Cortical projections and GABAergic organization of the cat medical geniculate body

The thalamus is the major source for ascending sensory information to the cerebral cortex, though it is often regarded as a passive relay nucleus. This thesis provides new neuroanatomical data about how the medial geniculate body (MGB) could process acoustic information. I will describe some anatomical substrates that could (1) underlie interactions between the corticothalamic system and intrinsic medial geniculate body circuitry; (2) explain how the neurochemical organization of neurons and axon terminals among medial geniculate body subdivisions may have a role in local processing; (3) show areal and laminar distribution patterns of thalamocortical projections which may be nucleus-specific.; The first study used immunocytochemical (γ-aminobutyric acid [GABA] and glutamic acid decarboxylase [GAD]) and connectional methods (biotinylated dextran amine [BDA], a highly sensitive axonal tracer). The novel result was a convergence of two complementary types of giant terminals in the dorsal division of the medial geniculate body. One type was excitatory and originates from the cerebral cortex, the other variety was inhibitory (GABAergic) and its source is unknown. The two kinds of giant terminals may provide powerful, and perhaps counterbalancing, influences that may converge on the same postsynaptic target. Such convergence of giant excitatory and inhibitory terminals onto principal cell dendrites could influence thalamic oscillations implicated in vigilance and attention.; A second study of GABA in the medial geniculate body showed a regional distribution of GABAergic neurons and puncta (axon terminals) that correspond to cytoarchitectonic and physiological boundaries. Each medial geniculate body nucleus had a unique set of inhibitory features. In particular, the dorsal division contained giant terminals as well as another population of GABAergic neurons, which are much larger than the small Golgi type II interneurons characteristic in the medial geniculate body and elsewhere in the sensory thalamus. These larger cells could participate in local circuit processing restricted to the dorsal division or be the source of the intrinsic giant terminals.; The last investigation examined thalamocortical projection patterns and axonal structure using BDA. Each division of the medial geniculate body had a distinct set of projections to auditory cortex. The projections from the ventral division were restricted to tonotopic fields, while the dorsal and medial divisions projected to tonotopic and nontonotopic areas. The laminar distribution of boutons, had three patterns: type 1 input terminated in cortical layers III/IV and was notably patchy in AI; type 2 projections terminated in layers I, III/IV and had a significant lateral component; and type 3 labeling was more complex, ended in layers I, III/IV, VI, and was more diffuse. The regional differences in areal and laminar patterns are specific to individual medial geniculate body nuclei. These patterns could contribute to thalamic-dependent developmental principles for laminar organization. A second finding is that some nonlemniscal neurons which project to layer I have unusually thick axons—up to 5 μm in diameter—perhaps for rapid transmission of acoustic information to supragranular cortex. The regional specialization of substrates in the medial geniculate body—giant axon terminals, GABA, and TC projections—may parcel out functional streams of auditory information and contribute to the complexity of its role in sensory processing.