The Senses

Dr. C. George Boeree

Taste (or gustation)

There are about 10,000 taste buds on the tongue, clustered in papillae (those bumps all over your tongue).  The taste buds are clusters of neuron bodies that line tiny pits in the papillae, and look sort of like a microscopic bunch of bananas.

Molecules from the food we eat get mixed with saliva and find their way into the little pits and onto the surfaces of the neurons.  Like a key fitting into a lock, these molecules open up tiny pores on the cell membranes and begin the process of firing the neuron very much the same way as the neurotransmitters do between neurons.

There are only four basic tastes -- that is, only four particular molecules that one or another neuron responds too on the tongue:

Bitter - alkaloids
Sweet - sugars
Salt - sodium chlorids
Sour - acids

There may also be a fifth taste:  Umami or savoriness, which involves a sensitivity to glutamate (which you may remember as one of the neurotransmitters).  You find it in aged cheese, tomatos, mushrooms, meat, and soy sauce. It is best known as monosodium glutamate, which is used to enhance the flavor of meat.

We experience salty and sour because the salt and acid ions directly open ion channels in the sensory neurons.  Sweet, bitter, and umami, on the other hand, bind to proteins that have receptor sites.  Our ability to taste bitter may have evolved in order to protect us from food poisoning.

Note also that the tongue is sensitive to touch (hence the idea of texture in food), and to temperature and, of course, pain.  Jalapeño peppers, for example, have a certain taste in the ordinary sense, but also provide us with delightful (!) sensations of pain.  You might find it useful to know that, if your mouth is burning from eating peppers, it helps to drink milk, because milk fats dissolve the active chemical (capsaicin) while water merely spreads it around.

In addition, the tongue is sensitive to cold, which can be triggered not only by actual low temperatures, but by chemicals such as menthol.

Recent research with mice shows that some mice also have taste buds that respond to fats.  Apparently, it is a genetic trait, one that may help us to understood why some people are more attracted to fatty foods than others.

There are people that cannot taste anything.  This is technically known as ageusia.  Fortunately, it is very rare.

Perhaps the biggest part of taste for us is, oddly, smell...

Smell (olfaction)

Smell works much like taste:  It is also a “lock and key” sense.  This time, it is a matter of moist air being drawn over a piece of specialized mucous membrane about the size of a dime at the top of your nasal cavity.

With smell, we seem to be responding to the presence of some combination of seven basic molecules:
Floral
Pepperminty
Musky
Pungent (like spices)
Camphoraceous (like mothballs or muscle liniments)
Ethereal (like dry-cleaning fluid)
Putrid (like rotten eggs)
These are the seven smells suggested by the researcher John Amoore in 1952, when he also outlined his "lock and key" theory of how smells work.  But it is far from certain that these are the fundamental scents -- some researchers believe there are many more.

The chemical senses are extremely sensitive, and this goes especially for smell.  It is believed to be thousands of times more sensitive than taste and actually accounts for as much as 80 or 90% of what we perceive as flavor. There are roughly 40 million smell receptor cells in humans. Dogs have us beat, paws down, with 100 million cells.  But they don't even rate compared with rabbits, with one billion cells.  Mammals tend to have a good sense of smell, especially carnivores and their prey (for very obvious reasons!)  Primates, however, have a relatively poor sense of smell.  On the other hand, cetaceans (whales and dolphins) have no sense of smell at all!  A lack of the sense of smell in humans is called anosmia.

There has been considerable debate over many years about the existence of a smell-like sense that can detect the presence of molecules called pheromones.  Many animals clearly can smell the presence of a potential mate over great distances.  Male silkworms can detect even a simgle molecule that indicates a female!  (It is the antennae that serve as smell organs in insects.)

People can certainly smell other people -- but is there a special smell that doesn’t really have a particular odor, but rather leads us to feel, well, those special “I want you” feelings?  I think not, but there are many who disagree with me.

Touch (tactile)

The skin actually has three types of sensation:  Pressure, temperature, and pain.

Pressure is a simple matter of mechanical distortion, the bending of the dendrites of a mechanoceptor.  When bent, the stress cause the opening of channels, the exchange of ions, and, of course, the firing of the neuron. In some cases, the dendrites are embedded in capsules which, when compressed, stimulate the neuron. Plus different neurons are sensitive to different kinds of pressure:  light touch, firm pressure, and vibration.

Temperature seems to be a very direct influence of the heat or cold opening certain ion gates.  So far, we have found three of them:  One for cold, one for hot, and one for very hot.  Perhaps there are also ones for very cold and even just plain warm.

It is interesting to note that menthol can also spark the cold receptors, and make us believe we are feeling cold when we are not!  It is also peculiar that, when we touch a thermal grill -- a surface that has alternating lines of cold and heat -- we feel neither heat nor cold, but pain!

