Projections of the lateral terminal accessory optic nucleus of the common marmoset (Callithrix jacchus)

A central mesencephalic reticular formation projection to the supraoculomotor area in macaque monkeys

The central mesencephalic reticular formation is physiologically implicated in oculomotor function and anatomically interwoven with many parts of the oculomotor system's premotor circuitry. This study in Macaca fascicularis monkeys investigates the pattern of central mesencephalic reticular formation projections to the area in and around the extraocular motor nuclei, with special emphasis on the supraoculomotor area. It also examines the location of the cells responsible for this projection. Injections of biotinylated dextran amine were stereotaxically placed within the central mesencephalic reticular formation to anterogradely label axons and terminals. These revealed bilateral terminal fields in the supraoculomotor area. In addition, dense terminations were found in both the preganglionic Edinger-Westphal nuclei. The dense terminations just dorsal to the oculomotor nucleus overlap with the location of the C-group medial rectus motoneurons projecting to multiply innervated muscle fibers suggesting they may be targeted. Minor terminal fields were observed bilaterally within the borders of the oculomotor and abducens nuclei. Injections including the supraoculomotor area and oculomotor nucleus retrogradely labeled a tight band of neurons crossing the central third of the central mesencephalic reticular formation at all rostrocaudal levels, indicating a subregion of the nucleus provides this projection. Thus, these experiments reveal that a subregion of the central mesencephalic reticular formation may directly project to motoneurons in the oculomotor and abducens nuclei, as well as to preganglionic neurons controlling the tone of intraocular muscles. This pattern of projections suggests an as yet undetermined role in regulating the near triad.

The extraocular motor nuclei: organization and functional neuroanatomy

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.

Present concepts of oculomotor organization

This chapter gives an introduction to the oculomotor system, thus providing a framework for the subsequent chapters. This chapter describes the characteristics, and outlines the structures involved, of the five basic types of eye movements, for gaze holding ("neural integrator") and eye movements in three dimensions (Listing's law, pulleys).

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): III. Projections of the translation zones of the ventral uvula and nodulus

Previous electrophysiological studies in pigeons have shown that the complex spike activity of Purkinje cells in the medial vestibulocerebellum (nodulus and ventral uvula) is modulated by patterns of optic flow that result from self-translation along a particular axis in three-dimensional space. There are four response types based on the axis of preferred translational optic flow. By using a three axis system, where +X, +Y, and +Z represent rightward, upward, and forward self-motion, respectively, the four cell types are t(+Y), t(-Y), t(-X-Z), and t(-X+Z), with the assumption of recording from the left side of the head. These response types are organized into parasagittal zones. In this study, we injected the anterograde tracer biotinylated dextran amine into physiologically identified zones. The t(-X-Z) zone projected dorsally within the vestibulocerebellar process (pcv) on the border with the medial cerebellar nucleus (CbM), and labeling was found in the CbM itself. The t(-X+Z) zone also projected to the pcv and CbM, but to areas ventral to the projection sites of the t(-X-Z) zone. The t(-Y) zone also projected to the pcv, but more ventrally on the border with the superior vestibular nucleus (VeS). Some labeling was also found in the dorsal VeS and the dorsolateral margin of the caudal descending vestibular nucleus, and a small amount of labeling was found laterally in the caudal margin of the medial vestibular nucleus. The data set was insufficient to draw conclusions about the projection of the t(+Y) zone. These results are contrasted with the projections of the flocculus, compared with the primary vestibular projection, and implications for collimotor function are discussed.

The accessory optic system contributes to the spatio-temporal tuning of motion-sensitive pretectal neurons

The nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow that results from self-motion and are important for oculomotor control. These neurons have large receptive fields and exhibit direction selectivity to large moving stimuli. In response to drifting sine wave gratings, LM and nBOR neurons are tuned to either low spatial/high temporal frequencies (SF, TF) or high SF/low TF stimuli. Given that velocity = TF/SF, these are referred to as "fast" and "slow" neurons, respectively. There is a heavy projection from the AOS to the pretectum, although its function is unknown. We recorded the directional and spatio-temporal tuning of LM units in pigeons before and after nBOR was inactivated by tetrodotoxin injection. After nBOR inactivation, changes in direction preference were observed for only one of 18 LM units. In contrast, the spatio-temporal tuning of LM units was dramatically altered by nBOR inactivation. Two major effects were observed. First, in response to motion in the preferred direction, most (82%) neurons showed a substantially reduced (mu = -67%) excitation to low SF/high TF gratings. Second, in response to motion in the anti-preferred direction, most (63%) neurons showed a dramatically reduced (mu = -78%) inhibition to high SF/low TF gratings. Thus the projection from the nBOR contributes to the spatio-temporal tuning rather than the directional tuning of LM neurons. We propose a descriptive model whereby LM receives inhibitory and excitatory input from "slow" and "fast" nBOR neurons, respectively.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): II. Projections of the rotation zones of the flocculus

