We have already discussed two of the special sensesÄhearing and vision. In this module we shall discuss two more special senses (i.e., taste and smell), as well as other senses in the skin, the internal organs, and in the muscles, joints and tendons.
Smell and taste are chemical senses. Smell results from the stimulation of olfactory receptors by chemical gas molecules. Taste results when taste receptors are stimulated by chemical molecules dissolved in saliva. In some as yet undetermined way, these two chemical senses, taste and smell, are related. Take away the sense of smell, and you have very little taste sensation left.
The olfactory (smell) receptors and the olfactory bulb are located deep in
the nasal passages. The olfactory receptors lie in two small patches on the
roof of each nasal passage. Figure 14 shows a cross section of the nose.
The receptors are stimulated by specific molecules, which initiate impulses
to the brain. The olfactory cortex, where the impulses are projected, is in the
frontal lobe of the brain.
Figure 14. Cross section of the nose (showing olfactory receptors on olfactory bulb)
Figure 14. Cross section of the nose (showing olfactory receptors on olfactory bulb)
The receptors are not on the main route of air as it moves through the nose in normal breathing. Thus the sense of smell is relatively dull unless we sniff, stirring air in the nasal passages and bringing more molecules in direct contact with the receptors. Because the smell receptors are so inaccessible, experimenters have found it difficult to devise reliable techniques for presenting stimuli. Normally, if we want to smell something more clearly, we sniff. This procedure is also used frequently in the laboratory. It is not very easy to standardize a sniff.
The logical step in testing ability to smell, then, is to make careful measurements of the amount of odorous material available. This procedure does not really tell how much of the available material stimulates the receptors, but it does identify a limit on what the subject can smell.
One of the most recent theories of smell is the stereo-chemical, or lock- and key, theory derived from research by Amoore et al. (1964). This theory suggests that airborne molecules of different shapes fit into sockets in the olfactory receptors. Five of the odors that Amoore lists as basicÄcamphor, musk, floral, peppermint, and etherÄarise from molecules having distinctive patterns of electrical charge. These molecules may correspond to specific olfactory receptors. One test of this theory would be to synthesize substances with particular molecular shapes and see if they have the odor predicted by the theory. In some tests they do, and research is continuing.
Smells may figure differentially in different forms of mental illness. Daniel S. O'Leary, of the University of Iowa Hospital, compared 18 schizophrenics and 16 normals (O'Neil, 2001). Pleasant (vanilla) and unpleasant (smilar to moldy socks) smells were presented to the subjects. Brain imaging devices tracked blood flow in the brain. For pleasant smells there was little difference between the groups, though the schizophrenics found the smells half as pleasant. There were big differences, however, for the unplesant smells. Healthy people used the limbic system in the brain (that which controls emotions like fear). The schizophrencis, however, barely used the limbic system but relied on the frontal cortex, that which is usually reserved for higher functions like decision making. Paranoid schizophrenics may respond to unpleasant stimuli as a threat which needs to be mentally evaluated. And they may also not really enjoy, as do normal persons, ordinary pleasant sensations.
The stimuli for taste are also theorized to be chemical molecules. The receptors for taste are specialized cells that are grouped together in little clusters known as taste buds. Most of these buds are located on the top and the sides of the tongue, but a few are located at the back of the mouth and throat. To stimulate the taste buds substances must be dissolved in saliva that washes around the taste buds and penetrates the cells within them.
Figure 15. Areas of the tongue
Experiments have identified four fundamentally different tastes. These are salt, sour, sweet, and bitter. Other tastes are said to be some combination of these four. Figure 15 shows that different areas are more sensitive to different tastes. There has been a great deal of experimental work devoted to mapping the areas of the tongue that are sensitive to sweet, salt, sour, and bitter stimuli (Pfaffmann, 1951). In general the back of the tongue is most sensitive to bitter stimuli, while the sides are sensitive to sour stimuli. The sensation of sweet comes primarily from the tip of the tongue, while taste buds sensitive to salt seem to be scattered all over the tongue, except in the center. The center of the tongue does not produce any sensation of taste.
Physiological psychologists also have difficulty trying to determine which chemical substances are responsible for sensations of taste. For example, sugar tastes sweet, but so do other substances like saccharin which are not chemically related to sugar at all. Quinine tastes bitter, but so do mineral salts, and these substances are not related chemically, either. The sensation of sour is usually associated pith acids. Ordinary table salt, however, is the only chemical substance which we taste as pure salt. Other salts produce experiences of bitter or sweet in addition to that of salt.
One problem that affects studies of both taste and smell has to do with adaptation. Nearly all our sense organs adapt to stimuliÄthey gradually become less sensitive during the course of stimulation. Some senses, such as hearing, adapt very little. Taste and smell, however, adapt very quickly. For example, when you walk into a room containing a strong smell, you become unaware of the smell after a few minutes. Because of this rapid adaptation, experiments on taste and smell are somewhat hampered. A subject can only be exposed to a few tastes and smells before becoming insensitive to the stimuli.
Through our skin we can feel itching, burning, and textures that are smooth or rough. We feel things as sticky, wet, or soft. We sense vibrations, cold, and warmth. These and other experiences are variations of three basic skin sensitivities -- pressure, temperature, and pain.
The experience a subject reports when he is touched lightly is called pressure or touch. The amount of pressure required to produce this experience varies greatly for different parts of the body. For example the tip of the tongue, the lips, fingers, and hands are more sensitive than arms and legs.
The part of the body that is most sensitive can be determined with a simple test called a two-point threshold. Place your thumb and forefinger about an inch apart on a friend's back. He will feel pressure but will probably not be able to tell you how many fingers you are using to touch him. Spread your fingers further apart and apply pressure to his back again. Repeat this procedure until you find the point where he can tell you how many fingers are touching him. This is a very rough approximation of the two-point threshold for the back.
