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Researchers identify previously unknown subtype of neuron in the visual cortex.

Neuroplasticity, or brain plasticity, is an umbrella term that describes lasting change to the brain throughout life.  One of the fundamental principles of how neuroplasticity functions is linked to the concept of synaptic pruning, the idea that individual connections within the brain are constantly being removed or recreated, depending on how they are used. Neuroplastic change can occur to individual neurons, or at whole-brain scales, such as cortical remapping in response to injury.
While previous in vitro studies have provided extensive information about the intrinsic physiology and local connectivity of specific cortical cell types, information about brain-wide connectivity and plasticity has been more elusive.  Now, a study from researchers at the Salk Institute has identified a new subtype of nerve cell, or neuron, in the visual cortex and how the new cell and two similar neurons process images and connect to other parts of the brain. The team state that learning how the brain analyzes visual information at such a detailed level may one day help doctors understand elements of disorders like schizophrenia and autism.  The opensource study is published in the journal Neuron.

Previous studies show that the cerebral cortex is populated by numerous types of excitatory and inhibitory neurons. Excitatory pyramidal neurons (PNs) are the source of nearly all cortical outputs and thus play an essential role in mediating interactions between brain areas. In contrast, cortical inhibitory neurons make primarily local connections and modulate cortical outputs. Many studies have capitalized on cell type specific mouse lines to explore the diversity of inhibitory neuron types and their unique roles in cortical computations.  In contrast, mouse lines for exploring the diverse contributions of different types of cortical PNs have only recently become available.  These lines have been used to investigate the functional properties and connections of layer 6 PN types with most previous studies of layer 5 (L5) PN types relying on more conventional cell targeting approaches.  The current study genetically manipulates specific cell types in animal models to visualize the size, shape and electrical activity unique to individual types of neurons.

The current study defines and characterizes three L5 neuron types in mouse primary visual cortex which display distinct morphology, physiology, brain-wide connectivity, and visual responses. Results show different L5 neurons comprise distinct output channels for sensory information processing.

Data findings show a new sub-type of neuron that, unlike previously described L5 cortico-cortical (CC) and cortico-subcortical (CS) neurons, doesn’t project to striatum and has distinct morphology, physiology, and visual responses. Monosynaptic rabies tracing shows that CC neurons preferentially receive input from higher visual areas, while CS neurons receive more input from structures implicated in top-down modulation of brain states. The lab observed that CS neurons are also more direction-selective and prefer faster stimuli than CC neurons.  The group conclude that these differences suggest distinct roles as specialized output channels, with CS neurons integrating information and generating responses more relevant to movement control and CC neurons being more important in visual perception.

The team surmise that understanding these cell types contributes another piece to the puzzle uncovering neural circuits in the brain, circuits that will ultimately have implications for neurological disorders.  They go on to add that the part of the brain investigated was the visual cortex, however, the cell types observed are also found in every other part of the cerebral cortex, such as areas involved in motor or cognitive functions.  For the future, the researchers state that experiments will likely determine even more functions of these neurons.

Source: The Salk Institute

Efr3a-Cre+ Cell Morphology and Axonal Projections in Visual Cortex.  Difference in morphologies of L5 Efr3a-Cre+ neurons labeled  by AAV-FLEX-tdTomato in V1 versus V2L or V2ML.  Three Types of Cortical Layer 5 Neurons That Differ in Brain-wide Connectivity and Function.   Callaway et al 2015.
Efr3a-Cre+ Cell Morphology and Axonal Projections in Visual Cortex. Difference in morphologies of L5 Efr3a-Cre+ neurons labeled by AAV-FLEX-tdTomato in V1 versus V2L or V2ML. Three Types of Cortical Layer 5 Neurons That Differ in Brain-wide Connectivity and Function. Callaway et al 2015.


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