Quantitative assessment of nociception in horses by use of the nociceptive withdrawal reflex evoked by transcutaneous electrical stimulation.

The research paper assesses pain response (nociception) in horses using a reflex test that uses transcutaneous electrical stimulation. The study shows it is possible to record nociceptive withdrawal reflex (NWR) in conscious standing horses and measure reactions to different stimulus intensities.

Research Methods

• The study involved 10 adult horses.
• The horses’ lateral palmar digital nerve on the forelimb was stimulated transcutaneously. In simpler terms, an electrical stimulus was applied through the skin to stimulate this nerve.
• Responses to this stimulus were recorded from various muscles in the forelimb using surface electromyographic recordings. This process records the electrical activity produced by skeletal muscles.
• The stimulation was computer-controlled and comprised a 25-millisecond train of 5 constant-current pulses.
• The researchers studied the 80- to 250-milliseconds post-stimulation interval to identify the nociceptive withdrawal reflex (NWR) — a defensive response to pain.
• The current intensity was increased gradually until the NWR threshold intensity (lt) was reached. The lt is the minimal current level at which the reflex was triggered.
• The researchers also observed the horses’ behavior in response to the electrical pulses and scored it on a scale of 0 (no reaction) to 5 (vigorous reaction).

Results

• The median threshold intensity to trigger NWR was found to be 2.5 mA in the tested horses.
• The median onset latency of the NWR, i.e., the time latency from the trigger to the reflex appearance, was 96.0 milliseconds at threshold intensity and a bit faster, at 89.6 milliseconds, for suprathreshold stimuli that exceed the NWR triggering threshold.
• As the stimulus intensity was increased beyond the threshold, the reflex’s amplitude, or magnitude, was found to be greater, indicating a stronger reflex response.
• Similarly, the horses’ behavioural reactions also strengthened slightly with the increase in stimulus intensity.

Conclusion

• The study concludes that it is possible to evoke and record the NWR in non-sedated, standing horses through transcutaneous electrical stimulation. It also establishes a reflex threshold and shows that reflexes can be measured in response to varying stimulus intensities.
• This study can contribute to a better understanding of pain processing in large animals like horses and form a basis for further scientific investigations into animal nociception.