sense
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{{this|the empirical or physical senses of living organisms (vision, taste, etc.)}}
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 system has a specific
sensory system, or organ, dedicated to each sense.
Definition
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.
(1) The traditional
five senses are sight, hearing, touch, smell and taste, a classification attributed to
Aristotle.
(2) Humans are considered to have at least five additional senses that include:
nociception (pain);
equilibrioception (balance);
proprioception and
kinaesthesia (joint motion and acceleration);
sense of time;
thermoception (temperature differences); and possibly an additional weak
magnetoception (direction)
(3), and six more if interoceptive senses (see
other internal senses below) are also considered.One commonly recognized categorisation for human senses is as follows:
chemoreception;
photoreception;
mechanoreception; and
thermoception. This categorisation has been criticized as too restrictive,{{By whom|date=March 2010}} however, as it does not include categories for accepted senses such as the
sense of time and sense of
pain.Non-human animals may possess senses that are absent in humans, such as
electroreception and detection of
polarized light.A broadly acceptable definition of a sense would be "A system that consists of a group of 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 typically arise around the classification of the various cell types and their
mapping to regions of the brain.
Senses
{{Citations missing|date=March 2008}}
Sight
Sight or
vision is the ability of the brain and eye to detect
electromagnetic waves within the visible range (
light) which is why people see 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 argue{{Citation needed|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
Hearing or
audition is the sense of
sound perception. 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 because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the
inner ear which detect mechanical motion of the fibers within a range of about 20 to 20,000
hertz,
(4) with substantial variation between individuals. Hearing at high frequencies declines with age. Sound can also be detected as vibrations conducted through the body by tactition. Lower frequencies than that can be heard are detected this way. The inability to hear is called
deafness.
Taste
Taste or
gustation is one of the two main "chemical" senses. There are at least four types of tastes
(5) that "buds" (receptors) on the
tongue detect, and hence there are anatomists who argue{{Citation needed|date=March 2008}} that these constitute five or more different senses, given that each receptor conveys information to a slightly different region of the brain{{Citation needed|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
(6). The umami receptor detects the
amino acid glutamate, 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
smell of a food as well as its taste.
Smell
Smell or
olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature. Odor 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
nose differ from most other neurons in that they die and regenerate on a regular basis. The inability to smell is called
anosmia. Some neurons in the nose are specialized to detect
pheromones.{{Citation needed|date=January 2010}}
Touch
Touch, also called
tactition or
mechanoreception, is a perception resulting from activation of neural receptors, generally in the
skin including
hair follicles, but also in the
tongue,
throat, and
mucosa. 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.
(7) The loss or impairment of the ability to feel anything touched is called tactile
anesthesia.
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 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 equilibrium and 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 of
angular momentum 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.
Temperature
Thermoception is the sense of
heat and the absence of heat (
cold) by the
skin and including 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 to heat. The cold receptors play an important part in the dogs 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.
Kinesthetic sense
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.
(8)Pain
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 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.
(9)Direction
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, though it is also present to a limited extent in humans. 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.
(10) One study has found that cattle make use of magnetoception, as they tend to align themselves in a north-south direction.
(11) Magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.{{Citation needed|21 July 2008|date=July 2008}}
Other internal senses
An
internal sense or interoception is "any sense that is normally stimulated from within the body".
(12) These involve numerous sensory receptors in internal organs, such as
stretch receptors that are neurologically linked to the brain.
Non-human senses
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.
Echolocation
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 by interpreting reflected sounds (esp. their own footsteps), a phenomenon which is known as
human echolocation.
Smell
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. Some animals have a
vomeronasal organ which is mainly used to detect
pheromones; the organ is
vestigial in humans and separate from the main olfactory system, analogous to the human sense of smell.
Vision
Cats have the ability to see in low light 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
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.
(13) It has been found that
birds and some other animals are
tetrachromats and have the ability to see in the
ultraviolet down to 300 nanometers.
Bees and
dragonflies(14) are also able to see in the ultraviolet.
Balance
Ctenophora have 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) 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
monotreme order. Among these mammals, the
platypus(15) has the most acute sense of electroception.
Body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense,
(16) 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.
- Pressure detection uses the organ of Weber, a system consisting of three appendages of vertebrae transferring changes in shape of the gas bladder to the middle ear. It can be used to regulate the buoyancy of the fish. Fish like the weather fish and other loaches are also known to respond to low pressure areas but they lack a swim bladder.
- Current detection The lateral line in fish and aquatic forms of amphibians is a detection system of water currents, mostly consisting of vortices. The lateral line is also sensitive to low frequency vibrations. The mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing. It is used primarily for navigation, hunting, and schooling. The receptors of the electrical sense are modified hair cells of the lateral line system.
- Polarized light direction/detection is used by bees to orient themselves, especially on cloudy days. Cuttlefish can also perceive the polarization of light. Most sighted humans can in fact learn to roughly detect large areas of polarization by an effect called Haidinger's brush, however this is considered an entoptic phenomenon rather than a separate sense.
- Slit sensillae of spiders detect mechanical strain in the exoskeleton, providing information on force and vibrations.
Plant senses
Some plants have sensory organs, for example the
Venus fly trap, that respond to vibration, light, water, scents, or other specific chemicals. Some plants sense the location of other plants and attack and eat part of them.
(17)Culture
{{See|Ṣaḍāyatana|Ayatana|Indriya}}The five senses are enumerated as the "five material faculties" (
{{IAST|pañcannaṃ indriyānaṃ avakanti}}) in Buddhist literature. They appear in allegorical representation as early as in the
Katha Upanishad (roughly 6th century BC), as five horses drawing the "
chariot" of the body, guided by the mind as "chariot driver".Depictions of the five senses as
allegory became a popular subject for seventeenth-century artists, especially among
Dutch and
Flemish Baroque painters. A typical example is
Gérard de Lairesse's
Allegory of the Five Senses (1668), in which each of the figures in the main group allude to a sense: sight is the reclining boy with a
convex mirror, hearing is the
cupid-like boy with a
triangle, smell is represented by the girl with flowers, taste by the woman with the fruit and touch by the woman holding the bird.
See also
{{Commons category|Senses}}{{Commons category|Allegories of senses}}{{Wikiversity|What is the sixth sense}}
Research centers
References
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[Senses]
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[JewishEncyclopedia.com - Senses, the five]
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[Magnetic fields and the central nervous system, Clinical Neurophysiology, Volume 111, Issue 11, Pages 1934 - 1935, A . Voustianiouk]
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[Frequency Range of Human Hearing, Physics Factbook by Glenn Elert (ed)]
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[The Sense of Taste]
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[Press Releases - Nature Neuroscience]
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[Science, August 6, 2009, Zhou-Feng Chen, et al. ]
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[Robles-De-La-Torre 2006]
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[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]
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[The Magnetic Sense of Animals]
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[BBC science news article]
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[Dorland's Medical Dictionary 26th edition, under sense]
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[WEB,weblink The illustrated story of the Vampire bat, 2007-05-25, ]
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[WEB,weblink Directional Selectivity in the Simple Eye of an Insect, 2009-05-20, ]
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[weblink" title="web.archive.org/web/19981206164009weblink">Electroreception in the Platypus]
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[WEB,weblink Implant gives artist the sense of "magnetic vision", 2007-05-25, ]
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["No brainer Behavior", Susan Milius, Science News, June 20, 2009, vol 175, no 13, pages 16-19. weblink]
External links
{{Sensory system}}
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