stimulus (physiology) stimulus (physiology) in physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. the ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be made, is called sensitivity (excitability). sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors. when a stimulus is detected by a sensory receptor, it can elicit a reflex via stimulus transduction. an internal stimulus is often the first component of a homeostatic control system. external stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response. in order for a stimulus to be detected with high probability, its level of strength must exceed the absolute threshold; if a signal does reach threshold, the information is transmitted to the central nervous system (cns), where it is integrated and a decision on how to react is made. although stimuli commonly cause the body to respond, it is the cns that finally determines whether a signal causes a reaction or not. the light from the lamp (1.) functions as a detectable change in the plant's environment. as a result, the plant exhibits a reaction of phototropism—directional growth (2.) toward the light stimulus part of a series onbiologyscience of life index outline glossary history (timeline) key components cell theory ecosystem evolution phylogeny properties of life adaptation energy processing growth order regulation reproduction response to environment domains and kingdoms of life archaea bacteria eukarya (animals, fungi, plants, protists) branches anatomy biotechnology botany cell biology ecology evolutionary biology genetics marine biology microbiology molecular biology mycology neuroscience paleontology phycology physiology protistology virology zoology research biologist (list) list of biology awards list of journals list of research methods list of unsolved problems applications agricultural science biomedical sciences health technology pharming biology portal category types homeostatic imbalances homeostatic outbalances are the main driving force for changes of the body. these stimuli are monitored closely by receptors 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 temperature changes. examples of mechanoreceptors include baroreceptors which detect changes in blood pressure, merkel's discs which can detect sustained touch and pressure, and hair cells which detect sound stimuli. homeostatic imbalances that can serve as internal stimuli include nutrient and ion levels in the blood, oxygen levels, and water levels. deviations from the homeostatic ideal may generate a homeostatic emotion, such as pain, thirst or fatigue, that motivates behavior that will restore the body to stasis (such as withdrawal, drinking or resting). blood pressure blood pressure, heart rate, and cardiac output are measured by stretch receptors found in the carotid arteries. nerves embed themselves within these receptors and when they detect stretching, they are stimulated and fire action potentials to the central nervous system. these impulses inhibit the constriction of blood vessels and lower the heart rate. if these nerves do not detect stretching, the body determines perceives low blood pressure as a dangerous stimulus and signals are not sent, preventing the inhibition cns action; blood vessels constrict 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 elicit a large response and cause neurological changes in the body. pain also causes a behavioral change in the body, which is proportional to the intensity of the pain. the feeling is recorded by sensory receptors on the skin and travels to the central nervous system, where it is integrated and a decision on how to respond is made; if it is decided that a response must be made, a signal is sent back down to a muscle, which behaves appropriately 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. two main types of nociceptors exist, a-fiber nociceptors and c-fiber nociceptors. a-fiber receptors are myelinated and conduct currents rapidly. they are mainly used to conduct fast and sharp types of pain. conversely, c-fiber receptors are unmyelinated and slowly transmit. these receptors conduct slow, burning, diffuse pain. the absolute threshold for touch is the minimum amount of sensation needed to elicit 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 onto a person's cheek from a distance of one centimeter. this value will change based on the body part being touched. vision vision provides opportunity for the brain to perceive and respond to changes occurring around the body. information, or stimuli, in the form of light enters the retina, where it excites a special type of neuron called a photoreceptor cell. a local graded potential begins in the photoreceptor, where it excites the cell enough for the impulse to be passed along through a track of neurons to the central nervous system. as the signal travels from photoreceptors to larger neurons, action potentials must be created for the signal to have enough strength to reach the cns. if the stimulus does not warrant a strong enough response, it is said to not reach absolute threshold, and the body does not react. however, if the stimulus is strong enough to create an action potential in neurons away from the photoreceptor, the body will integrate the information and react appropriately. visual information is processed in the occipital lobe of the cns, specifically in the primary visual cortex. the absolute threshold for vision is the minimum amount of sensation needed to elicit a response from photoreceptors in the eye. this amount of sensation has a definable value and is often considered to be the amount of light present from someone holding up a single candle 30 miles away, if one's eyes were adjusted to the dark. smell smell allows the body to recognize chemical molecules in the air through inhalation. olfactory organs located on either side 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 cribiform plate, the superior portion of the perpendicular plate, the superior nasal concha. only roughly two percent of airborne compounds inhaled are carried to olfactory organs as a