Ganong's Review of Medical Physiology, 23rd Edition

152
SECTION II
Physiology of Nerve & Muscle Cells

ending in a Pacinian corpuscle does not abolish the generator po-
tential. When the first node of Ranvier is blocked by pressure or
narcotics, the generator potential is unaffected but conducted im-
pulses are abolished (Figure 8–2). When the sensory nerve is sec-
tioned and the nonmyelinated terminal is allowed to degenerate,
no generator potential is formed. These and other experiments
have established that the generator potential is produced in the
unmyelinated nerve terminal. The receptor therefore converts
mechanical energy into an electrical response, the magnitude of
which is proportionate to the intensity of the stimulus. The gen-
erator potential in turn depolarizes the sensory nerve at the first
node of Ranvier. Once the firing level is reached, an action poten-
tial is produced and the membrane repolarizes. If the generator
potential is great enough, the neuron fires again as soon as it re-
polarizes, and it continues to fire as long as the generator poten-
tial is large enough to bring the membrane potential of the node
to the firing level. Thus, the node converts the graded response of
the receptor into action potentials, the frequency of which is pro-
portionate to the magnitude of the applied stimuli.

SENSORY CODING

Converting a receptor stimulus to a recognizable sensation is
termed
sensory coding.
All sensory systems code for four ele-
mentary attributes of a stimulus: modality, location, intensity,
and duration.
Modality
is the type of energy transmitted by the
stimulus.
Location
is the site on the body or space where the
stimulus originated.
Intensity
is signaled by the response am-
plitude or frequency of action potential generation.
Duration
refers to the time from start to end of a response in the receptor.

These attributes of sensory coding are shown for the modality

of touch in Figure 8–1.

MODALITY

Humans have four basic classes of receptors based on their sen-

sitivity to one predominant form of energy: mechanical, ther-

mal, electromagnetic, or chemical. The particular form of energy

to which a receptor is most sensitive is called its

adequate stim-

ulus.

The adequate stimulus for the rods and cones in the eye, for

example, is light (an example of electromagnetic energy). Recep-

tors do respond to forms of energy other than their adequate

stimuli, but the threshold for these nonspecific responses is

much higher. Pressure on the eyeball will stimulate the rods and

cones, for example, but the threshold of these receptors to pres-

sure is much higher than the threshold of the pressure receptors

in the skin.

LOCATION

The term

sensory unit

is applied to a single sensory axon and all

its peripheral branches. These branches vary in number but may

be numerous, especially in the cutaneous senses. The

receptive

field

of a sensory unit is the spatial distribution from which a

stimulus produces a response in that unit (Figure 8–1). Repre-

sentation of the senses in the skin is punctate. If the skin is care-

fully mapped, millimeter by millimeter, with a fine hair, a

sensation of touch is evoked from spots overlying these touch re-

ceptors. None is evoked from the intervening areas. Similarly,

temperature sensations and pain are produced by stimulation of

FIGURE 8–2
Demonstration that the generator potential in a Pacinian corpuscle originates in the unmyelinated nerve terminal.
(1)
The electrical responses to a pressure of 1
×
(record a), 2
×
(b), 3
×
(c), and 4
×
(d) were recorded. The strongest stimulus produced an action po-
tential in the sensory nerve (e).
(2)
Similar responses persisted after removal of the connective tissue capsule, except that the responses were more
prolonged because of partial loss of adaptation.
(3)
The generator responses persisted but the action potential was absent when the first node of
Ranvier was blocked by pressure or with narcotics (arrow).
(4)
All responses disappeared when the sensory nerve was cut and allowed to degen-
erate before the experiment.

1

2

3 4

d

e

e

c

b

a

dc

ba

dc

b

a

abcd