Perisynaptic astrocytic processes (PAPs) in excitatory tripartite synapses play an important role for the synaptic transmission through glutamate clearance, ions homeostasis, and synapse isolation. Here, we analyzed the proximity of PAPs to the synaptic interface at spines on the tuft dendritic branches of layer 5 pyramidal cells in the mouse primary motor cortex (M1) during a skillful motor seed reaching task (Sohn et al., 2022. Science Advances, eabm0531). Mice were trained daily for this task and in vivo imaging was conducted on days 0, 4, and 8. The animals were perfused after day 8 and tissue blocks containing the M1 cortex were processed for EM observations. Large volume EM of the cortex was obtained using automated tape-collecting ultramicrotome and scanning electron microscopy. Newly formed and stable spines with synapse, which were observed in the M1 cortex during the 8 days motor task,were identified with correlated light and electron microscopy analysis. Tripartite synaptic structures, each including a presynaptic terminal, a postsynaptic spine and PAP(s), in 5 cubic um volume were 3D-reconstructed. The structural plasticity of PAPs involves alterations in their distance to the synaptic interface, PAP coverage volume, coverage surface area, localization of mitochondria within PAPs in relation to synapse, and branching complexity. These morphological measures were obtained as a function of the distance from the synaptic interface perimeter (Villanueva et al., 2023. PNAS). Our preliminary results showed that the perisynaptic PAPs to the newly formed spines were located more distantly from the synaptic interface than those to the stable spines.