The Indirect Projection From The Spinal Vestibular Nucleus Via The Pontine Nucleus To The Paraflocculus In The Rat

The vestibular nuclei are the sites of the secondary neurons receiving the vestibular primary afferents, and integrate the information from the vestibular organ and other afferents, and regulate the body motions through the efferent projections. Variable motions, changes of gravity and posture or head position could excite vestibular receptors, and then the vestibular system could modulate body posture, eye-movement, muscle strain so as to maintain the body balance, at the same time, it could rapidly modulate the circulation, respiration and other visceral activity for keeping the stabilization of internal environment.The vestibular nucleus mainly receive the primary vestibular afferents, at the same time, they receive afferent signals from other sensory systems as well. The optic motion signals, as the primary vestibular stimulations do, can evoke most of the second vestibular neurons. The optokinetic signals from cerebral cortex and thalamus reach vestibular nucleus after their combination with the space signals, which triggers off the more complex vestibular reflex. The cerebellum has close relationship with vestibular nucleus. It does not only receive primary afferent signals from vestibular, but also connects with the second vestibular neurons bidirectionally to coordinate the vestibular reflex more precisely. The plastic changes of vestibular nucleus and cerebellar flocculus(FI) and cerebellar paraflocculus(PFI) neurons plays an important role in smooth pursuit and vestibuloocular reflex adaption. However, anatomical researches found that there is no obviously direct fiber connections between vestibular nucleus and PFI. So that, there must be an indirect pathway between them which provides basic structure for their coordination during swift ocular movement.Pontine nucleus(PN) is the most important relayed nucleus between cerebrum and cerebellum. Increasing recent researches showed that PN can converge signals from peripheral nerve and brainstem and spinal cord, then send the signals to the more superior nerve system via cerebellum after integrating the signals. Retrograde tracing researches found that there are projections from SpVe to PN, while PN send fibers toPFI. Therefore, we hypothesized that there might be an indirect pathway between SpVe and PFI via PN, which may be the important component in the vestibuloocular system.In present study, the following experiments were processed by using neural tracing technique, immunohistochemical technique and other technique in Sprague-Dawley rat. The indirect projection from SpVe to PFI via PN was studied in the follwing experiments. Two parts are included: (1) The direct projection from SpVe to PN;(2)The indirect projection from SpVe to PFI via PN. The results are as follows:(1) The direct projection from SpVe to PNWith the respective injection of anterograde tracer PHA-L into SpVe and retrograde tracer FG into LPN and MPN, the distribution of PHA-L-labeled fibers and terminals were observed in PN, and the FG-labeled neurons were observed in VN. The results show that: ? In VN, only SpVe and X group send fibers to LPN and MPN. ?There were projections from SpVe and X group to FI, but no projection to PFI. ?The dense PHA-L labeled fibers and terminals were localized in dorsal LPN and dorsal medial PN, and contralateral predominance respectively, mainly receives projections from X cell group, while MPN mainly receives projections from SpVe.(2) The indirect projection from SpVe to PFI via PNThere were two experimental groups. The first: Anterograde tracer PHA-L was injected in LPN and the distribution of PHA-L-labeled fibers and terminals were observed in cerebellum;the second: PHA-L was injected in SpVe and FG was injected in PFI in the same rat, then off-vertical axis rotation (OVAR) was used to activate the otolith organ for inducing c-fos expression in the central nerve system. Results: ?After PHA-L was injected in LPN, there were PHA-L-labled fibers in PFI, but no fibers distribute in FI. ?After FG was injected in PFI, the FG-labeled retrograde neurons were found in the LPN and the MPN. The results suggest that there is a pathway from LPN and MPN to PFI. ?After PHA-L was injected in SpVe and FG was injected in PFI respectively in the same rat and administered with OVAR, the overlap area of the PHA-L-labeled fibers and terminals and FG-labeled neurons and FG/c-Fos double labled neurons were only found in LPN. However there were only the overlap of the PHA-L-labeled fibers and terminals and FG-labeled neurons in MPN, the FG/c-Fos double labled neurons was not found in the MPN.The results above indicated: there are direct projections from SpVe and X -groupto FI, while the projections to PFI are indirect via LPN. MPN may also involved in the indirect pathway. To confirm this possible pathway still need further study by immunoelectron microscope method. The indirect projection from SpVe to PFI via PN was showed as the following figure:Left RightPFIFISpVe and X-group...