Electromyography and Fos immunostaining study establish a possible functional link between trigeminal proprioception and the oculomotor system in rats

The objective of this study was to explore whether there was a functional link between trigeminal proprioception and the oculomotor system mediated through jaw muscle afferents.Electromyography（EMG） was undertaken of the levator palpebrae（LP） and superior rectus（SR）,and Fos expression was detected in the brainstem following consecutive down-stretching of the lower jaw at 2-4 Hz in rats.Retrograde tracing was undertaken of the interstitial nucleus of Cajal and Darkschewitsch nucleus（INC/DN） pre-oculomotor neurons.FMG-like responses were recorded from the LP/SR during down-stretching of the lower jaw at 2-4 Hz in 3 out of 11 rats.Fos expression was induced by consecutive down-stretching of the lower jaw at 2-4 Hz for 20-30 seconds.Interestingly,Fos expression was distributed mainly in the bilateral INC/DN area.We also examined Fos-like immunoreactivity in central mesencephalic and paramedian pontine reticular formation that harbors premotor neurons controlling horizontal eye movement,but no Fos-like staining was observed therein.By injection of retrograde tracers into the oculomotor nucleus combined with Fos immunostaining,double labeled pre-oculomotor neurons were visualized to distribute in the INC/DN.In conclusions,there may exist a trigeminal proprioceptive-oculomotor system neural circuit through jaw muscle afferents in rats.Judging from Fos distribution pattern,this pathway might be related to vertical and torsional eye movements.

Is Marcus Gunn jaw winking a primitive reflex？ Rat neuroanatomy

AIM： To investigate a possible trigeminal proprioceptive- oculomotor neural pathway and explore possible synaptic connections between neurons in this pathway. Attempt to bring a new insight to mechanism of Marcus Gunn syndrome （MGS）. METHODS： Anterograde and retrograde tract tracing was applied and combined with immunofluorescent stain in rats. After electrophysiological identifying mesencephalic trigeminal nucleus （Vine） neurons, intracellular injection of tracer was performed to trace axon trajectory. RESULTS： Following injections of anterograde tracers into the Vine, labeled terminals were observed ipsilateral in oculomotor and trochlear nuclei （Ill/IV）, as well as in their premotor neurons in interstitial nucleus of Cajal and Darkschewitsch nucleus （INC/DN）. Combining with choline acetyltransferase （CHAT） immunofluorescent stain, it showed that Vme projecting terminals contact upon ChAT positive Ill/IV motoneurons under confocal microscope. By retrograde labeling premotor neurons of the III, it showed that Vme neuronal terminals contact with retrogradely labeled pre-oculomotor neurons in the INCIDN. Axons of intraceiiularly labeled Vme neurons that respond to electric stimuli of the masseter nerve traveled into the ipsilateral III. CONCLUSION： There may exist a trigeminal propdoceptive- oculomotor system neural circuit in the rat, which is probably related to vertical-torsional eye movements. Possible association of this pathway with MGS etiology was discussed.

Is there a primitive reflex residue underlying Marcus Gunn Syndrome? Rat electrophysiology

AIM: To make an electrophysiological demonstration of a possible jaw muscle afferents-oculomotor neural pathway that was proposed by our previous works on rats, which substantiates an early "release hypothesis" on pathogenesis of human Marcus Gunn Syndrome(MGS). METHODS: Extracellular unit discharge recording was applied and both orthodromic and spontaneous unitary firing were recorded in the oculomotor nucleus(III), and the complex of pre-oculomotor interstitial nucleus of Cajal and Darkschewitsch nucleus(INC/DN), following electric stimulation of the ipsilateral masseter nerve(MN) in rats. RESULTS: Extracellular orthodromic unit discharges, with latencies of 3.7±1.3 and 4.7±2.9 ms, were recorded unilaterally in the III, and the INC/DN neurons, respectively. Spontaneous unit discharges were also recorded mostly in the INC/DN and less frequently in the III. Train stimulation could prompt either facilitation or inhibition on those spontaneous unit discharges. The inhibition pattern of train stimulation on the spontaneous discharging was rather different in the III and INC/DN. A slow inhibitory pattern in which spontaneous firing rate decreased further and further following repeated train stimulation was observed in the III. While, some high spontaneous firing rate units, responding promptly to the train stimuli with a short-term inhibition and recovered quickly when stimuli are off, were recorded in the INC/DN. However, orthodromic unit discharge was not recorded in the III and INC/DN in a considerable number of experiment animals. CONCLUSION: A residual neuronal circuit might exist in mammals for the primitive jaw-eyelid reflex observed in amphibians, which might not be well-developed in all experimental mammals in current study. Nonetheless, this pathway can be still considered as a neuroanatomic substrate for development of MGS in some cases among all MGS with different kind of etiology.