D41 Electrophysiology of receptors

Topic

Electrophysiological recording of a sensory receptor generally measures the electrical activity of either an individual sensory neuron or a group of such neurons using a tungsten or glass microelectrode inserted into the nervous tissue or the receptor's axon.

This extracellular recording technique captures each action potential of the sensory neuron as a deflection in extracellular voltage—the very same electrical activity mentioned at the beginning of this paragraph. Bringing the microelectrode closer to the sensory neuron results in a sharper extracellular voltage deflection.

In contrast, the Hodgkin-Huxley model describes the ionic mechanisms underlying the action potentials introduced at the start of this document: the sodium and potassium currents of the sensory neuron, along with their specific activation and inactivation kinetics, generate the characteristic shape of the action potential. A faster activation of the sodium current results in a more abrupt action potential waveform. Stevens' intensity-response curve describes how the sensory neuron's final firing rate scales with the stimulus intensity, following Stevens' power law—a model also found elsewhere in this set—but applied here to the neuron's firing rate rather than to the subject's perceived magnitude of the stimulus. Each distinct sensory modality has its own characteristic exponent: this exponent is considerably lower for touch and considerably higher for pain. A higher characteristic exponent for a given sensory modality means that the neuron's firing rate increases more rapidly as stimulus intensity rises.

The sensory receptor's adaptation constant distinguishes between a *phasic* receptor—which responds primarily to changes in the stimulus and adapts quickly—and a *tonic* receptor—which responds to the sustained level of the stimulus. This distinction mirrors the one established elsewhere in this set between the Pacinian corpuscle (sensitive to much higher frequencies and thus markedly phasic) and the Merkel disc (sensitive to fine texture in a much more sustained manner and thus markedly tonic). Therefore, this model—utilizing the final electrical activity of an individual sensory neuron—allows the Shannon information established by another model in this set to quantify how much of this final information regarding the stimulus is transmitted by the sensory neuron per unit of time; this, combined with the final discriminative capacity established by that same model via signal detection theory, characterizes the sensory receptor's final discriminative capacity in the face of accompanying noise. Consequently, if the sensory neuron transmits a greater amount of this Shannon information per unit of time, the sensory receptor's final discriminative capacity is likewise increased.

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