Understanding the input-output transfer function of cortical neurons

Abstract

We aim to understand how brain circuits, learn, memorize or process information. To achieve this aim, we follow a bottomup approach by focusing on single neurons from rat brains and study how different synaptic inputs of a single neuron translate to an output or an action potential. We have custom-built a unique two-photon laser microscope that incorporates a holographic projector, which transforms the incident laser into multiple foci at the sample volume. The hologram is programmable so we can position the different foci anywhere around the neuron in 3D. Each focus can be used to trigger a synaptic input or used as an optical probe to record the activity of the neuron. We can therefore stimulate and probe the activity from multiple locations within the neuron's dendritic tree using light. For triggering inputs, a focal stimulation represents a synaptic input via two-photon photolysis of caged neurotransmitters. For recording, a laser focus excites a calcium indicator that changes in fluorescence whenever the neuron is active. Using these techniques, we have now identified a novel function of a specific set of dendrites that can have a significant role in learning and memory. The set of dendrites we are probing are currently unexplored due to their very thin morphology. We were able to observe unique properties that allow these dendrites to be more receptive to inputs whenever the neuron fires a series of action potentials. Hence, they have a functional role in the brain's capacity to learn and memorize.