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.

Date: 2023-01-01

Creator: Isabel Stella Petropoulos

Access: Open access

A substantial factor for behavioral flexibility is modulation — largely via neuropeptides — which occurs at multiple sites including neurons, muscles, and the neuromuscular junction (NMJ). Complex modulation distributed across multiple sites provides an interesting question: does modulation at multiple locations lead to greater dynamics than one receptor site alone? The cardiac neuromuscular system of the American lobster (Homarus americanus), driven by a central pattern generator called the cardiac ganglion (CG), is a model system for peptide modulation. The peptide myosuppressin (pQDLDHVFLRFamide) has been shown in the whole heart to decrease contraction frequency, largely due to its effects on the CG, as well as increase contraction amplitude by acting on periphery of the neuromuscular system, either at the cardiac muscle, the NMJ, or both. This set of experiments addresses the location(s) at which myosuppressin exerts its effects at the periphery. To elucidate myosuppressin’s effects on the cardiac muscle, the CG was removed, and muscle contractions were stimulated with L-glutamate while superfusing myosuppressin. Myosuppressin increased glutamate-evoked contraction amplitude in the isolated muscle, suggesting that myosuppressin exerts its peripheral effects directly on the cardiac muscle. To examine effects on the NMJ, excitatory junction potentials were evoked by stimulating of the motor nerve and intracellularly recording a single muscle fiber both in control saline and in the presence of myosuppressin. Myosuppressin did not modulate the amplitude of EJPs suggesting myosuppressin acts at the muscle and not at the NMJ, to cause an increase in contraction amplitude.

Date: 2025-01-01

Creator: Yasemin Altug

Access: Open access

In order to maintain circuit stability through environmental perturbations, such as increases in temperature, neural circuits are able to adjust their output via modulatory and ion channel regulation. For instance, peptide modulators enable the lobster cardiac neuromuscular system to sustain physiological function at temperatures that surpass the crash temperature of the organ in the absence of modulation. Crash temperature is defined as the temperature at which neural activity ceases. For a crash, this temperature induced loss of activity is recovered when temperature is returned within the permissible range. Thus, it is hypothesized that there are underlying physiological mechanisms employed by the nervous system that compensates for changes in temperature and provides stability within acute temperature fluctuations. Neuromodulatory mechanisms have been proposed as one hypothesis that provide this temperature compensation. In accordance with previously collected data (Lemus 2022), I hypothesized that myosuppressin, a crustacean neuropeptide, provides stability during acute temperature variations. Because myosuppressin acts on the cardiac neurons and muscles separately, we hypothesized that the myosuppressin-induced increase in heart contraction amplitude, and decrease in contraction period can offset each other to provide system stability as temperature is increased. To test whether or not myosuppressin stabilizes circuit output as temperature is increased, myosuppressin was applied to the lobster whole heart at 7ºC, 10ºC, 13ºC and 16ºC, for 20 minutes. Changes in cardiac output in response to temperature and modulation were assessed by measuring the contraction force, heart beat frequency, and minimum contraction force. Interestingly, and contrary to previous results, in this data set, the cardiac neuromuscular system was temperature compensated in saline alone (control), and was not temperature compensated when perfused with myosuppressin (10-6 M). These findings seemed to differ from Lemus’ data (2023), where the cardiac neuromuscular system was not temperature compensated in control conditions and became temperature compensated when perfused with myosuppressin. The seasons at which each data set was collected (June-August vs November-March) could underlie these observed discrepancies.

Date: 2020-01-01

Creator: Jacob Salman Kazmi

Access: Open access

Neuromodulation may be a substrate for the evolution of behavioral diversity. The extent to which a central pattern generator is modulated could serve as a mechanism that enables variability in motor output dependent on an organism’s need for behavioral flexibility. The pyloric circuit, a central pattern generator in the crustacean stomatogastric nervous system (STNS), stimulates contractions of foregut muscles in digestion. Since neuromodulation enables variation in the movements of pyloric muscles, more diverse feeding patterns should be correlated with a higher degree of STNS neuromodulation. Previous data have shown that Cancer borealis, an opportunistic feeder, is sensitive to a wider array of neuromodulators than Pugettia producta, a specialist feeder. The observed difference in modulatory capacity may be coincidental since these species are separated by phylogeny. We predict that the difference in modulatory capacity is a product of a differential need for variety in foregut muscle movements. This study examined two members of the same superfamily as P. producta, the opportunistically feeding snow crab (Chionoecetes opilio) and portly spider crab (Libinia emarginata). Using extracellular recording methods, the responses of isolated STNS preparations to various neuromodulators were measured. Initial qualitative results indicate that the STNS of C. opilio is sensitive to all of these neuromodulators. Additionally, previous data on the neuromodulatory capacity of L. emarginata was supported through similar electrophysiological analysis of the isolated STNS. As a first step in determining the mechanism of differential sensitivity between species, tissue-specific transcriptomes were generated and mined for neuromodulators.

Date: 2016-05-01

Creator: Michael M Kang

Access: Access restricted to the Bowdoin Community