Ultrastructural study of large efferent neurons in the superior colliculus of the cat after retrograde labeling with horseradish peroxidase

Abstract

The ultrastructure of large neurons in the stratum griseum intermedium of the cat superior colliculus was examined following injections of horseradish peroxidase (HRP) into the dorsal tegmental decussation. Four HRP-labeled cells were selected, and the synaptology of their cell bodies and selected regions of proximal and distal dendrites was examined. The four neurons represent four morphologically distinct cell types: multipolar radiating, tufted, large vertical, and medium-sized trapezoid radiating. These four neurons correspond with cell types X1, X2, X3, and T1 respectively, according to the recent classification of neurons in the superior colliculus of the cat by Moschovakis and Karabelas (J. Comp Neurol. 239:276-308, '85). The three X type neurons are similar in having 83% of their somata and over 74% of their proximal dendrites contacted by synaptic profiles. Distal dendrites of the X type neurons, however, receive fewer synaptic contacts. In contrast, in the T1 cell, only 69% of the soma membrane is contacted by synaptic profiles, and the synaptic coverage on proximal and distal dendrites does not vary much from this. Of the eight types of synaptic terminals described in the stratum griseum intermedium of the cat superior colliculus by Norita (J. Comp. Neurol. 190:29-48, '80), only five are found in contact with the X and T type efferent neurons described here. There are some regional differences in terminal distribution, although each terminal is represented on each cell. Type III terminals (small, contain mostly pleomorphic vesicles, and make symmetrical contacts) are the most abundant on cell bodies and dendrites of all four cell types. Terminal types II (medium-sized, containing round and flattened vesicles, and making asymmetrical contacts), and IV (medium to large in size, containing flattened vesicles, and making symmetrical contacts) are well represented. In general, terminal types I (small, containing densely packed round vesicles, and making asymmetrical contacts) and VI (small and irregular in shape, containing flattened vesicles and making symmetrical contacts) are found infrequently. The identity of different types of synaptic terminal is discussed.

Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection

“…All scale bars are in milliseconds, except where indicated otherwise its sensory inputs. Axons of the TRS tract arise from medium-large sized multipolar neurons Behan et al 1988), and the present results are consistent with previous anatomical descriptions of their widespread distribution in the intermediate and deep SC layers (Kawamura and Hashikawa 1978;Weber et al 1979;Murray and Coulter 1982;Nudo and Masterton 1989). The axons exit the SC medio-ventrally, course around the periaqueductal gray, and cross the midline at the dorsal tegmental decussation to join the predorsal bundle en route to the brainstem and spinal cord.…”

Cortical somatosensory and trigeminal inputs to the cat superior colliculus: Light and electron microscopic analyses

“…Although the postsynaptic elements were not established in these experiments, physiological data showed that SIV cortical terminals contacted tectospinal neurons (Wallace et al, 1993). The present results indicate that both the cortical and trigeminal pathways contact more than one class of SGI neuron, with some of the postsynaptic dendritic profiles showing characteristics of tectospinal cells (Moschovakis and Karabelas, 1985;Behan et al, 1988). Thus, our working hypothesis is that somatosensory related cortical inputs distribute to the proximal dendrites of tectospinal cells, whereas trigeminal inputs contact more peripheral parts of these cells.…”

“…These dendrites can be identified easily when they are sectioned longitudinally. Similar characteristics have been described for multipolar neurons within the SGI (Behan et al, 1988), and some of these cells are known to project to the spinal cord (Weber et al, 1979;Moschovakis and Karabelas, 1985;Behan et al, 1988). In the other category, dendritic profiles are spine-free and receive only sparse synaptic input.…”

Association of efferent neurons to the compartmental architecture of the superior colliculus.

“…compatible with differential integrative mechanisms inside different compartments. Such a mode of neuronal integration is likely to involve presynaptic dendrites (23), which have been seen in the deep layers of the feline colliculus, where they probably belong to interneurons (24,25). Alternatively, signal flow through the colliculus may be determined by the specific position of efferent neurons relative to the compartmental matrix.…”