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Central pattern generators are neuronal networks that produce reliable rhythmic motor output. A simple pattern generator, known as the cardiac ganglion (CG), controls the heart of the American lobster, Homarus americanus. Previous studies have suggested that stretch feedback relays information to the cardiac ganglion about the degree of filling in the heart, and that this feedback is mediated by stretch-sensitive dendrites extending from CG neurons. I sought to determine the mechanisms behind this stretch feedback pathway. One hundred second extension pyramids were applied to each heart while amplitude and frequency of contractions were recorded; 87% of hearts responded to stretch with a significant increase in frequency of contractions. To ascertain the role of dendrites in this feedback pathway, the accessible branches along the trunk of the CG were severed, de-afferenting the CG. In de-afferented hearts, stretch sensitivity was significantly less than in intact hearts, suggesting that the dendrites extending from the CG are essential for carrying stretch feedback information. To separate the effects of active and passive forces of heart contraction on stretch sensitivity, the CG was de-efferented by severing the motor nerves that induce muscle contraction. Hearts with only anterolateral nerves cut or with all four efferents cut were significantly less stretch sensitive than controls. These results indicate that the CG is sensitive to active stretch of each contraction. Hearts with reduced stretch feedback had more irregular frequency of contractions, indicating that a role of stretch feedback in the cardiac system may be to maintain a regular heart rate.
Available for download on Monday, May 15, 2017