to the darkness.


The smell allows the body to recognize chemical molecules in the air by inhalation. The olfactory organs located on both sides of the nasal septum consist of olfactory epithelium and lamina propria.

The olfactory epithelium, which contains olfactory receptor cells, covers the inferior surface of the cribriform plate, the upper portion of the perpendicular plate, the superior nasal concha.

Only about two percent of the inhaled airborne compounds are transported to the olfactory organs as a small sample of the air that is inhaled.

The olfactory receptors extend beyond the epithelial surface and provide a basis for many cilia that are found in the surrounding mucus.

Odorant binding proteins interact with these cilia by stimulating the receptors. Odors are usually small organic molecules. A greater solubility in water and lipids is directly related to stronger odorous odors.

The binding of Odorant to receptors coupled to protein G activates adenylate cyclase, which converts adenosine triphosphate into cyclic adenosine monophosphate.

Cyclic adenosine monophosphate, in turn, promotes the opening of sodium channels resulting in a localized potential.

The absolute threshold for smell is the minimum amount of sensation necessary to obtain a response from the receptors in the nose.

This amount of feeling has a definable value and is often considered a single drop of perfume in a six-bedroom house. This value will change depending on what substance is being smelled.


The taste registers the flavor of food and other materials that pass through the tongue and through the mouth. The taste cells are found on the surface of the tongue and in the adjacent portions of the pharynx and larynx.

Taste cells are formed in the taste buds, specialized epithelial cells, and usually come back every ten days. From each cell, the microvilli protrude, sometimes called taste hair, through the taste pore and into the oral cavity.

The dissolved chemicals interact with these receptor cells; Different tastes join specific receivers. The salt and bitter receptors are chemically closed ion channels, which depolarize the cell.

The receptors sweet, bitter and umami (tasty) are called gustducins, receptors coupled to specialized protein G. Both divisions of receptor cells release neurotransmitters to the afferent fibers causing activation of the action potential.

The absolute threshold for taste is the minimum amount of sensation needed to trigger a response from the receptors in the mouth. This amount of sensation has a definable value and is often considered a single drop of quinine sulfate in 250 gallons of water.


Changes in pressure caused by the sound reaching the outer ear resonate with the tympanic membrane, which articulates with the auditory ossicles or the bones of the middle ear.

These tiny bones multiply these pressure fluctuations as they pass through the disturbance in the cochlea, a spiral-shaped bone structure within the inner ear.

The hair cells in the cochlear duct, specifically the organ of Corti, deviate as fluid waves and movement of the membrane travel through the chambers of the cochlea.

The bipolar sensory neurons located in the center of the cochlea monitor the information of these receptor cells and transmit it to the brainstem through the cochlear branch of cranial nerve VIII.

The sound information is processed in the temporal lobe of the central nervous system, specifically in the primary auditory cortex.

The absolute threshold for sound is the minimum amount of sensation needed to obtain a response from the receivers in the ears. This amount of sensation has a definable value and is often considered a ticking in an environment without sound at 20 feet away.


The semicircular canals, which are directly connected to the cochlea, can interpret and transmit balance information to the brain using a method similar to that used for hearing.

The hair cells in these parts of the ear protrude kinocilia and stereocilia in a gelatinous material that lines the ducts of this channel.

In parts of these semicircular canals, specifically the macules, calcium carbonate crystals known as statoconia rest on the surface of this gelatinous material.

By tilting the head or when the body undergoes a linear acceleration, these crystals move and disturb the cilia of the hair cells and, consequently, affect the release of neurotransmitters that will absorb the surrounding sensory nerves.

In other areas of the semicircular canal, specifically the blister, a structure known as a dome, analogous to the gelatinous material in the maculae, distorts the hair cells in a similar manner when the fluid medium surrounding it causes the dome to move.

The ampulla communicates to the brain information about the horizontal rotation of the head. The neurons of the adjacent vestibular ganglia control the hair cells in these ducts. These sensory fibers form the vestibular branch of the 8th cranial nerve.

Cell response

In general, the cellular response to stimuli is defined as a change in the state or activity of a cell in terms of movement, secretion, enzyme production or gene expression.

The receptors on the surfaces of the cells are detection components that monitor the stimuli and respond to changes in the environment by transmitting the signal to a control center for further processing and response.

