FIGURE 1.

Schematic representation of a dendritic spine. The model of a dendritic spine is composed of a spherical head and a cylindrical neck that connects the head to the dendrite. The head contains an active network of protein molecules of myosin, actin, and calmodulin. Active pumps are located on the surface of the head, and calcium channels are situated on top. The solid black circles represent calcium ions. The springs connecting the circles and the membrane represent the actin network: when a protein contracts it affects the spine volume. The diagram is not drawn to scale. The spine is arbitrarily partitioned into four compartments, as indicated on the left part of the figure: compartment 1, from the top of the head to a distance of R/3; compartment 2, from R/3 to the middle of the head; compartment 3, from the middle of the head to a distance of l/4, where l is the length of the neck; and compartment 4, the bottom l/4 of the neck.