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Archived Newsletters 2010-08-30 Senses Senses - There is no firm
agreement among neurologists as to the number of senses because of differing
definitions of what constitutes a 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.The traditional five senses
are sight, hearing, touch, smell and taste, a classification attributed to
Aristotle. 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), 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,
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
Sight
Sight or vision is the ability of the
brain and eye to detect electromagnetic waves within the visible range of
(light) and, in some cases, determine between varying colors, hues, and
brightness. There is some 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 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, 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 that "buds"
(receptors) on the tongue detect, and hence there are anatomists who argue that
these constitute five or more different senses, given that each receptor
conveys information to a slightly different region of the brain. The inability to taste
is called ageusia.
The four well-known receptors detect sweet, salty,
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. 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.
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. 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, theheat 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.
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.
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. One study has found that cattle make use
of magnetoception, as they tend to align themselves in a north-south direction.
Magnetotactic bacteria build miniature magnets inside themselves and use them
to determine their orientation relative to the Earth's magnetic field.
Other
internal senses
An internal sense or interoception is
"any sense that is normally stimulated from within the body". 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
Most non-human mammals have a much keener sense of
smell than humans, although the mechanism is similar. Sharks combine their keen
sense of smell with timing to determine the direction of a smell. They follow
the nostril that first detected the smell. Insects have olfactory receptors on
their antennae.
Vomeronasal
organ
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 to mark their territory, trails, and sexual
state. Reptiles like snakes and monitor lizards make extensive use of it as a
smelling organ, transferring scent molecules to the vomeronasal organ with the
tips of the forked tongue. 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.
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. 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 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:
The
only order of mammals that is known to demonstrate electroception is the
monotreme order. Among these mammals, the platypus has the most acute sense of
electroception.
Body
modification enthusiasts have experimented with magnetic implants to attempt to
replicate this sense, 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.
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.
Culture
The five senses are enumerated as the "five
material faculties" (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.
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