W. Denk et al., 2 TYPES OF CALCIUM RESPONSE LIMITED TO SINGLE SPINES IN CEREBELLAR PURKINJE-CELLS, Proceedings of the National Academy of Sciences of the United Statesof America, 92(18), 1995, pp. 8279-8282
Of fundamental importance in understanding neuronal function is the un
ambiguous determination of the smallest unit of neuronal integration.
It was recently suggested that a whole dendritic branchlet, including
tens of spines, acts as the fundamental unit in terms of dendritic cal
cium dynamics in Purkinje cells. By contrast, we demonstrate that the
smallest such unit is the single spine. The results show, by two-photo
n excited fluorescence laser scanning microscopy, that individual spin
es are capable of independent calcium activation. Moreover, two distin
ct spine populations were distinguished by their opposite response to
membrane hyperpolarization. Indeed, in a subpopulation of spines calci
um entry can also occur through a pathway other than voltage-gated cha
nnels. These findings challenge the assumption of a unique parallel fi
ber activation mode and prompt a reevaluation of the level of function
al complexity ascribed to single neurons.