Abstract

• The caudate nucleus of the guinea pig was examined at the ultrastructural level. At least four types of neurons and seven types of synapses are distinguished. The large polygonal neurons are measured 18-25µm in diameter and have large amount of cytoplasm with many stacks of granular endoplasmic reticulum, concentrations of ribosomes, and rich Golgi cisternae. The medium-sized, ovoid neurons are the most frequent cells. These neurons are measured 10-15µm in diameter and contain a lot of organelles. The medium-sized, polygonal neurons are measured 11-18µm in diameter. These cells have cytoplasm with moderate amount of granular endoplasmic reticulum and Golgi cisternae. The small neurons are the least common never cells in the caudate nucleus. They have a nucleus with indentations, little Golgi apparatus and few cisternae of granular endoplasmic reticulum. The axon terminals containing small round vesicles are the most frequent type of synapses. The majority of them make asymmetric contact on dendritic spines of neurons. Some of remainders make symmetric contact with dendrites and somata and others asymmetric contact with the same structures. Few of terminals 'en passant' with small round vesicles contact on dendrites and somata. The terminals containing elongated vesicles contact symmetrically on the dendrits and somata. All of the terminals containing pleomorphic or large round vesicles make synapses in the same manner of elongated vesicle terminal.