American Society for Peripheral Nerve

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Sensorimotor Connections between CN V and CN VII May Provide a Possible Anatomical and Histologic Basis for Synkinetic Buccinator Hypertonicity
Ahneesh J Mohanty, BA1, Renee ML Misere, MD2, Arno Lataster, MSc.3, Shai M. Rozen, M.D.4 and Stefania Tuinder, MD, PhD3, (1)University of Texas Southwestern Medical Center Department of Plastic Surgery, Dallas, TX, (2)Maastricht University Medical Centre, Maastricht, Netherlands, (3)Plastic and Reconstructive surgery, MUMC+ Maastricht University Medical Centre, Maastricht, Netherlands, (4)Department of Plastic Surgery, University of Texas Southwestern Medical Center Department of Plastic Surgery, Dallas, TX

Buccinator muscle hypertonicity, a primarily masticatory muscle that may contribute to resting facial tone, is a disfiguring sequela often observed in patients with post-paretic synkinesia. The buccinator muscle is uniquely described as a dually innervated muscle through plexus-like connections between the trigeminal and facial nerves- receiving motor innervation from the facial nerve (CN VII) and sensory innervation from the buccal branch of the trigeminal nerve (V3). However, it is currently unknown why this specific muscle receives its innervation through this sensorimotor network. Therefore, this study aims to elucidate histologically the innervation of this muscle and provide suppositions on how its dysregulation may underlie buccinator hypertonicity observed in post-paretic synkinesia.

5 formalin-fixed and fresh-frozen hemifaces were dissected for this study. After exposing distal branches of the facial nerve, the mandible was resected in order to expose branches of the mandibular nerve and the foramen ovale. CN V/CN VII anastomoses were meticulously dissected under loupe magnification, and verified by histology using S100 (myelin), H&E, VAchT (motor axons), and TH (autonomic axons) staining. Biopsies of the buccinator, masseter, and orbicularis oculi muscle were also analyzed using the same stains. Histologic images were objectively quantified using ImageJ.

Plexiform anastomoses between the buccal nerve (V3) and buccal branches of the facial nerve were reliably found in all hemifaces superficial to the buccinator. Histologic analysis of these anastomoses revealed a fusion of CN V3 and CN VII fibers into a single nerve characterized by a single continuous epineurium. Quantification of VAchT staining showed a linear decrease in mean staining intensity along the length of the anastomosis (R2=0.67) toward the V3 connection point, indicating fusion of sensory and motor fibers. A higher concentration of sensory fibers and a co-localization of sensory nerve endings with motor fibers were also observed in buccinator muscle biopsies, but notably absent in those of the masseter and orbicularis oculi.

The close coupling of sensory and motor innervation was found to be a unique feature of the buccinator muscle. Anatomical and histologic findings seem to suggest CN V and CN VII are involved in a concerted feedback system with the buccal nerve possibly contributing proprioceptive information from the buccinator muscle while the facial nerve contributes motor response. Proprioceptive feedback seems to be essential for the maintenance of resting tone. An imbalance between motor and proprioceptive feedback may underlie buccinator hypertonicity, however, further studies are needed to elucidate this concept.

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