Michael Conley

Organization of the Visual Sector of the Thalamic Reticular Nucleus in Galago

Projections to and from the visual sector of the thalamic reticular nucleus were studied in the prosimian primate genus Galago by anterograde and retrograde transport of WGA‐HRP injected into the dorsal lateral geniculate nucleus (GLd), pulvinar nucleus, and their cortical targets. Contrary to the idea that thalamic connections with the reticular nucleus are not delimited sharply between nuclei associated with the same modality, our results show a distinct laminar segregation of the projections from the GLd and pulvinar nuclei. The GLd is connected reciprocally with the lateral 2/3 of the caudal part of the reticular nucleus, and the striate cortex sends projections to the same lateral tier. Both sets of projections are organized topographically, lines of projection taking the form of slender elongated strips that run from caudo‐dorsal to rostro‐ventral within the nucleus. The pulvinar nucleus, which projects to several areas of the temporal, parietal, and occipital lobes, including the striate cortex, is connected reciprocally with the medial 1/3 of the caudal part of the reticular nucleus. Every injection into the pulvinar nucleus labelled a wide area of the medial tier, with no indication of visuotopic organization. The projections from the middle temporal area, one of the principal targets of the pulvinar nucleus, also terminate only in the medial tier of the visual sector. And we would expect that, in general, a thalamic nucleus and its cortical target would project to the same part of the reticular nucleus. The case of the striate area is an exception but only in the sense that it projects to the pulvinar nucleus as well as GLd. Thus an injection into a single locus in area 17 produces two parallel strips in the visual sector of the reticular nucleus, but both are in the lateral tier. We propose that each strip arises from a separate population of cells with cortical layer VI, one with an allegiance to the GLd and the other to the pulvinar nucleus.

The Organization of Projections from Subdivisions of the Auditory Cortex and Thalamus to the Auditory Sector of the Thalamic Reticular Nucleus in Galago

Anterograde and retrograde transport techniques were used to study the connexions between different subdivisions of the auditory cortex and thalamus with the thalamic reticular nucleus in the prosimian, Galago. In particular, the goal was to determine whether the primary auditory nucleus, GMv, and its cortical target, area I of the auditory cortex (A I), project to a different region of the auditory sector of the reticular nucleus from the secondary auditory nuclei, GMmc and Po and their cortical targets outside A I. The results show that the projections to and from the auditory sector are indeed segregated: injections of wheatgerm agglutinin‐conjugated horseradish peroxidase into either GMmc or Po labelled cells and terminals along the medial, lateral and ventral borders of the auditory sector, forming a U‐shaped pattern. Projections from area II of the auditory cortex produced almost an identical pattern of the terminal labelling in the auditory sector. In contrast, injections into GMv‐labelled cells and terminals in the centre region of the auditory sector, in the ‘interior’ of the U‐shaped region. Projections from A I were distributed to both the U‐shaped border region and the central core of the auditory sector probably because A I received projections from GMmc, Po and GMv. The significance of these results depends on a comparison between the auditory and visual sectors of the reticular nucleus. Both sectors are divided into tiers or subsectors–one related to the primary relay nucleus, i.e. GLd or GMv, and the other related to the secondary relay nuclei, i.e. pulvinar nucleus, GMmc, Po, etc.

Different distributions of large and small retinal ganglion cells in the cat after HRP injections of single layers of the lateral geniculate body and the superior colliculus

Retinal ganglion cells were labeled with HRP after injecting single layers of GL or single strata within the stratum griseum superficiale (SGS). Only small cells were labeled after injecting small cell C layers and upper SGS. Only large cells were labeled after injecting lower SGS. Small and large cells were labeled after injecting medial interlaminar nucleus (MIN) and layers A and A1.

Laminar asymmetry in the distribution of choline acetyltransferase-immunoreactive neurons in the retina of the tree shrew (Tupaia belangeri)

Cholinergic neurons in the retina of the tree shrew were identified immunocytochemically using a monoclonal antibody directed against choline acetyltransferase (ChAT). The chief result is that roughly 4 times as many ChAT-immunoreactive neurons are found in the inner nuclear layer (INL) as in the ganglion cell layer (GCL). In the INL, two classes of cholinergic neuron can be distinguished on the basis of soma size, one large and one small. The large neurons correspond closely in size and number to the displaced cholinergic neurons in the GCL, suggesting that these are the matching populations of cholinergic amacrine cells reported in other species. The small ChAT-immunoreactive neurons, on the other hand, which make up 60% of the total number of ChAT-positive neurons in the retina, appear to have no counterpart in the GCL. Whether these small neurons are a separate class of amacrine cell or some other cell type (e.g. bipolar, interplexiform, etc.) remains to be determined.

Morphology of retinogeniculate axons in the macaque.

The size, pattern of terminal arborizations, and laminar specificity of individual retinogeniculate axons were studied in the macaque following injections of HRP into the optic tract. Axons that terminated in the magnocellular layers had significantly larger fiber diameters and wider terminal fields than those that terminated in the parvocellular layers. Terminal fields of magnocellular fibers spanned most of the width of their target layer, whereas those of parvocellular fibers were restricted to approximately one-half the width of their target layers; almost all terminal fields were oriented along lines of projection. All of the optic tract fibers that we examined terminated in only one layer of the lateral geniculate nucleus (GL), including a population of fine caliber fibers that project to the intercalated layers, and none had collateral projections outside the GL. The results suggest that each layer--magnocellular, parvocellular, and intercalated--receives projections from a morphologically distinct population of optic tract fibers.

Thalamic and basal forebrain cholinergic connections of the rat posterior parietal cortex

The thalamic connectivity and basal forebrain cholinergic input to the posterior parietal cortex (PPC) of Long-Evans rats was examined using combined retrograde tracing and immunocytochemical methods. As in previous studies, the PPC could be distinguished by its input from the lateral posterior, lateral dorsal, and posterior nuclei of the thalamus, but not the lateral geniculate nucleus or ventrobasal complex. These nuclei were also observed to receive reciprocal projections from the ipsilateral PPC. Cholinergic neurons innervating the PPC were primarily localized to the substantia innominata/nucleus basalis region. The implications of these data for possible functions of the cholinergic input to PPC are discussed.

Differential distribution of somatostatin-like immunoreactivity in the visual sector of the thalamic reticular nucleus in Galago

Immunocytochemical methods were used to compare the distributions of somatostatin‐14 (SOM) and glutamic acid decarboxylase (GAD) in the medial and lateral tiers of the visual sector of the thalamic reticular nucleus in the bushbaby, Galago. As expected, all of the neurons in the visual sector were immunoreactive for GAD, the synthesizing enzyme for GABA, but the distribution of SOM‐immunoreactive cells was not uniform. It appeared that every cell in the medial tier was immunoreactive for SOM, but that very few cells in the lateral tier contained this neuropeptide. The significance of the difference in reticular neuron SOM content could be related to the functional differences between the dorsal lateral geniculate nucleus, which is connected reciprocally with the lateral tier, and the pulvinar nucleus, which is connected reciprocally with the medial tier.