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There is no firm agreement amongst neurologists as to exactly how many senses there are. The disagreements stem from a lack of consensus as to what the definition of a sense should be. Although school children are still routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification first devised by Aristotle), it is generally agreed that there are at least nine different senses in humans, and a minimum of two more observed in other organisms.
A broadly acceptable definition of a sense would be "a system that consists of a sensory cell type (or group of cell types) that respond to a specific kind of physical energy, and that correspond to a defined region (or group of regions) within the brain where the signals are received and interpreted". Where disputes arise is with regard to the exact classification of the various cell types and their mapping to regions of the brain.
Using this definition several senses can be identified. This list begins with those five senses defined by Aristotle and hence probably most familiar to the reader.
Seeing or vision describes the ability to detect light by the eye and the brain to interpret the image as "sight". There is disagreement as to whether or not this constitutes one, two or even three distinct senses. Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of colour (the frequency of light) and brightness (the energy of light). Some argue that the perception of depth also constitutes a sense, but it is generally regarded that this is really a cognitive (that is, post-sensory) function derived from having stereoscopic vision (two eyes) and is not a sensory perception as such.
Hearing or audition is the sense of sound perception and results from tiny hair fibres in the inner ear detecting the motion of atmospheric particles within (at best) a range of 20 to 20000 Hz. Sound can also be detected as vibration by tactition. Lower and higher frequencies than can be heard are detected this way only.
Taste or gustation is one of the two "chemical" senses. It is well-known that there are at least four types of taste "bud" (receptor) on the tongue and hence, as should now be expected, there are anatomists who argue that these in fact constitute four or more different senses, given that each receptor conveys information to a slightly different region of the brain.
The four well-known receptors detect sweet, salt, sour, and bitter, although the receptors for sweet and bitter have not been conclusively identified. A fifth receptor, for a sensation called " umamiUmami is the name for the taste sensation produced by some substances--more specifically, monosodium glutamate, which is an amino acid. This is one of the five basic tastes. It was discovered in 1907 by Kikunae Ikeda. This flavor is considered basic in Ja", was first theorised in 1908 and its existence confirmed in 2000 (see [1]). The umami receptor detects the amino acidIn chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. In biochemistry, this shorter and more general term is frequently used to refer to alpha amino acids: those amino acids in which the amino and carb glutamate, a flavor commonly found in meat, and in artificial flavourings such as monosodium glutamate.
SmellOlfaction the sense of smell, is the detection of chemicals dissolved in air (or, by animals that breathe water, in water). In vertebrates smells are sensed by the olfactory epithelium located in the nose and processed by the olfactory system. How smell w or olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature, according to current theory. The combination of features of the odor molecule makes up what we perceive as the molecule's smell. In the brain, olfaction is processed by the olfactory systemThe olfactory system is the sensory system used for olfaction. The accessory olfactory system senses pheromones. The olfactory system is often spoken of along with the gustatory system as the chemosensory senses because both transduce chemical signals int. Olfactory receptor neuronAn olfactory receptor neuron also called an olfactory sensory neuron is the primary transduction cell for olfaction in the olfactory system. Humans have about 40 million olfactory receptor neurons. Olfactory receptor neurons reside on the olfactory epithes in the noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration. In most mammals, it also houses the nosehairs, which catch airborne particles and prevent them from reaching the lungs. differ from most other neurons in that they die and regenerate on a regular basis.
Note that in aquatic organisms there is essentially no difference between smell and taste.
The 2004 Nobel PrizeThe Nobel Prizes (pronounced no-BELL or no-bell are awarded annually to people who have done outstanding research, invented groundbreaking techniques or equipment, or made outstanding contributions to society. It is generally regarded as the supreme comme in Physiology or MedicineList of Nobel Prize laureates in Physiology or Medicine from 1901 to the present day. 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s External links http://www. se/medicin (announced 4 October 2004) was won by Richard Axel and Linda Buck for their work explaining olfaction, published first in a joint paper in 1991 that described the very large family of about one thousand genes for odorant receptors and how the receptors link to the brain.
The remaining senses can be considered types of touch or physical feeling.
Tactition is the sense of pressure perception, generally in the skin.
Thermoception is the sense of heat and the absence of heat (cold), also by the skin and including internal skin passages. It is also the first of the group of senses not identified explicitly by Aristotle. Again there is some disagreement about how many senses this actually represents--the thermoceptors in the skin are quite different from the homeostatic thermoceptors which provide feedback on internal body temperature. How warm or cold something feels does not only depend on temperature, but also on specific heat capacity and heat conductance; e.g., warm metal feels warmer than warm wood, and cold metal feels colder than cold wood, because metal has a higher thermal conductivity than wood. Wind feels cold because of the heat withdrawn for evaporation of sweat or other moisture, and because an isolating layer of warm air around the body blows away; however, in the case of hot air, wind makes it feel hotter, for a similar reason as the latter.
Nociception is the perception of pain. It can be classified as from one to three senses, depending on the classification method. The three types of pain receptors are cutaneous (skin), somatic (joints and bones) and visceral (body organs).
Equilibrioception is the perception of balance and is related to cavities containing fluid in the inner ear. There is some disagreement as to whether or not this also includes the sense of "direction" or orientation. However, as with depth perception earlier, it is generally regarded that "direction" is a post-sensory cognitive awareness.
Proprioception is the perception of body awareness and is a sense that people rely on enormously, yet are frequently not aware of. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. (This can be demonstrated by anyone closing their eyes and waving their hand around. 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).
Based on this outline and depending on the chosen method of classification, somewhere between 9 and 21 human senses have been identified. Additionally, there are some other candidate physiological experiences which may or may not fall within the above classification (for example the sensory awareness of hunger and thirst).