The skin is sensitive to temperature changes of even a few degrees above and below its own temperature. In mapping areas of the body that are sensitive to heat or cold, tiny cold spots and warm spots can be detected. Different receptors for heat and cold are probably present in the skin.
A number of very different stimuli produce the experience of painÄa hard blow to the skin, scalding steam, a needle prick, or acid. One laboratory procedure used to produce painful sensations involves a device which radiates heat to small areas of the skin. As the heat increases, the subject experiences first warmth and then pain. Experimenters have also used pinpricks and chemical solutions to induce painÄbut these methods are not as precise as heat. Information about the skin receptors is being obtained today by recording the electrical activity of individual nerve fibers. Some fibers have been found that correspond to our sensations of touch, temperature, and pain. Although we do not have complete information about which receptors are responsible for our sensations of temperature and touch, fibers responsive to touch stimulation and to iricreases and decreases in temperature have been located.
The visceral senses are located in the viscera, or internal organs, of the body. We know very little about the receptors of our internal organs, primarily because these areas are so inaccessible to experimental studies. There must be receptors for heat and cold in the throat and stomach because we experience these sensations when we eat or drink. There must also be receptors for pressure and pain, since pressure, or even pain, is felt when the stomach or the bladder is full.
Hidden away in our muscles, joints, and tendons are a variety of sense organs. These sense organs provide us with information about body movement.
This sense of body movement is called kinesthesis. In some ways it is the most important sense we have, because it provides an automatic system for coor- dinating our muscles when we walk, talk, or use our hands.
Kinesthetic receptors are found in three distinct areas: the muscles, the ten- dons, and the joints. Those in the muscles signal relaxation of the muscles. Those located in the tendons signal contractions of the muscle. The receptors in the joints are stimulated whenever a limb moves, changing the positions of the bones in the joint. Together these deep receptors make us aware of the position and the movement of all parts of our bodies.
Now test yourself without looking back.
1. The two chemical senses are_______________________and ____________________.
2. Receptors for the sense of smell are:
a. Iocated in the roof of the nasal cavity.
b. Iocated in the floor of the nasal cavity.
c. stimulated by molecules of liquids.
d. stimulated by molecules of gases.
3. Receptors for the sense of taste are:
a. Iocated primarily in the center of the tongue.
b. Iocated primarily at the tip, back, and sides of the tongue.
c. stimulated when certain molecules are dissolved in saliva.
d. stimulated when gas molecules reach the taste buds.
4. List the four basic tastes.___________________ _____________________
1 ) Skin sense________
2) Visceral sense __________
3) Deep sense________________
a. Pressure -
6. List three locations of kinesthetic receptors.____________________________________________________
ANSWER KEY PAGE 142
5 OR MORE CORRECT PAGE 105
FEWER THAN 5 CORRECT PAGE 102
|Type of Sense||Sense||Location of Receptors||Stimulated by||Result|
|Chemical||Smell||roof of nasal cavity||gaseous molecules||smell distinct ordors of aromas|
|Taste||back, sides, and tip of tongue; back of mouth cavity||molecules dissolved in saliva||taste, sweet, sour, salty and bitter substances|
|Skin||Pressure||in skin||touch||awareness of touch|
|Pain||in skin||extreme heat, extreme pressure, needle prick, acid, and so on||painful sensations|
|Temperature||in skin||temperature changes||cold or warm feeling|
|Visceral||Pressure||in internal organs||internal pressure (full bladder)||discomfort|
|Pain||in internal organs||extreme pressure (very full bladder); acid (stomach hyperacidity)||pain|
|Temperature||in internal organs||heat or cold||awareness of temperature|
|Deep||Kinesthesis||in muscles, tendons and joints||stretching, relaxing, or change in position||awareness of body movement|
Read the section of the exercise reference chart that deals with chemical senses. Then answer the following questions.
A fruity aroma results from the stimulation of receptors in the_____________________________6
A sweet taste results from the stimulation of receptors in the____________________________________4
The center of the tongue contains receptors for:
Taste buds give us four basic tastes. They are: ___________________________________________ 3
The stimulation of receptors for smell comes from what kind of molecules?
2 gas molecules
3 sweet, sour, salty, bitter
5 roof of nasal cavity
Read the sections of the exercise reference chart that refer to skin and visceral senses. Then do the following exercises.
1 ) Skin sense________
2) Visceral sense_____________
Which of the following is normally a stimulus for pain?
a. Mild cold
b. Extreme heat
c. A sharp object
As a visceral sense, pain receptors may be stimulated by:
a. normal digestive processes.
b. increased acid in the stomach.
c. extreme pressure inside the bladder.
A feeling of heat in the abdomen is ______________________ a_____________________________ sense.
The awareness of touch on the left leg is a ____________________________ sense.
Read the section of the exercise reference chart that deals with deep senses. Then complete this exercise set.
The sense of kinesthesis gives us an awareness of ______________________________6
Kinesthetic receptors are located in:
A kinesthetic receptor in a joint is stimulated by_____________________________________2
The chemical senses are _________________ and________________________5
Pain can be either a_____________________ sense or a___________________ sense.
NOW TAKE PROGRESS CHECK 2
1 b, c
2 change in position
4 1) a c d
2) a c d
6 body movement
8 b, c, d
9 b, c, e
10 skin; visceral
5 OR MORE CORRECT PAGE 105 UNIT 6 Table of Contents
FEWER THAN 5 CORRECT INSTRUCTOR CONFERENCE
UNIT 6 Table of Contents
July 31, 2001