Comparison of nociceptive withdrawal reflexes and recruitment curves between the forelimbs and hind limbs in conscious horses.
OBJECTIVE
To compare nociceptive withdrawal reflexes (NWRs) evoked from the distal aspect of the left forelimb and hind limb in conscious standing horses and to investigate NWR recruitment for graded electrical stimulation intensities.


METHODS
20 adult horses.


METHODS
Surface electromyographic (EMG) activity evoked by transcutaneous electrical stimulation of the digital palmar (or plantar) nerve was recorded from the common digital extensor and cranial tibial muscles. Stimuli consisted of 25-millisecond train-of-5 constant current pulses. Current intensity was gradually increased until NWR threshold intensity was reached. The EMG signal was analyzed for quantification of the NWR. Behavioral responses accompanying the reflex were scored (scale, 0 to 5). The NWR recruitment curves were determined at 0.9, 1.1, 1.2, and 1.3 times the NWR threshold intensity.


RESULTS
The NWR threshold was significantly higher for the hind limb (median value, 6.6 mA; range, 3 to 10 mA) than the forelimb (median, 3 mA; range, 1.7 to 5.5 mA). The NWR of the hind limb had a significantly longer latency (median, 122.8 milliseconds; range, 106 to 172 milliseconds), compared with the forelimb (median, 98 milliseconds; range, 86 to 137 milliseconds), and it was associated with significantly stronger behavioral reactions. Gradual increase of NWR amplitude was evident at increasing stimulation intensities and supported by the behavioral observations.


CONCLUSIONS
We documented NWRs evoked from the forelimb and hind limb and their recruitment with stimuli of increasing intensity in horses. These results provide a basis for use of NWRs in studies on nociceptive modulation in horses.
Spadavecchia
Claudia
C
Department of Clinical Veterinary Sciences, University of Berne, Switzerland.
Arendt-Nielsen
Lars
L
Andersen
Ole K
OK
Spadavecchia
Luciano
L
Doherr
Marcus
M
Schatzmann
Urs
U
D000818:Animals; D003243:Consciousness; D005260:Female; D005552:Forelimb / Q000502:physiology*; D006614:Hindlimb / Q000502:physiology*; D006736:Horses / Q000502:physiology*; D008297:Male; D009619:Nociceptors / Q000502:physiology; D010146:Pain / Q000503:physiopathology* / Q000662:veterinary*; D017288:Pain Threshold / Q000502:physiology; D011999:Recruitment, Neurophysiological / Q000502:physiology; D012018:Reflex / Q000502:physiology*