The lateral geniculate nucleus (LGN) receives neural inputs from several subcortical sources including the pedunculopontine nucleus in the brainstem and the tuberomammillary area of the hypothalamus. The terminals of these efferents to the LGN use acetylcholine (ACh) and histamine, respectively, as transmitters. Using immunohistochemistry to label the terminals, their morphology was examined using the electron microscope with particular emphasis on their synaptic contacts. Terminals with ACh made 2-3 synaptic contacts for each varicosity onto the dendrites of relay neurons and interneurons. Histamine-containing terminals did not make synaptic contacts, and therefore probably use histamine as a general modulator affecting all LGN neurons having the appropriate receptors, whereas ACh from the brainstem may have a specific function such as saccade-related modifications.
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