The stimuli are always converted into electrical signals through transduction. This electrical signal, or potential receptor, takes a specific route through the nervous system to initiate a systematic response.

Each type of receptor is specialized to respond preferentially to a single type of stimulus energy, called an adequate stimulus.

Sensory receptors have a well-defined range of stimuli to which they respond, and each one is attuned to the particular needs of the organism.

The stimuli are transmitted throughout the body by mechanotransduction or chemotransduction, depending on the nature of the stimulus.

Mechanical stimulation

In response to a mechanical stimulus, it is proposed that cellular force sensors are extracellular matrix molecules, cytoskeleton, transmembrane proteins, proteins at the membrane-phospholipid interface, elements of the nuclear matrix, chromatin and the lipid bilayer.

The answer can be twofold: the extracellular matrix, for example, is a conductor of mechanical forces but its structure and composition is also influenced by cellular responses to those same forces applied or generated endogenously.

Mechanosensitive ion channels are found in many types of cells and it has been shown that the permeability of these channels to the cations is affected by stretch receptors and mechanical stimuli.

This permeability of the ion channels is the basis for the conversion of the mechanical stimulus into an electrical signal.

Chemical stimulus

Chemical stimuli, such as odorants, are received by cell receptors that are often coupled to ion channels responsible for chemo transduction. Such is the case in olfactory cells.

Depolarization in these cells results from the opening of non-selective cation channels when the odorant binds to the specific receptor.

The G protein-coupled receptors in the plasma membrane of these cells can initiate second messenger pathways that cause the opening of the cation channels.

In response to stimuli, the sensory receptor initiates sensory transduction by creating potential gradients or action potentials in the same or adjacent cell.

The sensitivity to the stimuli is obtained by chemical amplification through the second messenger pathways, in which enzymatic cascades produce a large number of intermediate products, which increases the effect of a receptor molecule.

Systematic response

Nervous system response

Although the receptors and stimuli are varied, most of the extrinsic stimuli first generate graduated potentials located in the neurons associated with the sensory organ or specific tissue.

In the nervous system, internal and external stimuli can elicit two different categories of responses: an excitatory response, usually in the form of an action potential, and an inhibitory response.

When a neuron is stimulated by an excitatory impulse, the neuronal dendrites are linked by neurotransmitters that cause the cell to become permeable to a specific type of ion; the type of neurotransmitter determines to which ion the neurotransmitter will become permeable.

In excitatory postsynaptic potentials, an excitatory response is generated. This is caused by an excitatory neurotransmitter, usually glutamate that binds to the dendrites of a neuron, causing an influx of sodium ions through channels located near the binding site.

This change in the permeability of the membrane in the dendrites is known as local graduated potential and causes the membrane voltage to change from a negative resting potential to a more positive voltage, a process known as depolarization.

The opening of the sodium channels allows nearby sodium channels to open, allowing the change in permeability to extend from the dendrites to the body of the cell.

If a graduated potential is strong enough, or if several potentials graduate at a sufficiently fast frequency, depolarization can extend through the body of the cell to the axon mound.

From the axon mound, an action potential can be generated and propagated by the axon of the neuron, which causes the sodium ion channels in the axon to open as the impulse travels.

Once the signal begins to travel through the axon, the membrane potential has already passed the threshold, which means that it can not be stopped. This phenomenon is known as an all-or-nothing response.

Groups of sodium channels opened by the change in membrane potential strengthen the signal as it moves away from the mound of the axon, allowing it to move along the axon.

As the depolarization reaches the end of the axon, or the terminal of the axon, the end of the neuron becomes permeable to calcium ions, which enter the cell through calcium ion channels.

Calcium causes the release of neurotransmitters stored in synaptic vesicles, which enter the synapse between two neurons known as presynaptic and postsynaptic neurons.

If the signal of the presynaptic neuron is excitatory, it will cause the release of an excitatory neurotransmitter, which will cause a similar response in the postsynaptic neuron.

These neurons can communicate with thousands of other receptors and target cells through extensive and complex dendritic networks. The communication between recipients in this way allows discrimination and the more explicit interpretation of external stimuli.

Indeed, these potential localized graduates trigger action potentials that communicate, in their frequency, along the nerve axons that eventually reach the specific cortex of the brain.

In these highly specialized parts of the brain, these signals coordinate with others to possibly trigger a new response.

