Stimulus Location

A third type of information to be signaled is the location of the stimulus—in other words, where the stimulus is being applied. (It should be noted that in vision, hearing, and smell, stimulus location is interpreted as arising from the site from which the stimulus originated rather than the place on our body where the stimulus was actually applied. For example, we interpret the sight and sound of a barking dog as occurring in that furry thing on the other side of the fence rather than in a specific region of our eyes and ears. More will be said of this later; we deal here with the senses in which the stimulus is located to a site on the body.)

The main factor coding stimulus location is the site of the stimulated receptor. The precision, or acuity, with which one stimulus can be located and differentiated from an adjacent one depends upon the amount of convergence of neuronal input in the specific ascending pathways. The greater the convergence, the less the acuity. Other factors affecting acuity are the size of the receptive field covered by a single sensory unit and the amount of overlap of nearby receptive fields. For example, it is easy to discriminate between two adjacent stimuli (two-point discrimination) applied to the skin on a finger, where the sensory units are small and the overlap considerable. It is harder to do so on the back, where the sensory units are large and widely spaced. Locating sensations from internal organs is less precise than from the skin because there are fewer afferent neurons in the internal organs and each has a larger receptive field.

It is fairly simple to see why a stimulus to a neuron that has a small receptive field can be located more precisely than a stimulus to a neuron with a large receptive field (Figure 9-10). The fact is, however, that even in the former case one cannot distinguish exactly where within the receptive field of a single neuron a

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

9. The Sensory Systems

© The McGraw-Hill Companies, 2001

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Receptive Fields Skin

FIGURE 9-10

The information from neuron a indicates the stimulus location more precisely than does that from neuron b because a's receptive field is smaller.

Central nervous system

FIGURE 9-10

The information from neuron a indicates the stimulus location more precisely than does that from neuron b because a's receptive field is smaller.

stimulus has been applied; one can only tell that the afferent neuron has been activated. In this case, receptive-field overlap aids stimulus localization even though, intuitively, overlap would seem to "muddy" the image. Let us examine in the next two paragraphs how this works.

An afferent neuron responds most vigorously to stimuli applied at the center of its receptive field because the receptor density—that is, the number of receptors in a given area—is greatest there. The response decreases as the stimulus is moved toward the receptive-field periphery. Thus, a stimulus activates more receptors and generates more action potentials if it occurs at the center of the receptive field (point A in Figure 9-11). The firing frequency of the afferent neuron is also related to stimulus strength, however, and a high frequency of impulses in the single afferent nerve fiber of Figure 9-11 could mean either that a moderately intense stimulus was applied to the center at A or that a strong stimulus was applied to the periphery at B. Thus, neither the intensity nor the location of the stimulus can be detected precisely with a single afferent neuron.

Since the receptor endings of different afferent neurons overlap, however, a stimulus will trigger activity in more than one sensory unit. In Figure 9-12, neurons A and C, stimulated near the edge of their receptive fields where the receptor density is low, fire at a lower frequency than neuron B, stimulated at the center of its receptive field. In the group of sensory

Stimulus B

FIGURE 9-11

Two stimulus points, A and B, in the receptive field of a single afferent neuron. The density of nerve endings around area A is greater than around B, and the frequency of action potentials in response to a stimulus in area A will be greater than the response to a similar stimulus in B.

- Stimulus A

Stimulus B

FIGURE 9-11

Two stimulus points, A and B, in the receptive field of a single afferent neuron. The density of nerve endings around area A is greater than around B, and the frequency of action potentials in response to a stimulus in area A will be greater than the response to a similar stimulus in B.

Receptive Field And Location Stimulus

FIGURE 9-12

A stimulus point falls within the overlapping receptive fields of three afferent neurons. Note the difference in receptor response (that is, the action-potential frequency in the three neurons) due to the difference in receptor distribution under the stimulus (low receptor density in A and C, high in B).

FIGURE 9-12

A stimulus point falls within the overlapping receptive fields of three afferent neurons. Note the difference in receptor response (that is, the action-potential frequency in the three neurons) due to the difference in receptor distribution under the stimulus (low receptor density in A and C, high in B).

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Afferent neurons

Action potentials _

in interneuron

Action potentials in afferent neuron

-< Excitatory synapses -< Inhibitory synapses

FIGURE 9-13

Afferent pathways showing lateral inhibition. The central fiber at the beginning of the pathway (bottom of figure) is firing at the highest frequency and inhibits, via inhibitory neurons A, the lateral neurons more strongly than the lateral pathways inhibit it, via inhibitory neurons B.

units in Figure 9-12, a high action-potential frequency in neuron B occurring simultaneously with lower frequencies in A and C permits a more accurate localization of the stimulus near the center of neuron B's receptive field. Once this location is known, the firing frequency of neuron B can be used to indicate stimulus intensity.

Lateral Inhibition The phenomenon of lateral inhibition is, however, far more important in localization of the stimulus site than are the different sensitivities of receptors throughout the receptive field. In lateral inhibition, information from afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus's center. Thus, lateral inhibition increases the contrast between relevant and irrelevant information, thereby increasing the effectiveness of selected pathways and focusing sensory-processing mechanisms on "important" messages. Figure 9-13 shows one neuronal arrangement that accomplishes lateral inhibition. Lateral inhibition can occur at different levels of the sensory pathways but typically happens at an early stage.

Lateral inhibition can be demonstrated by pressing the tip of a pencil against your finger. With your eyes closed, you can localize the pencil point precisely, even though the region around the pencil tip is also indented and mechanoreceptors within this region are activated (Figure 9-14). Exact localization occurs because the information from the peripheral regions is removed by lateral inhibition.

Lateral inhibition is utilized to the greatest degree in the pathways providing the most accurate localization. For example, movement of skin hairs, which we can locate quite well, activates pathways that have significant lateral inhibition, but temperature and pain, which we can locate only poorly, activate pathways that use lateral inhibition to a lesser degree.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • janet rivera
    Where is stimulus located?
    8 years ago
  • archibald
    How is a stimulus of greater intensity porcessed as compared to a stimulus of lesser intensity?
    7 years ago
  • SELENE
    Where are the stimulus points on the body?
    6 years ago
  • preston
    What is stimmilus andwhere located?
    6 years ago
  • Jaxon
    Why stimulus to a neuron with small receptive field is more precisely located?
    3 years ago
  • clelia
    Where can we locate stimulus?
    1 year ago
  • JAMES
    Why is location of stimulus important?
    1 year ago
  • Why vary the region where the stimulus was applied?
    1 month ago

Post a comment