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Isthmotectal projections in turtles were examined by making serial section reconstructions of axonal and dendritic arborizations that were anterogradely or retrogradely filled with HRP. Two prominent tectal-recipient isthmic nuclei--the caudal magnocellular nucleus isthmi (Imc) and the rostral magnocellular nucleus isthmi (Imr)--exhibited strikingly different patterns of organization. Imc cells have flattened, bipolar dendritic fields that cover a few percent of the area of the cell plate constituting the nucleus and they project topographically to the ipsilateral tectum without local axon branches. The topography was examined explicitly at the single-cell level by using cases with two injections at widely separated tectal loci. Each Imc axon terminates as a compact swarm of several thousand boutons placed mainly in the upper central gray and superficial gray layers. One Imc terminal spans less that 1% of the tectal surface. Imr cells, by contrast, have large, sparsely branched dendritic fields overlapped by local axon collaterals while distally, their axons nontopographically innervate not only the deeper layers of the ipsilateral tectum but also ipsilateral Imc. Imr receives a nontopographic tectal input that contrasts with the topographic tectal input to Imc. Previous work on nucleus isthmi emphasized the role of the contralateral isthmotectal projection (which originates from a third isthmic nucleus in turtles) in mediating binocular interactions in the tectum. The present results on the two different but overlapping ipsilateral tecto-isthmo-tectal circuits set up by Imc and Imr are discussed in the light of physiological evidence for selective attention effects and local-global interactions in the tectum.

Dorsal ventricular ridge (DVR) is a thalamorecipient, subcortical telencephalic structure in reptiles and birds. Although there is a fair amount of information about sources of afferents to DVR, little is known about the relationship of projections from individual thalamic nuclei to the organization of the structure. This study examines the relationship between thalamic projections and both areal and zonal divisions of anterior DVR (ADVR; Balaban, '78a) of emydid turtles with orthograde degeneration, autoradiographic and horseradish peroxidase techniques. Individual thalamic nuclei contribute either a diffuse or a restricted projection to ADVR. Diffuse projections arise primarily from the dorsomedial anterior nucleus. These fine-caliber axons distribute bilaterally over a wide region of the telencephalon via both medial and lateral thalamotelencephalic pathways. The terminal regions include septum, striatum and the medial bank of cortex caudal to the lamina terminalis. In ADVR, the fibers are distributed sparsely in zones 2-4 of dorsal, medial and ventral areas. Restricted projections to ADVR originate in nucleus rotundus, nucleus reuniens and nucleus caudalis. They ascend ipsilaterally in the lateral thalamotelencephalic pathway (lateral forebrain bundle), and enter ADVR rostral to the anterior commissure. Nucleus rotundus projects to zone 4 of dorsal area, nucleus caudalis projects to zones 2-4 of dorsal division of medial area, and nucleus reuniens projects to zones 2-4 of both the ventral division of medial area and the ventral area. Comparison of these results with thalamotelencephalic projections in mammals suggests that diffuse and restricted thalamic projection systems are a common feature of both groups. Restricted thalamic projections in reptiles, birds and mammals terminating in anatomically distinct regions, also appear to be associated with different sensory modalities. The significance of diffuse systems is not clear.

Evoked potential studies (Lende, '64) suggest that echidnas have a single, topographically organized somatosensory area (SMI) that spans a mediolaterally oriented sulcus called sulcus alpha. A motor area (MI) is situated on the prealpha gyrus. This study examines the cytoarchitecture and thalamic afferents of SMI in the echidna, Tachyglossus aculeatus. SMI contains two cytoarchitectonic fields. A caudal field extends across the postalpha gyrus and onto the floor of sulcus alpha. It has a well-developed layer 4 and a relatively small number of medium-sized pyramidal cells in layer 5. The rostral field extends from the floor of sulcus alpha onto its rostral bank. It also has a well-developed layer 4 but has a large number of large pyramidal cells in layer 5. Layer 4 thins as it is followed onto the crown of the prealpha gyrus. The remainder of this gyrus contains a single cytoarchitectonic field with a thin layer 4 and a layer 5 heavily populated with larger pyramidal cells. This field corresponds to the physiologically defined motor area MI. Thalamic afferents to SMI were examined by placing pressure injections of horseradish peroxidase into the two cytoarchitectonic fields. An injection that involved both fields retrogradely labeled neurons throughout the ventral posterior nucleus of the thalamus. An injection restricted to the caudal field labeled a band of neurons that extends rostrocaudally throughout the ventral part of the ventral posterior nucleus. An injection restricted to the rostral field labeled a band of neurons situated dorsally in the ventral posterior nucleus. No other thalamic groups contained labeled neurons comparable to the labeling seen in the intralaminar or posterior nuclei following a horseradish peroxidase injection into SI of marsupial or placental mammals. These results indicate that SMI in Tachyglossus contains two cytoarchitectonic fields that resemble areas 3a and 3b in some placental mammals, suggesting that the constellation of cytoarchitectonic fields corresponding to areas 4, 3a, and 3b is a basic mammalian character which has been modified in marsupial and many placental mammals.

The projection from the dorsal lateral geniculate complex to the visual cortex in Pseudemys and Chrysemys turtles was examined by using the anterograde transport of horseradish peroxidase (HRP) in vitro and the retrograde transport of HRP in vivo. In vitro HRP injections into the lateral forebrain bundle were used to fill geniculocortical axons anterogradely, which were then analyzed in cortical wholemount preparations. Geniculocortical axons gain access to the visual cortex along its entire rostral-caudal extent. They course in slightly curved trajectories for up to 2 mm from the lateral edge of the cortex through both the lateral (or pallial thickening) and medial parts of Desan's cortical area D2. Single axons are of fine caliber. They tend to cross each other and sometimes branch in the pallial thickening, but are generally unbranched in the medial part of D2. They bear small, fusiform varicosities at irregular intervals along their lengths. Although axons show small variations in the number of varicosities per 100 microns segment, no consistent variation in varicosity number as a function of distance could be detected. These results indicate that geniculocortical axons project to the visual cortex in an orderly pattern. The retrograde transport experiments provide some clue as to the significance of this pattern. Small, ionotophoretic injections of HRP in the visual cortex retrogradely labeled neurons in the dorsal lateral geniculate complex. Injections in the rostral visual cortex retrogradely labeled neurons in the caudal pole of the geniculate complex. Injections at progressively more caudal loci within the visual cortex labeled neurons at progressively more rostral loci within the geniculate complex. Thus, there is a representation of the rostral-caudal axis of the geniculate complex along the caudal-rostral axis of the visual cortex. Consistent with the anterograde transport experiments that showed individual geniculocortical axons coursing through both lateral and medial parts of the visual cortex, HRP injections restricted to the medial edge of the visual cortex retrogradely labeled neurons along the entire dorsal-ventral axis of the geniculate complex at the appropriate rostral-caudal position. The neurophysiological studies of Mazurskaya ('72: J. Evol. Biochem. Physiol. 8:550-555; respond to a small, moving stimulus anywhere in visual space, implying a convergence of inputs from all points in visual space somewhere along the retinogeniculocortical pathway. The experiments reported here suggest a convergence in the geniculocortical projections of information along the vertical meridians, or azimuth lines, of visual space onto neurons lying along lateral to medial transects through the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)