If a signal from the presynaptic neuron is inhibitory, inhibitory neurotransmitters, normally gamma-aminobutyric acid will be released at the synapse. This neurotransmitter causes a postsynaptic inhibitory potential in the postsynaptic neuron.

This response will cause the postsynaptic neuron to become permeable to chloride ions, making the membrane potential of the cell negative; a negative membrane potential makes it more difficult for the cell to trigger an action potential and prevents any signal from being transmitted through the neuron.

Depending on the type of stimulus, a neuron can be excitatory or inhibitory.

Muscular system response

Nerves in the peripheral nervous system extend to various parts of the body, including muscle fibers. A muscle fiber and the motor neuron to which it is connected.

The point at which the motor neuron joins the muscle fiber is known as the neuromuscular junction. When muscles receive information from internal or external stimuli, motor neurons stimulate muscle fibers.

The impulses are transmitted from the central nervous system to the neurons until they reach the motor neuron, which releases the neurotransmitter acetylcholine at the neuromuscular junction.

Acetylcholine binds nicotinic acetylcholine receptors on the surface of the muscle cell and opens the ion channels, allowing sodium ions to flow into the cell and potassium ions to flow; This movement of ions causes a depolarization, which allows the release of calcium ions inside the cell.

Calcium ions bind to proteins within the muscle cell to allow muscle contraction; the final consequence of a stimulus.

Endocrine system response


The endocrine system is affected to a large extent by many internal and external stimuli.

An internal stimulus that causes the release of the hormone is blood pressure. The hypotension , or low blood pressure, is a major driving force for the release of vasopressin, a hormone which causes water retention in the kidneys. This process also increases a person’s thirst.

Through fluid retention or fluid intake, if an individual’s blood pressure returns to normal, the release of vasopressin decreases and the kidneys retain less fluid. The hypovolemia , or low levels of fluids in the body, can also act as a stimulus to cause this response.


Epinephrine, also known as adrenaline, is also commonly used to respond to internal and external changes. A common cause of the release of this hormone is the fight or flight response.

When the body finds an external stimulus that is potentially dangerous, epinephrine is released from the adrenal glands.

Epinephrine causes physiological changes in the body, such as constriction of the blood vessels, dilation of the pupils, increased heart and respiratory rate, and glucose metabolism .

All these responses to a single stimulus help protect the individual, whether the decision is made to stay and fight, or to flee and avoid danger.

Digestive system response

Cephalic phase

The digestive system can respond to external stimuli, such as the sight or smell of food, and cause physiological changes before food enters the body. This reflex is known as the cephalic phase of digestion.

The sight and smell of food are stimuli strong enough to cause salivation, secretion of gastric and pancreatic enzymes and endocrine secretion in preparation for incoming nutrients.

By beginning the digestive process before the food reaches the stomach, the body can metabolize nutrients more effectively and efficiently in necessary nutrients.

Once the food reaches the mouth, the taste and information of the receptors in the mouth add to the digestive response. Chemoreceptors and mechanoreceptors, activated by chewing and swallowing, further increase the release of enzymes in the stomach and intestine.

Enteric nervous system

The digestive system can also respond to internal stimuli. The digestive tract or enteric nervous system only contains millions of neurons.

These neurons act as sensory receptors that can detect changes, such as food entering the small intestine, in the digestive tract.

Depending on what these sensory receptors detect, certain enzymes and digestive juices of the pancreas and liver can be secreted to aid in the metabolism and breakdown of food.

Methods and techniques

Clamping techniques

Intracellular measurements of the electrical potential across the membrane can be obtained by registering microelectrodes.

Patch clamping techniques allow the manipulation of the intracellular or extracellular ionic or lipid concentration while recording the potential. In this way, the effect of various conditions on threshold and propagation can be evaluated.

Non-invasive neuronal exploration

Positron emission tomography and magnetic resonance imaging allow the non-invasive visualization of the activated regions of the brain while the subject of the test is exposed to different stimuli. The activity is controlled in relation to the blood flow to a particular region of the brain.

Extraction time of the hind limbs

Sorin Barac et al . In a recent article published in the Journal of Reconstructive Microsurgery, the response of test rats to pain stimuli was controlled by inducing an acute external heat stimulus and measuring abstinence times of the hind legs.