Other Senses
The previous lecture notes covered vision and hearing. This one covers the remaining senses, including the cutaneous senses (touch), the kinesthetic senses, and the chemical senses.
The Cutaneous Senses
The cutaneous senses, or sense of touch, really include a variety of qualitatively different sensations: light pressure, deep pressure, hot, cold, pain, and some others. The sensors of these qualities are specialized nerve endings, sometimes with an associated structure, as in the case of pressure receptors. Temperature is sensed, interestingly enough, by a pair of receptor types, which I have labeled "hot" and "cold" receptors. These labels are not strictly accurate, however. So-called "hot" receptors respond best to increasing skin temperature, "cold" receptors to decreasing skin temperature. Extreme hot or cold stimulates the "cold" receptors; the simultaneous activity of both hot and cold receptors gives rise to the sensation of "burning" hot.
The Kinesthetic Senses
The kinesthetic senses are those which provide the brain with information about the body's positions and movement. They include sensors for joint angle (in the joints), muscle tension and muscle length (in the muscles), head rotational acceleration (in three dimensions), and gravity, the latter two provided by the semicircular canals attached to the bony structure of the cochlea in the inner ear. Together these senses inform you about the relative position of various body parts (e.g., are your arms in front of you or behind you?), and how the body is moving with respect to the world outside (rotation, position with respect to the pull of gravity).
The Chemical Senses
The chemical senses include taste and smell. Both depend on a chemical analysis of molecules. In the case of taste, chemicals in the food dissolve in saliva and are conveyed to the taste buds, where taste receptors respond to characteristics of the chemical in much the same way that receptor protiens in a synapse respond to the chemical signature of a neurotransmitter. There are four types of taste receptor, detecting four distinct types of substance:
- salty -- salts (such as sodium chloride)
- sweet -- sugars
- sour -- acids (hydrogen ion)
- bitter -- alkaloids
In the case of smell, odorant molecules are conveyed to receptors in the upper part of the nose, where they dissolve in mucus and are conveyed to the smell receptors. At one time it was proposed that there may be perhaps seven types of smell receptor (with names like fruity, floral, camphoraceous, pungent, and putrid) but currently it is suspected that there may be dozens and perhaps even hundreds of types.