Thalamotelencephalic projections in the turtle (Pseudemys scripta)

Abstract

The telencephalic projections of the turtle thalamus were studied using the Fink-Heimer ('67) technique for staining dezenerated axons and their terminals.Large thalamic lesions produced terminal degeneration i n the basal telencephalic nuclei, the core of the dorsal ventricular ridge and the outer half of layer I i n general cortex. A variety of control lesions confirmed that these projections arisc i n the thalamus. Circumscribed thalamic lesions revealed first, that there is some degree of spatial organization in the turtlc's thalamocortical projection system and second, that at least one sensory relay nucleus, the dorsal lateral geniculate, projects to general cortex. Detailed comparisons of the turtle's thalamotelencephalic projections with thosc present i n two primitive mammalian species, the hedgehog and the opossum, provided a basis for identifying probablc homologies in thc forebrains of reptilcs and mnmmals.

Dendritic Properties of Turtle Pyramidal Neurons

“…In mammals it survives in both the piriform cortex and the hippocampal formation, often termed "archicortex" or "paleocortex." In turtles (and other reptiles) the three-layered pallium processes not only olfactory but also other sensory information relayed via the thalamus (Desan 1984;Hall and Ebner 1970;Mulligan and Ulinski 1990). There is a broad range of evidence establishing that the more elaborate sixlayered neocortex was derived from the pallium (e.g., Mulligan and Ulinski 1990;Reiner 2000).…”

“…There is a broad range of evidence establishing that the more elaborate sixlayered neocortex was derived from the pallium (e.g., Mulligan and Ulinski 1990;Reiner 2000). In particular, there is evidence that projections onto the dendrites of turtle pyramidal neurons from other cortical and subcortical regions are more laminar and organized than in mammalian pyramidal neurons (Desan 1984;Hall and Ebner 1970;Mulligan and Ulinski 1990;Smith et al 1980).…”

Visual stimuli induce waves of electrical activity in turtle cortex

“…The flow of visual information through the retinogeniculocortical pathway in turtles projects to a rostral area of its dorsal cortex (1,2), whose cellular and synaptic physiology share basic features with mammalian neocortex (3,4). Anatomical studies indicate that the geniculocortical afferents traverse the lateral part of this area of dorsal cortex with few collaterals, and their ending defines the border between a lateral, solely visual area (D2) and a medial area (D1) that receives input from multiple sensory modalities (2,5) (Fig.…”

Dorsal Cortex Lesions Impair Habituation in Turtles <i>(Chrysemys picta)</i>

“…It has no clear homologue in mammals but possesses cell types similar to those found in layers 5 and 6 of mammalian isocortex [Reiner, 1991]. The dorsal cortex receives projections from the dorsal thalamus [Hall et al, 1977], including a projection from the lateral geniculate nucleus [Hall and Ebner, 1970], and a projection from the basal forebrain [Ouimet et al, 1985], which contains cholinergic cells [Hohmann et al, 1983;Mufson et al, 1984;Powers and Reiner, 1993]. Lesions of the dorsal cortex do not impair retention of visual discriminations [Reiner and Powers, 1983;Grisham and Powers, 1989], but they do produce deficits in learning [Blau and Powers, 1989;Grisham and Powers, 1989;Avigan and Powers, 1995].…”