Identification and distribution of retinocollicular terminals in the cat: An electron microscopic autoradiographic analysis

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Abstract
The electron microscopic (EM) autoradiographic method has been used to determine the size, distribution, and ultrastructure of retinocollicular axon terminals following intraocular injection of either (3H)‐leucine or (3H)‐proline. In the contralateral colliculus, retinal terminals are most numerous in the uppermost sublamina of the superficial grey, SGS1. The mean diameter of terminals in the contralateral SGS1 and SGS2 is 1.3 μm ± 0.33, and 1.45 μm ± 0.37, respectively. The mean diameter of terminals in the deepest contralateral sublamina, SGS3 (1.85 μm ± 0.62), is significantly larger than the mean diameter of those in the contralateral SGS1 or SGS2. By contrast, the mean diameter of terminals in different sublayers of the ipsilateral superficial grey is relatively constant, ranging from 1.56 μm ± 0.45 to 1.63 μm ± 0.60. Most ipsilateral retinal terminals are located in the uppermost part of SGS2. It is postulated that there are two populations of retinal terminals in the superficial grey: a population of smaller terminals present in the contralateral SGS1 and SGS2, and a population of larger terminals present in the contralateral SGS3. Both populations appear to be present in the ipsilateral sublaminae. While all labeled retinocollicular terminals contain round vesicles and asymmetric membrane thickenings, two morphological features appear to be correlated with retinal terminal size: (1) the population of smaller retinal terminals more frequently contacts postsynaptic profiles containing scattered round vesicles; and (2) large, dense‐core vesicles are more commonly associated with the presynaptic profiles of the population of larger terminals. A comparison of these morphological findings with available physiological data (Hoffman, 1973; Fukuda and Stone, 1974; McIlwain and Lufkin, 1976 McIlwain, 1978) suggests that the smaller terminals may be those of W‐retinal ganglion cells, and the larger terminals those of Y‐retinal ganglion cells.