Afferent Vestibular Nerve And The Efferent Vestibular Neurons In The Central Nervous Pathway

Objective: To explored the direct central neural pathway between afferent and efferent vestibular neurons in rats and to show the cell bodies of neurons and their axons in this pathway. Methods: In part one, anterograde tracers that allow quantitation of fibers and terminals combined with retrograde tracers that can be used for staining dendritic arbors and cell bodies of neurons were conducted to examine the possible anatomical connection. The anterograde tracer, PHA-L, was injected into the medial vestibular nucleus (MVN) or lateral vestibular nucleus (LVN) of rats. 5%MFS was injected into contralateral vestibular cavity after a week. Five days later, frozen sections for the brainstem were cut on a slice microtome and stained cells and axons were visualized with the aid of confocal laser scanning microscope and fluorescent microscope. In part two, postsynaptic currents (EPSCs) in the efferent vestibular neurons medial to the genu of facial nerve were recorded by whole cell patch clamp technique with the stimulations of MVN. In part three, the action potentials (APs) of MVN neurons were recorded by antidromic stimulation of the efferent vestibular neurons medial to the genu of facial nerve. The morphology of the recorded neurons in MVN was visualized by biocytin staining. The cell body of neuron in MVN and its axon which innervate the efferent vestibular neurons were reconstructed by Photoshop 7.0 software.Results: In part one, the efferent vestibular neurons were labeled by 5%MFS, which appeared red fluorescence. These neurons were located medial and lateral to the genu of facial nerve and in reticulation structure bilaterally. Some neurons distributed ventral to the genu of facial nerve. They presented all kinds of morphologies according to their locations. The labeled neurons in MVN or LVN and their axons distributed near the genu of facial nerve as well as reticulation structure bilaterally by PHA-L appeared green fluorescence. Double labeled neurons by two different fluorescent dyes , which appeared yellow fluorescence, were visualized in the medial and lateral to the genu of facial nerve and reticulation structure bilaterally. In part two, the resting membrane potentials of efferent vestibular neurons located medial to the genu of facial nerve (medial E group neurons) ranged between -55 mv and -70 mv in current clamp recordings. Single pulse (0.08mA, 0.1 Hz, 100μs) electrical stimulation of MVN evoked excitatory postsynaptic currents (EPSCs) in these neurons. In part three, the antidromic action potentials were recorded in MVN evoked by electrical stimulation of the medial E group neurons. They were spikes, whose amplitudes were about 62.5～65mv and cycle durations were about 82～90ms.Biocytin stainings indicated that the recorded neurons located in MVN and the axons' terminals went into the area of the medial E group neurons. Conclusions: There are direct central neural pathways between the afferent and efferent vestibular neurons in rats. The results provide direct physiological evidence that the afferent vestibular neurons located in MVN project with excitatory synaptes to the efferent vestibular neurons medial to the genu of facial nerve and these cell bodies in MVN and there axons could be visualized by intracellular biocytin staining.