John K. Harting

The efferent projections of the pretectal complex: an autoradiographic and horseradish peroxidase analysis

Anterograde autoradiographic data reveal that neurons within the pretectal complex of the tree shrew possess axons which terminate within three general categories of targets. First, there are targets of a major ipsilateral descending pathway which include: the dorsal cap of Kooy of the inferior olivary complex, the dorsolateral and dorsomedial regions of the griseum pontis, the mesencephalic reticular formation which lies immediately dorsal and lateral to the red nucleus, the medial terminal nucleus and the superficial layers of the superior colliculus. A second category of targets receive their pretectal input from a large ascending bundle which projects ipsilaterally to: the reticular and lateral nuclei of the thalamus, the zona incerta, the central lateral and paracentral intralaminar nuclei of the thalamus and bilaterally to the ventral lateral geniculate nucleus. A third category of targets include cranial nerve and closely associated nuclei which play a role in eye movements. Pretectofugal fibers projecting to nuclei in this third category terminate ipsilaterally within the nucleus of Darkschewitsch, bilaterally within the nucleus of the posterior commissure and the interstitial nucleus of Cajal, and contralaterally within the somatic cell column of the oculomotor and trochlear nuclei. There are also commissural projections to contralateral pretectal cell groups. Injections of horseradish peroxidase were placed within 11 pretectal targets. These data, which confirm and extend our autoradiographic findings, show that the majority of pretectal targets receive input from several pretectal nuclei, and that the size of pretectal neurons, rather than the cell groups in which they are located, dictates the termination site(s) of their axons.

Tectal Control of Spinal Cord Activity:Neuroanatomical Demonstration of Pathways Connecting the Superior Colliculus with theCervical Spinal Cord Grey

Publisher Summary This chapter discusses the neuroanatomical pathways connecting the superior colliculus with the cervical spinal cord grey. The superior colliculus plays an important role in head and eye movements. The electrophysiological methods have been in use to reveal significant data regarding the role of the superior colliculus in the production of eye and head movement related activity. Upon reaching the spinal cord, tectospinal axons pass into the ventral luniculus, from which they exit to terminate within the gray matter of the cervical spinal cord. In addition to these numerous anterograde studies, recent investigations utilizing the retrograde transport of horseradish peroxidase (HRP) have revealed that the cells of origin of the tectospinal tract lie only within the intermediate and deep layers of the contralateral superior colliculus. These investigators found that stimulation of the intermediate and deep tectal layers results in excitatory postsynaptic potentials (EPSPs) within contralateral extensor motoneurons at upper cervical levels.

Structure and connections of the thalamic reticular nucleus: Advancing views over half a century.

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowans contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections. J. Comp. Neurol. 463:360–371, 2003.

The trigeminocollicular projection in the cat: Patch-like endings within the intermediate gray

We have used two neuroanatomical tracing techniques to study the trigeminocollicular projection in the cat. In one series of experiments we injected [3H]proline into the alaminar division of the spinal trigeminal nucleus and analyzed the distribution and pattern of anterogradely transported label within the superior colliculus. These autoradiographic data reveal that the trigeminocollicular projection: (1) is primarily contralateral; (2) reaches only the rostral 60-70% of the colliculus; and (3) terminates in a discontinuous, patch-like tier within the middle of the dorsal-ventral axis of the stratum griseum intermediale (SGI). The patches of label measure approximately 330 micrometer in the medial-lateral dimension and 250 micrometer in the dorsal-ventral extent. There seems to be an alignment of the patches in the rostral-caudal direction, suggesting that the trigeminal input forms longitudinal columns in the colliculus. Anterogradely transported protein is also present within the stratum griseum profundum (SGP). In contrast to the patch-like pattern present in the SGI, label in the SGP is more diffusely distributed. In a second series of experiments we injected horseradish peroxidase-wheat germ agglutinin (HRP-WGA) into the spinal trigeminal nucleus. While the distribution and pattern of the contralateral trigeminocollicular axons is similar to that in the autoradiographic experiments, patches of anterogradely transported HRP-WGA are also present within the ipsilateral SGI and SGP. Furthermore, the HRP-WGA data reveal groups of retrogradely labeled collicular neurons which lie in intimate association with the patches of anterogradely transported tracer. Our findings are discussed in relation to other collicular afferents which terminate in a patch-like manner within the SGI. We hypothesize that the colliculus contains many vertically oriented modules. Each module consists of tectopetal axon terminals -- arranged in sandwich fashion -- and functionally related collicular (tectofugal) neurons.

