Electron microscopic observations on recurrent axon collateral boutons of a triceps surae γ-motoneuron in the cat

Boutons from the recurrent axon collaterals of an adult cat gastrocnemius γ-motoneuron were studied after intracellular labelling with horseradish peroxidase (HRP). Light and electron microscopic observations revealed that the studied γ-motoneuron possessed 6 dendrites with 39 dendritic end branches totally and that its axon gave off two axon collaterals with together 8 synaptic boutons of either en passant or terminal type. Both collateral trees were confined to the ventral part of lamina VII. Four of the synaptic boutons were studied electron microscopically. They were all found to be S-type boutons containing spherical vesicles. In two cases the bouton could be further subclassified as a type T bouton. All of the studied boutons made synaptic contact with thin dendrites of unknown origin.