The relationship between axonal spike shape and functional modality in cutaneous C-fibres in the pig and rat
Section snippets
Anaesthesia and surgery
Pigs (Deutsches Landschwein, n=40; weights 12–35 kg) were premedicated [azaperone (Stresnil, Janssen, Belgium), 2 mg/kg, i.m.] and then anaesthetized with halothane (1.5–2%) in 60% N2O/40% O2. The electrocardiogram, rectal temperature and expired CO2 were monitored continuously. The room temperature was kept between 24 and 26°C, the pig was covered with paper drapes and a heating pad was switched on if necessary to prevent any fall in temperature. Under these conditions, rectal temperature was
Results
Recordings of axonal spike shape have been made from C-fibres dissected from the pig and rat saphenous nerve. The 109 C-fibres studied in the pig comprised 32 heat nociceptor units, 32 polymodal nociceptor units, 24 low-threshold mechanoreceptor units and 21 inexcitable C-fibres (see Table 1). The 180 C-fibres studied in the rat comprised 70 polymodal nociceptor units, 14 mechanical nociceptor units, three heat nociceptor units, 23 mechanoreceptor units, 20 cold thermoreceptor units, 28
Variability of spike shapes
Recording extracellularly from fine filaments is not the ideal way to get information about action potential shape. Many factors related to the recording conditions will affect the amplitude and time-course of the action potential recorded in this way. Comparing spikes recorded from the same filament at the same time reduced the variability considerably, but only small numbers of such recordings were achieved, and only for the more numerous classes. A major variable will be the extent to which
Conclusions
This study has shown that, in two different species, axonal spikes of nociceptive C-fibres are of longer duration than those of C-mechanoreceptors. In the pig, where two major sub-classes of C-fibre nociceptor are found, the heat nociceptors have wider spikes than the polymodal nociceptors. In the rat, sympathetic postganglionic axons have spikes of longer duration than afferent axons. None of the differences in spike duration can be explained by differences in conduction velocity. The
Acknowledgements
M.D.G. was funded by a Bayliss Starling studentship from University College London. Funding for B.L. to visit Bad Nauheim was provided by the Max-Planck-Gesellschaft.
References (34)
- et al.
Activity-dependent slowing of conduction velocity provides a method for identifying different functional classes of C-fibre in the rat saphenous nerve
Neuroscience
(1996) - et al.
Co-expression of nociceptor properties in dorsal root ganglion neurons from the adult rat in vitro
Neuroscience
(1996) - et al.
Capsaicin blocks tetrodotoxin-resistant sodium potentials and calcium potentials in unmyelinated C fibres of biopsied human sural nerve in vitro
Neurosci. Lett.
(1996) - et al.
Ionic currents in the somatic membrane of rat dorsal root ganglion neurons—I. Sodium currents
Neuroscience
(1981) - et al.
Ontogenic development of the TTX-sensitive and TTX-insensitive Na+ channels in neurons of the rat dorsal root ganglia
Brain Res. devl Brain Res.
(1992) - et al.
Differential expression of membrane currents in dissociated mouse primary sensory neurons
Neuroscience
(1994) - et al.
Calcium potentials and tetrodotoxin-resistant sodium potentials in unmyelinated C fibres of biopsied human sural nerve
Neuroscience
(1995) - et al.
Changes in axonal impulse conduction correlate with sensory modality in primary afferent fibers in the rat
Brain Res.
(1990) - et al.
Somal action potential duration differs in identified primary afferents
Neurosci. Lett.
(1986) Ca2+-activated K+ currents in neurones: types, physiological roles and modulation
Trends Neurosci.
(1996)