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Respiratory pattern formation in the bullfrog (Rana castebieana) Chatburn, Jonathon William

Abstract

This study examined the stability of the bullfrog in vitro brainstem-spinal cord preparation over time, and the influence of the midbrain on fictive breathing, chemosensitivity, and respiratory pattern formation. Fictive breathing was measured over an 8-hour recording period from both the in vitro brainstem-spinal cord preparation (midbrain intact) and the isolated medulla preparation (midbrain removed). Preparations were exposed randomly to aCSF equilibrated at three different pH levels (8.0, 7.8, 7.6) prior to and following removal of the midbrain. The fictive breathing pattern was analyzed using Poincare pot distributions following progressive rostro-caudal transections within the midbrain and rostral medulla of the in vitro brainstem-spinal cord preparation. Following setup, the fictive breath frequency progressively declined over the first 2-3 hours, leveled off and stayed constant for the remainder of the recording period in both the in vitro brainstem-spinal cord and the isolated medulla preparations. Lowering the pH of the aCSF caused significant increases in breath frequency prior to and following removal of the midbrain, but elicited a larger chemoresponse in preparations without the midbrain intact. Following transection at the midbrain medulla border the fictive breath frequency decreased, the average inter-breath interval (IBI) length increased, and the average breath duration increased. Fictive episodic breathing was not eliminated following removal of the midbrain, however, the distances between breaths within episodes, and between episodes dramatically increased in size. The spatio-temporal coordination of breaths within the breathing pattern became less precise following removal of the midbrain, resulting in episodes that were less discrete and inconsistent in size and occurrence. Preparations that exhibited consistent episodic breathing produced Poincare distributions on which the inter-breath intervals fell into three distinct size groupings. These groupings corresponded to the distance between breaths, episodes, and episode clusters within the breathing pattern. These preparations also produced harmonic distribution patterns on the Poincare plots where the inter-breath interval groupings that occurred were related to each other in size by whole number ratios. These findings suggest that transections through the brainstem-spinal cord initially stimulate breathing, and that there is a period of time following in which neural activity stabilizes. The results also suggest that the midbrain contains a site, or sites that moderate the chemoresponse in vitro, while independently supplying a source of tonic drive that increases the number of breaths per unit time, and increases the spatio-temporal coordinating of breaths within the breathing pattern. We feel that the central pattern generator (CPG) that is responsible for the anuran respiratory pattern continuum could consist of a multiple-coupled oscillator network. Two of the oscillators may lie bilaterally within the medulla, while one lies bilaterally within the caudal midbrain. We propose that it is the synchronizing interactions of these oscillators that are responsible for producing discontinuous breathing patterns in which breaths, episodes, and episode clusters occur with precise regularity.

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