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Dynamic compartmental computations in tuft dendrites of layer 5 neurons during motor behavior

Tuft dendrites of layer 5 pyramidal neurons form specialized compartments important for motor learning and performance, yet their computational capabilities remain unclear. Structural-functional mapping of the tuft tree from the motor cortex during motor tasks revealed two morphologically distinct populations of layer 5 pyramidal tract neurons (PTNs) that exhibit specific tuft computational properties. Early bifurcating and large nexus PTNs showed marked tuft functional compartmentalization, representing different motor variable combinations within and between their two tuft hemi-trees. By contrast, late bifurcating and smaller nexus PTNs showed synchronous tuft activation. Dendritic structure and dynamic recruitment of the N-methyl-d-aspartate (NMDA)–spiking mechanism explained the differential compartmentalization patterns. Our findings support a morphologically dependent framework for motor computations, in which independent amplification units can be combinatorically recruited to represent different motor sequences within the same tree. Description Independent computations within dendrites Cortical pyramidal neurons typically have an elaborate dendritic tree that receives and integrates the many synaptic inputs targeting the neuron. An open question is how information is represented in dendrites in vivo. Otor et al. investigated synaptic computations in the apical tuft of layer 5 pyramidal neurons in the mouse motor cortex using two-photon calcium imaging, behavioral analysis, and cable modeling. Early-branching layer 5 pyramidal neurons showed marked compartmentalization of dendritic calcium signaling, whereas late-branching pyramidal neurons had synchronous tuft activation. N-methyl-d-aspartate spikes and cable properties could explain the varying compartmentalization patterns. Compartmentalized activity between hemi-trees was correlated with behavioral outcome. These results indicate a cell-type-dependent dynamic combinatorial code for motor representation. —PRS Cell type–dependent dynamic combinatorial code for motor representation in tuft dendrites of M1 layer 5 pyramidal neurons is discussed.