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4 Graded Responses of Sensory Neurons
Learning Objective 4: To appreciate how the intensity and duration of a stimulus are reflected in the receptor potential and action potential discharge rate of a sensory afferent neuron.
Unlike the all-or-none action potential, receptor potentials are graded and are proportional to the strength of the stimulus that evokes them. Further, receptor potentials normally do not display refractoriness: They persist as long as the stimulus is present. The magnitude of the receptor potential is then reflected in either the rate or action potential generation (i.e., discharge or firing rate) in the sensory neuron or in the amount of neurotransmitter released in a specialized sensory receptor cell that lack axons. The relationship between the magnitude of the receptor potential and the firing rate of the sensory neuron (or the amount of neurotransmitter released for a specialized receptor cell that does not produce action potentials) forms the basis of the neural code, the firing rate or amount of transmitter released ‘reports’ the strength of a stimulus.
Each of the neuron’s four signaling components produces a characteristic signal. The figure shows a sensory neuron activated by stretching of a muscle, which the neuron senses through a specialized receptor, the muscle spindle. A. The input signal, called a receptor potential, is graded in amplitude and duration, proportional to the amplitude and duration of the stimulus. B. The trigger zone sums the depolarization generated by the receptor potential. An action potential is generated only if the receptor potential exceeds a certain voltage threshold. Once this threshold is surpassed, any further increase in amplitude of the receptor potential can only increase the frequency with which the action potentials are generated, because action potentials have a constant amplitude. The duration of the receptor potential determines the duration of the train of action potentials. Thus the graded amplitude and duration of the receptor potential is translated into a frequency code in the action potentials generated at the trigger zone. All action potentials produced are propagated faithfully along the axon. C. Action potentials are all-or-none. Because all action potentials have a similar amplitude and duration, the frequency and duration of firing represents the information carried by the signal. D. When the action potential reaches the synaptic terminal, it initiates the release of a neurotransmitter, the chemical substance that serves as the output signal. The frequency of action potentials determines how much neurotransmitter is released by the cell.
