OBSERVATION: The fundamental basis of all science is observation. And ALL observation is made with the senses. Instruments (such as meter sticks, thermometers, Voltmeters, and Geiger counters) are often used to augment the senses.
Traditionally there are five senses:
But despite the importance of the senses for life, science, and knowledge of all variety, much about the senses remains obscure. For example, touch may actually be several distinct senses.
SIGHT: Even the most important sense, vision, is only partly understood. It has been long understood that light, diverging from its point of origin (or reflection), is focused by the convex lense at the front of the eye forming a real image on the light sensitive back surface of the transparent spherical eye. This retina is densely populated with several distinct kinds of nerve sensors: Most abundant by far, the long, slender rods are (therefore?) very sensitive to dim light (coupled together, they may detect single photons) and are responsible for black and white night vision; it is generally believed that three pigmented variations of shorter, larger diameter cones results in sensitivity to different color ranges, thereby provide information for color vision. Gerald Huth has noted that the rods and cones form a detector array. Huth proposes that rather than each rod or cone being an isolated detector, they work together as a spatial array to detect light with the spacing between rod-rod, cone-cone, and rod-cone being responsible for the three different color detection ranges. Whether distinguishing color is a matter of pigments or spacing, the traditional notion of three primary colors is a certainly a simplified description of color processing by human eyes and brains rather than a description of fundamental aspects of color or light. (See a dialog on colored vision and Primary colors: No for a further explanations of color vision and color printing.) The retina consists of 5 layers of cells with extensive interconnections between each layer. There are about 125 times as many receptors in the retina as there are ganglion cells in the optic nerve. So some processing must occur in the eye prior to transmitting the information to the brain. The signals from different cone types are segregated in opponent pairs during this processing so that ganglion cells have specific receptive fields that are excited by one cone type in the center and inhibited by those further away. This organization has been extensively investigated but the perception of color does not relate easily with this organization.
Only the front part of human eyes have similarity to cameras that use film. More similar are digital cameras using CCDs (charge coupling devices) containing millions of distinct pixel light detectors, each of which sends electronic signals to a computer chip for processing such as color correction and jpeg compression. Rods and cones use spatial, molecular, and electrical processes to detect the light. 11-cis-retinal (an aldehyde) is an oxidized form of Vitamin A (an alcohol often derived from half of a β-carotene molecule). Absorbed light partially dissociates electrons in the cis double bond (in middle of a series of resonating double bonds), allowing the normally stiff 11-cis-retinal molecule to briefly rotate. The bond reforms 50% cis and 50% trans isomers. The differently shaped trans-retinal forces a shape change in the larger enclosing protein, rhodopsin, releasing an electron signal. Nearby proteins called transducin amplify the electric signal. Recent experiments with rodents suggest that when light is brighter the transducin move away from the rhodopsin reducing the signal amplification. Transparent ganglions pre-process information from clusters of cones and rods, perhaps distinguishing color, pattern, and motion before transmission to the brain for further processing. Despite the pre-processing, vision is data intense, requiring the largest nerve bundle in the body to transport electrical information to the brain. Much about the recognition of patterns (such as with letters and faces) and motion remains less than completely understood. Conceivably there may be other aspects of vision beyond color, pattern, and motion.
HEARING: Like vision, the initial mechanism of hearing is understood. Sound vibrations cause the membrane in the ear (ear drum) to push on a series of three bony levers (hammer, anvil, stirrup) transferring vibrations through an inner membrane to a liquid in the Cochlea, the snail shaped inner ear. The motion of the liquid causes the sway of a series of hairs. The motion of hairs is detected by nerves much like nerves can feel the motion of hairs on other body surfaces. (A similar system of fluid moving hairs is used in the nearby semicircular canals to maintain balance by detecting head motions.) The detection of tones, rhythms, and harmonies are presumably functions of the brain. Like motion and patterns in vision, they remain shrouded in mystery.
TASTE: Classically the tongue detects four flavors: sweet, salty, sour and its chemical opposite, bitter. Recently a fifth taste of glutamate (msg) has been proposed. There are proposals for the mechanism of taste involving a chemical
lock and key process where there is a match of molecular shape and functional groups with larger receptor molecules. Taste buds mostly located along the edges of the tongue (from front to back in the order listed above) contain taste pores which house taste cells attached to nerves.
SMELL: The detection of odors is done by a series of nerve cells at the top most part of the nasal passage. Tiny nerves fibers lead directly from this olfactory bulb into the central part of the brain. The sense of smell may use similar chemical
lock and key mechanisms to those of taste; if that is true, there may need to be as many as 1000 distinct nerve receptors. Apparently a single substance at different concentrations causes different combinations of receptors to report detection; the result is perception of distinct odors from different concentrations of the same substance. Perception also varies with time. Attempts to classify odors into primary smells has generally been unsatisfactory.
The taste of a particular food is typically a combination of information from the taste buds and the olfactory bulb. The possibility of patterns and motions in tastes and odors similar to visual and sound
harmonies and time
rhythms remains beyond current investigation.
TOUCH: The sense of touch seems to involve several types of sensors, or perhaps sensors that can report differing types of information. The skin can report temperature (or at least warmth or cold), pressure (such as touch), the motion of skin hairs and perhaps even their position as when they are more erect when cold, and pain of excess heat, cold, pressure, cut, or puncture. Possible connections between temperature, pressure, pain and pleasure remain nearly at the level of cult!
Some areas of the body are more heavily populated with nerves: at other locations the body is unable to distinguish two touches some distance apart. A kiss is thought to create much pleasure because the face and lips are very sensitive and seem to involve many nerves. Other sexual pleasures are probably similar.
Much of the time the senses report monotonously duplicate information that the nervous system generally ignores. Small background sounds, peripheral vision where little changes, and constant reports of the pressure of clothes and moderate temperatures generally go unnoticed. But at any moment a person can usually choose to monitor information from a given sensor. A person can feel their shirt collar, the motion of a toe, and sometimes even hear their heart beat. But usually a person's brain takes note of only the most interesting of the thousands of bits of information from their senses. A study of fruit fly photoreceptors may reveal part of the mechanism used to discount nerve signals. Fruit fly detectors involves a protein channel that regulates the flow of ions through cells. The protein is abundant when there has been no stimulation. But after stimulation 70% of the protein is moved to a storage compartment reducing the signals.
Internal organs also contain nerves that typically are continually ignored unless a problem occurs. But any person who has ever experienced pressure caused by child birth, appendicitis, a gall stone, or kidney stone knows how intense that pain can be. Sometimes it is as if those nerves have never been calibrated and their location mapped so that the pain seems to originate some distance from the actual source.