Presynaptic Dendrites : Implications of Spikeless Synaptic Transmission and Dendritic Geometry

Two recent developments have modified our traditional concept of the neuron: output synapses have been discovered intermixed with input synapses and synaptic transmission has been shown to grade with presynaptic voltage. In a number of invertebrate spikeless and spiking neurons, synaptic transmission lasts for the duration of the presynaptic depolarization . There is a threshold presynaptic voltage, but it is sometimes below the spike threshold, or even below the resting potential . Above the release threshold, the postsynaptic potential (PSP) grades over a wide range of presynaptic voltages . The steady-state cable equation can be used to predict how PSPs spread within a complex dendritic geometry . Input synapses located on long, thin processes develop large PSPs, although these PSPs attenuate markedly before reaching central structures. Whereas a proximal dendrodendritic output synapse would see proximal and distal input synapses as approximately equal, a distal output synapse would see many-fold differences in the relative PSP sizes . Thus regional computations could take place in distal dendritic trees ; a given neuron could compute many different functions of' its inputs . Models are made of Aplysia and lobster neurons, of a cat spinal motoneuron, and of a neuron in the rat superior colliculus .