Modeling of axonal actin hotspots and trails. (A) Left: Stationary hotspots (yellow spheres) are localized uniformly along the axon. Actin trails nucleate on the hotspots, with barbed ends facing the hotspots. Note that as new monomers (colored arrowheads) are incorporated at the barbed end, there is progressive translocation of individual monomers toward the pointed end (bottom box). Right: Known assembly, disassembly, and summary of reaction rates of actin. (B) Basal actin monomer concentration in the axon. Average elongation velocity of actin trails increases linearly (red line) with increase in basal monomer concentration. At a basal concentration of 47 µM, the average elongation velocity matches the experimentally observed average trail elongation velocity of ∼1 µm/s (blue lines). (C) Actin monomer concentration profiles along the axon during the growth of an actin trail. Multiple colored lines show declining concentration of actin monomers along the axon shaft at successive time points after trail nucleation. Note that the hotspot is located at x = 0, and the trail grows with its barbed end attached to this location. The trail collapses ∼9 s after nucleation, growing to a length of 8.8 µm. This wide concentration gradient drives in monomer from regions of higher concentration toward the hotspot. (D) Actin monomer depletion during trail elongation. An actin trail is randomly nucleated at t = 22.4 s, the trail grows (bottom, red trace), and monomer depletion from the barbed end (top, black trace) slows down the rate of trail elongation. Note that fluctuations of actin-monomer concentration reflect monomers randomly attaching/detaching from the barbed end. As the trail collapses at t = 30.7 s, actin-monomer concentration spikes back up.