In physiology, a stimulus is a change in a living thing's internal or external environment. This change can be detected by an organism or organ using sensitivity, and leads to a physiological reaction. Sensory receptors can receive stimuli 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 adequate stimulus is a property of a sensory receptor that determines the type of energy to which a sensory receptor responds with the initiation of sensory transduction. Sensory receptor are specialized to respond to certain types of stimuli. The adequate stimulus is the amount and type of energy required to stimulate a specific sensory organ.
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal root ganglia of the spinal cord.
In cellular biology, mechanotransduction is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number of senses and physiological processes in the body, including proprioception, touch, balance, and hearing. The basic mechanism of mechanotransduction involves converting mechanical signals into electrical or chemical signals.
Thigmomorphogenesis the phenomenon by which plants alter their growth and development in response to mechanical stimuli, exemplifies their remarkable adaptability to fluctuating environmental conditions. From subtle forces such as wind or rain to deliberate touch, these stimuli trigger a cascade of cellular and molecular responses, shaping plant architecture to optimize survival and ecological fitness. Botanists first observed these changes when greenhouse-grown plants were found to be taller and more slender compared to stockier plants grown outdoors, where they were exposed to natural mechanical stresses. At its core, thigmomorphogenesis involves the perception of mechanical forces by cellular mechanosensors, their transduction into biochemical signals, and subsequent changes in gene expression and hormone activity. This response integrates diverse molecular players, including mechanosensitive ion channels, receptor-like kinases, cytoskeletal elements, phytohormones, and transcription factors, which collectively drive both immediate and long-term morphological adaptations.
Merkel nerve endings are mechanoreceptors situated in the basal epidermis as well as around the apical ends or some hair follicles. They are slowly adapting. They have small receptive fields measuring some milimeters in diameter. Most are associated with fast-conducting large myelinated axons. A single afferent nerve fibre branches to innervate up to 90 such endings. Merkel nerve endings respond to light touch. They respond to sustained pressure, and are sensitive to edges of objects. Their exact functions remain controversial.
In biology, thigmonasty or seismonasty is the nastic (non-directional) response of a plant or fungus to touch or vibration. Conspicuous examples of thigmonasty include many species in the leguminous subfamily Mimosoideae, active carnivorous plants such as Dionaea and a wide range of pollination mechanisms.
Guard cells are specialized cells in the epidermis of leaves, stems and other organs of land plants that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells become turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.
Plant perception is the ability of plants to sense and respond to the environment by adjusting their morphology and physiology. Botanical research has revealed that plants are capable of reacting to a broad range of stimuli, including chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch. The scientific study of plant perception is informed by numerous disciplines, such as plant physiology, ecology, and molecular biology.
A pulvinus may refer to a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent movement. Pulvinus is also a botanical term for the persistent peg-like bases of the leaves in the coniferous genera Picea and Tsuga. Pulvinar movement is common, for example, in members of the bean family Fabaceae (Leguminosae) and the prayer plant family Marantaceae.
Mechanosensation is the transduction of mechanical stimuli into neural signals. Mechanosensation provides the basis for the senses of light touch, hearing, proprioception, and pain. Mechanoreceptors found in the skin, called cutaneous mechanoreceptors, are responsible for the sense of touch. Tiny cells in the inner ear, called hair cells, are responsible for hearing and balance. States of neuropathic pain, such as hyperalgesia and allodynia, are also directly related to mechanosensation. A wide array of elements are involved in the process of mechanosensation, many of which are still not fully understood.
Mechanosensitive channels (MSCs), mechanosensitive ion channels or stretch-gated ion channels are membrane proteins capable of responding to mechanical stress over a wide dynamic range of external mechanical stimuli. They are present in the membranes of organisms from the three domains of life: bacteria, archaea, and eukarya. They are the sensors for a number of systems including the senses of touch, hearing and balance, as well as participating in cardiovascular regulation and osmotic homeostasis (e.g. thirst). The channels vary in selectivity for the permeating ions from nonselective between anions and cations in bacteria, to cation selective allowing passage Ca2+, K+ and Na+ in eukaryotes, and highly selective K+ channels in bacteria and eukaryotes.
