The Sweetest Scent (Senses and Sensations)
A variety of pressure receptors respond to variations in pressure firm, brushing, sustained, etc. The touch sense of itching caused by insect bites or allergies involves special itch-specific neurons in the skin and spinal cord. Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary. Balance , equilibrioception , or vestibular sense is the sense that allows an organism to sense body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance.
The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears. In technical terms, this organ is responsible for two senses of angular momentum acceleration and linear acceleration which also senses gravity , but they are known together as equilibrioception. The vestibular nerve conducts information from sensory receptors in three ampulla that sense motion of fluid in three semicircular canals caused by three-dimensional rotation of the head. The vestibular nerve also conducts information from the utricle and the saccule , which contain hair-like sensory receptors that bend under the weight of otoliths which are small crystals of calcium carbonate that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force.
Thermoception is the sense of heat and the absence of heat cold by the skin and internal skin passages, or, rather, the heat flux the rate of heat flow in these areas. There are specialized receptors for cold declining temperature and for heat increasing temperature. The cold receptors play an important part in the animal's sense of smell, telling wind direction.
The heat receptors are sensitive to infrared radiation and can occur in specialized organs, for instance in pit vipers. The thermoceptors in the skin are quite different from the homeostatic thermoceptors in the brain hypothalamus , which provide feedback on internal body temperature. Proprioception , the kinesthetic sense , provides the parietal cortex of the brain with information on the movement and relative positions of the parts of the body.
Neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in surprising and deep deficits in perception and action. Nociception physiological pain signals nerve-damage or damage to tissue.
The three types of pain receptors are cutaneous skin , somatic joints and bones , and visceral body organs.
Sensory Perception – Anatomy and Physiology
It was previously believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century indicated that pain is a distinct phenomenon that intertwines with all of the other senses, including touch. Pain was once considered an entirely subjective experience, but recent studies show that pain is registered in the anterior cingulate gyrus of the brain.
For example, humans avoid touching a sharp needle, or hot object, or extending an arm beyond a safe limit because it is dangerous, and thus hurts. Without pain, people could do many dangerous things without being aware of the dangers.
Sensational scents: a sensory journey to the 17th century
Magnetoception or magnetoreception is the ability to detect the direction one is facing based on the Earth's magnetic field. Directional awareness is most commonly observed in birds , which rely on their magnetic sense to navigate during migration.
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Cattle make use of magnetoception to align themselves in a north-south direction. Sexual stimulation is any stimulus including bodily contact that leads to, enhances and maintains sexual arousal , and may lead to orgasm. Distinct from the general sense of touch, sexual stimulation is strongly tied to hormonal activity and chemical triggers in the body.
Although sexual arousal may arise without physical stimulation , achieving orgasm usually requires physical sexual stimulation stimulation of the Krause-Finger corpuscles  found in erogenous zones of the body. An internal sense also known as interoception  is "any sense that is normally stimulated from within the body". Interoception is thought to be atypical in clinical conditions such as alexithymia.
Chronoception refers to how the passage of time is perceived and experienced. Although the sense of time is not associated with a specific sensory system , the work of psychologists and neuroscientists indicates that human brains do have a system governing the perception of time ,   composed of a highly distributed system involving the cerebral cortex , cerebellum and basal ganglia.
One particular component, the suprachiasmatic nucleus , is responsible for the circadian or daily rhythm , while other cell clusters appear to be capable of shorter-range ultradian timekeeping. One or more dopaminergic pathways in the central nervous system appear to have a strong modulatory influence on mental chronometry , particularly interval timing.
The sense of agency refers to the subjective feeling of having chosen a particular action.
Some conditions, such as schizophrenia, can lead to a loss of this sense, causing a person to feel like a machine or even leading to delusions of being controlled from some outside source. The opposite extreme occurs too, with some people experiencing everything in their environment as if they had decided that it would happen. Even in non-pathological cases, there is a measurable difference between making a decision and the feeling of agency. Through methods such as the Libet experiment , a gap of half a second or more can be detected from the time when there are detectable neurological signs of a decision having been made to the time when the subject actually becomes conscious of the decision.
There are also experiments in which an illusion of agency is induced in psychologically normal subjects. In Wegner and Wheatley , subjects were given instructions to move a mouse around a scene and point to an image about once every thirty seconds. However, a second person—acting as a test subject but actually a confederate—had their hand on the mouse at the same time, and controlled some of the movement.
Experimenters were able to arrange for subjects to perceive certain "forced stops" as if they were their own choice. Recognition memory is sometimes divided into two functions by neuroscientists: familiarity and recollection. The temporal lobe, in particular the perirhinal cortex , responds differently to stimuli which feel novel than to things which feel familiar.
