| Research Abstract |
Connections from saccular afferents to vestibular neurons were studied by means of intracellular recordings of excitatory (E) and inhibitory (I) postsynaptic potentials (PSPs) in vestibular neurons after focal stimulation of the saccular macula in decerebrated cats. Focal stimulation was given to the saccular macula in two ways, in which the polarity of stimulus current via a pair of electrodes was changed. In group A, one of the electrodes was inserted into the ventral and the other into the dorsal edge of the saccular macula. The focal stimulation was across the striola, so that the reversal of morphological polarization in hair cells was bridged by the pulse stimulus. Mort vestibular neurons tested, the stimulation of the saccular macula evoked monosynaptic EPSPs, including EPSP-IPSP sequences, with one polarity of stimulation, and disynaptic IPSPs when the polarity of the stimulus current was changed. In group B, a pair of electrodes was inserted into the dorsal edge of the saccula
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r macula, so that the striola was not bridged by the current stimmulus. In all of the vestibular neurons tested, the response pattern was always the same regardless of the polarity; mono- and disynaptic EPSPs or disynaptic IPSPs. The results support the hypothesis that saccular afferents from one population of hair cells activate vestibular neurons monosynaptically, and that afferents from another population of hair cells located on the opposite side of the striola appear to project to the same vestibular neurons disynaptically via inhibitory interneurons. Neural circuits from saccular afferents to vestibular neurons, which we term cross-striolar inhibition, may thus provide a mechanism for increasing the sensitivity to vertical linear acceleration. The circuit described is provided not only with high sensitivity, but also with input noise-resistant characteristics.
Connections from the otolith organs to neck extensor, neck flexor and sternocleidomastoid (SCM) motoneurons were studied in the cat. Electrical stimulation of the utricular nerve evoked EPSPs with disynaptic latency in ipsilatera (I-) extensor, I- flexor and contralateral (c-) SCM motoneurons. The same stimulation evoked disynaptic IPSPs in I-SCM and e-extensor, and trisynaptic IPSPs in c- flexor motoneurons. Electrical stimulation of the saccular nerve evoked EPSPs with disynaptic latency in bilateral extensor flexor motoneurons. The same stimulation evoked disynaptic IPSPs in I-flexor and, and trisynaptic IPSPs in c-flexor motoneurons. The same stimulation evoked no visible potentials in c-SCM motoneurons. Thus, the otolith-neck connectivities play an important role in maintaining the relative position of the head against horizontal and vertical linear acceraration and tilting of the head. Less
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