Human Frontalis Muscle Innervation and Morphology

Area of Science:

• Anatomy
• Histology
• Neuroscience

Background:

• The frontalis muscle's clinical significance necessitates understanding its microscopic structure and innervation.
• Previous suggestions regarding afferent innervation prompted a detailed microscopic investigation.

Purpose of the Study:

• To investigate the microscopic appearance of the frontalis muscle.
• To clarify the presence and location of neuromuscular junctions and collagen XXII expression.

Main Methods:

• Serial sections of the frontalis muscle from seven human cadavers were examined using light microscopy.
• Immunohistochemistry was performed using antibodies for collagen XXII and neurofilament.

Main Results:

• Collagen XXII, a marker for myotendinous junctions, was expressed at muscle fiber endings at both insertion sides, despite the absence of tendons at the origin.
• Neuromuscular junctions were identified in the muscle belly and cranial portion, suggesting potential afferent nerve fiber pathways.

Conclusions:

• This study provides a detailed microscopic description of the frontalis muscle.
• It demonstrates collagen XXII expression in the absence of tendon formation, a novel finding.
• The study confirms the lack of corpuscular afferent neuronal structures within the frontalis muscle.