Showing 1 - 4 of 4 Items

Date: 2021-01-01

Creator: Barry Logan

Access: Open access

Date: 2020-01-01

Creator: William Allen

Access: Open access

Central pattern generators (CPGs) are neural circuits whose component neurons possess intrinsic properties and synaptic connections that allow them to generate rhythmic motor outputs in the absence of descending inputs. The cardiac ganglion (CG) is a nine-cell CPG located in the American lobster, Homarus americanus. Stretch of the myocardium feeds back to the CG through mechano-sensitive dendrites and is thought to play a role in maintaining regularity in the beating pattern of the heart. The novel peptide AMGSEFLamide has been observed to induce irregular beating patterns when applied at high concentrations. This study investigated the interaction between stretch-related feedback and AMGSEFLamide modulation in generating irregular beating patterns in the whole heart of Homarus americanus. It was hypothesized that greater longitudinal stretch of the heart would result in greater regularity in the instantaneous beat frequency, based on previous findings that stretch-sensitive dendrites play a role in the regulation of the heartbeat. Furthermore, it was predicted that the elimination of stretch feedback via deafferentation of the heart would augment the irregularity induced by AMGSEFLamide. Data showed significantly increased irregularity in beating in response to 10-6 M AMGSEFLamide application. Longitudinal stretch did not reliably alter baseline variability in frequency, nor did it influence the modulatory effect of AMGSEFLamide. Deafferentation did not significantly alter baseline irregularity. Deafferented preparations did exhibit a trend of responding to AMGSEFLamide with a greater percent increase in irregularity compared to when afferents were intact, suggesting a potential role of stretch-stabilization in response to modulatory perturbations in the Homarus heart.

Date: 2021-01-01

Creator: Grace Marie Hambelton

Access: Open access

Baroreceptors are stretch receptors located in the aorta of mammals; in response to increased afterload, they elicit a decrease in heart rate, creating a negative feedback loop that lowers blood pressure. Although lobsters (Homarus americanus) do not have baroreceptors like mammals, closely related land crabs have been shown to have baroreceptor-like responses. Heart contraction is also regulated by the Frank-Starling response, where increasing stretch or preload increases the contractile force of the heart. In addition to these types of biomechanical modulations, lobsters use a central pattern generator, the cardiac ganglion, to maintain synchronicity of the heartbeat. The heart is also controlled by the central nervous system via neuromodulators, such as myosuppressin, which has been shown to increase active force and decrease frequency in isolated lobster hearts. We performed experiments on a lobster heart with the main arteries still intact, and varied the preload by stretching anterior arteries, and the afterload by elevating the dorsal abdominal artery. We added myosuppressin to modulate the cardiac ganglion output and muscle contraction. We found that the baroreceptor-like response is most directly modulated by active force, whereas frequency could be a secondary control. Increasing preload does increase active force, but that does not correlate to a higher cardiac output, which shows that how hard the heart pumps is not what determines how effectively it is pumping. Additionally, we found that myosuppressin has a much stronger effect on frequency than active force, and so with myosuppressin, frequency becomes the main determinant of cardiac output.

Date: 2024-01-01

Creator: Andre Eden

Access: Open access

During every second of a human’s life, the cardiovascular system is modulated by factors both intrinsic and extrinsic to the physiology of the heart. We can uncover new insights regarding the nature of our system through investigations of similar systems in other model species. One example materializes itself in the form of the American Lobster (Homarus americanus) whose single-chambered heart finds resemblance to the function and anatomy to that of humans. The lobster heart is powered by the cardiac ganglion (CG), a group of neurons that drive contractions of surrounding heart muscles, known as the myocardium. Both the CG and myocardium work in a feedback loop, with both intrinsic (afterload and preload) and extrinsic (temperature and neuropeptides) factors affecting cardiac output (CO) or the overall ability of the heart to carry out its primary function of nutrient distribution. In this paper, we examine how the addition of these factors into in vitro whole heart preparations affect CO and other associated variables. From experimentation, we conclude that the neuropeptide SGRNFLRFamide (SGRN) increases the heartbeat frequency and the active force exerted by the heart. We also conclude that increases in temperature decrease CO as higher temperatures decrease heartbeat frequency and the active force exerted by the heart. Lastly, we conclude that the effect of preload and afterload combined produce more robust effects on the CO and active force of the heart, potentially painting a better picture of what may happen in vivo.