Observations on the somatodendritic morphology and axonal trajectory of intracellularly HRP‐Labeled efferent neurons located in the deeper layers of the superior colliculus of the cat

Abstract

Efferent neurons of the deeper layers of the cat's superior colliculus were stained with horseradish peroxidase (HRP) to demonstrate patterns of somatodendritic morphology and axonal trajectory. A combination of somatodendritic and axonal features of the HRP-labeled cells revealed the existence of three major groups of tectal efferent neurons (X, T, and I). X neurons are mostly large and multipolar and participate in the crossed descending and ipsilateral ventral ascending projections of the superior colliculus. The X group includes multipolar radiating (X1), tufted (X2), large vertical (X3), medium-sized vertical (X4), and medium-sized horizontal (X5) neurons. T neurons participate in one or two of the major tectofugal bundles (medial descending ipsilateral, lateral descending ipsilateral, medial dorsal ascending, crossed descending) besides providing a commissural branch. They also issue recurrent collaterals distributed within a more or less restricted area of the deeper layers. The T group includes medium-sized, trapezoid, radiating (T1) and small or medium-sized, ovoid, vertical (T2) neurons. I neurons participate in the ipsilateral descending projection of the superior colliculus. They are small, triangular or ovoid, sparsely ramified cells that provide long, varicose collaterals irregularly distributed within the deeper layers. The majority of T neurons are located in the ventral stratum opticum or dorsal stratum griseum intermediale; X3 and X5 neurons are situated immediately below in the dorsal stratum griseum intermediale, while X1, X2, X4, and I neurons are indiscriminately distributed within the deeper layers. The polythetic classification presented here provides a conceptual framework for the description of tectal efferent neurons. It is open-ended and can thereby accommodate new cells types as indicated by the disclosure of a small horizontal (A) and a small radiating (unclassified) neuron. Moreover, it does not preclude the construction of alternate taxonomies. A dendro-architectonic classification into four groups [vertical (X3, X4, T2, I), horizontal (X5, A), radiating (X1, T1, I), and tufted (X2)] can be made and would relate to the mode of integration of various tectopetal inputs. A classification based on the dorsoventral location of tectal efferent neurons is also possible and would relate to the dorsoventral distribution of neurons with specific response properties.

Topographic Organization of Excitatory and Inhibitory Commissural Connections in the Superior Colliculi and Their Functional Roles in Saccade Generation

“…Specifically, the cell bodies of tectotectal CNs were observed primarily in the intermediate gray layer [stratum griseum intermedium (SGI)] and the deep gray layer [stratum griseum profundum (SGP)] (Edwards 1977;Fish et al 1982;May 2006;Olivier et al 1998;Yamasaki et al 1984). Similarly, the main terminal field of tectotectal neurons arborized in the SGI and SGP (Behan and Kime 1996;Moschovakis and Karabelas 1985). However, no detailed data are available to show the correlation between this laminar arrangement and the topographical distribution of excitatory and inhibitory commissural neurons in the SC.…”

Nigral Inhibition of GABAergic Neurons in Mouse Superior Colliculus

“…The SGI receives GABAergic inputs arising from the substantia nigra pars reticulata (SNr), one of two output nuclei of the basal ganglia (Jayaraman et al, 1977;Chevalier et al, 1984;May and Hall, 1984;Karabelas and Moschovakis, 1985;Moschovakis and Karabelas, 1985;Williams and Faull, 1988;Harting and Van Lieshout, 1991;Bickford and Hall, 1992;Mana and Chevalier, 2001;Cebrián et al, 2005). The SNr inhibits SGI neurons with high-frequency tonic input (Hikosaka and Wurtz, 1985a,b).…”

Maturation of multisensory integration in the superior colliculus: Expression of nitric oxide synthase and neurofilament SMI-32

“…Of particular interest was determining the spatial relationship between these two neuron populations. In order to do so, the large deep layer neurons that send their descending axons to the brainstem and spinal cord (Moschovakis and Karabelas, 1985;Munoz and Guiton, 1991;Guitton and Munoz, 1991;Meredith et al, 2001;Fuentes-Santamaria et al, 2006) were labeled using the neurofilament protein SMI-32. This protein is preferentially expressed in SC output neurons and facilitates detailing their somatic and dendritic morphology (Fuentes-Santamaria et al, 2006).…”