  Dendritic spines are small membranous structures that protrude from the
neuronal dendrite. Each spine contains a synaptic contact site that may connect
its parent dendrite to the axons of neighboring neurons. Dendritic spines are
markedly distinct in shape and size, and certain types of stimulation prompt
spines to evolve, in fairly predictable fashion, from thin nascent morphologies
to the mushroom-like shapes associated with mature spines. This striking
progression is coincident with the (re)configuration of the neuronal network
during early development, learning and memory formation, and has been
conjectured to be part of the machinery that encodes these processes at the
scale of individual neuronal connections. It is well established that the
structural plasticity of spines is strongly dependent upon the actin
cytoskeleton inside the spine. A general framework that details the precise
role of actin in directing the transitions between the various spine shapes is
lacking. We address this issue, and present a quantitative, model-based
scenario for spine plasticity validated using realistic and physiologically
relevant parameters. Our model points to a crucial role for the actin
cytoskeleton. In the early stages of spine formation, the interplay between the
elastic properties of the spine membrane and the protrusive forces generated in
the actin cytoskeleton propels the incipient spine. In the maturation stage,
actin remodeling in the form of the combined dynamics of branched and bundled
actin is required to form mature, mushroom-like spines. Our model identifies
additional factors that plausibly aid the stabilization and maintenance of
spine morphology. Taken together, our model provides unique insights into the
fundamental role of actin remodeling and polymerization forces during spine
formation and maturation.
