Generator Receptor Potential

The electrical behavior of sensory nerve endings is similar to that of the dendrites of other neurons. In response to an environmental stimulus, the sensory endings produce local graded changes in the membrane potential. In most cases, these potential changes are depolarizations that are analogous to the excitatory postsynaptic potentials (EPSPs) described in chapter 7. In the sensory endings, however, these potential changes in response to environmental stimulation are called receptor, or generator, potentials because they serve to generate action potentials in response to the sensory stimulation. Since sensory neurons are pseudounipolar (chapter 7), the action potentials produced in response to the generator potential are conducted continuously from the periphery into the CNS.

The pacinian, or lamellated, corpuscle, a cutaneous receptor for pressure (see fig. 10.4), can serve as an example of sensory transduction. When a light touch is applied to the receptor, a small depolarization (the generator potential) is produced. Increasing the pressure on the pacinian corpuscle increases the magnitude of the generator potential until it reaches the threshold depolarization required to produce an action potential (fig. 10.2). The pacinian corpuscle, however, is a phasic receptor; if the pressure is maintained, the size of the generator potential produced quickly diminishes. It is interesting to note that this phasic response is a result of the onionlike covering around the dendritic nerve ending; if the layers are peeled off and the nerve ending is stimulated directly, it will respond in a tonic fashion.

When a tonic receptor is stimulated, the generator potential it produces is proportional to the intensity of the stimulus. After a threshold depolarization is produced, increases in the amplitude of the generator potential result in increases in the frequency with which action potentials are produced (fig. 10.3). In this way, the frequency of action potentials that are conducted into the central nervous system serves as the code for the strength of the stimulus. As described in chapter 7, this frequency code is needed because the amplitude of action potentials is constant (all or none). Acting through changes in action potential frequency, tonic receptors thus provide information about the relative intensity of a stimulus.

Table IG.I

Classification of Receptors Based on Their Normal (or "Adequate") Stimulus

Receptor

Normal Stimulus

Mechanisms

Examples

Mechanoreceptors

Mechanical force

Deforms cell membranes of sensory dendrites or

Cutaneous touch and pressure receptors; vestibular

deforms hair cells that activate sensory nerve endings

apparatus and cochlea

Pain receptors

Tissue damage

Damaged tissues release chemicals that excite sensory

Cutaneous pain receptors

endings

Chemoreceptors

Dissolved chemicals

Chemical interaction affects ionic permeability of

Smell and taste (exteroceptors) osmoreceptors and

sensory cells

carotid body chemoreceptors (interoceptors)

Photoreceptors

Light

Photochemical reaction affects ionic permeability of

Rods and cones in retina of eye

receptor cell

Initial segment of axon

Receptor; dendrites

Initial segment of axon

Receptor; dendrites

Threshold

Threshold

■ Figure 10.2 The receptor (generator) potential. Sensory stimuli result in the production of local graded potential changes known as receptor, or generator, potentials (numbers 1-4). If the receptor potential reaches a threshold value of depolarization, it generates action potentials (number 5) in the sensory neuron.

Action potentials

Generator potentials

Stimuli

-Threshold

-Threshold

Time

■ Figure 10.3 The response of tonic receptors to stimuli. Three successive stimuli of increasing strengths are delivered to a receptor. The increasing amplitude of the generator potential results in increases in the frequency of action potentials, which persist as long as the stimulus is maintained.

Test Yourself before You Continue

1. Our perceptions are products of our brains; they relate to physical reality only indirectly and incompletely. Explain this statement, using examples of vision and the perception of cold.

2. Explain what is meant by the law of specific nerve energies and the adequate stimulus, and relate these concepts to your answer for question no. 1.

3. Describe sensory adaptation in olfactory and pain receptors. Using a line drawing, relate sensory adaptation to the responses of phasic and tonic receptors.

4. Explain how the magnitude of a sensory stimulus is transduced into a receptor potential and how the magnitude of the receptor potential is coded in the sensory nerve fiber.

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