Membrane Properties

Nociceptors exhibit specialized electrical properties that set them apart from low-threshold mechanore-ceptors, whose cell bodies also are located in sensory ganglia. At a descriptive level, the action potentials in nociceptors recorded intrasomatically have consistently different shapes, suggestive of the contribution of different ion channels (Fig. 2). Specifically, the spike in nociceptors tends to be larger and broader than spikes in low-threshold mechanoreceptors. A shoulder is often observed on the descending limb of the spike. Spikes in these cells also exhibit a longer lasting after-hyperpolarization. It has been suggested that the differences in spike properties between nociceptors and nonnociceptors are caused by a reduced delayed rectifier, leading to a larger upstroke and a larger Ca channel conductance. This would account for both the shoulder and the longer postspike after-hyperpolar-ization (AHP), the latter mediated by an enhanced Ca-mediated K conductance. These differences in spike configuration have been useful in studies carried out in dissociated cell culture where other criteria to classify nociceptors (e.g., response threshold) are unavailable.

Nociceptors also have a unique complement of Na channels that distinguishes them from nonnociceptors. Na channels are highly differentiated and can be classified in part on the basis of their sensitivity to the neurotoxin tetrodotoxin (TTX). Some Na channels can be blocked by TTX concentrations in the nano-molar range, whereas others are insensitive to TTX concentrations as high as 10 mM. Several sensory-neuron-specific Na channels have been identified, including the TTX-sensitive channel called PN1 and the TTX-insensitive channels called SNS/PN3 and SNS2/NaN. These Na channels have been cloned, and the expression of SNS/PN3 and SNS/NaN has been found to be restricted to small somata, i.e., putative nociceptors. In agreement with these studies, the somatic spike of nociceptors recorded in vivo is

Figure 2 Examples of action potentials recorded intracellularly from the soma of a nociceptor and a low-threshold mechanor-eceptor. Note the difference in spike configuration. Adapted from Traub, R. J. and Mendell, L. M. (1988). J. Neurophysiol. 59, 41-55, with permission. Further details are given in the text.

Figure 2 Examples of action potentials recorded intracellularly from the soma of a nociceptor and a low-threshold mechanor-eceptor. Note the difference in spike configuration. Adapted from Traub, R. J. and Mendell, L. M. (1988). J. Neurophysiol. 59, 41-55, with permission. Further details are given in the text.

resistant to TTX, in contrast to the somatic impulses in low-threshold afferents that are blocked by TTX. However, axonal spikes in both classes of afferents can be blocked by TTX. Thus, the membrane of individual DRG cells is not necessarily homogeneous with respect to Na channel expression. Significantly, there is also evidence for TTX-resistant Na channels in the peripheral terminals of nociceptive neurons. Thus, any physiological role for TTX-resistant channels obtained from studies of the cell body can be applied to generation of the impulse in the sensory terminals (see Section VIII.A on peripheral sensitization).

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