The time course of synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses.

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Abstract
Excitatory post-synaptic potentials (EPSP) were evoked in motoneurons by impulses in single group Ia axons. After recording the EPSP, the motoneuron and the group Ia axon were injected with horseradish peroxidase. The morphological details of the connection formed by each group Ia axon with a motoneuron were subsequently reconstructed. Four Ia axon-motoneuron pairs were obtained. The electrotonic distance from the soma to each synaptic bouton in the connection was calculated. The electrotonic lengths of those dendrites on which synaptic connections were found were calculated. The shape indices of each recorded EPSP and the standard cable model of the motoneuron were used to calculate the electrotonic distance from the soma to the point on the equivalent dendritic cable at which the EPSP originated. This distance was compared with the distance obtained from the reconstruction of the synaptic connection at which the EPSP was generated. For 2 of the 4 connections, the locations calculated by both methods agreed to within 0.1 .lambda.. Similar agreement could only be obtained for the other 2 connections if synaptic transmission did not occur at some of the boutons in the termination.