Synaptic integration in hippocampal CA1 pyramids.

Excitatory synapses on hippocampal pyramids are exclusively located to dendritic spines, usually in a 1:1 proportion. The number of spines indicates a convergence of as many as 25,000-30,000 excitatory boutons per CA1 pyramidal cell in rats. Activation of a single afferent fibre produces a unitary excitatory postsynaptic potential (EPSP) of about 150 microV, probably produced by a single quantum of transmitter. The release probability is normally low, but may be increased by facilitatory processes. On the average, each afferent fibre has few boutons (mostly 1, but up to 5) in contact with a given CA1 pyramid. Surprisingly, in view of the large synaptic convergence, only 100-300 synchronously active excitatory synapses seem necessary to make the cell discharge. Synapses in various parts of the dendritic tree are nearly equally effective in this regard. Excitatory postsynaptic potentials produced by neighbouring synapses sum linearly, both with each other and with hyperpolarizing, inhibitory potentials. Cable theoretical considerations suggest that the summation effect will be greater for synapses contacting the same secondary dendrite than for more distributed dendritic contacts. Three types of inhibitory neurones provide different classes of interference. The chandelier cells (axoaxonic cells) terminate upon the initial axons of a large number of pyramidal cells, and are thus capable of producing a wide-spread and effective inhibition. By hyperpolarizing the somata of a smaller number of cells, basket cells counteract all excitatory inputs to these cells, irrespective of synaptic location. In contrast to these two forms of global inhibition, stellate cells may cause a shunting form of inhibition at specific dendritic sites.(ABSTRACT TRUNCATED AT 250 WORDS)