Comparison of Threshold and Tolerance Nociceptive Withdrawal Reflexes in Horses.

Authors: Mühlemann Selina, Leandri Massimo, Risberg Åse Ingvild, Spadavecchia Claudia

Journal: Animals : an open access journal from MDPI

DOI: 10.3390/ani11123380 PubMed: 34944157Log in to save

Summary

# Editorial Summary: Nociceptive Withdrawal Reflexes in Horses Researchers investigating pain perception in horses used electrical stimulation of the digital nerve combined with electromyography to characterise both the threshold at which withdrawal reflexes first appear and the tolerance point at which animals exhibit maximal behavioural response. In 12 horses, median NWR threshold was established at 7.0 mA, with tolerance occurring at substantially higher intensity (10.7 mA), demonstrating a clear measurable gap between initial nociceptive detection and full pain response. Two distinct reflex components emerged at threshold—an early R1 (44 ms latency) and delayed R2 (81 ms)—but higher-intensity stimulation produced marked changes in both components: amplitudes and durations increased significantly whilst latencies shortened, with 65% of tolerance-level stimulations generating a single ultrafast burst of activity at 39 ms latency. These findings validate the feasibility of measuring NWR tolerance as a complement to threshold measurements and provide quantitative evidence for the rapid, high-amplitude reflex responses that equine professionals observe clinically during acute pain responses, offering a more complete neurophysiological model for understanding equine nociception beyond simple threshold data.

Read the full abstract on PubMed

Practical Takeaways

•NWR tolerance can be reliably measured in horses and provides additional information beyond threshold alone for assessing nociceptive responses
•The reflex pattern changes dramatically at high stimulus intensities, with faster, more intense bursts appearing—this may explain acute behavioral responses seen in practice during painful stimuli
•Measuring both threshold and tolerance could improve detection sensitivity and characterization of nociception in clinical pain assessment protocols

Key Findings

•Median NWR threshold was 7.0 mA with two main reflex components (R1 at 44 ms, R2 at 81 ms latency)
•NWR tolerance was 10.7 mA, significantly higher than threshold
•Increasing stimulation intensity from threshold to tolerance increased R1 and R2 amplitude and duration while decreasing latency
•High-intensity stimuli triggered ultrafast reflex bursts (39 ms latency) in 65% of tolerance-level stimulations

Conditions Studied

nociception assessmentnociceptive withdrawal reflex characterization

Related References

Validation of mechanical, electrical and thermal nociceptive stimulation methods in horses.

Luna S P L, Lopes C, Rosa A C, Oliveira F A, Crosignani N, Taylor P M, Pantoja J C(2015)Equine veterinary journal

Remote Controlled Nociceptive Threshold Testing Systems in Large Animals.

Taylor Polly(2020)Animals : an open access journal from MDPI

Mechanical nociceptive thresholds in the axial skeleton of horses.

Haussler K K, Erb H N(2006)Equine veterinary journal

Thoracolaryngeal reflex latencies in Thoroughbred horses with recurrent laryngeal neuropathy.

Curtis R A, Hahn C N, Evans D L, Williams T, Begg L(2005)Veterinary journal (London, England : 1997)

The effect of regional hypothermia on mechanical nociceptive thresholds in the equine distal forelimb.

Rainger J E, Wardius S, Medina-Torres C E, Dempsey S M, Perkins N, van Eps A W(2021)Veterinary journal (London, England : 1997)