Understanding sensory-motor circuits, the hierarchical level of the cerebral cortex.

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In a recent study, Suter and Sheperd identify many shared features in the circuits of primary motor (M1) and secondary somatosensory (S2) cortices and show that these areas communicate via mutual projections that give each area monosynaptic access to the other one. These interareally corticoespinal (CSPs) neuron circuits may enable M1 and S2 to operate in a coordinated manner for sensorimotor integration.

M1 and S2 cortices, although anatomically and functionally distinct, share an intriguing cellular component: corticospinal neurons in layer 5B. Their functions are diverse and complex, with M1 generally involved in movement-related functions and S2 in higher-order somatosensory processing. But how these areas fit into the corticocortical hierarchy? One intriguing possibility is that, whereas M2–>M1 and S1–>M1 projections follow feedback (“top-down”) and feed forward (“bottom-up”) patterns, respectively, the bidirectional connections between M1 and S2 may be reciprocally organized, implying a shared hierarchical level.

The authors explored these issues by adapting photostimulation-based tools combined with retrograde and anterograde tracers to characterize the reciprocal interareal excitatory inputs to CSPs in mouse M1 and S2. They focused on the lateral agranular region of M1containing the forelimb representation area, and the corresponding forelimb area within S2. These methods allowed them to analyze monosynaptic inputs from long-range axonal projections at cellular and subcellular (dendritic) resolution.  They found that the mutual connections S2–>M1-CSP and M1–>S2-CSP closely resemble one another, albeit with several area-specific differences. Thus, the circuits of M1 and S2 are not only organized in parallel but are synaptically linked through interareal connections that directly innervate CSPs in each area.

These results may be relevant to the concept of a “sensorimotor interface” in cortical networks involved in sensory perception, decision-making, and motor control which suggests that the ability to make decisions occurs at the sensory-motor interface. Sensory responses are often observed in M1 neurons and may depend on S2 in particular. Indeed, in a recent study in the rat, the S2–>M1 projection was proposed to subdivide M1 into sensory-input and motor-output areas. The converse possibility, that M1–>S2 projections endow S2 with “motor” properties, has not been functionally assessed but would be consistent with the M1–>S2 projections reported by Suter and Shepherd.

Rafael Olivares Moreno

Reference:
Benjamin A. Suter and Gordon M.G. Shepherd. Reciprocal Interareal Connections to Corticospinal Neurons in Mouse M1 and S2.X The Journal of Neuroscience. 2015  doi: 10.1523/JNEUROSCI.4287-14.2015

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