The tectopontine projection the the rat with comments on visual pathways to the basilar pons

Electrophysiological analysis of the trigemino-tecto-olivo-cerebellar (crus II) projection in the rat

In albino rats the whisker area was electrically stimulated while climbing fiber responses were surveyed in the cerebellar cortex of the tecto-olivo-recipient zone in the medial region of crus II. Climbing fiber responses were evoked by the vibrissal stimulation only in the medial portion of the zone. Based both on lesion experiments of the superior colliculus and on recording multiunit potentials in the inferior olive it is suggested that the trigeminal climbing fiber responses are evoked via the superior colliculus.

Contralateral head movements produced by microinjection of glutamate into superior colliculus of rats: evidence for mediation by multiple output pathways

One of the major efferent pathways of the superior colliculus crosses midline to run caudally in the contralateral predorsal bundle, innervating targets in the brain stem and eventually reaching the cervical spinal cord. A variety of evidence suggests that this tecto-reticulo-spinal pathway may mediate the orienting movements that can be evoked by tectal stimulation. However, we have recently found that orienting head movements can still be obtained in rats after section of the tecto-reticulo-spinal pathway, implying that additional pathways are also involved. The present study sought to test this implication, by taking advantage of the fact that in rats the cells of origin of the tecto-reticulo-spinal pathway are largely segregated within the lateral part of the stratum album intermediate. It is thus possible to find out whether orienting head movements can be produced by a cell-excitant from tectal regions that contain few cells of origin of the tecto-reticulo-spinal pathway. Hooded rats in an open field were filmed during microinjections of sodium L-glutamate (50 mM, 200 nl) into the superior colliculus, and the films analysed for the appearance of contralaterally directed movements of the head and body. Subsequent histological reconstruction of the injection sites indicated that such movements could be obtained from widespread areas within the superior colliculus, including not only lateral stratum album intermediale but also the deep layers, and parts of the medial superficial and intermediate layers. Moreover, sites in or close to lateral stratum album intermediate often gave circling movements with downward pointing head, whereas some sites outside lateral stratum album intermediale gave sustained immobility with the head pointing contralaterally and upwards. This evidence supports the view that tectal efferent pathways besides the tecto-reticulo-spinal pathway are involved in the control of head movement. In addition, at least some of these pathways are not collaterals of the tecto-reticulo-spinal pathway, since the movements were obtained from collicular regions with few tecto-reticulo-spinal pathway cells. Finally, the results are consistent with the view that different collicular output pathways mediate movements that have different functions.

Electrophysiological analysis of the tecto-olivo-cerebellar (lobulus simplex) projection in the rat

In albino rats the deep layers of the superior colliculus were stimulated, and climbing fiber responses of Purkinje cells were explored in the medial region of the lobulus simplex. They were identified in a strip (ca. 0.7-1.0 mm wide) in the most medial region of folium b. In the tectorecipient zone of the medial accessory olive antidromically evoked potentials from the lobulus simplex were recorded laterally to those from lobule VII. Evidence is presented that climbing fibers innervating the zone in the lobulus simplex are axon collaterals of the inferior olivary neurons which project to crus II.

Further evidence for segregated output channels from superior colliculus in rat: ipsilateral tecto-pontine and tecto-cuneiform projections have different cells of origin

Two of the targets of the ipsilateral descending pathway from the superior colliculus are the cuneiform area (immediately ventral to the inferior colliculus), and the dorsolateral basilar pons. The cells of origin of the projections to these targets in rat were studied with a retrograde double-labelling technique, using the fluorescent tracers True blue and Diamidino yellow. Although many tectal cells were single-labelled by injections into basilar pons or the cuneiform area, less than 5% were double-labelled. The two projections thus appear to arise mainly from separate populations of cells within the superior colliculus.

Afferent connections of the nuclei reticularis pontis oralis and caudalis: a horseradish peroxidase study in the rat

