An ultrastructural study of the synaptic contacts of alpha 1-motoneuron axon collaterals. II. Contacts in lamina VII

Horseradish peroxidase (HRP) was injected intracellularly in triceps surae alpha-motoneurons. The axons and axon collaterals of these neurons were traced light and electron microscopically. Synaptic boutons of collaterals in the ventral part of Rexed's lamina VII were studied ultrastructurally. The boutons exhibited spherical synaptic vesicles and made synaptic contacts with cell bodies and proximal dendrites of neurons assumed to be Renshaw cells and with dendrites of unknown origin. The observations are discussed in relation to earlier qualitative and quantitative studies on the other known synaptic contacts of the alpha-motor axons, both in the central and peripheral nervous system.