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Abstract

Thalamic fibers in the cortex of Pseudemys turtles were studied with the electron microscope to determine the type of synaptic vesicle they contain, the type of membrane differentiation they form, and the type of processes they contact. Following unilateral removal of the thalamus, all degenerating thalamic axon terminals are located in the outer third of the molecular layer in the rostral half of general cortex. In the middle of this zone they constitute as much as 25% of all vesicle-containing profiles. The degenerated terminals appear as electron opaque profiles, most commonly with a uniform opacity. They contain round agranular vesicles and form synapses with asymmetrical membrane differentiations. They synapse mainly on dendritic spines containing mitochondria and/or membranous sacs, although some thalamic fibers contact small clear spines, dendrites, and, rarely, cell bodies. Counts show that 86% of degenerated contacts are on dendritic spines and 14% on dendritic shafts. The spines probably all belong to the dendrites of the pyramidal cells, whose somata are located in the deep cellular layer. The dendritic shafts and somata are most likely those of the aspinous stellate neurons located in the molecular layer. Although these stellate cells are not sufficiently numerous to form a cell “layer,” each transverse section through thalamic recipient cortex contains about nine of these cells and they occur in a ratio of 1:37 to pyramidal cells in the underlying main cell layer. We have calculated that in a rectangular solid of turtle cortex whose dimensions are 1 mm × 1 mm × the depth from pial surface to the underlying ventricle, there are 5.2 million thalamic fiber contacts (all in the outer 100 μm), 15,000 pyramidal neurons in the main cell layer, and 400 stellate cells in the molecular layer. Of the 5.2 million thalamic synapses, 0.7 million contact stellate cells and 4.5 million contact pyramidal cells. Thus each stellate cell in the molecular layer receives on the average 1,800 thalamic fiber contacts, while each pyramidal cell receives only 300 thalamic fiber synapses on the distal portion of its dendrites. The calculations lead to the conclusion that individual stellate cells receive at least six times more thalamic fiber synapses than individual pyramidal cells in turtle cortex. We suggest that the stellate cells in the thalamic input zone are inhibitory and that each thalamic volley not only excites efferent pyramidal cells but is also a powerful activator of inhibitory interneurons.