Localization of synaptic input on dendrites of a lamprey spinal cord neurone from physiological measurements of membrane properties.

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Abstract
Composite excitatory post-synaptic potentials (EPSP) resulting from electrotonic and chemical synaptic junctions were recorded from 18 interneurons following stimulation of the I2 burster axon in the isolated lamprey spinal cord. In each cell, the half-width of the electrotonic EPSP was measured and used, together with the cable parameters estimated for the same neuron, to locate the position of synaptic contact made by the I2 axon on the dendrites of the interneuron. The synaptic location ranged from 0.05-1.35 space constants, with a mean of 0.46. The synaptic potential was simulated using the Rall model of the neuron. When compared with the experimentally recorded EPSP with the same half-width, the rise-time of the simulated synaptic potential was faster. By changing the value of synaptic distance and/or synaptic current duration, the half-width, rise-time and decay of the simulated synaptic potential fit closely the experimental EPSP. The range of synaptic distance estimated from the simulation decreased considerably (0.2-0.7 space constants; mean 0.52). Direct comparison of synaptic location estimated from histological tracings of dendritic trees from these same cells injected with horseradish peroxidase compared favorably with synaptic location estimated from the simulations. Functionally similar presynaptic axons that make synaptic connections at the same electrotonic distance from the soma on functionally similar post-synaptic cells is proposed. This occurs in the face of large variations in physical distance for these same synaptic contacts.