Stimulus Duration

Receptors differ in the way they respond to a constantly maintained stimulus—that is, in the way they undergo adaptation.

The response—the action-potential frequency—at the beginning of the stimulus indicates the stimulus strength, but after this initial response, the frequency differs widely in different types of receptors. Some receptors respond very rapidly at the stimulus onset, but, after their initial burst of activity, fire only very slowly or stop firing all together during the remainder of the stimulus. These are the rapidly adapting receptors; they are important in signaling rapid change (for example, vibrating or moving stimuli). Some receptors adapt so rapidly that they fire only a single action potential at the onset of a stimulus—an on response—

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

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Lateral Inhibition Receptor

FIGURE 9-14

(a) A pencil tip pressed against the skin depresses surrounding tissue. Receptors are activated under the pencil tip and in the adjacent tissue. (b) Because of lateral inhibition, the central area of excitation is surrounded by an area where the afferent information is inhibited. (c) The sensation is localized to a more restricted region than that in which mechanoreceptors are actually stimulated.

FIGURE 9-14

(a) A pencil tip pressed against the skin depresses surrounding tissue. Receptors are activated under the pencil tip and in the adjacent tissue. (b) Because of lateral inhibition, the central area of excitation is surrounded by an area where the afferent information is inhibited. (c) The sensation is localized to a more restricted region than that in which mechanoreceptors are actually stimulated.

Action potentials

Stimulus intensity

Rapidly adapting

Action potentials

Stimulus intensity

Time

Slowly adapting

Time

FIGURE 9-15

Rapidly and slowly adapting receptors. The top line in each graph indicates the action-potential firing of the afferent nerve fiber from the receptor, and the bottom line, application of the stimulus.

from higher centers in the brain. The reticular formation and cerebral cortex, in particular, control the input of afferent information via descending pathways. The inhibitory controls may be exerted directly by synapses on the axon terminals of the primary afferent neurons (an example of presynaptic inhibition) or indirectly via interneurons that affect other neurons in the sensory pathways (Figure 9-16).

Descending Pathways

FIGURE 9-16

Descending pathways may control sensory information by directly inhibiting the central terminals of the afferent neuron (an example of presynaptic inhibition) or via an interneuron that affects the ascending pathway by inhibitory synapses. Arrows indicate the direction of action-potential transmission.

while others respond at the beginning of the stimulus and again at its removal—so-called on-off responses. The rapid fading of the sensation of clothes pressing on one's skin is due to rapidly adapting receptors.

Slowly adapting receptors maintain their response at or near the initial level of firing regardless of the stimulus duration (Figure 9-15). These receptors signal slow changes or prolonged events, such as occur in the joint and muscle receptors that participate in the maintenance of upright posture when standing or sitting for long periods of time.

Essentials of Human Physiology

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  • olo
    What are stimulus of the body?
    7 years ago

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