Dendritic spines: role of active membrane in modulating synaptic efficacy.

Dendritic spines have been increasingly implicated as sites for neuronal plasticity. Earlier-theoretical studies of dendritic-spine function have assumed passive membrane, and have consequently predicted that postsynaptic potentials in the dendrite are attenuated when the synapse is located on the spine head rather than on the dendritic shaft. Our studies show that active membrane in the spine head (e.g. voltage-dependent Na+ or Ca2+ channels) can produce amplification rather than attenuation of the postsynaptic potential. The presence and amount of amplification depend on the density of active channels and on the spine-neck resistance. For a given type of spine head, there is an optimal spine-neck resistance; a given change in neck resistance can therefore either increase or decrease the amplitude of postsynaptic potentials. These results support the idea that spines mediate synaptic plasticity and suggest a variety of modulatory mechanisms.