The afferent connections of the nuclei reticularis pontis oralis and caudalis were studied experimentally in the rat by the aid of either free horseradish peroxidase or horseradish peroxidase conjugated with wheat germ agglutinin used as retrograde tracers. The results suggest that the nucleus reticularis pontis oralis receives its main input from the zona incerta and field H1 of Forel, the superior colliculus, the central gray substance, and the mesencephalic and magnocellular pontomedullary districts of the reticular formation. Many other structures seem to represent modest additional sources of projections to the nucleus reticularis pontis oralis; these structures include numerous cortical territories, the nucleus basalis, the central amygdaloid nucleus, hypothalamic districts, the anterior pretectal nucleus, the substantia nigra, the cuneiform, the accessory oculomotor and the deep cerebellar nuclei, trigeminal, parabrachial and vestibular sensory cell groups, the nuclei raphe dorsalis and magnus, the locus coeruleus, the dorsolateral tegmental nucleus, and the spinal cord. While the afferentation of the rostral portion of the nucleus reticularis pontis caudalis appears to conform to the general pattern outlined above, some deviations from that pattern emerge when the innervation of the caudal district of the nucleus reticularis pontis caudalis is considered; the most striking of these differences is the fact that both spinal and cerebellar inputs seem to distribute much more heavily to the referred caudal district than to the remaining magnocellular pontine reticular formation. The present results may contribute to the elucidation of the anatomical substrate of the functionally demonstrated involvement of the nuclei reticularis pontis oralis and caudalis in several domains that include the regulation of the sleep-waking cycle and cortical arousal, somatic motor mechanisms and nociceptive behavior.

Midbrain stimulation in the anesthetized rat: direct locomotor effects and modulation of locomotion produced by hypothalamic stimulation

The midbrain contains circuits that modulate locomotion. To delineate some of the involved regions, low-level stimulation (25 microA, 10 s train of 0.5 ms pulses at 50 Hz) was applied to the midbrain during locomotor stepping. Stepping was elicited in the anesthetized (pentobarbital, 40 mg/kg) rat by stimulating the hypothalamus with 0.5 ms pulses at 40 Hz at various currents. The rat was held in a stereotaxic apparatus such that locomotor stepping movements turned a wheel. Facilitation of locomotion was produced by stimulation in the anterior ventromedial midbrain and in the posterodorsal midbrain. When presented alone, such stimulation produced locomotion. Inhibition of locomotion was produced by stimulation of the superior colliculus (ventral layers) and the ventromedial midbrain. Additional inhibitory sites were found in the central gray and the lateral tegmentum. Inhibitory collicular stimulation, when presented alone, was characterized by the absence of any hindlimb response. Inhibitory ventromedial stimulation, when presented alone, frequently produced poststimulation locomotion and when presented with hypothalamic stimulation was characterized by postinhibitory increases in locomotion. These results indicate that: (1) the locomotor effects of stimulation in midbrain and hypothalamic sites can summate: (2) multiple locomotor suppressive systems are present in the midbrain and among them are a collicular system and a ventromedial system.

Two converging brainstem pathways mediating circling behavior

Ipsiversive circling results from stimulation of the rostromedial tegmentum (RMT) or medial pons (PONS), and contraversive circling results from stimulation of the superior colliculus (SC). To determine whether these sites are functionally connected, the collision method of Shizgal, Bielajew, Corbett, Skelton and Yeomans (1980) was used in rats. Pairs of stimulation pulses were presented to two sites, and the degree of collision between stimulation-evoked action potentials was assessed by measuring the frequency required for circling at short and long intrapair conditioning-testing (C-T) intervals. Collision was evidenced when the required frequencies were higher at short C-T intervals than at long C-T intervals. Collision of 46-62% was observed between RMT and PONS, and collision of 15-29% was observed between SC and PONS. Sites from which collision was obtained were located along the trajectories of the medial tegmental tract and the crossed tectospinal pathway. Refractory periods in all sites were similar, ranging from 0.3 to 1.7 ms. Conduction velocities of axons connecting RMT and PONS and SC and PONS were comparable, ranging from 0.8 to 13.3 m/s and 1.7 to 13.8 m/s, respectively, with lower conduction velocities associated with more ventral pontine sites. Thus, RMT and PONS, and SC and PONS are connected by myelinated axons that mediate circling.

Head and body movements produced by electrical stimulation of superior colliculus in rats: effects of interruption of crossed tectoreticulospinal pathway

Stimulation of the superior colliculus in rats produces movements of the head and body that resemble either orientation and approach towards a contralateral stimulus, or avoidance of, or escape from, such a stimulus. A variety of evidence indicates that the crossed descending pathway, which runs in the contralateral predorsal bundle to the pontomedullary reticular formation and the spinal cord, is involved in orienting movements. The nature of this involvement was investigated, by assessing the effects on tectally-elicited movements of midbrain knife-cuts intended to section the pathway as it crosses midline in the dorsal tegmental decussation. As expected, ipsilateral movements resembling avoidance or escape were little affected by dorsal tegmental decussation section, whereas contralateral circling movements of the body were almost abolished. However, contralateral movements of the head in response to electrical stimulation were not eliminated, nor were orienting head movements to visual or tactile stimuli. There was some suggestion that section of the dorsal tegmental decussation increased the latency of head movements from electrical stimulation at lateral sites, and decreased the accuracy of orienting movements to sensory stimuli. These results support the view that the crossed tectoreticulospinal system is concerned with approach rather than avoidance movements. However, it appears that other, as yet unidentified, tectal efferent systems are also involved in orienting head movements. It is possible that this division of labour may reflect functional differences between various kinds of apparently similar orienting responses. One suggestion is that the tectoreticulospinal system is concerned less in open-loop orienting responses (that are initiated but not subsequently guided by sensory stimuli), than in following or pursuit movements.

