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Modulation of respiratory rhythm and pattern in rana catesbeiana the bullfrog Meier, Janice T.

Abstract

The present study examined the role of neurons in the optic tectum in the formation of the periodic, episodic breathing pattern in the bullfrog, Rana catesbeiana. The first objective of the present study was to search for the presence of an "episodic centre", via progressive transections through the optic tectum of a decerebrate, artificially-ventilated in situ preparation. The results of these experiments revealed that the rostral optic tectum provides inhibitory, and the mid-optic tectum excitatory inputs to medullary centres, with respect to breathing frequency. Furthermore, the latter region also modulates burst pattern (providing an inhibitory input), as well as respiratory pattern, in concert with peripheral feedback from the vagus nerve. More specifically, following transections through the mid-optic tectum, the episodic breathing pattern was converted to one of evenly-spaced single breaths. In animals with at least one vagus intact, lung inflation restored the episodes. The caudal optic tectum, like the mid-region, appeared to influence both breathing pattern, as well as a component of burst pattern. Following transections at this level, the average burst duration increased significantly, while the integrated activity was not significantly altered. With respect to breathing pattern, the episodic breathing pattern was converted to one of evenly-spaced single breaths, although the overall breathing frequency was not changed significantly. The episodes could not be restored by lung inflation, suggesting that the vagal input which resulted in the reappearance of episodes following transections in the mid-optic tectum, acted rostral to the site of the caudal transection of the optic tectum. The possibility that an "episodic . centre" was located in the caudal optic tectum was refuted in one preparation in which episodes were observed following a transection at both the caudal optic tectum and rostral spinal cord. This suggested the presence of a neuronal input arising caudal to the site of transection in the spinal cord. Episodes re-occurred when this input was removed with only the medulla intact. These results implied that the centre(s) responsible for the formation of episodes exists not within the optic tectum, but rather within the medulla. A second objective of the present study was to examine the medulla of the bullfrog brain for the presence of multiple central rhythm generators for breathing. While recording from both the Vth and Xth cranial nerves, transections were made between the two, following which all neural activity from both nerves ceased. This suggested the presence of a centre, at the site of the transections, which is essential for the production of respiratory rhythm in the bullfrog, Rana catesbeiana.

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