Characteristics of Excitatory and Inhibitory Synapses in the Central Nervous System of the Cat

Abstract

ELECTRON microscopic investigations by Gray^1,2, De Lorenzo³ and Hamlyn⁴ suggest that there are three types of patches of membrane density in the synaptic contacts of the central nervous system, of which only two are functional (Gray's type 1 and type 2). However, no criterion has yet been established to distinguish the excitatory synapses from the inhibitory ones. In the peripheral autonomic nervous system two types of synaptic vesicles (cholinergic and adrenergic) have been found, the former having a diameter of 500 Å with clear content, while the latter have a diameter of 1000 Å with granular cores^5–7. So-called granulated vesicles have also been discovered by Shimizu⁸ in the hypothalamus of mammals. Gray's classification hints that the type 1 synapse is excitatory, whereas the type 2 is inhibitory⁹.