A genuine layer 4 in motor cortex with prototypical synaptic circuit connectivity

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Abstract

Motor cortex (M1) is classically considered an agranular area, lacking a distinct layer 4 (L4). Here, we tested the idea that M1, despite lacking a cytoarchitecturally visible L4, nevertheless possesses its equivalent in the form of excitatory neurons with input-output circuits like those of L4 neurons in sensory areas. Consistent with this idea, we found that neurons located in a thin laminar zone at the L3/5A border in the forelimb area of mouse M1 have multiple L4-like synaptic connections: excitatory input from thalamus, largely unidirectional excitatory outputs to L2/3 pyramidal neurons, and relatively weak long-range corticocortical inputs and outputs. M1-L4 neurons were electrophysiologically diverse but morphologically uniform, with pyramidal-type dendritic arbors and locally ramifying axons including branches extending into L2/3. Our findings therefore identify pyramidal neurons in M1 with the expected prototypical circuit properties of excitatory L4 neurons, and question the traditional assumption that motor cortex lacks this layer.

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Naoki Yamawaki

Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States

Competing interests
The authors declare that no competing interests exist.
Katharine Borges

Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States

Competing interests
The authors declare that no competing interests exist.
Benjamin A Suter

Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States

Competing interests
The authors declare that no competing interests exist.
Kenneth D Harris

Institute of Neurology, Department of Neuroscience, Physiology, and Pharmacology, University College London, London, United Kingdom

Competing interests
The authors declare that no competing interests exist.
Gordon M G Shepherd

Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States

For correspondence
g-shepherd@northwestern.edu

Competing interests
The authors declare that no competing interests exist.

Ethics

Animal experimentation: This study was performed in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. All of the animals were handled according to approved institutional animal care and use committee (IACUC) protocols (1248, 1331, 3310) of Northwestern University.

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This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.