Question: 

Painful sensations caused by noxious mechanical stimuli usually increase during repetitive or prolonged stimulation (Adriansen et al. Hum. Neurobiol. 3: 53–58, 1984). In contradiction to this observation most mechanically sensitive C- and Ad fibre nociceptors show adaptive behaviour to repetitive stimulation. The neural basis of this discrepancy is unclear at date. For the observed adaptive process changes in the transduction of mechanical stimuli or in the induced discharge activity may be responsible.

Methods: 

Single afferent C- and Ad-fibres were recorded from a meningeal nerve in the hemisected rat cranial preparation using suction glass electrodes. To stimulate meningeal receptive fields of identified afferent fibres we used a combined mechano-electrostimulator that allows applying stimuli of both modalities to the same neuron (De Col et al. J Physiol. 2011 Dec 5. [Epub ahead of print]). Ongoing stimulation with single electrical pulses was used to induce discharge activity at different levels while responses to a 250 ms sinusoid mechanical stimulus were analysed.

Results: 

Increasing discharge activity of afferents led to a decrease in conduction velocity accompanied by a decrease in the response frequency as well as the number of action potentials evoked by the mechanical stimulus.

Conclusion: 

The history of preceding discharge activity is an important factor that determines the mechanical response properties of C- and Ad-fibers. Higher discharge frequencies limit responses to mechanical stimuli, which may contribute to the adaptive behaviour of mechanonociceptive afferents.