Qualitative and quantitative analyses of the patterns of retinal input to neurons in the dorsal lateral geniculate nucleus of the cat.

In the visual system of the cat the projection from the retina to the lateral geniculate nucleus has been studied extensively. However, the patterns of synaptic contacts made by individual axons onto individual cells have not been described. In this study these patterns have been examined for class 1 cells (Guillery: J Comp Neurol 128:21, '66). Retinogeniculate axons and lateral geniculate neurons are labeled with horseradish peroxidase (HRP) via injections into the optic tracts and optic radiations, respectively. Sections are then processed for combined light and electron microscopic analysis. They are examined with the light microscope to identify labeled lateral geniculate neurons that appear to be contacted by labeled retinal axons. These cells and axons are then analyzed by a computerized microscope system, and sites of apparent synaptic contact are recorded. This light microscopic analysis indicates that individual class 1 cells are contacted by many retinogeniculate axons (> 10) and that each of these axons contacts many lateral geniculate neurons (> 20). Some axons make numerous contacts that are concentrated onto a few dendrites, while others make only a few contacts, which are spread over several dendrites. In all cases, the majority of contacts are on the dendritic shafts of relatively thick secondary and tertiary dendrites. Electron microscopic analysis confirms that most of the contacts identified with the light microscope are synaptic. It also reveals that labeled and unlabeled retinal axons can innervate the same dendritic segment. Finally, one cell was studied that had its soma and most of its dendrites in lamina A1 but some of its dendrites extended into lamina A. This cell received input from retinal axons in both layers, thus suggesting that it may have been binocularly excitable.