Structure and function of neuronal dendrites

Abstract

Neurons represent the cellular substrate for information processing in the nervous system. Already around 1900 the Spanish neuroanatomist Ramón y Cajal proposed that neurons possess two discrete functional domains, the axonal and the somatodendritic compartment. Cajal established the foundation for the neuron doctrine by suggesting dendrites to be the synaptic input regions of neurons, and that information processing travels from dendritic regions towards axon terminals and output synapses (“the theory of dynamic polarization”, Shepherd, 1991). Despite a number of exceptions, for most neurons this rule prevails to the present. Therefore, dendritic architecture has two fundamental functions in the nervous system. First dendrites expand the receptive surface of neurons, and their shape dictates how many and which presynaptic neurons can contact a postsynaptic dendritic arbor. Thus, dendritic structure influences the number of synapses as well as the wiring logic within neuronal networks. Second dendritic structure impacts the temporal and spatial integration of postsynaptic potentials. Accordingly, in different types of neurons with different functions dendritic gestalt differs significantly, and dendritic architecture often serves to classify neuron types. In most cases, however, the specific function of dendritic architecture remains largely elusive. Dendritic structure analysis is further bedeviled by dendrites exhibiting voltage-gated ion channels which themselves vastly modify function and computing power. Although a multitude of neurodevelopmental and neurodegenerative disorders coincides with dendritic defects, it often remains unclear whether these structural defects are the cause or a consequence of the dysfunction. Therefore, on the one hand it is important to determine the contribution of dendritic structure to the function of different types of healthy neurons. On the other hand the question arises whether dendritic defects impact neuronal function qualitatively and to what degree of dendritic defect neuronal function can be maintained. This article will first summarize basic functions of passive dendritic architecture which applies for most neurons but confers variable characteristics to different types of neurons. It will be discussed how the location of input synapses in a passive electrical structure affects the integration of postsynaptic potentials. Then principles will be introduced how this localization-dependence of synaptic inputs into dendrites can be compensated for. And finally, an identified Drosophila motoneuron will serve as an example that at least in specific types of neurons basic function can be maintained with a minimum number of dendrites and input synapses. By contrast, in this example dendritic structure is imperative for fine tuning of adaptive behavioral functions which are essential for survival and reproduction. These findings will then be discussed in the context of other neuronal functions.