Electron microscopy: two major synaptic types on spinal motoneurons.

Two major types of synaptic bulbs are defined on the motoneuron surface of the monkey, on the basis of content of agranular "synaptic" vesicles of two distinct kinds. Both types are present on dendritic as well as perikaryal surface. Because of the approximately equal numbers, the hypothesis that one type is excitatory and the other inhibitory naturally arises.