An autoradiographic analysis of the tecto-olivary projection in primates

Autoradiographic tracing methods were used to demonstrate a well-defined projection from the superior colliculus to the inferior olivary complex in the monkey. This projection originates within the deep layers of the superior colliculus, descends within the contralateral tecto-spinal tract, and terminates within the caudal 1/3 of the medial accessory nucleus. The terminal field is restricted to a densely packed, darkly stained group of cells located in the most dorsal segment of subnucleus b. In one animal, another group of olivary afferents was identified. These fibers also descend within the contralateral tecto-spinal tract, and terminate within the dorsal cap of Kooy. While it was not possible to determine the origin of this projection, our data suggest that it arises within a region adjacent to the rostral pole of the superior colliculus. The present study further indicates that in the monkey relatively few axons which course within the classical tecto-spinal tract pass caudal to the medulla.

The precise origin of the tectospinal pathway in three common laboratory animals: A study using the horse-radish peroxidase method

The horseradish peroxidase tracing method has been used to study the cells of origin of the tectospinal projections in the opossum, the tree shrew, and the cat. The present data show that only those collicular neurons which occupy the deep (ventral to the stratum opticum) tectal laminae send axons to the cervical spinal cord. In particular, layer IV contains the greatest number of spinal projecting neurons. Our results also reveal that while only the large sized collicular neurons project upon the cervical spinal cord in the opossum and the tree shrew, neurons comprising several different size categories do so in the cat. We thus suggest that several different descending channels exist over which the superior colliculus can influence the neck musculature in the cat.

Corticocortical communication via the thalamus: Ultrastructural studies of corticothalamic projections from area 17 to the lateral posterior nucleus of the cat and inferior pulvinar nucleus of the owl monkey

Electron microscopic anterograde autoradiography has been used to analyze the morphology and postsynaptic relationships of area 17 cortical terminals in the lateral division of the lateral posterior nucleus (LPl) of the cat and medial division of the inferior pulvinar nucleus (IPm) of the owl monkey. Such terminals are thought to arise exclusively from layer 5 in the cat and primate (Lund et al. [1975] J. Comp. Neurol. 164:287–304; Abramson and Chalupa [1985] Neuroscience 15:81–95). All labeled terminals in both nuclei exhibited the morphology of ascending “lemniscal” afferents. That is, they contained round vesicles, were large, made asymmetrical synaptic and filamentous nonsynaptic contacts, and were classified as RLs. These cortical RLs also exhibited the postsynaptic relationships of lemniscal afferents. Thus, they were presynaptic to large dendrites within glial encapsulated glomeruli, where a majority was involved in complex synaptic arrangements called triads. They also were found adjacent to terminal profiles with pleomorphic vesicles but never adjacent to small terminals containing round vesicles.

Sublamination within the superficial gray layer of the squirrel monkey: an analysis of the tectopulvinar projection using anterograde and retrograde transport methods

Anterograde and retrograde tracing methods have been used to analyze the cells of origin and the axonal distribution of the tectopulvinar projection in the squirrel monkey. Our most interesting finding is that tectopulvinar neurons occupy a cytoarchitecturally distinct sublamina of the stratum griseum superficiale (SGS) called the lower SGS (SGSL). The distinction between the SGSL and the upper SGS (SGSU) is further indicated by the findings of others that the SGSL receives different amounts of retinal and cortical input compared to the SGSU. Previous physiological studies have also shown that cells in the SGSL possess different response characteristics than those in the SGSU. Differences in cytoarchitecture, afferent and efferent connections, and physiological properties of the SGSL versus the SGSU indicate that sublaminae are the anatomical mechanism which enables different information channels to maintain some degree of autonomy within the SGS, and at the same time use the same topographic map within this layer.

Projections of the superior colliculus to the supraspinal nucleus and the cervical spinal cord gray of the cat

Anterograde transport experiments reveal two novel findings regarding the distribution of descending tectofugal axons. First, such axons project to the supraspinal nucleus of the caudal medulla; this nucleus is known to project to the upper cervical spinal cord gray. Second, some tectospinal axons ramify within Rexeds lamina IX of the first 5 cervical spinal cord segments. This zone contains motoneurons which innervate neck musculature. Retrograde data reveal that tectospinal neurons occur in clusters within the intermediate and deep gray layers. A close relationship between the clusters of tectospinal neurons and a modular type of connectional organization of the intermediate and deep gray layers is suggested.

Representation of the complete retina in the contralateral superior colliculus of some mammals

Abstract ‘Our data on the rat include one case, number 2, with a lesion restricted to the extreme temporal retina, entirely on the temporal side of the fixation point. In this case the great majority of the degenerated fibers are uncrossed. None of the crossed fibers can be seen to terminate in the lateral geniculate nucleus and certainly most of them pass over the nucleus and terminate in the lateral margin of the superior colliculus. The uncrossed fibers, on the other hand, all enter the lateral geniculate nucleus and terminate within its caudal third.’ K. S. Lashley, 1934 (ref. 7) .