I will talk about pain separately, but basically, pain is a matter of detecting certain chemicals indicative of tissue damage.  With pain is classified itching and tickling.  It is interesting that there is a chemical called capsaicin that acts on pain receptors just like “real” damage does.  It is found in such things as jalapeño peppers, as mentioned above.

Kinesthetic sense

The kinesthetic sense is based on receptor neurons in the muscles and joints that basically work on the mechanical distortion principle.  Some of these receptors are mechanoceptors just like those in the skin; others are spindles that begin to fire when stretched.

Vestibular sense

The vestibular sense tells you which way is up, how your body is oriented in relation to up, and how your body is moving in space.  The sensations are based on hair cells - mechanoceptors with dendrites that resemble brushes.  In the inner ear, there is a special arrangement of three semicircular canals around a central area.  In the semicircular canals, the motion of the fluid as you spin causes gelatinous lumps called cupulas to bend one way or the other, which in turn causes the hair cells to bend.  The three canals are oriented at roughly 90º to each other, and so give you spinning information in all three dimensions.

The vestibular sense is also connected to parts of the brain that tell you when it is time to vomit.  This is the cause of motion sickness.

If you spin hard enough and then suddenly stop, the tiny current keeps going for a little bit, and gives you the sensation that you are still spinning, but in the opposite direction.  Your brain may try to compensate for this, and cause you to fall or at very least feel dizzy.

You can also confuse these canals when you take a shower and allow hot or cold water into your ear.  The temperature changes can cause currents to develop that wind up feeling just like spinning, and you may get dizzy.

The two central areas of this organ also have hair cells.  The hair cells are embedded in gelatinous lumps called maculas which pulls them in one direction or another, depending on whether you are standing upright, bent over one way or another, or standing on your head.  The bending of the hair cells again sends signals to the brain which interprets them accordingly.

Hearing (audition)

Hearing is also a matter of hair cells!  You recall, I’m sure, the basic structure of the ear:  The outer ear canal leads to the ear drum, a thin tissue stretched across the opening.  Behind the ear drum, there is a sequence of three tiny bones that slightly amplify the vibrations of the ear drum.  They end at another thin tissue that closes the true organ of hearing, called the cochlea.  It is actually a tube, first bent in half, then wound up into a coil, and filled with fluid.

Along this tube, there is a membrane that moves according to the wave patterns set up in the fluid.  It has hair cells growing below it, and those hair cells send messages to the brain as to the wave patterns and changes they detect.  That may sound complicated enough, but this description is actually highly simplified!

Vision

Vision is different from all the other senses.  It involves receptor neurons that are sensitive to light.  Light enters through the pupil and lens of the eye and is projected onto the back surface of the eye called the retina.  The retina is composed of, among many other things, receptor neurons called rods and cones

The rods are sensitive to a broad range of light, i.e. they tell us about “white.”  They contain what is called visual purple (rhodopsin), a chemical that is sensitive to light.  Note that a crucial part of this chemical is derived from vitamin A -- so eat your carrots!  The chemical breaks down when exposed to light and releases a protein (opsin) which eventually releases a neurotransmitter to send messages to the brain that “there is light.”  Then the breakdown products are re-assembled back into rhodopsin.

Cones are similar, but involve a chemical called iodopsin that is sensitive to more specific wavelengths of light, depending on pigments associated with the chemical.  One kind of cone responds to red, one to green, and one to blue.  Again, a protein (retinene) leads to the release of neurotransmitters, etc.

Rods are far more sensitive than cones.  This is why you see in “black and white” when there isn’t much light.  Nocturnal animals tend to be color-blind, that is, they don’t have any cones, since color is of little use to them while high sensitivity is.  Also, nocturnal animals usually have a shiny backing to their retina that reflects light back to the rods called a tapetum.  It is usually made up of tiny crystals.  This is why cats and other animals reflect light from their eyes!

The great majority of color-blind people suffer from red-green color blindness.  This comes about because they lack either red or green cones, so that red and green are indistinguishable.  Everything is in shades of blue and yellow.  It is much more common in men than women, occuring in about 1 in every 20 men.  This is because the genes for red-green colorblindness are on the X chromosome of the 23rd pair.  Since women have two X chromosomes, they must inherit the problem from both parents.  On the other hand, a man with red-green colorblindness will not transmit the gene to his sons - only to his daughters (who will probably not be color blind!).

Some people suffer from blue-yellow color blindness, which means that there is something wrong with their cones for blue.  They see the world in shades of green and red.  Since the gene for this kind of color blindness is on chromosome 7, it is equally distributed between men and women.  It is, however, extremely rare.

Also very rare is complete color blindness, which can mean that the person only has one kind of cone or none at all.

As with all the senses, there is a great deal more to vision that this, but this will do for us.

© Copyright 2002, C. George Boeree