The Information Content of Action Potential Trains - A Synaptic Basis

Electrical recordings from three neurons revealed that the same spike train emitted by one neuron had markedly different effects on two target neurons. A spike train from a single neocortical pyramidal neuron produced synaptic responses in two target pyramidal neurons that differed in response strength and rates of activity-dependent depression of synaptic transmission. When a pyramidal neuron targeted another pyramidal neuron as well as an interneuron, then responses were also qualitatively different. The responses onto the pyramidal neuron displayed marked activity-dependent depression while those onto the interneuron displayed marked activity-dependent facilitation. The results suggest that each target could have a unique response to the same presynaptic signal. The information contained within the spike train therefore appears to be fragmented and re-integrated into the network at specific locations. The degree to which the specific fragment extracted by each synapse, will influence the spiking activity of the neuron, depends the ongoing integration of input from other presynaptic neurons. It is therefore proposed that differential synaptic transmission enables the neocortex to encode and decode the information contained within spike trains in an associative manner.