Is the function of dendritic spines to concentrate calcium?

Although dendritic spines are thought to play an important role in synaptic transmission and plasticity, their function remains unknown. Theoretical investigations of spine function have focused on the large electrical resistance provided by the narrow constriction of the spine neck. However, this narrow constriction is also thought to provide a large diffusional resistance. The importance of this diffusional resistance was investigated theoretically with models. When calcium currents were activated on dendritic spines, peak spine head Ca2+ concentration was an order of magnitude larger in 'long-thin' spines than in 'mushroom-shaped' or 'stubby' spines. The same currents activated on dendrites produced even smaller local Ca2+ concentration changes. Although the diffusional resistance of the spine neck was important for producing these differences in [Ca2+], the amplitude and duration of the Ca2+ current relative to the number of Ca2+ binding sites determined whether Ca2+ would be concentrated near synapses. Given the importance of Ca2+ for long-term potentiation, the ability of spines to concentrate Ca2+ may play a key role in processes leading to learning and memory storage.