Multiple dynamic representations in the motor cortex during sensorimotor learning.

The mechanisms linking sensation and action during learning are poorly understood. Layer 2/3 neurons in the motor cortex might participate in sensorimotor integration and learning; they receive input from sensory cortex and excite deep layer neurons, which control movement. Here we imaged activity in the same set of layer 2/3 neurons in the motor cortex over weeks, while mice learned to detect objects with their whiskers and report detection with licking. Spatially intermingled neurons represented sensory (touch) and motor behaviours (whisker movements and licking). With learning, the population-level representation of task-related licking strengthened. In trained mice, population-level representations were redundant and stable, despite dynamism of single-neuron representations. The activity of a subpopulation of neurons was consistent with touch driving licking behaviour. Our results suggest that ensembles of motor cortex neurons couple sensory input to multiple, related motor programs during learning.

Separation of voltage- and ligand-gated calcium influx in locust neurons by optical imaging.

Calcium ions can enter neurons through either ionotropic transmitter receptors or through voltage-gated calcium channels. Thus, an observed rise in intracellular calcium concentration upon synaptic stimulation can be due to either one of these mechanisms or to both of them. We analyzed the individual contribution of transmitter- and voltage-gated calcium entry in non-spiking somata, acutely dissociated from thoracic ganglia of the locust Locusta migratoria. By optically recording the calcium signal following different stimulation protocols, we isolated the voltage- and the transmitter-gated component and found that these components indeed summate to the total rise in calcium observed under control conditions.

Mechanisms of dendritic calcium signaling in fly neurons.

We examined the mechanisms underlying dendritic calcium accumulation in lobula plate tangential cells of the fly visual system using an in vitro preparation of the fly brain. Local visual stimulation evokes a localized calcium signal in the dendrites of these cells in vivo. Here we show that a similar localized calcium accumulation can be elicited in vitro by focal iontophoretic application of the cholinergic agonist carbachol. The calcium signal had at least two sources: first, voltage-dependent calcium channels contributed to the carbachol-induced signal and were concentrated on the dendrite, the soma, and the terminal ramification of the axon. However, the dendritic calcium signal induced by carbachol stimulation was only weakly dependent on membrane depolarization. The most likely explanation for the second, voltage-independent part of the dendritic calcium signal is calcium entry through nicotinic acetylcholine receptors. We found no indication of second-messenger or calcium-mediated calcium release from intracellular stores. In summary, the characteristic spatiotemporal calcium signals in the dendrites of lobula plate tangential cells can be reproduced in vitro, and result from a combination of voltage- and ligand-gated calcium influx.