The fine structure of the subthalamic nucleus in the cat. I. Neuronal perikarya

The fine structure of the neurons in the subthalamic nucleus (Sth) of the cat has been studied. On the basis of the following criteria: the size and the shape of the neuronal perikarya, the ultrastructural characteristics of their organelles, the presence and quantity of somatic spines and their afferentation, three categories of neuronal perikarya were discriminated: large, medium and small neurons. The large neurons have a diameter above 18 microns and nucleocytoplasmic ratio above 1:1,8. These neurons are the richest in organelles, possess the most numerous spines, and are most afferented. According to their shape, the large neurons were subdivided in multipolar, fusiform, and irregular types. The medium sized neurons have a mean diameter between 16 and 13 microns and nucleocytoplasmic ratio 1:1,5-1:1,3. Their cytoplasm contains a lesser number of organelles that are more loosely distributed. The medium neurons possess a small number of somatic spines and are less afferented. The small neurons have a mean diameter below 12 microns and their nucleocytoplasmic ratio is below 1:1,2. Their cytoplasm is poor in organelles and they usually lack spines. The small neurons are remarkably oligoafferented. The large neurons constitute approximately 25%, the medium sized - about 50%, and the small neurons - less than 5% of the neuronal population. The remaining neuronal perikarya do not offer reliable electron microscopical criteria that could allow an unequivocal discrimination.