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

Excitatory post‐synaptic potentials (e.p.s.p.s) were evoked in motoneurones by impulses in single group Ia axons. After recording the e.p.s.p., the motoneurone and the group Ia axon were injected with horseradish peroxidase. The morphological details of the connexion formed by each group Ia axon with a motoneurone were subsequently reconstructed. Four Ia axon‐motoneurone pairs were obtained. The electrotonic distance from the soma to each synaptic bouton in the connexion was calculated. The electrotonic lengths of those dendrites on which synaptic connexions were found were also calculated. The shape indices of each recorded e.p.s.p. and the standard cable model of the motoneurone were used to calculate the electrotonic distance from the soma to the point on the equivalent dendritic cable at which the e.p.s.p. originated. This distance was compared with the distance obtained from the reconstruction of the synaptic connexion at which the e.p.s.p. was generated. For two of the four connexions, the locations calculated by both methods agreed to within 0.1 lambda. Similar agreement could only be obtained for the other two connexions if synaptic transmission did not occur at some of the boutons in the termination. Evidence that some boutons were not involved in transmission is presented in the following paper (Redman & Walmsley, 1983).