title: A genuine layer 4 in motor cortex with prototypical synaptic circuit connectivity.
abstract: The 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 the 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.
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States.
authors.forename: Naoki
authors.initials: N
authors.lastname: Yamawaki
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States.
authors.forename: Katharine
authors.initials: K
authors.lastname: Borges
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States.
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Suter
authors.affiliation: Institute of Neurology, University College London, London, United Kingdom.
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Harris
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, United States.
authors.forename: Gordon M G
authors.initials: GM
authors.lastname: Shepherd
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure; D003712:Dendrites / Q000502:physiology / Q000648:ultrastructure; D005456:Fluorescent Dyes; D005822:Genetic Vectors; D051379:Mice; D008863:Microspheres; D008867:Microtomy; D009044:Motor Cortex / Q000502:physiology* / Q000648:ultrastructure; D017966:Pyramidal Cells / Q000502:physiology* / Q000648:ultrastructure; D013238:Stereotaxic Techniques; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure; D055366:Synaptic Potentials / Q000502:physiology*; D009435:Synaptic Transmission; D013788:Thalamus / Q000502:physiology / Q000648:ultrastructure; D046509:Tissue Culture Techniques
chemical_list: D005456:Fluorescent Dyes
keywords: layer 4; microcircuit; mouse; neocortex; neuroscience; pyramidal neuron; thalamocortical