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Anatomia, histologia, embryologia2024; 53(5); e13099; doi: 10.1111/ahe.13099

Nerve entry points in the mimic musculature of the horse head.

Abstract: Facial expressions are important in pain recognition in horses, but current observation-based pain scales remain subjective. A promising technique to quantitatively measure subtle changes in expression patterns, including changes invisible to the human eye, is surface electromyography (sEMG). To achieve high-quality and reliable sEMG signals, unilateral placement of bipolar electrodes is required in relation to the motor endplates (MEP). We aimed to localize the nerve entry points (NEPs; where the nerve branch first pierced the muscle belly) and the direction of the terminal nerve endings to estimate MEP locations of the innervating nerves in five equine facial muscles involved in pain expression. Three cadaveric Dutch Warmblood horse heads were dissected to identify the NEPs in the musculi caninus, levator anguli oculi medialis, nasolabialis, masseter and zygomaticus. These points were marked with pins and measured in relation to a reference line between two anatomical landmarks near the origin and insertion of the respective muscle. Relative distances were calculated from the most caudally situated landmark. NEPs were located at 33%-38% (caninus), 69%-86% (levator anguli oculi medialis) and 0%-18% (zygomaticus) from the caudal landmark. The nasolabialis showed two innervations zones. Its NEPs were located at 47%-72% (dorsal muscle branch) and 52%-91% (ventral branch). All terminal nerve endings were found to run in rostral direction. The masseter showed numerous NEPs diffusely spread within the muscle belly. Therefore, calculation of relative positions was not performed. These results could form the basis for feasibility studies and standardization of bipolar electrode positioning in vivo to measure facial muscle activity patterns in horses.
© 2024 The Author(s). Anatomia, Histologia, Embryologia published by Wiley‐VCH GmbH.
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Publication Date: 2024-08-05 PubMed ID: 39099214DOI: 10.1111/ahe.13099Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research focuses on understanding facial expressions in horses, specifically pinpointing nerve entry points (NEPs) in their facial muscles to enable more accurate surface electromyography (sEMG) measurements. The goal is to use these electromyography metrics to provide an objective assessment of pain levels in horses, thus standardizing the currently subjective pain scales based on observational data.

Objective of the Study

  • The study aims to investigate the NEPs of nerves that innervate facial muscles in horses. These facial muscles have been identified as significant in expressing pain in these organisms.
  • The researchers hope that by understanding the exact neuroanatomical locations of where nerve branches enter the facial muscle belly, they can improve sEMG readings. This is based on the theory that getting the electrodes of sEMG devices closer to the motor endplates (MEPs; the part of the muscle receiving innervation from the nerve) enhances the quality of the electrical signal picked up.

Methodology of the Study

  • The study was performed on three cadaveric Dutch Warmblood horse heads. The team dissected these specimens to find out and mark the NEPs in five muscle types: the musculi caninus, levator anguli oculi medialis, nasolabialis, masseter, and zygomaticus.
  • After identifying the NEPs, researchers measured their locations relative to a reference line drawn between two anatomical landmarks for each muscle. The point found deepest within each muscle served as the caudal landmark.

Findings of the Study

  • The research found distinct NEPs for the musculi caninus (33%-38% from the caudal landmark), levator anguli oculi medialis (69%-86%), and zygomaticus (0%-18%). The nasolabialis was observed to have two innervation zones, with NEPs along both dorsal (47%-72%) and ventral (52%-91%) muscle branches.
  • All terminal nerve endings were found to direct towards the rostral, or frontward, direction.
  • The masseter muscle showed a significant number of NEPs spread within the muscle belly. Hence, relative position calculations were not possible in this case.

Implications of the Study

  • This research aids in refining the process of bipolar electrode placement for sEMG in horses, ultimately improving the understanding and treatment of pain in these creatures.
  • The findings can guide future studies and standardization protocols for the measurement of facial muscle activity patterns in horses. This can serve as a foundation for quantitative pain recognition in horses, reducing subjectivity in pain evaluation.

Cite This Article

APA
Wolschrijn CF, Smit IH, Schouten J, Moller Te NCR. (2024). Nerve entry points in the mimic musculature of the horse head. Anat Histol Embryol, 53(5), e13099. https://doi.org/10.1111/ahe.13099

Publication

Anatomia, histologia, embryologia
ISSN: 1439-0264
NlmUniqueID: 7704218
Country: Germany
Language: English
Volume: 53
Issue: 5
Pages: e13099

Researcher Affiliations

Wolschrijn, C F
  • Section of Anatomy and Physiology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Smit, I H
  • Section of Equine Musculoskeletal Biology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Schouten, J
  • Section of Anatomy and Physiology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Moller Te, N C R
  • Section of Equine Musculoskeletal Biology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.

MeSH Terms

  • Animals
  • Horses / anatomy & histology
  • Facial Muscles / innervation
  • Facial Muscles / anatomy & histology
  • Electromyography / veterinary
  • Head / innervation
  • Head / anatomy & histology
  • Facial Expression
  • Motor Endplate / anatomy & histology
  • Cadaver

Grant Funding

  • Utrecht University Faculteit diergeneeskunde

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