Objective: 

Recent results suggest that trigeminal mesencephalic nucleus (TMnu) also contains first-order neurons specialized in relaying spatial information, related to whisker location and their movement, to trigeminal-cortical pathways. Retrograde tracers revealed in fact neurons innervating i) the neuromuscular spindles of the masseter muscle, ii) the mystacial pad, as well as iii) neurons innervating both structures. Electrophysiological findings demontrated that mesencephalic neurons (TMne) respond to spontaneous movements of the macrovibrissae. Here we analyze the peripheral distribution of TMne terminals to the whisker pad structures to locate their targets.

Methods: 

Under aseptic conditions, in anaesthetized rat, the fluorescent carbocyanine dye Dil was unilaterally injected into the TMnu. After seven days, the animals were deeply anaesthetized and transcardially perfused. The brain and the mystacial pad were then removed, post-fixed and frozen sectioned. Brain sections were histologically processed, to assess the tracer injection site, and the fluorescence analysis. The ipsilateral whole pad was subdivided into four portions each containing two rows of vibrissae. Alternate sections parallel to the skin were mounted on gelatine-coated slides for fluorescence detection of anterograde Dil-labelled terminals in the pad structures.

Results: 

Results showed that TMne directly innervate the macrovibrissae. Anterograde Dil-labelled terminals were in fact found around the upper level of the ring sinus of the vibrissae follicle-sinus complex. These were constituted of circumferential fine- and small-caliber fibers with lanceolate endings.

Conclusions: 

With concern to their functional roles, it is possible that TMne may convey spatial information related to the macrovibrissae location and displacement to the trigeminal brainstem nuclei.