small sample of the air being inhaled. olfactory receptors extend past the epithelial surface providing a base for many cilia that lie in the surrounding mucus. odorant-binding proteins interact with these cilia stimulating the receptors. odorants are generally small organic molecules. greater water and lipid solubility is related directly to stronger smelling odorants. odorant binding to g protein coupled receptors activates adenylate cyclase, which converts atp to camp. camp, in turn, promotes the opening of sodium channels resulting in a localized potential. the absolute threshold for smell is the minimum amount of sensation needed to elicit a response from receptors in the nose. this amount of sensation has a definable value and is often considered to be a single drop of perfume in a six-room house. this value will change depending on what substance is being smelled. taste taste records flavoring of food and other materials that pass across the tongue and through the mouth. gustatory cells are located on the surface of the tongue and adjacent portions of the pharynx and larynx. gustatory cells form on taste buds, specialized epithelial cells, and are generally turned over every ten days. from each cell, protrudes microvilli, sometimes called taste hairs, through also the taste pore and into the oral cavity. dissolved chemicals interact with these receptor cells; different tastes bind to specific receptors. salt and sour receptors are chemically gated ion channels, which depolarize the cell. sweet, bitter, and umami receptors are called gustducins, specialized g protein coupled receptors. both divisions of receptor cells release neurotransmitters to afferent fibers causing action potential firing. the absolute threshold for taste is the minimum amount of sensation needed to elicit a response from receptors in the mouth. this amount of sensation has a definable value and is often considered to be a single drop of quinine sulfate in 250 gallons of water. sound changes in pressure caused by sound reaching the external ear resonate in 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 the disturbance into the cochlea, a spiral-shaped bony structure within the inner ear. hair cells in the cochlear duct, specifically the organ of corti, are deflected as waves of fluid and membrane motion travel through the chambers of the cochlea. bipolar sensory neurons located in the center of the cochlea monitor the information from these receptor cells and pass it on to the brainstem via the cochlear branch of cranial nerve viii. sound information is processed in the temporal lobe of the cns, specifically in the primary auditory cortex. the absolute threshold for sound is the minimum amount of sensation needed to elicit a response from receptors in the ears. this amount of sensation has a definable value and is often considered to be a watch ticking in an otherwise soundless environment 20 feet away. equilibrium semi circular ducts, which are connected directly to the cochlea, can interpret and convey to the brain information about equilibrium by a similar method as the one used for hearing. hair cells in these parts of the ear protrude kinocilia and stereocilia into a gelatinous material that lines the ducts of this canal. in parts of these semi circular canals, specifically the maculae, calcium carbonate crystals known as statoconia rest on the surface of this gelatinous material. when tilting the head or when the body undergoes linear acceleration, these crystals move disturbing the cilia of the hair cells and, consequently, affecting the release of neurotransmitter to be taken up by surrounding sensory nerves. in other areas of the semi circular canal, specifically the ampulla, a structure known as the cupula—analogous to the gelatinous material in the maculae—distorts hair cells in a similar fashion when the fluid medium that surrounds it causes the cupula itself to move. the ampulla communicates to the brain information about the head's horizontal rotation. neurons of the adjacent vestibular ganglia monitor the hair cells in these ducts. these sensory fibers form the vestibular branch of the cranial nerve viii. cellular response main article: cell signaling in general, cellular response to stimuli is defined as a change in state or activity of a cell in terms of movement, secretion, enzyme production, or gene expression. receptors on cell surfaces are sensing components that monitor stimuli and respond to changes in the environment by relaying the signal to a control center for further processing and response. stimuli are always converted into electrical signals via transduction. this electrical signal, or receptor potential, takes a specific pathway through the nervous system to initiate a systematic response. each type of receptor is specialized to respond preferentially to only one kind of stimulus energy, called the adequate stimulus. sensory receptors have a well-defined range of stimuli to which they respond, and each is tuned to the particular needs of the organism. stimuli are relayed throughout the body by mechanotransduction or chemotransduction, depending on the nature of the stimulus. mechanical in response to a mechanical stimulus, cellular sensors of force are proposed to be extracellular matrix molecules, cytoskeleton, transmembrane proteins, proteins at the membrane-phospholipid interface, elements of the nuclear matrix, chromatin, and the lipid bilayer. response can be twofold: the extracellular matrix, for example, is a conductor of mechanical forces but its structure and composition is also influenced by the cellular responses to those same applied or endogenously generated forces. mechanosensitive ion channels are found in many cell types and it has been shown that the permeability of these channels to cations is affected by stretch receptors and mechanical stimuli. this permeability of ion channels is the basis for the conversion of the mechanical stimulus into an electrical signal.. chemical chemical stimuli, such as odorants, are received by cellular receptors that are often coupled