The submicroscopic organization of the rod and cone synapses of the albino rabbit has been investigated with the use of the electron microscope. The most common rod synapse consists of an enlarged expansion of the rod fiber (the so called spherule) into which the dendritic postsynaptic fiber of the bipolar cell penetrates and digitates. The membrane surrounding the terminal consists of a double layer, the external of which is interpreted as belonging to the intervening glial cells. The synaptic membrane has a pre- and a postsynaptic layer with a total thickness of 180 to 300 A. The presynaptic layer is frequently denser and is intimately associated with the adjacent synaptic vesicles. The synaptic membrane shows processes constituted by foldings of the presynaptic layer. The entire spherule is filled with synaptic vesicles varying in diameter between 200 and 650 A with a mean of 386 A. In addition, the spherule contains a few large vacuoles near the rod fiber, interpreted as endoplasmic reticulum, and a matrix in which with high resolution a fine filamentous material can be observed. The postsynaptic fiber is homogeneous and usually does not show synaptic vesicles.

In animals maintained in complete darkness for 24 hours vesicles appear to accumulate near the synaptic membrane and its processes. After 9 days there is a sharp decrease in size of the synaptic vesicles.

A special rod synapse in which the dendritic postsynaptic expansion penetrates directly into the rod cell body has been identified. In line with Cajal's classification this type of synapse could be considered as a somatodendritic one.

The cone synapse has a much larger terminal with a more complex relationship with the postsynaptic fiber. However, the same components recognized in the rod synapse can be observed. In animals maintained for 9 days in complete darkness there is also a considerable diminution in size of the synaptic vesicles.

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