Why so many synapses? | ¿Para qué tantas sinapsis?

Abstract

Introduction and development. Synapses are the most abundant structures in the body of all animals. This number appears to be exceedingly large considering their apparent function: excite or inhibit the postsynaptic cell. In the few cases for which data are available, the number of synapses that two identified cells establish is reasonably constant between individuals although variable during development. Observations in a variety of sensory systems and animal species indicate that synapse number is important for normal physiology. Recent data from the olfactory system in Drosophila show that, if this number increases threefold, the sensitivity of perception increases up to three orders of magnitude. Similarly, the loss of perception sensitivity to a given odorant correlates with a loss of sensory synapses in selected neurons. These changes require proper regulation of the cAMP pathway. Conclusions. It seems reasonable to assume that the postsynaptic effects in the case of N versus N′ synapses should be different. Since the action potential parameters in the postsynaptic cell are the same in both cases, we can propose, as a working hypothesis, that the functional differences rely in the differential calcium dynamics throughout the postsynaptic dendritic branches.