Thermographic analysis of the mechanism of involuntary contraction of the facial mimic muscles

面部模拟肌肉是自发或反射性收缩的。虽然这些肌肉由快肌纤维和慢肌纤维组成，但它们缺乏固有的肌纺锤体，通常需要诱导慢肌纤维反射性收缩。我们发现，位于提肌和跗骨之间的米勒肌中的机械感受器作为外源性机械感受器，诱导面部慢肌纤维的反射性收缩。为了强烈地收缩上眼睑，提上睑肌非骨骼快肌纤维的自愿收缩增加，拉伸穆勒肌的机械感受器，强烈地唤起三叉神经本体感觉。这种本体感觉刺激面部运动神经元，通过中脑三叉神经和蓝斑神经来诱导面部慢肌纤维的反射性收缩，两者都有间隙连接。面部慢肌纤维的反射性收缩在热成像上产生热量。

Facial mimic muscles are contracted voluntarily or reflexively. Although these muscle consist of fast- and slow-twitch fibers, they lack intrinsic muscle spindles normally required to induce reflex contraction of slow-twitch fibers. We found that the mechanoreceptors in Mueller's muscle, which locates between the levator muscle and the tarsus, function as extrinsic mechanoreceptors to induce reflex contraction of the facial slow-twitch fibers. To strongly retract the upper eyelid, increased voluntary contraction of the levator non-skeletal fast-twitch fibers stretches the mechanoreceptors in Mueller's muscle to strongly evoke trigeminal proprioception. This proprioception stimulates the facial motor neurons to induce reflex contraction of the facial slow-twitch fibers via the mesencephalic trigeminal neurons and the locus ceruleus, both of which are connected with gap junctions. This reflex contraction of the facial slow-twitch fibers thermographically produces heat.