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First published online March 12, 2009
Journal of Experimental Biology 212, 906-913 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.020529

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The basis of vagal efferent control of heart rate in a neotropical fish, the pacu, Piaractus mesopotamicus

E. W. Taylor^1,2,*, C. A. C. Leite¹, L. H. Florindo¹, T. Beläo¹ and F. T. Rantin¹

¹ Department of Physiological Sciences, Federal University of São Carlos (UFSCar), São Carlos, SP, Brazil
² School of Biosciences, University of Birmingham, Birmingham B15 2TT, UK

^* Author for correspondence (e-mail: e.w.taylor{at}bham.ac.uk)

Accepted 6 January 2009

The role of the parasympathetic nervous system, operating via the vagus nerve, in determining heart rate (f_H) and cardiorespiratory interactions was investigated in the neotropical fish Piaractus mesopotamicus. Motor nuclei of branches of cranial nerves VII, IX and X, supplying respiratory muscles and the heart, have an overlapping distribution in the brainstem, while the Vth motor nucleus is more rostrally located. Respiration-related efferent activity in the cardiac vagus appeared to entrain the heart to ventilation. Peripheral stimulation of the cardiac vagus with short bursts of electrical stimuli entrained the heart at a ratio of 1:1 over a range of frequencies, both below and sometimes above the intrinsic heart rate. Alternatively, at higher bursting frequencies the induced f_H was slower than the applied stimulus, being recruited by a whole number fraction (1:2 to 1:6) of the stimulus frequency. These effects indicate that respiration-related changes in f_H in pacu are under direct, beat-to-beat vagal control. Central burst stimulation of respiratory branches of cranial nerves VII, IX and X also entrained the heart, which implies that cardiorespiratory interactions can be generated reflexly. Central stimulation of the Vth cranial nerve was without effect on heart rate, possibly because its central projections do not overlap with cardiac vagal preganglionic neurons in the brainstem. However, bursts of activity recorded from the cardiac vagus were concurrent with bursts in this nerve, suggesting that cardiorespiratory interactions can arise within the CNS, possibly by irradiation from a central respiratory pattern generator, when respiratory drive is high.

Key words: Piaractus mesopotamicus, neuranatomy, neurophysiology, cranial nerves, vagus, cardiorespiratory interactions

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