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CHAPTER

8

Properties of Sensory

Receptors

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the classification of sensory receptors.

■

Name the types of sensory receptors found in the skin, and discuss their relation to

touch, cold, warmth, and pain.

■

Define generator potential.

■

Explain the essential elements of sensory coding.

INTRODUCTION

Information about the internal and external environment

activates the CNS via a variety of
sensory receptors.
These

receptors are transducers that convert various forms of energy

in the environment into action potentials in neurons. The

characteristics of these receptors, the way they generate

impulses in afferent neurons, and the general principles or

“laws” that apply to sensation are considered in this chapter.

Emphasis is placed on receptors mediating the sensation of

touch, and later chapters focus on other sensory processes.

We learn in elementary school that there are “five senses,” but

this dictum takes into account only some of the senses that

reach our consciousness. In addition, some sensory receptors

relay information that does not reach consciousness. For exam-

ple, the muscle spindles provide information about muscle

length, and other receptors provide information about arterial

blood pressure, the temperature of the blood in the head, and

the pH of the cerebrospinal fluid. The list of senses in Table 8–1

is somewhat simplified. The rods and cones, for example,

respond maximally to light of different wavelengths, and three

different types of cones are present, one for each of the three

primary colors. There are five different modalities of taste:

sweet, salt, sour, bitter, and umami. Sounds of different pitches

are heard primarily because different groups of hair cells in the

cochlea are activated maximally by sound waves of different

frequencies. Whether these various responses to light, taste,

and sound should be considered separate senses is a semantic

question that in the present context is largely academic.

SENSE RECEPTORS

& SENSE ORGANS

It is worth noting that the term
receptor
is used in physiology
to refer not only to sensory receptors but also, in a very differ-
ent sense, to proteins that bind neurotransmitters, hormones,
and other substances with great affinity and specificity as a
first step in initiating specific physiologic responses.

CLASSIFICATION OF

SENSORY RECEPTORS

Numerous attempts have been made to classify sensory recep-

tors, but none has been entirely successful. One classification

divides them into (1) teleceptors (“distance receivers”), which

are concerned with events at a distance; (2) exteroceptors,

which are concerned with the external environment near at

hand; (3) interoceptors, which are concerned with the internal