Assessment of the wind-up phenomenon in the equine nociceptive trigeminal system.

This study investigated the response of horses’ nerves to repeated low-intensity electrical stimulation. The researchers found that unlike spinal nerves, the horses’ facial nerves did not show an increase in response over time, suggesting a different underlying mechanism.

Objective

• The research aimed to investigate the wind-up phenomenon in horses using a non-invasive model. The wind-up phenomenon refers to the process where the frequency or intensity of a monotonous stimulus causes an increase in the perception of pain. Using repeated sub-threshold nociceptive electrical stimulation to understand this behavior is an established practice. Nociception is the neural processing of harmful stimuli with the potential to cause tissue damage, resulting in the sensation of pain.

Methodology

• The study involved 10 horses. The researchers applied repeated transcutaneous electrical stimulation to the supraorbital and infraorbital nerves of the horses. Transcutaneous refers to the application of any medication or procedure across the skin. The stimulation intensities varied between 0.5 and 1.3 times the trigemino-cervical reflex threshold defined for single stimulation.
• During the stimulation, the researchers recorded the evoked electromyographic activity of orbicularis oculi, splenius, and cleidomastoideus muscles. This activity was then analyzed in the early and late stages of the blink reflex and the trigemino-cervical reflex. Electromyographic activity refers to the electrical activity of muscles.
• Moreover, behavioral reactions were evaluated using a numerical rating scale, a common tool to assess the severity of pain.

Results

• The results revealed that the late nociceptive component and the trigemino-cervical reflex could not be elicited by sub-threshold intensity repeated transcutaneous electrical stimulation. In other words, the repeated low-intensity electrical stimulation did not cause an increase in the response of these nerves.
• Also, the median reflex amplitude of all the horses showed a tendency to decline over time during the stimulation train, meaning there was no temporal summation of afferent trigeminal inputs. Afferent refers to conducting or conducted inward or toward something (for nerves, the central nervous system; for blood vessels, the organ supplied).

Conclusion

• These results suggest that the modulation of trigeminal nociceptive processing resulting from repeated Aδ fiber stimulations is different from spinal processing. This indicates that the processing done by facial nerves as a response to painful stimuli deviates from the one executed by spinal nerves. Particularly, it seems that this processing has more of an inhibitory than a facilitatory effect on the stimuli.
• In conclusion, further evaluation is necessary to highlight the underlying mechanisms that cause these differences.