Topography of corticopontine remodelling after cortical lesions in newborn rats

Autoradiographic and axonal degeneration staining techniques were combined in individual animals to study the distribution of corticopontine fibers. In normal animals, forelimb and hindlimb motor cortical projections terminated somatotopically within the ipsilateral pontine nuclei. Sparse crossed projections also displayed a somatotopic pattern. After unilateral sensorimotor cortical lesions in newborn rats, an increase in the crossed corticopontine fibers arising from the opposite unablated motor cortex was observed at maturity. These fibers distributed in a topographic pattern similar to the normal ipsilateral corticopontine pattern; forelimb motor cortical projections terminated rostral to hindlimb motor cortical fibers. The specific distribution of the anomalous fibers suggests that they constitute a functional pathway.

Certain basilar pontine afferent systems are GABA-ergic: combined HRP and immunocytochemical studies in the rat

Injection of the tracer substance wheat germ agglutinin-horseradish peroxidase (WGA-HRP) directly into the basilar pontine nuclei using a ventral surgical approach resulted in the labeling of somata in many areas both rostral and caudal to the basilar pons. Certain of the sections that had been reacted for HRP were also incubated in antiserum prepared against glutamic acid decarboxylase (GAD) and processed according to routine peroxidase anti-peroxidase immunocytochemical procedures. Neuronal somata exhibiting both HRP and GAD reaction products were considered to represent GABA-ergic neurons that provide axonal projections to the basilar pontine nuclei. Such double-labeled neurons were observed within the zona incerta, anterior pretectal nucleus, lateral cerebellar nucleus, perirubral area, and the pontine and medullary reticular formation.

Electrophysiological analysis of the tecto-olivo-cerebellar (crus II) projection in the rat

In albino rats the superior colliculus was stimulated and evoked potentials were explored in the cerebellar hemisphere including the paramedian lobule, and crura I and II. Climbing fiber responses were identified in a strip (ca. 1 mm wide) in the medial region of crus II, extending to both folia a and b. In the tectorecipient zone in subnucleus c of the medial accessory olive, antidromically evoked potentials from crus II were recorded lateral to those from lobule VII. Evidence is presented that inferior olivary neurons projecting to crus II are different from those projecting to lobule VII.

Hypothalamic and ventral thalamic projections to the superior colliculus in the cat

The present report describes the organization of collicular afferents that arise within either the hypothalamus or the ventral thalamus. Following the placement of large injections of WGA-HRP into the superior colliculus of the cat, retrogradely labeled neurons are located within the reticular nucleus of the thalamus, the zona incerta, the fields of Forel, and throughout the hypothalamus. Although the dorsal hypothalamic area contains the largest number of labeled hypothalamic neurons, labeled cells are also found within the periventricular, paraventricular, dorsomedial, ventromedial, posterior, lateral, and anterior hypothalamic nuclei. A strikingly similar pattern of distribution of labeled neurons is also observed following placement of small injections of WGA-HRP that are restricted within the stratum griseum intermedium (SGI). In contrast, hypothalamic and ventral thalamic labeling is not seen after placement of injections within the stratum griseum superficiale. Following the placement of injections of tritiated anterograde tracers within the dorsal hypothalamic area, transported label is organized in two bands of clusters over the SGI. When injections of tritiated tracers are placed within the zona incerta, terminal label is also located over the SGI; however, the distribution of silver grains does not appear as clusters or distinct puffs. On the basis of the comparison of the cellular types that give rise to these projections and the differences in terminal distribution, we suggest that the hypothalamic and ventral thalamic projections to the superior colliculus are totally separate and unrelated pathways. The functional implications of the hypothalamotectal pathway are also discussed.

