Neuromuscular specializations within human pharyngeal constrictor muscles

Area of Science:

• Anatomy and Physiology
• Neuroscience
• Muscle Biology

Background:

• Current understanding posits pharyngeal constrictor (PC) muscles are innervated by the vagus (X) nerve and possess homogeneous muscle fiber content.
• This study challenges the homogeneous model, proposing distinct functional layers within the PC muscles.

Purpose of the Study:

• To test the hypothesis that adult human PCs are divided into two distinct, specialized layers: a slow inner layer (SIL) and a fast outer layer (FOL).
• To determine the specific nerve innervating each layer (glossopharyngeal (IX) for SIL, vagus (X) for FOL).
• To characterize the muscle fiber types within each layer.

Main Methods:

• Human pharynges (n=8) were analyzed using Sihler's stain for gross motor innervation, and acetylcholinesterase (AChE) and silver stains for terminal axonal branching.
• Myosin heavy chain (MHC) expression in PC muscle fibers was assessed via immunocytochemical and immunoblotting techniques.
• Developmental, pathological (idiopathic Parkinson's disease), and comparative (macaque monkey) specimens were examined to assess layer specialization.

Main Results:

• Nerve IX innervated the SIL, while nerve X innervated the FOL in adult human PCs.
• The SIL contained slow-tonic and alpha-cardiac MHC isoforms, indicating a slow-contracting muscle.
• The FOL contained developmental MHC isoforms, indicative of fast-contracting muscle; this layer was obscured in elderly individuals and those with idiopathic Parkinson's disease.

Conclusions:

• Human PCs exhibit functional fiber layering with distinct innervation and specialized muscle fibers, with the SIL appearing unique to normal humans.
• The SIL's specialized MHC isoforms suggest a role in sustained contractions for human speech and respiration.
• The FOL's adaptation for rapid movement supports its role in swallowing, and observed fiber shifts in senescence and IPD may explain dysphagia susceptibility.