Mechanothermal nociceptors in the scaly skin of the chicken leg

Electrophysiological recordings were made from single sensory mechanothermal nociceptive afferent fibres in dissected nerve filaments of the parafibular nerve innervating the scaly skin of the lower leg of the chicken. Two classes of mechanothermal nociceptors were identified consisting of 34 C fibres (conduction velocities 0.45-1.5 m/s, mean 1.08) and nine A-delta fibres (3-15 m/s. mean 6.34). The C fibre afferents had receptive fields which were circular or elliptical in shape and ranged in size from I mm in diameter to 4X3 mm. Thresholds to mechanical stimulation in the C fibre afferents ranged from 0.3 to 33 g (median 1.5 g) and thermal thresholds were in the range 39-61 degreesC (median 49.4 degreesC). Stimulus-response curves to thermal and/or mechanical stimulation were recorded from 28 C fibre afferents and subjected to a linear regression analysis to determine whether they were best fitted by a linear, log or power function. The results were variable and no single function provided the best fit for all the responses. Of the fibres tested with both stimulus modalities (n = 17). only 12 fibres showed the same best fit for both stimuli; in the others the best fit regression lines differed between stimuli. The response of the A-delta fibres to mechanical and thermal stimulation was very similar to the C fibres but the small number of A-delta fibres precluded any detailed statistical analysis. Comparison of the physiological properties of the C fibres in the leg with those previously identified in the beak showed that those in the leg had significantly lower thermal thresholds, but higher mechanical thresholds. The possible functional significance of these differences is discussed. These findings are also discussed in a comparative context to identify similarities and differences between mechanothermal nociceptors in birds and other vertebrates, relating these to their possible evolutionary and functional significance. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.