Aim: 

It has been recently shown that rat extratrigeminal fibers, originating from the hypoglossal nucleus, travel along the infraorbital division of the trigeminal nerve (ION) to join the extrinsic musculature of the mistacial pad (Mameli et al 2006 and 2008). Aim of the present study was to investigate their functional role.

Methods: 

In the rat, Dil and True Blue tracers were injected into vibrissal pad and masseter muscle respectively. The mesencephalic trigeminal nucleus, which receives proprioceptive information from the chewing muscle was then analysed in order to investigate possible correlations among mimetic and chewing muscles. Furthermore, mesencephalic neurons evoked responses following mandibular stretch and vibrissal movements were also electrophysiologically analysed.

Results: 

The mesencephalic trigeminal nucleus showed: 1) Dil labelled neurons following pad injection, 2) True Blu labelled neurons following masseter injection, as well as, 3) double labelled neurons. As for the electrophysiological results, mesencephalic trigeminal neurons showed three different patterns of response, i.e.: 1) neurons that responded to mandibular stretch alone, 2) to vibrissal movements alone, as well as 3) neurons that responded to both stimuli.

Conclusion: 

It can be speculated that, given the lack of pad neuromuscular spindles, the ION could carry out proprioceptive messages to the mesencephalic nucleus. The hypoglossal neurons, which receive monosynaptic information from the mesencephalic nuclei may in turn modulate the response to this afference.