Previous neurophysiologic research in birds and mammals has shown that there are two types of Purkinje cells in the flocculus. The first type shows maximal modulation in response to rotational optokinetic stimulation about the vertical axis (rVA neurons). The second type shows maximal modulation in response to rotational optokinetic stimulation about a horizontal axis oriented 45 degrees to contralateral azimuth (rH45c neurons). In pigeons, the rVA and rH45c are organized into four alternating parasagittal zones. In this study we investigated the projections of Purkinje cells in the rVA and rH45c zones by using the anterograde tracers biotinylated dextran amine and cholera toxin subunit B. After iontophoretic injections of tracers into the rH45c zones, heavy anterograde labeling was found in the infracerebellar nucleus and the medial margin of the superior vestibular nucleus. Some labeling was also consistently observed in the lateral cerebellar nucleus and the dorsolateral vestibular nucleus. After injections into the rVA zones, heavy anterograde labeling was found in the medial and descending vestibular nuclei, the nucleus prepositus hypoglossi, and the central region of the superior vestibular nucleus. Less labeling was seen in the tangential nucleus, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. These results are compared and contrasted with findings in mammalian species.

Intravitreal injection of the attenuated pseudorabies virus PRV Bartha results in infection of the hamster suprachiasmatic nucleus only by retrograde transsynaptic transport via autonomic circuits

Intravitreal injection of the attenuated strain of pseudorabies virus (PRV Bartha) results in transneuronal spread of virus to a restricted set of central nuclei in the rat and mouse. We examined the pattern of central infection in the golden hamster after intravitreal inoculation with a recombinant strain of PRV Bartha constructed to express enhanced green fluorescent protein (PRV 152). Neurons in a subset of retinorecipient nuclei [i.e., suprachiasmatic nucleus (SCN), intergeniculate leaflet, olivary pretectal nucleus (OPN), and lateral terminal nucleus] and autonomic nuclei [i.e., paraventricular hypothalamic nucleus and Edinger-Westphal nucleus (EW)] are labeled by late stages of infection. Infection of the EW precedes infection in retinorecipient structures, raising the possibility that the SCN becomes infected by retrograde transsynaptic infection via autonomic (i.e., EW) circuits. We tested this hypothesis in two ways: (1) by removing the infected eye 24 hr after PRV 152 inoculation, well before viral infection first appears in the SCN; and (2) by examining central infection after intravitreal PRV 152 injection in animals with ablation of the EW. The pattern and time course of central infection were unchanged after enucleation, whereas EW ablation before intravitreal inoculation eliminated viral infection in the SCN. The results of EW lesions along with known connections between EW, OPN, and SCN indicate that intravitreal injection of PRV Bartha produces a retrograde infection of the autonomic innervation of the eye, which subsequently labels a restricted set of retinorecipient nuclei via retrograde trans-synaptic infection. These results, taken together with other genetic data, indicate that the mutations in PRV Bartha render the virus incapable of anterograde transport. PRV Bartha is thus a retrograde transsynaptic marker in the CNS.

Projections from the nucleus of the basal optic root and nucleus lentiformis mesencephali to the inferior olive in pigeons (Columba livia)

The nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow and the generation of the optokinetic response. Previous studies have shown that the nBOR projects bilaterally to the medial column (mc) of the inferior olive (IO) and the LM projects to the ipsilateral mc. In the present study the retrograde tracer cholera toxin subunit B was injected into either the caudal or rostral mc. From all injections, retrogradely labeled cells were seen in the ipsilateral pretectum along the border of the medial and lateral subnuclei of the LM. Cells were also seen in bilaterally in the nBOR. On the contralateral side, a discrete group of cells was labeled in the rostral margin of the nBOR. These cells were localized in the dorsal portion of the nBOR proper and some were found in the adjacent nBOR dorsalis. On the ipsilateral side, a diffuse group of cells was seen in the caudal nBOR. Most of these cells were in the nBOR dorsalis and outside the nBOR complex in the area ventralis of Tsai and the reticular formation. From the injections into the caudal mc, a greater proportion of labeled cells was found in the LM, whereas a greater proportion of cells was found in the nBOR from the injections into the rostral mc. This differential projection from LM and nBOR to the caudal and rostral mc is consistent with the optic flow preferences of neurons in the mc, and a similar pattern of connectivity has been found in mammalian species.

