|Title:||Intrathalamic interaction in the medial geniculate body|
Medial geniculate body.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Rehabilitation Sciences|
|Pages:||vii, 108 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||All sensory signals other than olfactory ones go through the thalamus before reaching to the cortex. The thalamus receives massive feedback from the cortex. Both thalamocortical and corticothalamic projects pass through a thin sheet structure in the ventral thalamus called thalamic reticular nucleus (TRN) and give collateral inputs to its neurons. It has been suggested that the thalamus together with the TRN is not a simple relay station but a complicated processor. Previous studies have shown that the corticofugal projections have a strong modulation on the thalamic relay neurons. In this study, we investigated the intrathalamic interaction between different nuclei in the auditory system of rats and guinea pigs. Specifically, we investigated the gain adjustment in medial geniculate body (MGB) by TRN and the cross modality modulation via the TRN, and confirmed the pathway though which the MGB was modulated, by in-vivo extracellular and intracellular recording. In the first experiment, we examined the modulating effect of electrical activation of one site in the MGB on neuronal responses to acoustic stimuli in another site of the same nucleus. The auditory cortex was ablated to minimize the recurrent activation of the thalamocortical loop. Electrical activation of the MGB preceded the acoustic stimulus and would activate the corresponding TRN neurons, which in turns inhibited the MGB neuronal responses to the acoustic stimulus. Both facilitating and inhibitory modulation effects were found in the MGB, and the majority of the modulation was inhibition. The facilitation appeared as excitatory postsynaptic potentials or increased spike number in auditory responses. The inhibitory modulation appeared as inhibitory postsynaptic potentials (IPSPs) or decreased spike number in auditory responses. In the second experiment, we inactivated TRN by lidocaine after inhibitory modulation was found in MGB, and then repeated the experimental procedures as in the first experiment. We found that the inhibitory modulation was diminished after TRN inactivation.|
In the third experiment, we inactivated the inferior colliculus (IC) after we recorded the auditory responses of the MGB neurons. Electrical stimulation at the neighboring MGB site triggered similar IPSPs in the recorded MGB neuron. The IPSPs, the representation of inhibition modulation, still existed, which means that IC was not involved in this modulating pathway. In the fourth experiment, we examined the cross modality modulating effect between the visual and auditory system. Auditory response of MGB neurons were modulated by preceding electrically stimulation of the lateral geniculate nuclei (LGN). Both inhibitory and facilitating modulations were observed in this experiment. In the fifth experiment, we examined the spatial range in the MGB that was modulated by electrical stimulation in another MGB site. Modulations of auditory responses of MGB neurons on the same coronal plane were recorded when the electrical stimulation site in the MGB was fixed. We also investigated the modulations to a point of MGB unit from different stimulation sites of a coronal plane in the MGB. We found that in the randomly sampled coronal plane, the modulation range was 2.8-4.0mm laterally to the bregma and depth was from 5.0-6.5mm vertically to the brain surface. 52%-56% points of all points tested in the plane caused modulation effects. No tonotopic correlation in the modulation map was found. The present results indicate that a strong intrathalamic modulation occurs in the auditory thalamus. The modulation is via the TRN in the guinea pig. The results suggest that intrathalamic interaction between the TRN and MGB is likely an additional gain processor in the auditory signal pathway. The modulation via the TRN has cross-modal influences. The cross-modal influences and the results that each MGB receives modulations from a large thalamic area within the MGB suggest that the intrathalamic interaction may perform a global modulation function on the top of gain adjustment.
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