Stretching of Mechanoreceptors in Superior Tarsal Muscle Reflexively Contracts Slow-Twitch Fibers in Facial Expression Muscles: A Case Series.

Levator palpebrae superioris muscle (LPSM) and facial muscles comprise fast-twitch fibers (FTFs) and slow-twitch fibers (STFs) but lack muscle spindles required to contract STFs reflexively. Voluntary contractions and microsaccades of FTFs in LPSM stretch mechanoreceptors in superior tarsal muscle (STM) to induce phasic contractions of STFs in LPSM and frontalis muscle via mesencephalic trigeminal nucleus (MTN). They also induce prolonged contractions of STFs in bilateral frontalis and orbital orbicularis oculi muscles and physiological arousal via MTN and rostral locus coeruleus (LC). We hypothesized that stretching of mechanoreceptors in STM also induces prolonged contractions of STFs in other facial expression muscles (FEMs) via rostral LC. To verify this hypothesis, we reported a case series of abnormal contractions of FEMs due to aponeurosis disinsertion and disordered mechanoreceptor stretching. The first and second cases, which showed unilaterally and bilaterally sensitized mechanoreceptors, respectively, recorded increased prolonged contractions of ipsilateral and bilateral grimacing muscles, respectively. The third and fourth cases with asymmetrically and bilaterally desensitized mechanoreceptors experienced asymmetrically and bilaterally decreased prolonged contractions of grimacing and smiling muscles, respectively. Preoperatively and after surgery was performed to adjust mechanoreceptor stretching and reinsert aponeuroses into tarsi, we evaluated prolonged contractions of grimacing and smiling muscles during primary gazing and facial expression movements. Surgery satisfactorily cured abnormal prolonged contractions of grimacing and smiling muscles. Stretching of mechanoreceptors in STM by microsaccades or voluntary contractions of FTFs in LPSM might activate rostral LC via MTN, which tonically or phasically stimulates FEM motor neurons to reflexively contract their STFs, respectively.