DENDRITIC SPINES AS BASIC FUNCTIONAL UNITS OF NEURONAL INTEGRATION

MOST excitatory synaptic connections occur on dendritic spines(1). Cal cium imaging experiments have suggested that spines constitute individ ual calcium compartments(2,3), but recent results have challenged this idea(4,5). Using two-photon microscopy(6) to image fluorescence with high resolution in strongly scattering tissue, we measured calcium dyn amics in spines from CA1 pyramidal neurons in slices of rat hippocampu s. Subthreshold synaptic stimulation and spontaneous synaptic events p roduced calcium accumulations that were localized to isolated spines, showed stochastic failure, and were abolished by postsynaptic blockers , Single somatic spikes induced fast-peaking calcium accumulation in s pines throughout the cell. Pairing of spikes with synaptic stimulation was frequently cooperative, that is, it resulted in supralinear calci um accumulations. We conclude: (1) calcium channels exist in spine hea ds; (2) action potentials invade the spines; (3) spines are individual calcium compartments; and (4) spines can individually detect the temp oral coincidence of pre- and postsynaptic activity, and thus serve as basic functional units of neuronal integration.