The retinal projections to the ventral and dorsal divisions of the medial terminal nucleus and mesencephalic reticular formation in the Japanese monkey (Macaca fuscata): a reinvestigation with cholera toxin B subunit as an anterograde tracer

After a monocular injection of the cholera toxin B subunit (CTB) into the vitreous chamber of the eye, retinal projections to the medial terminal nucleus (MTN) of the accessory optic system (AOS) were studied in the Japanese monkey. The anterogradely transported tracer was visualized with the peroxidase antibody technique by using an anti-cholera toxin antibody. One small accumulation of the CTB-immunopositive retinofugal terminals was located in a small area just medial to the medial edge of the cerebral peduncle and anterior to the attachment of the oculomotor nerve, suggesting the existence of a ventral division of the MTN of the AOS. Caudally, one very small bundle of the retinofugal fibers extending dorsally from this accumulation was seen running along the medial edge of the cerebral peduncle and substantia nigra to the small region corresponding to the dorsal division of the MTN. A few small bundles of CTB-immunopositive retinal fibers were observed to leave the superior fasciculus of the AOS at various points. These fibers coursed medially through the cerebral peduncle and substantia nigra to reach some restricted areas of the mesencephalic reticular formation between the medial lemniscus and the substantia nigra.

Projections of the nucleus of the basal optic root in pigeons (Columba livia) revealed with biotinylated dextran amine

The nucleus of the basal optic root (nBOR) of the accessory optic system is known to be involved in the analysis of the visual consequences of self-motion. Previous studies have shown that the nBOR in pigeons projects bilaterally to the vestibulocerebellum, the inferior olive, the interstitial nucleus of Cajal, and the oculomotor complex and projects unilaterally to the ipsilateral pretectal nucleus lentiformis mesencephali and the contralateral nBOR. By using the anterograde tracer biotinylated dextran amine, we confirmed these projections and found (previously unreported) projections to the nucleus Darkshewitsch, the nucleus ruber, the mesencephalic reticular formation, and the area ventralis of Tsai as well as ipsilateral projections to the central gray, the pontine nuclei, the cerebellar nuclei, the vestibular nuclei, the processus cerebellovestibularis, and the dorsolateral thalamus. In addition to previous studies, which showed a projection to the dorsomedial subdivision of the contralateral oculomotor complex, we found terminal labelling in the ventral and dorsolateral subdivisions. Individual fibers were reconstructed from serial sections, and collaterals to various nuclei were demonstrated. For example, collaterals of fibers projecting to the vestibulocerebellum terminated in the vestibular or cerebellar nuclei; collaterals of fibers to the inferior olive terminated in the pontine nuclei; many individual neurons projected to the interstitial nucleus of Cajal, the nucleus Darkshewitsch, and the central gray and also projected to the nucleus ruber and the mesencephalic reticular formation; collaterals of fibers to the contralateral nucleus of the basal optic root terminated in the mesencephalic reticular formation and/or the area ventralis of Tsai; neurons projecting to the nucleus lentiformis mesencephali also terminated in the dorsolateral thalamus. The consequences of these data for understanding the visual control of eye movements, neck movements, posture, locomotion, and visual perception are discussed.

Efferent pathways of the nucleus of the optic tract in monkey and their role in eye movements

To clarify the role of the pretectal nucleus of the optic tract (NOT) in ocular following, we traced NOT efferents with tritiated leucine in the monkey and identified the cell groups they targeted. Strong local projections from the NOT were demonstrated to the superior colliculus and the dorsal terminal nucleus bilaterally and to the contralateral NOT. The contralateral oculomotor complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, also received inputs. NOT efferents terminated in all accessory optic nuclei (AON) ipsilaterally; contralateral AON projections arose from the pretectal olivary nucleus embedded in the NOT. Descending pathways contacted precerebellar nuclei: the dorsolateral and dorsomedial pontine nuclei, the nucleus reticularis tegmenti pontis, and the inferior olive. Direct projections from NOT to the ipsilateral nucleus prepositus hypoglossi (ppH) appeared to be weak, but retrograde tracer injections into rostral ppH verified this projection; furthermore, the injections demonstrated that AON efferents also enter this area. Efferents from the NOT also targeted ascending reticular networks from the pedunculopontine tegmental nucleus and the locus coeruleus. Rostrally, NOT projections included the magnocellular layers of the lateral geniculate nucleus (lgn); the pregeniculate, peripeduncular, and thalamic reticular nuclei; and the pulvinar, the zona incerta, the mesencephalic reticular formation, the intralaminar thalamic nuclei, and the hypothalamus. The NOT could generate optokinetic nystagmus through projections to the AON, the ppH, and the precerebellar nuclei. However, NOT also projects to structures controlling saccades, ocular pursuit, the near response, lgn motion sensitivity, visual attention, vigilance, and gain modification of the vestibulo-ocular reflex. Any hypothesis on the function of NOT must take into account its connectivity to all of these visuomotor structures.