Human Physiology, 14th edition (2016)

Sensory Physiology 269

( fig. 10.2 ). The pacinian corpuscle, however, is a phasic receptor;

if the pressure is maintained, the size of the generator potential

produced quickly diminishes. It is interesting to note that this

phasic response is a result of the onionlike covering around the

dendritic nerve ending; if the layers are peeled off and the nerve

ending is stimulated directly, it will respond in a tonic fashion.

When a tonic receptor is stimulated, the generator poten-

tial it produces is proportional to the intensity of the stimu-

lus. After a threshold depolarization is produced, increases

in the amplitude of the generator potential result in increases

in the frequency with which action potentials are produced

( fig. 10.3 ). In this way, the frequency of action potentials that

are conducted into the central nervous system serves as the

code for the strength of the stimulus. As described in chapter 7,

this frequency code is needed because the amplitude of action

potentials is constant (all or none). Acting through changes in

action potential frequency, tonic receptors thus provide infor-

mation about the relative intensity of a stimulus.

The effect of paradoxical cold provides another example of
the law of specific nerve energies. When the tip of a cold metal
rod is touched to the skin, the perception of cold gradually disap-
pears as the rod warms to body temperature. Then, when the tip
of a rod heated to 45 8 C is applied to the same spot, the sensation
of cold is perceived once again. This paradoxical cold is produced
because the heat slightly damages receptor endings, and by this
means produces an “injury current” that stimulates the receptor.
Regardless of how a sensory neuron is stimulated, there-
fore, only one sensory modality will be perceived. This speci-
ficity is due to the synaptic pathways within the brain that are
activated by the sensory neuron. The ability of receptors to
function as sensory filters so that they are stimulated by only
one type of stimulus (the adequate stimulus) allows the brain
to perceive the stimulus accurately under normal conditions.

Generator (Receptor) Potential

The electrical behavior of sensory nerve endings is similar to that
of the dendrites of other neurons. In response to an environmental
stimulus, the sensory endings produce local graded changes in the
membrane potential. In most cases, these potential changes are
depolarizations that are analogous to the excitatory postsynaptic
potentials (EPSPs) described in chapter 7. In the sensory endings,
however, these potential changes in response to stimulation are
called receptor, or generator, potentials because they serve to
generate action potentials in response to the sensory stimulation.
Because sensory neurons are pseudounipolar (chapter 7), the
action potentials produced in response to the generator potential
are conducted continuously from the periphery into the CNS.
The pacinian, or lamellated, corpuscle, a cutaneous recep-
tor for pressure (see fig. 10.4 ), can serve as an example of sen-
sory transduction. When a light touch is applied to the receptor,
a small depolarization (the generator potential) is produced.
Increasing the pressure on the pacinian corpuscle increases
the magnitude of the generator potential until it reaches the
threshold depolarization required to produce an action potential

Figure 10.2 The receptor (generator)
potential. Sensory stimuli result in the production of local
graded potential changes known as receptor, or generator,
potentials (numbers 1–4). If the receptor potential reaches a
threshold value of depolarization, it generates action potentials
(number 5) in the sensory neuron.

Initial

segment

of axon

Receptor;

dendrites

Threshold

5

4

3

2

1

Figure 10.3 The response of tonic receptors to

stimuli. Three successive stimuli of increasing strengths

are delivered to a receptor. The increasing amplitude of the

generator potential results in increases in the frequency of action

potentials, which persist as long as the stimulus is maintained.

Action

potentials

Generator

potentials

Stimuli

Threshold

Time

| CHECKPOINT

1a. Our perceptions are products of our brains;

they relate to physical reality only indirectly and

incompletely. Explain this statement, using examples

of vision and the perception of cold.

1b. Explain what is meant by the law of specific nerve

energies and the adequate stimulus, and relate these

concepts to your answer for question 1a.

1c. Describe sensory adaptation in olfactory and pain

receptors. Using a line drawing, relate sensory

adaptation to the responses of phasic and tonic

receptors.

2. Explain how the magnitude of a sensory stimulus
   is transduced into a receptor potential and how the
   magnitude of the receptor potential is coded in the
   sensory nerve fiber.