Wednesday, May 22, 2013

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Humans have 9 senses not 5

1. Seeing or vision describes the ability to detect light and interpret it 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. 

2. 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. 

3. 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) 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 "umami", was first theorised in 1908 
and its existence confirmed in 2000. The umami receptor detects 
the amino acid glutamate, a flavor commonly found in meat, and in artificial 
flavourings such as monosodium glutamate. 

4. Smell or olfaction is the other "chemical" sense. Olfactory neurons differ 
from most other neurons in that they die and regenerate on a regular basis. 

The remaining senses can be considered types of physical feeling. 

5. Tactition is the sense of pressure perception. 

6. Thermoception is the sense of heat and the absence of heat (cold). 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. 

7. 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). 

8. 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. 

9. 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). 


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