Senses are the physiological methods of
perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology(or cognitive science), and philosophy of perception. The nervous systemhas a specific sensory system, or organ, dedicated to each sense.
Definition of sense
There is no firm agreement among neurologists as to the number of senses because of differing definitions of what constitutes a sense. One definition states that an exteroceptive sense is a faculty by which outside stimuli are perceived. [ [http://www.eioba.com/a70192/senses Senses ] ] The traditional five senses are sight, hearing, touch, smell, taste: a classification attributed to
Aristotle. [ [http://www.jewishencyclopedia.com/view.jsp?artid=479&letter=S JewishEncyclopedia.com - SENSES, THE FIVE ] ] Humans also have at least six additional senses (a total of eleven including interoceptive senses) that include: nociception(pain), equilibrioception(balance), proprioception& kinesthesia(joint motion and acceleration), sense of time, thermoception(temperature differences), and in some a weak magnetoception(direction) [Magnetic fields and the central nervous system, "Clinical Neurophysiology", Volume 111, Issue 11, Pages 1934 - 1935, A . Voustianiouk] .
One commonly recognized catagorisation for human senses is as follows:
chemoreception; photoreception; mechanoreception; and thermoception. Indeed, all human senses fit into one of these four categories.
Different senses also exist in other organisms, for example
A broadly acceptable definition of a sense would be "a system that consists of a group sensory cell types that responds to a specific physical phenomenon, and that corresponds to a particular group of regions within the brain where the signals are received and interpreted." Disputes about the number of senses arise typically regarding the classification of the various cell types and their mapping to regions of the brain.
Sight or vision is the ability of the brain and eye to detect electromagnetic waves within the visible range (
light) interpreting the image as "sight." There is disagreement as to whether this constitutes one, two or three senses. Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of colour (the frequency of photons of light) and brightness (amplitude/intensity - number of photons of light). Some argueFact|date=December 2007 that stereopsis, the perception of depth, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of brain to interpret sensory input and to derive new information. The inability to see is called blindness.
Hearing or audition is the sense of
soundperception. Since sound is vibrations propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense akin to a sense of touch, albeit a very specialized one. In humans, this perception is executed by tiny hair fibres in the inner earwhich detect the motion of a membrane which vibrates in response to changes in the pressure exerted by atmospheric particles within a range of 20 to 22000 Hz, with substantial variation between individuals. Sound can also be detected as vibrations conducted through the body by tactition. Lower and higher frequencies than that can be heard are detected this way only. The inability to hear is called deafness.
Tasteor gustation is one of the two main "chemical" senses. There are at least four types of tastes [ [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.html The Sense of Taste] ] that "buds" (receptors) on the tonguedetect, and hence there are anatomists who argueFact|date=March 2008 that these constitute five or more different senses, given that each receptor conveys information to a slightly different region of the brainFact|date=March 2008. The inability to taste is called ageusia.
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 "
umami", was first theorised in 1908 and its existence confirmed in 2000 [ [http://www.nature.com/neuro/press_release/nn0200.html Press Releases - Nature Neuroscience ] ] . The umami receptor detects the amino acidglutamate, a flavour commonly found in meat and in artificial flavourings such as monosodium glutamate.
Note that taste is not the same as
flavour; flavour includes the smellof a food as well as its taste.
Smell or olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature. Odour molecules possess a variety of features and thus excite specific receptors more or less strongly. This combination of excitatory signals from different receptors makes up what we perceive as the molecule's smell. In the brain, olfaction is processed by the
olfactory system. Olfactory receptor neurons in the nosediffer from most other neurons in that they die and regenerate on a regular basis. The inability to smell is called anosmia.
Touch, also called
tactition, mechanoreception or somatic sensation, is the sense of pressure perception, generally in the skin. There are a variety of nerve endings that respond to variations in pressure (e.g., firm, brushing, and sustained). The inability to feel anything or almost anything is called anesthesia. Paresthesiais a sensationof tingling, pricking, or numbnessof a person's skinwith no apparent long term physical effect.
Balance and acceleration
Balance, Equilibrioception, or vestibular sense, is the sense which allows an organism to sense body movement, direction, and acceleration, and to attain and maintain postural equilibriumand balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears. Technically this organ is responsible for two senses, angular momentumand linear acceleration(which also senses gravity), but they are known together as equilibrioception.
vestibular nerveconducts information from the three semicircular canals, corresponding to the three spatial planes, the utricle, and the saccule. The ampulla, or base, portion of the three semicircular canals each contain a structure called a crista. These bend in response to angular momentum or spinning. The saccule and utricle, also called the " otolithorgans", sense linear acceleration and thus gravity. Otoliths are small crystals of calcium carbonatethat provide the inertia needed to detect changes in acceleration or gravity.
Thermoceptionis the sense of heat and the absence of heat (cold) by the skinand including internal skin passages. 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 relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch the tip of a finger to their nose. 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. [http://www.roblesdelatorre.com/gabriel/GR-IEEE-MM-2006.pdf Robles-De-La-Torre 2006]
Nociception (physiological pain) signals near-damage or damage to tissue. The three types of pain receptors are cutaneous (skin), somatic (joints and bones) and visceral (body organs). It was 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 gyrusof the brain. [ [http://www.ajronline.org/cgi/content/full/177/5/1205 Functional MR Imaging of Regional Brain Activation Associated with the Affective Experience of Pain -- Robert K. Fulbright et al., American Journal of Roentgenology, 2001; vol. 177, pages 1205-1210] ]
Other internal senses
An internal sense or interoception is "any sense that is normally stimulated from within the body." [Dorland's Medical Dictionary 26th edition, under sense] These involve numerous sensory receptors in internal organs, such as
stretch receptors that are neurologically linked to the brain.
