Convergent Innervations in the Orosensory Nucleus of the Solitary Tract

Much of the current understanding of how the brain processes sensory information has been gleamed from the functional organization of the primary sensory nucleus. The rostral nucleus of the solitary tract, the first-order orosensory relay, is innervated by four separate nerves, each conveying a subset of orosensory modalities and topographies to distinct populations of NTS neurons. However, the convergent innervation of these neurons by multi-topographic and/or multi-modal primary afferent fibers is not known. The goal of this dissertation is to investigate the possible convergence of oral topography and modality in the orosensory NTS, revealing information essential to the understanding of how the NTS processes incoming sensory information. Examining the fine distribution of each orosensory nerve in the NTS in Chapter 2 has revealed a differential subdivisional distribution for each nerve. The chorda tympani nerve innervates the rostrocentral, rostrolateral, and ventrolateral subdivisions. The greater superficial petrosal nerve innervates the rostrocentral and ventrolateral subdivisions. The glossopharyngeal nerve innervates the rostrocentral, rostrolateral, and intermediate lateral subdivisions. The lingual branch of the trigeminal nerves innervates the rostrolateral, intermediate lateral, and ventrolateral subdivisions. Considering the multi-modal and multi-topographic nature of each nerve, experiments in Chapter 3 investigated the convergence of orosensory modality and topography in these subdivisions using multiunit electrophysiology. This experiment revealed that multitopographic, uni-modal (taste) convergence of the anterior and posterior tongue occurs primarily along the rostrocentral, rostrolateral border. Multi-modal, uni-topographic convergence occurred in different NTS regions depending on the portion of the tongue stimulated. Multi-modal convergence of the anterior tongue occurred primarily in the medial half of the rostrolateral subdivision, while multi-modal convergence of the posterior tongue occurred in the caudal-most portions of the rostrocentral subdivision. Finally, experiments in Chapter 4 utilized high-magnification confocal microscopy to reveal convergent innervation between the chorda tympani and glossopharyngeal nerves onto ascending projection neurons. These cells were located in the rostrocentral subdivision in regions responding to uni-modal (taste), multi-topographic convergence of the anterior and posterior tongue as well as multi-modal, uni-topographic convergence of the posterior tongue. The above results demonstrate the heterogeneity and complexity of convergent innervation onto NTS projection neurons.

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