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High Speed Two-Photon Imaging of Calcium Dynamics in Dendritic Spines: Consequences for Spine Calcium Kinetics and Buffer CapacityRapid calcium concentration changes in postsynaptic structures are crucial for synaptic plasticity. Thus far, the determinants of postsynaptic calcium dynamics have been studied predominantly based on the decay kinetics of calcium transients. Calcium rise times in spines in response to single action potentials (AP) are almost never measured due to technical limitations, but they could be crucial for synaptic plasticity. With high-speed, precisely-targeted, two-photon point imaging we measured both calcium rise and decay kinetics in spines and secondary dendrites in neocortical pyramidal neurons. We found that both rise and decay kinetics of changes in calcium-indicator fluorescence are about twice as fast in spines. During AP trains, spine calcium changes follow each AP, but not in dendrites. Apart from the higher surface-to-volume ratio (SVR), we observed that neocortical dendritic spines have a markedly smaller endogenous buffer capacity with respect to their parental dendrites. Calcium influx time course and calcium extrusion rate were both in the same range for spines and dendrites when fitted with a dynamic multi-compartment model that included calcium binding kinetics and diffusion. In a subsequent analysis we used this model to investigate which parameters are critical determinants in spine calcium dynamics. The model confirmed the experimental findings: a higher SVR is not sufficient by itself to explain the faster rise time kinetics in spines, but only when paired with a lower buffer capacity in spines. Simulations at zero calcium-dye conditions show that calmodulin is more efficiently activated in spines, which indicates that spine morphology and buffering conditions in neocortical spines favor synaptic plasticity.
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Characterization of Cardiac L-Type and T-Type Calcium Channels During Normal and Defective Chick Heart Development(First Paragraph) The human heart is vital for survival from early in embryonic development throughout life. It begins developing around the third week of gestation from a pair of endocardial tubes that fuse to form a single primitive heart tube. The single-lumen heart tube develops a series of expanded areas and infoldings that divide it into four presumptive chambers. As the embryo grows, the heart begins looping. This looping process serves to bring the four presumptive chambers into the appropriate orientation for septation. The developing heart remodels itself into four separated chambers (two atria or holding chambers, two ventricles or pumping chambers) which provide for separate systemic and pulmonary circulation at birth. In most mammals, oxygenated blood enters the left atrium through four pulmonary veins. The blood is forced into the left ventricle when the left atrium contracts. When the left ventricle contracts, blood is pumped through the aorta and carried throughout the body. Deoxygenated blood returns to the right atrium via the superior and inferior vena cavae. Blood is forced into the right ventricle by contraction of the right atrium. Blood is then pumped through the pulmonary trunk and arteries to the lungs to be re-oxygenated. The four- chambered heart is formed by the eighth week o f gestation. (Larsen, 1997; de la Cruz & Markwald, 1998).
