Integration of Visual and Nonvisual Information in Nucleus Reticularis Thalami of the Cat
In cats with grey für extracellular recordings from 83 neurons of the nucleus reticularis thalami were obtained. Spontaneous activity and responses to electrical and light stimuli were investigated. Electrical stimuli were applied to each optic nerve, optic chiasm, visual cortex, superior colliculus, and to the mesencephalic reticular formation. Stationary light spots of various diameters were presented monocularly and binocularly. The corticofugal influence was assessed by reversibly inactivating the exposed parts of visual areas 17 and 18. Anatomical material (Szentágothai et al., 1972) and recent neurophysiological data (Dubin and Cleland, 1977) suggest that neurons of the perigeniculate nucleus are involved in inhibition of geniculate relay cells; therefore, special attention has been given to the possible role of nucleus reticularis thalami neurons as extrinsic interneurons of the dorsal lateral geniculate nucleus (LGN).
The perigeniculate nucleus apparently is a functional visual substructure of the nucleus reticularis thalami, receiving predominantly monosynaptic input from Y-cells of both retinae, whereas the rest of the nucleus reticularis thalami above the LGN is mainly innervated polysynaptically. Neurons of the nucleus reticularis thalami are identified as interneurons by their transsynaptic activation due to visual cortical stimulation and their typical response scatter to optic nerve, optic chiasm, visual cortex, and superior colliculus stimulation. Each of these stimuli usually elicits a burst of spikes (primary excitation) which, in most cases, is followed by a period of inhibition. Stimulation of the mesencephalic reticular formation causes inhibition in 63% (n = 52) and activation of 11% (n = 9) of nucleus reticularis thalami cells. Inactivation of the visual cortex prolongs the primary excitation caused by optical chiasm, visual cortex, and superior colliculus stimulation and reduces the strength of postexcitatory inhibition reversibly.
The model that is proposed explains binocular inhibition and disinhibitory effects of mesencephalic reticular formation stimulation in LGN relay cells via a pathway including the nucleus reticularis thalami. In addition, corticofugal effects that were described recently (Schmielau and Singer, 1977a; Schmielau and Singer, 1977b) are in accordance with the hypothesis.
KeywordsVisual Cortex Superior Colliculus Lateral Geniculate Nucleus Optic Chiasm Nucleus Reticularis Thalamus
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