|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.
Hearingor 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.
Tasteor 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 "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.
Smellor 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 system. Olfactory receptor neurons in the nose 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 remaining senses can be considered types of touch or physical feeling.
Feelingor Tactition is the sense of pressure perception, generally in the skin.
Thermoceptionis 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.
Nociceptionis 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).
Equilibrioceptionis 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.
Proprioceptionis 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).