The frog brainstem preparation as a model for studying the central control of breathing in tetrapods

RESUMO

Our frog brainstem preparation revealed mechanisms for the central control of breathing that are in many ways similar to those of mammals. Thus, the basic control mechanisms for air-breathing appear to have been present in the Devonian common ancestors of frogs and mammals and may be common to all lung-breathing vertebrates. Location The in vitro frog brainstem, including motor nuclei of cranial nerves V to X, maintains frequency and ratio of fictive buccal oscillations to fictive lung inflation episodes comparable with that of the living animal. In this preparation, transaction caudal to V abolishes spontaneous discharge in X but slow, spontaneous discharge in V may remain. Independent central pattern generation is present in the left and right half-brainstems. Chemosensitivity The frequency of fictive lung inflation increases with decrease in pH within the physiological range. Response to glutamate Biphasic response, consisting of a pause, followed by a dramatic increase in the frequency of fictive inspirations and positive baseline deflection, followed, in turn, by slow return of the baseline to the control level with frequency remaining above control as long as glutamate is applied. Local application reveals glutamate-sensitive sites in the ventral reticular formation. Response to substance P and physalaemin Similar to glutamate but the frequency of fictive inspirations decreases below control values. Response to strychnine The normal temporal sequence in firing of motor neurons of cranial nerves is disrupted and all nerves are synchronously active. The firing sequence of respiratory neurons is consistent with a grouping possibly homologous to the mammalian inspiratory, post-inspiratory and expiratory phases.