Different discharge properties of facial nucleus motoneurons following neurotmesis in a rat model

• There are two types of facial nucleus motoneurons.
• Nerve injury can affect the discharge properties of motoneurons.
• Change of firing pattern was distinct in different motoneurons.

Facial nucleus motoneurons innervating the facial expressive muscles are involved in a wide range of motor activities, however, the types of movement related neurons and their electrophysiological transformation after peripheral facial nerve injury haven’t been revealed. This study was designed to elucidate the types of facial nucleus motoneurons and their alterations of discharge parameters following peripheral facial nerve injury in vivo. Here we set up a rat model by implanting electrode arrays into the brainstem and recorded the electrophysiological signals of facial nucleus neurons in the intact rats for 5 days, then transected the trunk of facial nerve (TF), and continued the record for 4 weeks. At the 4th week post-surgery, the morphological changes of TFs were analyzed. In this paper, we described two types of putative facial nucleus motoneurons based on their electrophysiological properties and their firing frequency adaptation. Type I motoneurons (n = 57.6%) were characterized by a sustained spike adaptation, Type II motoneurons (n = 26.2%) were identified by a phasic fast spike firing. Facial palsy and synkinesia, caused by neurotmesis of TF, were accompanied by firing rates reduction and firing pattern alteration of motoneurons. Our findings suggest the presence of two types of facial nucleus motorneurons, and their response patterns after neurotmesis support the notion that the discharge pattern of motorneurons may play an important role in the facial nerve function.