Patterns of synaptic interconnections in the dorsal lateral geniculate nucleus of cat and monkey: a brief review.

Abstract Electron micrographs of the dorsal lateral geniculate nucleus of cat and monkey demonstrate three major axon types. With suitable fixation each type shows a characteristic preservation of organelles. One type has large terminals which contain round vesicles and pale mitochondria (RLP axons); the second type also contains round vesicles but has small terminals and dark mitochondria (RSD axons); while the third type contains flattened vesicles (F axons). This classification is valid in terms of connections. Comparisons with Golgi sections and with fiber degeneration studies allow tentative identification of RLP as retinogeniculate, of some RSD as corticogeniculate, and of F as intrageniculate axons. Further, several “connectivity rules” that apply to both species and all laminae can be formulated. For example: RSD axons do not make axosomatic contacts; postsynaptic elements are either dendrites of F axons, never RLP or RSD axons. In addition to these major axon types other presynaptic profiles are recognizable occasionally. These include dendrites which contain synaptic vesicles and very fine axons that contact the largest perikarya. Differences between species and between laminae are seen in terms of the part of the geniculate cell that receives the retinogeniculate input and in terms of the complexity of the synaptic zones associated with the retinogeniculate contacts.