Human Physiology, 14th edition (2016)

268 Chapter 10

Receptors also can be grouped according to the type of
sensory information they deliver to the brain. Proprioceptors
include the muscle spindles, Golgi tendon organs, and joint
receptors. These provide a sense of body position and allow
fine control of skeletal movements (as discussed in chapter 12).
Cutaneous (skin) receptors include (1) touch and pressure
receptors, (2) heat and cold receptors, and (3) pain receptors.
The receptors that mediate sight, hearing, equilibrium, taste,
and smell are grouped together as the special senses.
In addition, receptors can be grouped into exteroceptors,
which respond to stimuli from outside of the body (such as those
involved in touch, vision, and hearing), and interoceptors, which
respond to internal stimuli. Interoceptors are found in many
organs, and include mechanoreceptors and chemoreceptors. An
example of mechanoreceptors are those in blood vessels that
respond to stretch induced by changes in blood pressure, and che-
moreceptors include those that monitor blood pH or oxygen con-
centration in the regulation of breathing.

Tonic and Phasic Receptors:

Sensory Adaptation

Some receptors respond with a burst of activity when a stimu-
lus is first applied, but then quickly decrease their firing rate—
adapt to the stimulus—if the stimulus is maintained. Receptors
with this response pattern are called phasic receptors. An
example of a phasic receptor that responds with a pattern like
that shown in figure 10.1 a is a pacinian corpuscle (a pressure
receptor—see fig. 10.4 ). Some other phasic receptors respond
with a quick, short burst of impulses when a stimulus is first
applied, and then with another quick short burst of impulses
when the stimulus is removed. These phasic receptors thus pro-
vide information regarding the “on” and “off ” of a stimulus.
Those receptors that maintain their higher firing rate the entire
time that a stimulus is applied are known as tonic receptors
( fig. 10.1 b ).
Phasic receptors alert us to changes in sensory stimuli and
are in part responsible for our ability to cease paying attention
to constant stimuli. This ability is called sensory adaptation.
Odor, touch, and temperature, for example, adapt rapidly; bath

water feels hotter when we first enter it. Sensations of pain, by

contrast, adapt little, if at all.

Law of Specific Nerve Energies

Stimulation of a sensory nerve fiber produces only one

sensation—touch, or cold, or pain, and so on. According to

the law of specific nerve energies, the sensation character-

istic of each sensory neuron is that produced by its normal

stimulus, or adequate stimulus ( table 10.1 ). Also, although a

variety of different stimuli may activate a receptor, the ade-

quate stimulus requires the least amount of energy to do so.

The adequate stimulus for the photoreceptors of the eye, for

example, is light, where a single photon can have a measur-

able effect. If these receptors are stimulated by some other

means—such as by the high pressure produced by a punch

to the eye—a flash of light (the adequate stimulus) may be

perceived.

Figure 10.1 A comparison of phasic and tonic

receptors. Phasic receptors ( a ) respond with a burst of

action potentials when the stimulus is first applied, but then

quickly reduce their firing rate if the stimulus is maintained. This

produces fast-adapting sensations. Tonic receptors ( b ) continue

to fire at a relatively constant rate as long as the stimulus is

maintained. This produces slow-adapting sensations.

(a)

(b)

Stimulus

applied

Stimulus

withdrawn

Phasic receptor —

fast-adapting

Stimulus

applied

Stimulus

withdrawn

Tonic receptor —

slow-adapting

Resting

membrane

potential

Action potentials

Receptor Normal Stimulus Mechanisms Examples

Mechanoreceptors Mechanical force Deforms cell membranes of sensory

dendrites or deforms hair cells that

activate sensory nerve endings

Cutaneous touch and pressure receptors;

vestibular apparatus and cochlea

Pain receptors Tissue damage Damaged tissues release chemicals

that excite sensory endings

Cutaneous pain receptors

Chemoreceptors Dissolved chemicals Chemical interaction affects ionic

permeability of sensory cells

Smell and taste (exteroceptors)

osmoreceptors and carotid body

chemoreceptors (interoceptors)

Photoreceptors Light Photochemical reaction affects ionic

permeability of receptor cell

Rods and cones in retina of eye

Table 10.1 | Classification of Receptors Based on Their Normal (or “Adequate”) Stimulus