A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of stimuli. Although, in some cultures, five human senses were traditionally identified as such, many more are now recognized. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.
Electrocochleography is a technique of recording electrical potentials generated in the inner ear and auditory nerve in response to sound stimulation, using an electrode placed in the ear canal or tympanic membrane. The test is performed by an otologist or audiologist with specialized training, and is used for detection of elevated inner ear pressure or for the testing and monitoring of inner ear and auditory nerve function during surgery.
Plant bioacoustics refers to the creation of sound waves by plants. Measured sound emissions by plants as well as differential germination rates, growth rates and behavioral modifications in response to sound are well documented. Plants detect neighbors by means other than well-established communicative signals including volatile chemicals, light detection, direct contact and root signaling. Because sound waves travel efficiently through soil and can be produced with minimal energy expenditure, plants may use sound as a means for interpreting their environment and surroundings. Preliminary evidence supports that plants create sound in root tips when cell walls break. Because plant roots respond only to sound waves at frequencies which match waves emitted by the plants themselves, it is likely that plants can receive and transduce sound vibrations into signals to elicit behavioral modifications as a form of below ground communication.
Plants are exposed to many stress factors such as disease, temperature changes, herbivory, injury and more. Therefore, in order to respond or be ready for any kind of physiological state, they need to develop some sort of system for their survival in the moment and/or for the future. Plant communication encompasses communication using volatile organic compounds, electrical signaling, and common mycorrhizal networks between plants and a host of other organisms such as soil microbes, other plants, animals, insects, and fungi. Plants communicate through a host of volatile organic compounds (VOCs) that can be separated into four broad categories, each the product of distinct chemical pathways: fatty acid derivatives, phenylpropanoids/benzenoids, amino acid derivatives, and terpenoids. Due to the physical/chemical constraints most VOCs are of low molecular mass, are hydrophobic, and have high vapor pressures. The responses of organisms to plant emitted VOCs varies from attracting the predator of a specific herbivore to reduce mechanical damage inflicted on the plant to the induction of chemical defenses of a neighboring plant before it is being attacked. In addition, the host of VOCs emitted varies from plant to plant, where for example, the Venus Fly Trap can emit VOCs to specifically target and attract starved prey. While these VOCs typically lead to increased resistance to herbivory in neighboring plants, there is no clear benefit to the emitting plant in helping nearby plants. As such, whether neighboring plants have evolved the capability to "eavesdrop" or whether there is an unknown tradeoff occurring is subject to much scientific debate. As related to the aspect of meaning-making, the field is also identified as phytosemiotics.
In plant biology, plant memory describes the ability of a plant to retain information from experienced stimuli and respond at a later time. For example, some plants have been observed to raise their leaves synchronously with the rising of the sun. Other plants produce new leaves in the spring after overwintering. Many experiments have been conducted into a plant's capacity for memory, including sensory, short-term, and long-term. The most basic learning and memory functions in animals have been observed in some plant species, and it has been proposed that the development of these basic memory mechanisms may have developed in an early organismal ancestor.
Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.
Calcium signaling in Arabidopsis is a calcium mediated signalling pathway that Arabidopsis plants use in order to respond to a stimuli. In this pathway, Ca2+ works as a long range communication ion, allowing for rapid communication throughout the plant. Systemic changes in metabolites such as glucose and sucrose takes a few minutes after the stimulus, but gene transcription occurs within seconds. Because hormones, peptides and RNA travel through the vascular system at lower speeds than the plants response to wounds, indicates that Ca2+ must be involved in the rapid signal propagation. Instead of local communication to nearby cells and tissues, Ca2+ uses mass flow within the vascular system to help with rapid transport throughout the plant. Ca2+ moving through the xylem and phloem acts through a “calcium signature” receptor system in cells where they integrate the signal and respond with the activation of defense genes. These calcium signatures encode information about the stimulus allowing the response of the plant to cater towards the type of stimulus.
