Putative nociceptor responses to mechanical and chemical stimulation in skeletal muscles of the chicken leg

Electrophysiological responses of nociceptive sensory afferent fibres in the skeletal muscle of the chicken (Gallus domesticus) were examined using mechanical and chemical stimulation. The activity of single nociceptive afferent fibres was recorded from micro-dissected filaments of the fibular and lateral tibial nerves, which innervate the fibularis longus and lateral gastrocnemius muscles. Seventeen putative nociceptive fibres were identified by mechanical stimulation (muscle compression). Conduction velocities (CVs) ranged from 2.8 to 11.3 m/s (mean 5.8; S.E.M.F0.9 m/s). Response thresholds to tissue compression ranged from 38 to 126 kPa (mean 81; S.E.M.±4 kPa). Increases in pressure intensity, above individual fibre thresholds (X2 moderate; X3 noxious), produced intensity dependent increases in discharge rates. Fibres exhibited slowly adapting, irregular discharges lasting the duration of the stimulus and showed no spontaneous activity in the absence of mechanical stimulation. Intramuscular injection of acetic acid (1% v/v in isotonic saline; pH 2.8) in to the receptive field area stimulated discharge activity in 13 of the 17 (76%) pressure sensitive fibres. Acid injection resulted in prolonged irregular single or intermittent clustered discharges, which continued beyond the 15-min recording period. This study demonstrates the existence of nociceptive sensory fibres in chicken skeletal muscle that are able to respond to and encode acute tissue threatening and subjectively painful stimuli. The physiological characteristics of these nociceptive afferents are consistent with mammalian group III skeletal muscle nociceptors. These findings support the suggestion of a common, acute nociceptive response function in skeletal muscle in avians and other vertebrate classes.