7 These GECIs can be introduced not only using viruses but are also available in a variety of transgenic animals. 6 Further work in protein engineering led to GECI variants with attractive properties such as a redshifted emission spectrum. The development of genetically encoded calcium indicators (GECIs)-fluorescent proteins that sense calcium and report via a change in emission amplitude or spectrum-enabled measurements with high signal-to-noise ratios. The rising popularity of calcium imaging is due in part to technical advances. 4 Calcium imaging is also the primary method behind a recent large-scale effort to survey neural responses in the mouse visual cortex. For example, the method has been applied to awake animals to study, at the subcellular level, the role of dendritic calcium signals during perceptual performance 1 at the single-neuron level, learning-related neural activity across the course of motor skill acquisition 2 at the local-circuit level, dynamics of frontal cortical ensembles during decision-making 3 and, at the network level, interactions between brain regions that underlie neurodevelopment. In neuroscience, there has been an explosion in the number of studies that employ calcium imaging. We address commonly asked questions such as: Can calcium imaging be used to characterize neurons with high firing rates? Can the fluorescent signal report a decrease in spiking activity? What is the evidence that calcium transients in subcellular compartments correspond to distinct presynaptic axonal and postsynaptic dendritic events? By reviewing the empirical evidence and limitations, we suggest that, despite some caveats, calcium imaging is a versatile method to characterize a variety of neuronal events in vivo.Ĭalcium imaging refers to optical methods for measuring the concentration of calcium ions in cells. We highlight experiments that have directly calibrated in vivo calcium signals recorded using fluorescent indicators against electrophysiological events.
However, calcium transients have complex spatiotemporal dynamics, and since most optical methods visualize only one of the somatic, axonal, and dendritic compartments, a straightforward inference of the underlying electrical event is typically challenging. For example, calcium influx in the soma and axonal boutons accompanies spiking activity, whereas elevations in dendrites and dendritic spines are associated with synaptic inputs and local regenerative events. A major reason is that intracellular calcium transients are reflections of electrical events in neurons. Calcium imaging is emerging as a popular technique in neuroscience.
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