Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 8

Properties of Sensory Receptors 151

GENERATION OF IMPULSES IN

CUTANEOUS RECEPTORS

PACINIAN CORPUSCLES

The way receptors generate action potentials in the sensory
nerves that innervate them varies with the complexity of the
sense organ. In the skin, the Pacinian corpuscle has been
studied in some detail. As noted above, the Pacinian corpus-
cles are touch receptors. Because of their relatively large size
and accessibility, they can be isolated, studied with microelec-
trodes, and subjected to microdissection. The myelin sheath
of the sensory nerve begins inside the corpuscle (Figure 8–2).
The first node of Ranvier is also located inside, whereas the
second is usually near the point at which the nerve fiber leaves
the corpuscle.

GENERATOR POTENTIALS

Recording electrodes can be placed on the sensory nerve as it

leaves a Pacinian corpuscle and graded pressure applied to the

corpuscle. When a small amount of pressure is applied, a non-

propagated depolarizing potential resembling an EPSP is re-

corded. This is called the

generator potential

or

receptor

potential

(Figure 8–2). As the pressure is increased, the mag-

nitude of the receptor potential increases. When the magni-

tude of the generator potential is about 10 mV, an action

potential is generated in the sensory nerve. As the pressure is

further increased, the generator potential becomes even larger

and the sensory nerve fires repetitively.

SOURCE OF THE GENERATOR POTENTIAL

By microdissection techniques, it has been shown that removal of

the connective tissue lamellas from the unmyelinated nerve

FIGURE 8–1
Sensory systems encode four elementary attributes of stimuli: modality, location (receptive field), intensity, and
duration (timing). A)
The human hand has four types of mechanoreceptors; their combined activation produces the sensation of contact with an
object. Selective activation of Merkel cells and Ruffini endings causes sensation of steady pressure; selective activation of Meissner’s and Pacinian
corpuscles causes tingling and vibratory sensation.
B)
Location of a stimulus is encoded by spatial distribution of the population of receptors ac-
tivated. A receptor fires only when the skin close to its sensory terminals is touched. These receptive fields of mechanoreceptors (shown as red
areas on fingertips) differ in size and response to touch. Merkel cells and Meissner’s corpuscles provide the most precise localization as they have
the smallest receptive fields and are most sensitive to pressure applied by a small probe.
C)
Stimulus intensity is signaled by firing rates of individ-
ual receptors; duration of stimulus is signaled by time course of firing. The spike trains indicate action potentials elicited by pressure from a small
probe at the center of each receptive field. Meissner’s and Pacinian corpuscles adapt rapidly, the others adapt slowly.
(From Kandel ER, Schwartz JH,
Jessell TM [editors]:
Principles of Neural Science,
4th ed. McGraw-Hill, 2000.)

Touch

Meissner’s

corpuscle

Stimulus

Neural

spike train

Receptors

A Modality

B Location

C Intensity and time course

Receptive

field

Merkel

cells

Pacinian

corpuscle

Ruffini

endings