The functional organization of afferent vagal mechanisms controlling special and general visceral reflex responses of the rat esophagus

Dong, Haiheng (2001) The functional organization of afferent vagal mechanisms controlling special and general visceral reflex responses of the rat esophagus. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The functional organization of esophageal afferent mechanisms controlling special (esophageal motility) and general (cardiovascular) visceral reflex responses was investigated in urethane-anesthetized rats. Techniques utilized included esophageal manometry, vagal nerve cooling, single nerve fiber recording, extracellular recording, and pharmacological receptor blockade and stimulation. -- Distal esophageal distension elicits esophageal reflex contractions, and both excitatory and inhibitory cardiovascular reflexes. Based upon the effects of vagal cooling, it is inferred that separate subpopulations of A? vagal mechanosensory afferent fibers mediate these reflexes. -- Single fiber recording experiments demonstrate that vagal mechanosensory afferent fibers innervating the distal esophagus respond to intraluminal pressure increases over a wide dynamic range and show little adaptation. -- The pattern and strength of vagal motor output to the distal esophagus depend on the intensity of vagal afferent input to interneurons at the level of nucleus tractus solitarii (NTS). These interneurons respond to esophageal distension with distinct firing patterns. Increasing strength of stimulation changes the firing pattern or intensifies the responses of these interneurons. Load-dependent changes in esophageal reflex motor activities persist after spinal afferent input is eliminated. -- In the striated muscle tunica muscularis propria of the rat esophagus, distal inhibition is an inhibitory motor reflex evoked by esophageal mechanosensory afferent input from the proximal esophagus. The chief underlying process is the activation of GABAa and/or glycine receptors associated with NTS subnucleus centralis (NTSc) esophageal premotoneurons. In contrast, deglutitive inhibition does not involve inhibitory amino acid mediated neurotransmission in this region. -- Vagal mechanosensory afferent fibers mediating the excitatory component of the esophageal cardiovascular reflex (ECVR) terminate in the immediate vicinity of esophageal premotor neurons comprising the NTSc and activate second-order neurons via glutamate receptors of both the NMD A and non-NMDA subtype. Glutamatergic synapses at the level of the rostral ventrolateral medulla are involved in the mediation of the vasomotor component of the ECVR. -- Taken together, the results of this thesis research lead to a more detailed understanding of the mechanisms by which vagal mechanosensory afferents innervating the rat esophagus evoke special and general visceral reflexes. Distension-evoked esophageal reflex contractions and the two components of the ECVR involve functionally distinct vagal mechanosensory afferent fibers and affect separate central pathways originating from NTS interneurons.

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