Preferential denervation of the adductor muscles of the equine larynx. II: Nerve pathology.

This study examined the nerve branches in the larynx of horses to better understand why certain muscles are more prone to nerve damage in a condition called idiopathic laryngeal hemiplegia (ILH). The findings indicated that medium/large sized nerve fibers in one specific area were more commonly depleted in horses with this condition, despite their overall nerve fiber density remaining constant.

Research Methodology

• The research was conducted on two groups of horses: three normal ponies and six horses suffering from sub-clinical laryngeal disease.
• The terminal branches of the horses’ recurrent laryngeal nerve (RLN) were scrutinized for both quantity and quality of nerve fibers. The aim here was to explain why laryngeal adductor muscles are more likely to lose nerve function in the face of ILH.
• The researchers measured the size spectrum of the myelinated (coated with a protective layer) motor nerve fibers in the branches of the RLN that cause muscle contraction (adductor) and relaxation (abductor).
• The density of these myelinated fibers was also computed.

Findings

• No significant difference was found in the sizes of the larger group of myelinated fibers in the adductor or abductor branches in the three normal ponies.
• However, in horses with laryngeal hemiparesis, or partial paralysis of the larynx, there was a pronounced loss of medium/large sized myelinated fibers in the left adductor branch, despite nerve fiber densities being comparable to normal counterparts.
• Therefore, while no single morphological feature was identified that could account entirely for the selective muscle denervation seen in ILH, the pronounced loss of larger diameter nerves in the adductor branches reaffirms prior observations attributing vulnerability to these muscles in horses affected by ILH.