Dendritic location of synapses and possible mechanisms for the monosynaptic EPSP in motoneurons.

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Abstract
The predictions of a mathematical neuron model have been compared with observations on monosynaptic EPSPs [excitatory postsynaptic potentials] recorded from cat spinal motoneurons. Quantitative comparisons have been made by defining EPSP shape indices, time-to-peak and half-width, and comparing the distributions of shape index values provided by our experimental and theoretical samples. Certain important experimental properties of EPSPs (e.g. effects of polarizing currents and presence or absence of impedance changes measured from the cell soma) correlate with these shape indices and, therefore, appear to be determined by synaptic location. The synapses of group Ia afferent fibers were widely distributed over the motoneuron surface and that even those synapses located on the distal dendrites produced significant depolarizations at the soma. Several alternative membrane mechanisms underlying the monosynaptic EPSP in spinal motoneurons were discussed and assessed.