Selective deafferentation of hand cutaneous territory is followed by changes in fibre type distribution of a forearm muscle in the horse.

This research investigates the effect of capsaicin (a chili pepper extract) injections on horses’ forelimb nerves, specifically studying the change in muscle fiber distribution in the forearm muscle associated with finger flexing. As a result of eliminating certain nerve groups with the capsaicin treatment, the study indicated a transformation in the muscle fibers and an increase in tonic discharge, suggesting a link with the central nervous system’s remodeling.

Objective of the Study

• The goal of the research was to analyze the effect of group III and IV nerve afferents elimination by capsaicin in a horse’s peripheral area, focusing on whether this intervention would lead to a change in muscle fiber distribution in associated muscles.

Methodology

• Capsaicin was used due to its known ability to selectively damage group III and IV afferents and induce muscle fiber transformation. The substance was injected into the palmar nerves of the horse’s forelimb, aiming to disable these specific nerve groups in the hand area.
• The flexor carpi radialis (FCR) muscle, which aids in the flexion of the horse’s fingers, was studied to observe potential muscle fiber transitions resulting from the capsaicin injections.

Results and Conclusions

• 120 days post-injection, the researchers noted an increase in type I slow fibers and a decrease in type IIA fast fibers within the FCR muscle.
• It is surmised that the de-sensitization of ergo-nociceptive fibers over an extended period contributed to a shift or remodeling in the central nervous system. This change, in turn, appears to indirectly impact motoneuron excitability via short or long loop pathways, leading to heightened tonic discharge.
• The study therefore suggests that the central nervous system’s plasticity and function are influenced by peripheral nerve activity, specifically the group III and IV afferents. By selectively damaging these nerve groups with capsaicin, consequential effects on muscle fiber composition and central nervous system activity can be observed.