Lateral Inhibition

When a blunt object touches the skin, a number of receptive fields are stimulated—some more than others. The receptive fields in the center areas where the touch is strongest will be stimulated more than those in the neighboring fields where the touch is lighter. Stimulation will gradually diminish from the point of greatest contact, without a clear, sharp boundary. What we perceive, however, is not the fuzzy sensation that might be predicted. Instead, only a single touch with well-defined borders is felt. This sharpening of sensation is due to a process called lateral inhibition (fig. 10.6).

Lateral inhibition and the resultant sharpening of sensation occur within the central nervous system. Those sensory neurons whose receptive fields are stimulated most strongly inhibit—via interneurons that pass "laterally" within the CNS—sensory neurons that serve neighboring receptive fields.

Lateral inhibition is a common theme in sensory physiology, though the mechanisms involved are different for each sense. In hearing, lateral inhibition helps to more sharply tune the ability of the brain to distinguish sounds of different pitches. In vision, it helps the brain to more sharply distinguish borders of light and darkness; and in olfaction, it helps the brain to more clearly distinguish closely related odors.

Perception of two points of touch

Perception of two points of touch

Perception of one point of touch

Sensory neuron

Perception of one point of touch

Sensory neuron

■ Figure 10.5 The two-point touch threshold test. If each point touches the receptive fields of different sensory neurons, two separate points of touch will be felt. If both caliper points touch the receptive field of one sensory neuron, only one point of touch will be felt.

Lateral inhibition within central nervous system

Blunt object

ofio

Lateral inhibition within central nervous system

Blunt object

ofio

Lateral inhibition sharpens perception

Skin location

Lateral inhibition sharpens perception

Skin location

■ Figure 10.6 Lateral inhibition. When an object touches the skin (a), receptors in the central area of the touched skin are stimulated more than neighboring receptors (b). Lateral inhibition within the central nervous system reduces the input from these neighboring sensory neurons. Sensation, as a result, is sharpened within the area of skin that was stimulated the most (c).

Table 10.3 The Two-Point Touch Threshold for Different Regions of the Body

Two-Point Touch Body Region Threshold (mm)

Big toe

10

Sole of foot

22

Calf

48

Thigh

46

Back

42

Abdomen

36

Upper arm

47

Forehead

18

Palm of hand

13

Thumb

3

First finger

2

Source: From S. Weinstein and D. R. Kenshalo, editors, The Skin Senses, © 1968. Courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois.

Source: From S. Weinstein and D. R. Kenshalo, editors, The Skin Senses, © 1968. Courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois.

Test Yourself Before You Continue

1. Using a flow diagram, describe the neural pathways leading from cutaneous pain and pressure receptors to the postcentral gyrus. Indicate where crossing-over occurs.

2. Define the term sensory acuity and explain how acuity is related to the density of receptive fields in different parts of the body.

3. Explain the mechanism of lateral inhibition in cutaneous sensory perception and discuss its significance.

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