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.
Highlights
If the Rorb-expressing zone in M1 is organized, neurons in that laminar location should receive strong TC input from the primary motor thalamic nuclei, the ventrolateral nucleus (VL). This is suggested by previous anatomical work (Strick and Sterling, 1974; Jones, 1975; Cho et al, 2004; Kuramoto et al, 2009; Kaneko, 2013); while monosynaptic VL input to pyramidal neurons in the upper layers of vibrissal M1 was recently demonstrated using an optogeneticelectrophysiological approach (Hooks et al, 2013), it was not possible to determine if this input terminated in a putative layer 4 (L4) or L2/3, as vibrissal M1 is highly compressed due to its location at a cortical flexure
We focused on the forelimb area of M1 (Weiler et al, 2008; Tennant et al, 2010), located in the lateral agranular cortex (Caviness, 1975) where the upper layers are not compressed in this manner, and putative L4 can be more distinguished from more superficial layers. (For convenience, we refer to this forelimb region as ‘M1’.) To map input connections, we used an optogenetic strategy (Hooks et al, 2013), injecting AAV-ChR2-Venus in VL and subsequently preparing coronal slices containing M1; recording conditions were set to isolate monosynaptic inputs (Petreanu et al, 2009)
We recorded from neurons at the L3/5A border and from additional neurons across other layers, thereby obtaining a laminar profile of the excitatory TC input from VL (Figure 2B,C)
Summary
‘Agranular’ cortical regions such as the primary motor cortex (M1; area 4) are so named as they are commonly held to lack layer 4 (L4) (Brodmann, 1909). Neurons in the different layers form distinct sets of connections, and the relative thickness of the layers has implications for the function carried out by that area It is thought, for example, that the motor cortex does not have a layer 4, which suggests that the neural circuitry that controls movement differs from that in charge of vision, hearing, and other functions. Neurons at the border between layer 3 and layer 5A in the motor cortex possess many of the same properties as the neurons in layer 4 in sensory cortex They receive inputs from a brain region called the thalamus, and send outputs to neurons in layers 2 and 3. Yamawaki et al go on to characterize some of the properties of the neurons in the putative layer 4 of the motor cortex, finding that they do not look like the specialized ‘stellate’ cells that are found in some other areas of the cortex. We found that mouse M1 contains pyramidal neurons in a thin laminar zone at the L3/5A border with all these properties
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