In neural integrators, transient inputs are accumulated into persistent firing rates that are a neural correlate of short-term memory. Integrators often contain two opposing cell populations that ...increase and decrease sustained firing as a stored parameter value rises. A leading hypothesis for the mechanism of persistence is positive feedback through mutual inhibition between these opposing populations. We tested predictions of this hypothesis in the goldfish oculomotor velocity-to-position integrator by measuring the eye position and firing rates of one population, while pharmacologically silencing the opposing one. In complementary experiments, we measured responses in a partially silenced single population. Contrary to predictions, induced drifts in neural firing were limited to half of the oculomotor range. We built network models with synaptic-input thresholds to demonstrate a new hypothesis suggested by these data: mutual inhibition between the populations does not provide positive feedback in support of integration, but rather coordinates persistent activity intrinsic to each population.
We quantitatively studied the ontogeny of oculomotor behavior in larval fish as a foundation for studies linking oculomotor structure and function with genetics. Horizontal optokinetic and ...vestibuloocular reflexes (OKR and VOR, respectively) were measured in three different species (goldfish, zebrafish, and medaka) during the first month after hatching. For all sizes of medaka, and most zebrafish, Bode plots of OKR (0.065-3.0 Hz, +/-10 degrees/s) revealed that eye velocity closely followed stimulus velocity (gain > 0.8) at low frequency but dropped sharply above 1 Hz (gain < 0.3 at 3 Hz). Goldfish showed increased gain proportional to size across frequencies. Linearity testing with steps and sinusoids showed excellent visual performance (gain > 0.8) in medaka almost from hatching; but zebrafish and goldfish exhibited progressive improvement, with only the largest equaling medaka performance. Monocular visual stimulation in zebrafish and goldfish produced gains of 0.5 versus <0.1 for the eye viewing a moving versus stationary stimulus pattern but 0.25 versus <0.1 in medaka. Angular VOR appeared much later than OKR, initially at only high accelerations (>200 degrees /s at 0.5 Hz), first in medaka followed by larger (8.11 mm) zebrafish; but it was virtually nonexistent in goldfish. Velocity storage was not observed except for an eye velocity build-up in the largest medaka. In summary, a robust OKR was achieved shortly after hatching in all three species. In contrast, larval fish seem to be unique among vertebrates tested in their lack of significant angular VOR at stages where active movement is required for feeding and survival.
Characterization of the diffusional and electrotonic coupling of spines to the dendritic shaft is crucial to understanding neuronal integration and synaptic plasticity. Two-photon photobleaching and ...photorelease of fluorescein dextran were used to generate concentration gradients between spines and shafts in rat CA1 pyramidal neurons. Diffusional reequilibration was monitored with two-photon fluorescence imaging. The time course of reequilibration was exponential, with time constants in the range of 20 to 100 milliseconds, demonstrating chemical compartmentalization on such time scales. These values imply that electrical spine neck resistances are unlikely to exceed 150 megohms and more likely range from 4 to 50 megohms.
This introduction describes a single-compartment model of calcium dynamics that has been applied to fluorescence measurements of intracellular free calcium concentration (Ca(2+)i) changes in neurons. ...The model describes intracellular calcium handling under simplified conditions, for which analytical expressions for the amplitude and the time constants of Ca(2+)i changes can be explicitly derived. In particular, it reveals the dependence of the measured Ca(2+)i changes on the calcium indicator concentration. Applied to experimental data from small cells or subcellular compartments, the model equations have been extremely useful for obtaining quantitative information about essential parameters of Ca(2+) influx, buffering, and clearance. We illustrate also several changes that occur when the basic assumptions do not hold (e.g., when calcium diffusion, dye saturation, or kinetic effects become significant). Finally, we discuss how the changes in calcium dynamics, which are explained by the model, have been exploited for measuring properties of calcium-driven reactions, such as those regulating short-term synaptic enhancement, vesicle recycling, and adaptation.
In layer 2/3 pyramidal neurons of barrel cortex in vivo, calcium ion concentration (Ca2+) transients in apical dendrites evoked by sodium action potentials are limited to regions close to the soma. ...To study the mechanisms underlying this restricted pattern of calcium influx, we combined two-photon imaging of dendritic Ca2+ dynamics with dendritic membrane potential measurements. We found that sodium action potentials attenuated and broadened rapidly with distance from the soma. However, dendrites of layer 2/3 cells were electrically excitable, and direct current injections could evoke large Ca2+ transients. The restricted pattern of dendritic Ca2+ transients is therefore due to a failure of sodium action-potential propagation into dendrites. Also, stimulating subcortical activating systems by tail pinch can enhance dendritic Ca2+ influx induced by a sensory stimulus by increasing cellular excitability, consistent with the importance of these systems in plasticity and learning.
The role of dendrites in neuronal signal processing has been a topic of research for more than a century. Early work started with Cajal's original postulate that dendrites constitute the input side ...of the neuron and culminated in the idea that dendrites are passive cables. Later studies, however, showed that dendrites could produce action potentials. Here, we review within a historical perspective recent experimental work on the function of mammalian dendrites that has used novel approaches such as imaging of calcium concentration and dendritic patch recording. We do not intend an exhaustive review of the field of dendritic physiology nor cover appropriately progress in invertebrate preparations, but instead focus on a few experimental studies from pyramidal and Purkinje neurons. These new findings have provided direct evidence for active conductances in dendrites and chemical compartmentalization in spines. Although the logic of dendritic processing is still unclear, new results show that mammalian dendrites have a rich repertoire of electrical and chemical dynamics, suggesting that individual neurons are capable of sophisticated information processing. Combining these novel technologies with studies in behaving animals will greatly help in understanding dendritic integration.
Short-term memory is often correlated with persistent changes in neuronal firing rates in response to transient inputs. We model the persistent maintenance of an analog eye position signal by an ...oculomotor neural integrator receiving transient eye movement commands. Previous models of this network rely on precisely tuned positive feedback with <1% tolerance to mistuning, or use neurons that exhibit large discontinuities in firing rate with small changes in eye position. We show analytically how using neurons with multiple bistable dendritic compartments can enhance the robustness of eye fixations to mistuning while reproducing the approximately linear and continuous relationship between neuronal firing rates and eye position, and the dependence of neuron pair firing rate relationships on the direction of the previous saccade. The response of the model to continuously varying inputs makes testable predictions for the performance of the vestibuloocular reflex. Our results suggest that dendritic bistability could stabilize the persistent neural activity observed in working memory systems.
Larval zebrafish offer the potential for large-scale optical imaging of neural activity throughout the central nervous system; however, several barriers challenge their utility. First, ~panneuronal ...probe expression has to date only been demonstrated at early larval stages up to 7 days post-fertilization (dpf), precluding imaging at later time points when circuits are more mature. Second, nuclear exclusion of genetically-encoded calcium indicators (GECIs) limits the resolution of functional fluorescence signals collected during imaging. Here, we report the creation of transgenic zebrafish strains exhibiting robust, nuclearly targeted expression of GCaMP3 across the brain up to at least 14 dpf utilizing a previously described optimized Gal4-UAS system. We confirmed both nuclear targeting and functionality of the modified probe in vitro and measured its kinetics in response to action potentials (APs). We then demonstrated in vivo functionality of nuclear-localized GCaMP3 in transgenic zebrafish strains by identifying eye position-sensitive fluorescence fluctuations in caudal hindbrain neurons during spontaneous eye movements. Our methodological approach will facilitate studies of larval zebrafish circuitry by both improving resolution of functional Ca(2+) signals and by allowing brain-wide expression of improved GECIs, or potentially any probe, further into development.
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