Firing rates in the perirhinal cortex are connected with the sense of familiarity in humans and other mammals. Recent studies on lesions in the area concluded that rats with a damaged perirhinal cortex were still more interested in exploring when novel objects were present, but seemed unable to tell novel objects from familiar ones—they examined both equally. Thus, other brain regions are involved with noticing unfamiliarity, but the perirhinal cortex is needed to associate the feeling with a specific source.
Other living organisms have receptors to sense the world around them, including many of the senses listed above for humans.
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However, the mechanisms and capabilities vary widely. An example of smell in non-mammals is that of sharks , which combine their keen sense of smell with timing to determine the direction of a smell. They follow the nostril that first detected the smell. Although it is unknown to the degree and magnitude which non-human animals can smell better than humans. Many animals salamanders , reptiles , mammals have a vomeronasal organ  that is connected with the mouth cavity.
In mammals it is mainly used to detect pheromones of marked territory, trails, and sexual state. Reptiles like snakes and monitor lizards make extensive use of it as a smelling organ by transferring scent molecules to the vomeronasal organ with the tips of the forked tongue. In reptiles the vomeronasal organ is commonly referred to as Jacobsons organ. In mammals, it is often associated with a special behavior called flehmen characterized by uplifting of the lips.
The organ is vestigial in humans , because associated neurons have not been found that give any sensory input in humans. Flies and butterflies have taste organs on their feet, allowing them to taste anything they land on. Catfish have taste organs across their entire bodies, and can taste anything they touch, including chemicals in the water.
Cats have the ability to see in low light, which is due to muscles surrounding their irides —which contract and expand their pupils—as well as to the tapetum lucidum , a reflective membrane that optimizes the image. Pit vipers , pythons and some boas have organs that allow them to detect infrared light, such that these snakes are able to sense the body heat of their prey.
The common vampire bat may also have an infrared sensor on its nose. Bees and dragonflies  are also able to see in the ultraviolet. Mantis shrimps can perceive both polarized light and multispectral images and have twelve distinct kinds of color receptors, unlike humans which have three kinds and most mammals which have two kinds. Cephalopods have the ability to change color using chromatophores in their skin. Researchers believe that opsins in the skin can sense different wavelengths of light and help the creatures choose a coloration that camouflages them, in addition to light input from the eyes.
Many invertebrates have a statocyst , which is a sensor for acceleration and orientation that works very differently from the mammalian's semi-circular canals. Some plants such as mustard have genes that are necessary for the plant to sense the direction of gravity. If these genes are disabled by a mutation, a plant cannot grow upright. Certain animals, including bats and cetaceans , have the ability to determine orientation to other objects through interpretation of reflected sound like sonar.
They most often use this to navigate through poor lighting conditions or to identify and track prey. There is currently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or it actually constitutes a separate sense. Resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice.
Blind people report they are able to navigate and in some cases identify an object by interpreting reflected sounds especially their own footsteps , a phenomenon known as human echolocation.
Electroreception or electroception is the ability to detect electric fields. Several species of fish, sharks , and rays have the capacity to sense changes in electric fields in their immediate vicinity. For cartilaginous fish this occurs through a specialized organ called the Ampullae of Lorenzini. Some fish passively sense changing nearby electric fields; some generate their own weak electric fields, and sense the pattern of field potentials over their body surface; and some use these electric field generating and sensing capacities for social communication.
The mechanisms by which electroceptive fish construct a spatial representation from very small differences in field potentials involve comparisons of spike latencies from different parts of the fish's body. The only orders of mammals that are known to demonstrate electroception are the dolphin and monotreme orders. Among these mammals, the platypus  has the most acute sense of electroception. A dolphin can detect electric fields in water using electroreceptors in vibrissal crypts arrayed in pairs on its snout and which evolved from whisker motion sensors.
This permits the dolphin to locate prey from the seafloor where sediment limits visibility and echolocation. Spiders have been shown to detect electric fields to determine a suitable time to extend web for 'ballooning'. Body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense.
An electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and identified as coming from a static charge and not from wind or the like. This is not electroreception, as it is a post-sensory cognitive action. Hygroreception is the ability to detect changes in the moisture content of the environment. The ability to sense infrared thermal radiation evolved independently in various families of snakes. Most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril Loreal pit , while boas and pythons have three or more comparatively smaller pits lining the upper and sometimes the lower lip, in or between the scales.
Those of the pitvipers are the more advanced, having a suspended sensory membrane as opposed to a simple pit structure.