Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 8

Properties of Sensory Receptors 153

the skin only over the spots where the receptors for these modal-
ities are located. In the cornea and adjacent sclera of the eye, the
surface area supplied by a single sensory unit is 50–200 mm
2
.
Generally, the areas supplied by one unit overlap and interdigi-
tate with the areas supplied by others.
One of the most important mechanisms that enable local-
ization of a stimulus site is
lateral inhibition.
Information
from sensory neurons whose receptors are at the peripheral
edge of the stimulus is inhibited compared to information
from the sensory neurons at the center of the stimulus. Thus,
lateral inhibition enhances the contrast between the center
and periphery of a stimulated area and increases the ability of
the brain to localize a sensory input. Lateral inhibition under-
lies
two-point discrimination
(see Clinical Box 8–1).

INTENSITY

The intensity of sensation is determined by the amplitude of the
stimulus applied to the receptor. This is illustrated in Figure 8–3.
As a greater pressure is applied to the skin, the receptor poten-
tial in the mechanoreceptor increases (not shown), and the fre-
quency of the action potentials in a single axon transmitting
information to the CNS is also increased. In addition to increas-
ing the firing rate in a single axon, the greater intensity of stim-
ulation also will recruit more receptors into the receptive field.

It has long been taught that the magnitude of the sensation felt

is proportional to the log of the intensity of the stimulus

(Weber–Fechner law).

It now appears, however, that a power

function more accurately describes this relation. In other words,

R = KS

A

where R is the sensation felt, S is the intensity of the stimulus,

and, for any specific sensory modality, K and A are constants.

The frequency of the action potentials generated in a sensory

nerve fiber is also related to the intensity of the initiating

stimulus by a power function. An example of this relation is

shown is shown in Figure 8–4, in which the calculated expo-

nent is 0.52. However, the relation between direct stimulation

of a sensory nerve and the sensation felt is linear. Conse-

quently, it appears that for any given sensory modality, the

relation between sensation and stimulus intensity is deter-

mined primarily by the properties of the peripheral receptors.

DURATION

When a maintained stimulus of constant strength is applied to

a receptor, the frequency of the action potentials in its sensory

nerve declines over time. This phenomenon is known as

adap-

tation

or

desensitization.

The degree to which adaptation oc-

curs varies from one sense to another. Receptors can be

classified into

rapidly adapting (phasic) receptors

and

slowly

adapting (tonic) receptors.

This is illustrated for different

types of touch receptors in Figure 8–1

.

Meissner and Pacinian

corpuscles are examples of rapidly adapting receptors, and

Merkel cells and Ruffini endings are examples of slowly adapt-

ing receptors. Other examples of slowly adapting receptors are

muscle spindles and nociceptors. Different types of sensory

adaptation appear to have some value to the individual. Light

touch would be distracting if it were persistent; and, converse-

ly, slow adaptation of spindle input is needed to maintain pos-

ture. Similarly, input from nociceptors provides a warning

that would lose its value if it adapted and disappeared.

SENSORY INFORMATION

The speed of conduction and other characteristics of sensory

nerve fibers vary, but action potentials are similar in all nerves.

The action potentials in the nerve from a touch receptor, for

example, are essentially identical to those in the nerve from a

warmth receptor. This raises the question of why stimulation

of a touch receptor causes a sensation of touch and not of

warmth. It also raises the question of how it is possible to tell

whether the touch is light or heavy.

LAW OF SPECIFIC NERVE ENERGIES

The sensation evoked by impulses generated in a receptor de-

pends in part on the specific part of the brain they ultimately

activate. The specific sensory pathways are discrete from sense

CLINICAL BOX 8–1

Two-Point Discrimination

The size of the receptive fields for light touch can be mea-

sured by the

two-point threshold test.

In this procedure,

the two points on a pair of calipers are simultaneously posi-

tioned on the skin and one determines the minimum dis-

tance between the two caliper points that can be perceived

as separate points of stimulation. This is called the

two-point

discrimination threshold.

If the distance is very small, each

caliper point is touching the receptive field of only one sen-

sory neuron. If the distance between stimulation points is

less than this threshold, only one point of stimulation can be

felt. Thus, the two-point discrimination threshold is a meas-

ure of

tactile acuity.

The magnitude of two-point discrimi-

nation thresholds varies from place to place on the body and

is smallest where touch receptors are most abundant. Stimu-

lus points on the back, for instance, must be separated by at

least 65 mm before they can be distinguished as separate,

whereas on the fingertips two stimuli are recognized if they

are separated by as little as 2 mm. Blind individuals benefit

from the tactile acuity of fingertips to facilitate the ability to

read Braille; the dots forming Braille symbols are separated

by 2.5 mm. Two-point discrimination is used to test the in-

tegrity of the

dorsal column (medial lemniscus) system,

the central pathway for touch and proprioception.