MODULE 1

Classification of the Senses

Philosophers have pointed out that we learn most of what we know about the world through our sense organs. Everything we do depends first on the information we receive through these senses. To what degree and under what conditions this information is distorted depends on the very structure of the senses themselves.

In this section, we will study human sensory capacities to learn what factors set limits on our ability to receive environmental information, and to find out just how many ways we have of obtaining this information.

The senses can be classified in many ways. Aristotle distinguished five: vision, hearing, smell, taste, and touch. We have learned a great deal since Aristotle's time. In 1906, Sir Charles Sherrington suggested that the senses might be classified in terms of the kind of information each sense organ gives us. This type of ordering is called a functional classification.

The traditionalal functional classif cation of five senses does not suit all types of inquiry

Sherrington distinguished four kinds of senses based on their functions. Teleceptors give us information about our distant environment in much the same way that a telescope brings distant information closer. Vision and hearing fall into this category. Exteroceptors give us information about our immediate external environment, that which we contact directly. Our skin senses and sense of taste provide this type of information. Interoceptors give us information about the bodily functions that make up our internal environment,and proprioceptors give us information about bodily position.

Another classification system, the one we will be using, is based on differences in anatomy. This system includes four basic classes of senses, depending on where the end organs or receptors are located in the body. These categories are special senses, skin senses, visceral senses and deep senses. The special senses are those with specialized receptors or end organs in the head. Our receptors for vision are the rods and cones of the eye; we hear because of the action of hair cells in the ear. Smell and taste are also special senses because they depend on specialized receptors located in the nose and on the tongue. The vestibular sense, which is the sense of balance or equilibrium, has its receptors in the innermost part of the ear.

The anatomical classification provides a more complete description

Senses that have their receptors in the skin are called the skin senses. They include touch, pain, and temperature.

Visceral senses give us information about our internal organs. For example these senses tell us if the stomach is empty or the bladder full.

Deep senses include muscle, tendon, and joint sensations together with deep pain and pressure.

What is a sensory system? In general a sensory system consists of an end organ or group of receptor cells which respond to specific kinds of physical energy. For example the eye responds to light. The receptors respond by sending electrical impulses along the sensory nerves or nerves that lead from the receptor cells in the eye to the brain.

The receptors, the sensory nerves, and those portions of the central nervous system which process information for a particular sense make up that sensory system.

Sensory information is constantly sent to the brain from receptors. However, this is not the only direction in which impulses travel between a receptor and the central nervous system. Neural paths called centrifugal fibers lead from the brain to the sense organs. These centrifugal fibers modify the response of the receptor to the physical stimuli. The brain is not simply a passive recipient of anything the sense organs transmit. The brain modifies information received from our environment by modifying the activity of the sensory nerves or by modifying the sensitivity of the receptors. In so doing, it modifies our perceptions of the world around us.

The specific energy theory states that each receptor conveys just one sensation

Our sensations also depend on the particular sensory system excited. Johannes Muller arguing this point in 1840, stated that although the nerves of each system are capable of responding to a number of different types of stimuli, they are most sensitive to a particular kind of stimulus, and however they are stimulated, their stimulation results in a specific sensation. For example, if one were to press on his closed eye, he would get sensations of "seeing."

Application of Muller's principle -- the doctrine of specific energy of nerves-- gives us what we call place theories the senses. We will encounter these theories in our discussions of each of the senses.

Sensation results from physical stimulation of sensory systems

Our sensory systems are capable of making fine discriminations within the range of physical stimuli we perceive. We are capable of distinguishing between a tremendous number of sounds, and we can see the difference between hundreds of hues. However, the actual range of physical stimuli to which any sensory system is sensitive is well defined and measurable. For example, compared to the possible range of wavelengths, the portion we see as different colors of light is very small. If human eyes were constructed so that we were sensitive to a different set of wavelengths of light, our world would seem very different to us.


MODULE 1
PROGRESS CHECK 1

Now test yourself without looking back.

1. End organs or groups of receptor cells, nerve fibers, and parts of the central nervous system that respond to specific kinds of physical energies make up a(n):
a. sensory system.
b. motor system.
c. functional sensory classification system.
d. anatomical system.

2. The system that classifies senses according to where the end organs or receptor cells are located in the body is based on:
a. individual senses.
b. anatomy.
c. function of the senses.
d. individual differences in sensitivity.

3. Muller's doctrine of the specific energy of nerves indicates that:
a. sensations depend on the particular sensory system excited.
b. each nerve of each system is capable only of responding to specific stimuli related to that system.
c. (neither)

ANSWER KEY


MODULE 1
EXERCISES

Reread the text, then do these exercises.
Senses Distinquished by Aristotle --

Vision
Hearing
Smell
Taste
Touch

Senses Distinquished by Sherrington (Functional)

Teleceptors
Exteroceptors
Interoceptors
Proprioceptors

Anatomical Classification
Special senses (Receptors or end organs in the head)Vision Hearing Taste Smell Vestibular (equilibrium)
Skin senses(Receptors or end organs in the skin) Touch, pressure Temperature Pain
Visceral senses(Receptors or end organs in the internal organs)stomach
bladder
Throat
Deep senses (Receptors or end organs in muscles, and so forth; also called kinesthesis, meaning sensitivity to body move- ments) Muscles (stretch):Tendons (contraction)
Joints (prerssure changes)

Using the information in the preceding charts, answer the following.

What are the four classes of senses based on anatomy?____________________________________________________ 2

Which classification system is based on function?_________________________________________________ 4

Vision, hearing, smell, taste, and touch are the classifications made by_____________________________________________________5

List four special senses.__________________________________________________________3

The deep sense that provides information about body movements is called_________________________________________________1

ANSWERS

1 kinesthesis

2 special senses
skin senses
visceral senses
deep senses

3 (Four of these)
sight
hearing
smell
taste
vesflbular

4 Sherrington's

5 Aristotle


The visceral senses provide information about____________________________________2

List three skin senses.____________________________________________1

ANSWERS

1 touch, pain, temperature

2 the internal organs

Information from the physical environment -- physical stimulli such as sound, smells, light, taste -- affect our receptors (the eyes, ears, receptors in the nose and on the tongue). These receptors send information along the nerve tracts to various structures in the brain. Different structures in the brain receive fibers from different nerves, and are therefore sensitive to different stimuli.

What are the components of a sensory system?____________________________________________2

One class of theories, the place theories, assumes that different sensory systems are most sensitive to different stimuli.. These theories stem from the doctrine of____________________________________________4

ANSWERS

2 receptors, nerve fibers, central

4 specific energeies

NOW TAKE PROGRESS CHECK 2


MODULE 1
PROGRESS CHECK 2

1. The four classes of senses based on differences in anatomy are:
a. vision senses, hearing senses, smell senses, and deep senses.
b. special senses, skin senses, visceral senses, and deep senses.
c. senses dealing with the distant environment, the immediate external environment, the internal environment, and those senses dealing with our body's spatial position in the environment.
d. (none of these)

2. Who developed the doctrine of specific energy of nerves?
a. Sherrington
b. Aristotle
c. Muller
d. (none of these)

3. Which of the following was not indicated by the doctrine of specific energy of nerves?

a. The sensory system that is excited determines the sensations we feel.
b. Each nerve of each system is capable of responding to many kinds of stimuli.
c. Place theories assume that different parts of the central nervous system are sensitive to different stimuli.
d. (none of these)

4. Name me three parts of a sensory system.

ANSWER KEY

3 OR MORE CORRECT Go to Module 2

UNIT 6 Table of Contents

Oct. 1, 2004