Objectives: 

In previous experiments we demonstrated that: i) retrograde tracers injected into the mystacial pad of the rat muzzle, extensively labelled the trigeminal mesencephalic nucleus (TMnu) neurons, and that ii) within this same nucleus were located whisking neurons responsive to spontaneous movement of the macrovibrissae (Mameli et al. 2010), similar to those described in the trigeminal Gasser ganglion. It has been concluded that TMnu enclosed primary neurons specifically dedicated to encode spatial information related to vibrissae movement. The present research was aimed at verify this hypothesis.

Materials: 

Electrophysiological responses of the trigeminal mesencephalic neurons (TMne) to mechanical deflection of the macrovibrissae and to the artificial whisking, induced by electrical stimulation of the peripheral stump of the facial nerve were analyzed, and, by using neurochemical procedures the peripheral terminals of the TMne, joining the mystacial pad structures, were localized.

Results: 

Electrophysiological results showed that TMne responded to mechanical deflection of the macrovibrissae with different patterns of response during vibrissae deflection in one or two specific directions. Furthermore, TMne responded to the artificial whisking. Evoked polyphasic potentials were recorded at 1.28±0.03 ms latency and excitatory responses of single TMne also appeared at 2.3±0.06 ms latency, usually followed by a delayed response at 5.2 ±0.15 ms latency. The neuroanatomical approach showed that TMne were directly connected to the upper part of the ring sinus of the macrovibrissae, by circumferential fine- and small-calibre fibres with lanceolate endings.

Conclusions: 

It is known that TMne, specialized in proprioceptive encoding, are extensively connected to the trigeminal nuclei complex, thus it is possible that these latter, by macrovibrissae inputs, identify and locate the objects in the environment by integrating touch and spatial information respectively received by the Gasser's neurons and the TMne.