| Nucleus principalis (PrV) of the brainstem trigeminal complex mediates the processing and transfer of low-threshold mechanoreceptor input en route to the ventroposterior medial nucleus of the thalamus (VPM). In rats this includes tactile information relayed from the large facial whiskers via primary afferent fibers originating in the trigeminal ganglion (NV). Despite its strategic position within the subcortical afferent pathway, little is known regarding the function of PrV or the physiology of its constituent neurons. In the present study, we used controlled whisker deflections in conjunction with extracellular recording techniques to characterize the response properties and receptive fields of individual, antidromically identified VPM-projecting neurons in PrV. In order to understand the broader role of PrV in subcortical signal processing, we compared the responses of PrV neurons to those of cells recorded in pre(NV) and postsynaptic (VPM) populations under identical experimental and stimulus conditions. Our data indicate that substantial integration of physiologically diverse inputs occurs at the first synaptic station of the trigeminal afferent pathway (NV → PrV), with PrV neurons exhibiting broader directional tuning and larger receptive fields than those of NV neurons. In contrast, less integration appears to occur at the PrV → VPM synapse, where VPM neurons were found to possess directional tuning properties and spatiotemporal receptive field structures similar to those observed in PrV. Using sinusoidal whisker deflections, we also examined how the responses of VPM-projecting PrV neurons vary in relation to stimulus dynamics (e.g. velocity). In accordance with similar studies conducted previously in NV and VPM, we found that higher velocity stimuli elicited greater peak firing rates in PrV, and that PrV population responses exhibited less temporal dispersion and greater overall synchrony with increasing stimulus velocity. Thus, despite its substantial integrative function, PrV circuitry preserves the population-based representation of an important stimulus parameter, i.e. velocity. |