Pulmonary stretch receptorsare found in the lungs and control the respiratory rate.
Cutaneous receptors in the skin not only respond to touch, pressure, and temperature, but also respond to vasodilation in the skin such as blushing.
* Stretch receptors in the
gastrointestinal tractsense gas distension that may result in colic pain.
* Stimulation of sensory receptors in the esophagus result in sensations felt in the throat when
swallowing, vomiting, or during acid reflux.
* Sensory receptors in
pharynxmucosa, similar to touch receptors in the skin, sense foreign objects such as food that may result in a gaggingreflex and corresponding gagging sensation.
* Stimulation of sensory receptors in the
urinary bladderand rectummay result in sensations of fullness.
* Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries.
Analogous to human senses
Other living organisms have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely.
Among non-human species,
dogs have a much keener sense of smell than humans, although the mechanism is similar. Insects have olfactory receptors on their antennae.
Catshave the ability to see in the dark due to muscles surrounding their irises to contract and expand pupils as well as the tapetum lucidum, a reflective membrane that optimizes the image.
Pitvipers, pythons and some boas have organs that allow them to detect
infraredlight, such that these snakes are able to sense the body heat of their prey. The common vampire batmay also have an infrared sensor on its nose. [cite web|url=http://www.pitt.edu/AFShome/s/l/slavic/public/html/courses/vampires/images/bats/vambat.html|title=The illustrated story of the Vampire bat|accessdate=2007-05-25] It has been found that birdsand some other animals are tetrachromatsand have the ability to see in the ultravioletdown to 300 nanometers. Beesare also able to see in the ultraviolet.
Ctenophoreshave a balance receptor (a statocyst) that works very differently from the mammalian's semi-circular canals.
Not analogous to human senses
In addition, some animals have senses that humans do not, including the following:
Electroception(or "electroreception"), the most significant of the non-human senses, 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. 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 order of mammals that is known to demonstrate electroception is the
monotremeorder. Among these mammals, the platypus[ [http://web.archive.org/web/19981206164009/http://instruct1.cit.cornell.edu/courses/bionb420.07/anelson/platypus.html Electroreception in the Platypus ] ] has the most acute sense of electroception.
Body modificationenthusiasts have experimented with magnetic implants to attempt to replicate this sense, [cite web|url=http://www.boingboing.net/2005/05/05/implant_gives_artist.html|title=Implant gives artist the sense of "magnetic vision" |accessdate=2007-05-25] however in general humans (and probably other mammals) can detect electric fields only indirectly by detecting the effect they have on hairs. 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 however not electroception as it is a post-sensory cognitive action.
*Echolocation is the ability to determine orientation to other objects through interpretation of reflected sound (like
sonar). Bats and cetaceans are noted for this ability, though some other animals use it, as well. It is most often used 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 by interpreting reflected sounds (esp. their own footsteps), a phenomenon which is known as Human echolocation.
Magnetoception(or "magnetoreception") is the ability to detect fluctuations in magnetic fields and is most commonly observed in birds, though it has also been observed in insects such as bees. Although there is no dispute that this sense exists in many avians (it is essential to the navigational abilities of migratory birds), it is not a well-understood phenomenon [ [http://www.ks.uiuc.edu/Research/magsense/ms.html The Magnetic Sense of Animals ] ] . One study has found that cattle make use of magnetoception, as they tend to align themselves in a North-South direction [ [http://news.bbc.co.uk/1/hi/sci/tech/7575459.stm BBC science news article] ] . Magnetotactic bacteriabuild miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.Fact|21 July 2008|date=July 2008
*Pressure detection uses the
lateral line, which is a pressure-sensing system of hairs found in fish and some aquatic amphibians. It is used primarily for navigation, hunting, and schooling. Humans have a basic relative-pressure detection ability when eustachian tube(s) are blocked, as demonstrated in the ear's response to changes in altitude.
Polarized lightdirection / detection is used by beesto orient themselves, especially on cloudy days. Cuttlefishcan also perceive the polarization of light.
Auditory illusion, Optical illusion, Touch illusion
Sense of time
Howard Hughes Medical Institute(HHMI)
Institute for Advanced Science & Engineering(IASE)
* The 2004
Nobel Prizein Physiology or Medicine ( [http://nobelprize.org/medicine/laureates/2004/index.html announced] 4 October 2004) was won by Richard Axeland Linda Buckfor 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.
* [http://www.hhmi.org/senses/ HHMI - World renowned centre of medical excellence]
* [http://www.senses.info IASE - senses.info] - A research center that focuses on the basis of senses.
* [http://www.scribd.com/doc/32909/Sensing-or-feeling/ Answers to several questions related to senses and human feeling from curious kids]
* [http://www.physpharm.fmd.uwo.ca/undergrad/sensesweb The Physiology of the Senses tutorial] 12 animated chapters on vision, hearing, touch, balance and memory.
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