The Receptor Potential

The transduction process in all sensory receptors involves the opening or closing of ion channels that receive—either directly or through a second-messenger system—information about the outside world. The ion channels occur in a specialized receptor membrane and not on ordinary plasma membranes. The gating of these ion channels allows a change in the ion fluxes across the receptor membrane, which in turn produces a change in the membrane potential there. This change in potential is a graded potential called a receptor potential. The different mechanisms by which ion channels are affected in the various types of sensory receptors are described throughout this chapter.

The specialized receptor membrane where the initial ion-channel changes occur, unlike the axonal plasma membrane, does not generate action potentials. Instead, local current from the receptor membrane flows a short distance along the axon to a region where the membrane can generate action potentials. In myelinated afferent neurons, this region is usually at the first node of Ranvier of the myelin sheath (Figure 9-2).

In the case where the receptor membrane is on a separate cell, the receptor potential there causes the release of neurotransmitter, which diffuses across the extracellular cleft between the receptor cell and the afferent neuron and binds to specific sites on the afferent neuron. Thus, this junction is like a synapse. The combination of neurotransmitter with its binding sites on the afferent neuron generates a graded potential in the neuron's end analogous to either an excitatory post-synaptic potential or, in some cases, an inhibitory post-synaptic potential.

FIGURE 9-2

An afferent neuron with a receptor ending. The receptor potential arises at the nerve ending 1, and the action potential arises at the first node of the myelin sheath 2.

FIGURE 9-2

An afferent neuron with a receptor ending. The receptor potential arises at the nerve ending 1, and the action potential arises at the first node of the myelin sheath 2.

Receptor Potentials Action Potentials

PART TWO Biological Control Systems

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

PART TWO Biological Control Systems

Action

potential

response

([J|] 1 J J J J 1

Stimulus on

Stimulus off

FIGURE 9-3

Action potentials in a single afferent nerve fiber showing adaptation to a stimulus of constant strength.

As is true of all graded potentials, the magnitude of a receptor potential (or a graded potential in the axon adjacent to the receptor cell) decreases with distance from its origin. However, if the amount of depolarization at the first node in the afferent neuron is large enough to bring the membrane there to threshold, action potentials are initiated, which then propagate along the nerve fiber. The only function of the graded potential is to trigger action potentials. (See Figure 8-16 to review the properties of graded potentials.)

As long as the afferent neuron remains depolarized to or above threshold, action potentials continue to fire and propagate along the afferent neuron. Moreover, for complex reasons, an increase in the graded-potential magnitude causes an increase in the action-potential frequency in the afferent neuron (up to the limit imposed by the neuron's refractory period). Although the graded-potential magnitude determines action-potential frequency, it does not determine action-potential magnitude. Since the action potential is all-or-none, its magnitude is independent of the strength of the initiating stimulus.

Factors that control the magnitude of the receptor potential include stimulus strength, rate of change of stimulus strength, temporal summation of successive receptor potentials (see Figure 8-16), and a process called adaptation. This last process is a decrease in receptor sensitivity, which results in a decrease in the frequency of action potentials in an afferent neuron despite maintenance of the stimulus at constant strength (Figure 9-3). The degrees of adaptation vary widely between different types of sensory receptors. We shall see the significance of these differences later when we discuss the coding of stimulus duration.

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • gaudenzio
    Where does a receptor potential occur?
    3 months ago
  • adelgrim
    How degree of adaptation affects receptor potential?
    2 months ago

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