Anatomic and neuromuscular characterisation of the equine cricothyroid muscle.

The research seeks to offer detailed anatomy and functionality of the equine cricothyroid muscle and how it contributes to enlarging the laryngeal diameter in horses afflicted with recurrent laryngeal neuropathy for better electrode placement and muscle recruitment through functional electrical stimulation.

Study Methodology

• The study was based on seventeen equine larynges sourced during necropsy.
• Twelve of these larynges were cross-examined visually or ‘grossly dissected’.
• Seven larynges were prepared for immunohistochemistry (study of the tissue through a microscope) post dissection.
• Five remaining larynges were specially stained for viewing under a microscope, with three for acetylcholinesterase staining of motor endplates and two for Sihler’s staining.
• Further, three larynges were electrically stimulated following in situ (in the body) cadaver dissection and two larynges were removed and stimulated ex vivo (outside the body).

Results and findings

• Three distinctive neuromuscular compartments were discovered, each regulated by a primary nerve branch of the external branch of the cranial laryngeal nerve.
• These three compartments showed unique innervation, fibre type distribution and muscle fibre dimensions.
• When each compartment was electrically stimulated, it led to the thyroid cartilage moving ventrally (towards the belly) in relation to the cricoid cartilage, thus increasing the dorsoventral height (front to back height) of the rima glottis (the opening between the true vocal cords and arytenoids).
• All of these compartments help in straining the vocal cords by extending the front to back height of the rima glottis.

Conclusions and Implications

• Functional electrical stimulation of both cricothyroid and dorsal cricoarytenoid muscles could potentially improve voice quality in horses with recurrent laryngeal neuropathy by enhancing laryngeal cross-sectional area.
• The next phase of clinical applications would be to implement these findings in the treatment procedures for horses, helping in better electrode placement and more effective muscle recruitment.