The role of dendritic spines: comparing the complex with the simple

Since Cajal first observed dendritic spines as 'small thorns' projecting from the dendrites of cerebellar Purkinje cells over a hundred years ago, there has been continued debate on the role of these structures. Over 90% of excitatory synapses connect dendritic spines within the central nervous system, implying their functional importance. No fewer than 20 hypotheses have been proposed for the function of dendritic spines. These range from a simple mechanism for increasing the surface area of excitatory synaptic contacts, to a neuroprotective role. This review compares the two morphologically distinct spine types found on CA3 pyramidal neurons. We compare the similarities and differences displayed by these spines in compartmentalising Ca(2+) and discuss a potential role for the morphologically complex spines found on CA3 pyramidal neurons.