Dendrites of Cardiac Ganglion Regulate Heartbeat of American Lobster, Homarus americanus, Through Stretch Feedback

Date: 2014-05-01

Creator: Mara R Chin-Purcell

Access: Open access

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.

The Effects of Temperature on the Cardiac System of the American Lobster, Homarus americanus

Date: 2014-05-01

Creator: Elizabeth A Owens

Access: Open access

The American lobster, Homarus americanus, inhabits a large oceanic range spanning from Labrador, Canada to North Carolina, USA. This geographic range varies in temperature by as much as 25ºC, and daily temperature fluctuations of up to 12ºC may occur at a single location depending on season, water depth, and tides. The cardiac system of the lobster is sensitive to these temperature changes, and has been shown to adjust its functioning over a large temperature range. A previous study showed that various functional parameters respond differently to temperature changes, but a stable cardiac output can be maintained over the range of 2-20ºC. The current study showed that the effects of temperature were exerted primarily through changes in the lobster heart central pattern generator, the cardiac ganglion. Similar patterns of change were seen in both semi-intact hearts and isolated cardiac ganglion preparations in response to increasing temperature. Specifically, with increasing temperature, the burst frequency showed a biphasic pattern in which frequency initially increased, then decreased rapidly at high temperatures. The burst duration, duty cycle, and number of spikes per burst generally decreased with increasing temperature, and spike frequency increased over the entire temperature range. Semi-intact hearts and isolated cardiac ganglia showed similar “crash” patterns, characterized by complete loss of function at high temperatures and complete recovery of function when temperature was returned to baseline. Feedback in the semi-intact heart provided some stabilization of bursting activity, but it did not provide the expected protection from high temperatures. The isolated CG had a significantly higher crash temperature than did the semi-intact system. This discrepancy in crash temperatures may be explained by considering factors at the level of the muscle and neuromuscular junction (NMJ), such as stretch and nitric oxide (NO) feedback and the balance of facilitation and depression at the NMJ. Stimulated preparations showed defacilitation of contraction amplitude at high temperatures despite the maintenance of constant burst parameters of stimulation. Therefore, several factors contributing to the relatively low crash temperature of the intact system may be a shift in the balance of facilitation and depression at the NMJ, a depression in ganglion function due to the release of NO by the muscle, or a combination of the two mechanisms.