A light- and electron-microscopic investigation of the optic tectum of the frog, Rana pipiens, II: The neurons that give rise to the crossed tecto-bulbar pathway

Abstract

The superficial layers of the frog's optic tectum, Potter's (1969) layers A-G, comprise a complex neuropil made up of many afferent axons, the somata of a few neurons, and many dendrites from the neurons located in the deeper layers. Different types of retinal axons are believed to terminate in different layers (Maturana et al., 1960; Kuljis & Karten, 1988; Sargent et al., 1989), but little is known about the relationships between each type of input and the dendrites of the deep tectal neurons that extend into these superficial layers. The present study used the method of retrograde transport of horseradish peroxidase to study the synaptic contacts on the dendrites of the neurons that give rise to the crossed tecto-bulbar pathway. These cells have apical dendrites that ascend through the superficial retino-recipient layers.The somata of the cells that give rise to the crossed tecto-bulbar pathway are located in the superficial half of layer 6, preferentially clustered along the caudal, lateral, and rostral margins of the tectum. The somata of these cells range from 8−30 ¼m in diameter. Their axons are large (2−4 ¼m in diameter) myelinated fibers that arise from either their somata or proximal dendrites. Their axons travel within the deep medullary layer to leave the tectum at the lateral margin. Their dendritic arbors extend obliquely through the superficial layers to reach layer B where they turn and extend within the layer for up to 0.5 mm. The somata of these cells receive only a scant synaptic input. In contrast, their dendrites receive input in every layer, but the nature of this input varies from layer to layer. Synaptic terminals that resemble retinal ganglion cell boutons contact the labeled dendrites in layers B, F, and G. This indicates that the dendrites may receive monosynaptic input from several types of retinal ganglion cells. Terminals with small, flattened vesicles also contact the dendrites of these cells in each layer. In layer F and below, the terminals with flattened vesicles constitute 15% of the contacts; above layer F they constitute only 5−8% of the contacts. Terminals with medium-sized, flattened vesicles also contact the dendrites of these cells in every layer and constitute a large proportion of their input (33−95%). The latter terminals resemble those that are often postsynaptic to retinal terminals.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. I: Tracer study of projection neurons and their pathways

“…This type was not found in the present study. These findings in Discoglossus and Eleutherodactylus are consistent with studies in ranid frogs (Rana esculenta, R. ridibunda, R. catesbeiana, R. pipiens) when using HRP or cobaltic lysine as a retrograde and anterograde tracer (Lázá r et al, 1983;Lázár, 1984Lázár, , 1989Tóth et al, 1985;Antal et al, 1986;Hughes, 1990b). In these frog studies, ganglionic cells, pyramidal cells, and large pear-shaped cells were identified as descending projection neurons.…”

The effects of nicotinic and muscarinic receptor activation on patch-clamped cells in the optic tectum of rana pipiens

“…There are a number of different retinal ganglion cell classes in the frog and their terminals end in different tectal layers. Not all classes of retinal ganglion cells project to every tectal cell although it is thought that all layer 6 cells do receive some direct retinal input (Hughes, 1990;Nakagawa et al, 1994). Therefore, if all retinal ganglion cell terminals had nicotinic receptors on them, we would have expected to have seen a nicotine-evoked increase in sPSCs in every tectal cell from which we recorded.…”

An intracellular study of pretectal influence on the optic tectum of the frog, Rana catesbeiana

“…In anurans the optic tectum provides the main interface between afferent retinal messages and efferent motor commands [1][2][3][4][5][6] . It not only receives retinal signals via direct projections through the optic nerve but also receives them indirectly through various visual pathways, for example, posterior thalamus nucleus, nucleus lentiformis mesencephali (NLM), posterodorsal and posteroventral tegmental nuclei, dorsal horn of the cervical spinal cord, nucleus isthmi and contralateral tectum [7][8][9][10][11] .…”