Corticotectal terminals in the superior colliculus of the rabbit: A light‐ and electron microscopic analysis using horseradish peroxidase (HRP)‐tetramethylbenzidine (TMB)

Abstract

The projection from the striate cortex to the superior colliculus was studied light- and electron microscopically by means of anterogradely transported horseradish peroxidase and tetramethylbenzidine histochemistry. Labeled boutons were found in the stratum zonale (SZ) and in the stratum griseum superficiale (SGS), not in stratum opticum (SO). There are two maxima of frequency of labeled boutons, one in middle SGS at about 500 microns depth, and a smaller one in upper SGS at about 200 microns depth. Such a bimodal distribution of corticotectal terminals has not been described in any species before. Labeled myelinated axons were found in SGS and SO with a maximal frequency in middle SGS at about 400 microns depth. The myelinated axons in SZ, which are commonly considered to be of cortical origin, were not labeled. The labeled cortical terminals contained numerous round synaptic vesicles and predominantly dark mitochondria. They formed usually asymmetrical synapses and contacted dendrites, some of which contained synaptic vesicles. Occasionally, labeled boutons were observed which definitely did not belong to the type that is generally considered to be of cortical origin.

The Impact of a Corticotectal Impulse on the Awake Superior Colliculus

“…This result is consistent with morphological and physiological (electrical stimulation) studies indicating a retinotopic projection from V1 to SC (McIlwain, 1973;Hollander and Schonitzer, 1983;Hofbauer and Hollander, 1986;Harting et al, 1992). Axonal conduction time was another powerful predictor of postsynaptic impact within the SC.…”

“…Synaptic currents generated by most of our V1 CTect neurons reached peak values 500 -700 m beneath the surface of the SC. This depth is consistent with a dense band of V1 CTect terminals ϳ500 m beneath the surface of the SC (Hollander and Schonitzer, 1983), just beneath the focus of retinal input (Hofbauer and Hollander, 1986). However, Hollander and Schonitzer (1983) also reported a secondary band of CTect terminals ϳ200 m beneath the surface, but we detected no evidence of this superficial input.…”

“…This depth is consistent with a dense band of V1 CTect terminals ϳ500 m beneath the surface of the SC (Hollander and Schonitzer, 1983), just beneath the focus of retinal input (Hofbauer and Hollander, 1986). However, Hollander and Schonitzer (1983) also reported a secondary band of CTect terminals ϳ200 m beneath the surface, but we detected no evidence of this superficial input. It may be that our single-axon CSD method is sensitive only to retinotopically focused and powerful synaptic inputs generated by fastconducting CTect neurons.…”

“…We do not know, therefore, whether single CTect neurons generate a dense or sparse terminal arborization within the SC, the retinotopic precision of single-axon arborizations, or the postsynaptic impact generated by single CTect neurons. CTect neurons of V1 terminate in superficial, retinal recipient layers of the SC in a roughly topographic manner (McIlwain, 1973;Hollander and Schonitzer, 1983;Harting et al, 1992). Moreover, many superficial SC neurons respond to both retinal and CTect stimulation (Berson, 1988), and some receptive field properties are lost or modified after V1 inactivation (Rizzolatti et al, 1970;Rosenquist and Palmer, 1971;Ogasawara et al, 1984).…”

Original article Age-related dendritic and spinal alterations of pyramidal cells of the human visual cortex

“…In the pri mary visual cortex (V1), the great majority of cor ticotectal neurons are members of this distinctive subpopulation of pyramidal neurons [25,47,58]. Cor ticotectal neurons of V1 terminate in superficial, retinal recipient layers of the superior colliculus in a roughly topographic manner [16,17].…”