Calcium from internal stores modifies dendritic spine shape

Since their discovery at the turn of the century by Ramon y Cajal (1), dendritic spines have fascinated scientists by their unusual structure and by their likely role in cognitive functions such as learning and memory. Spines are protrusions from dendrites having diverse shapes that range more than 100-fold in size. Larger spines have proportionately larger synapses and more diversity in subcellular organelles and molecular composition than do smaller spines. These differences in spine morphology and composition might account for differences in the function of the synapses located on them. Existing data suggest that spines are maintained by optimal activation. More spines form when neurons have less excitatory activation, and spines are lost when activation is too high, or when presynaptic axons degenerate (for review, see ref. 2). This pattern suggests that neurons may homeostatically regulate input by means of the number of axospinous synapses.