Anatomical studies on the nucleus reticularis tegmenti pontis in the pigmented rat. II. Subcortical afferents demonstrated by the retrograde transport of horseradish peroxidase

The subcortical nuclear groups projecting to the nucleus reticularis tegmenti pontis (NRTP) were studied in pigmented rats with the aid of the retrograde horseradish peroxidase (HRP) technique. Small iontophoretic injections of HRP were placed in the medial regions of the NRTP, an area that has been shown in several species to be involved in eye movements. Other large injections in the NRTP or small injections placed just outside the nucleus were used to clarify the projections to the NRTP. Results indicate that the NRTP receives afferents from visual relay nuclei, including the nucleus of optic tract, the superior colliculus, and the ventral lateral geniculate nucleus; oculomotor-associated structures including the zona incerta, the H1 and H2 fields of Forel, the nucleus subparafasciculus, the interstitial nucleus of Cajal, the visual tegmental relay zone of the ventral tegmental area of Tsai, the mesencephalic, pontine, and medullary reticular formations, the nucleus of the posterior commissure, and a portion of the periaqueductal gray termed the supra-oculomotor periaqueductal gray; cerebellar and pontomedullary nuclei, including the superior, lateral, and medial vestibular nuclei, the deep cerebellar nuclei, and NRTP interneurons, and nuclei related to limbic functions including the lateral habenula, the mammillary nuclei, the hypothalamic nuclei, the preoptic nuclei, and the nucleus of diagonal band of Broca. A surprisingly large number of afferents to the medial regions of the NRTP arise from visual- or eye-movement-related nuclei. The projection from the nucleus of the optic tract (NOT) confirms previous anatomical and physiological studies on the pathways involved in horizontal optokinetic nystagmus, but the number of NOT afferents is small in relation to other areas potentially related to visuomotor pathways such as the zona incerta, ventral lateral geniculate nucleus, fields of Forel, perirubral area, and subparafasciculus. The NRTP may also relay information related to vertical visuomotor reflexes (e.g., vertical optokinetic nystagmus) given the strong projections from the medial terminal nucleus of the accessory optic system, visual tegmental relay zone, supra-oculomotor periaqueductal gray, interstitial n. of Cajal, and midbrain reticular formation. The presence of significant NRTP projections from the superior colliculus and the mesencephalic and pontine reticular formations suggests that these nuclei may provide the pathways for the noted saccade-related activity of NRTP neurons. In addition, projections from the vestibular nuclei were found that provide the anatomical basis for head velocity signals recorded in NRTP neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Observations on the somatodendritic morphology and axonal trajectory of intracellularly HRP-labeled efferent neurons located in the deeper layers of the superior colliculus of the cat

Efferent neurons of the deeper layers of the cat's superior colliculus were stained with horseradish peroxidase (HRP) to demonstrate patterns of somatodendritic morphology and axonal trajectory. A combination of somatodendritic and axonal features of the HRP-labeled cells revealed the existence of three major groups of tectal efferent neurons (X, T, and I). X neurons are mostly large and multipolar and participate in the crossed descending and ipsilateral ventral ascending projections of the superior colliculus. The X group includes multipolar radiating (X1), tufted (X2), large vertical (X3), medium-sized vertical (X4), and medium-sized horizontal (X5) neurons. T neurons participate in one or two of the major tectofugal bundles (medial descending ipsilateral, lateral descending ipsilateral, medial dorsal ascending, crossed descending) besides providing a commissural branch. They also issue recurrent collaterals distributed within a more or less restricted area of the deeper layers. The T group includes medium-sized, trapezoid, radiating (T1) and small or medium-sized, ovoid, vertical (T2) neurons. I neurons participate in the ipsilateral descending projection of the superior colliculus. They are small, triangular or ovoid, sparsely ramified cells that provide long, varicose collaterals irregularly distributed within the deeper layers. The majority of T neurons are located in the ventral stratum opticum or dorsal stratum griseum intermediale; X3 and X5 neurons are situated immediately below in the dorsal stratum griseum intermediale, while X1, X2, X4, and I neurons are indiscriminately distributed within the deeper layers. The polythetic classification presented here provides a conceptual framework for the description of tectal efferent neurons. It is open-ended and can thereby accommodate new cells types as indicated by the disclosure of a small horizontal (A) and a small radiating (unclassified) neuron. Moreover, it does not preclude the construction of alternate taxonomies. A dendro-architectonic classification into four groups [vertical (X3, X4, T2, I), horizontal (X5, A), radiating (X1, T1, I), and tufted (X2)] can be made and would relate to the mode of integration of various tectopetal inputs. A classification based on the dorsoventral location of tectal efferent neurons is also possible and would relate to the dorsoventral distribution of neurons with specific response properties.

Projections to the basilar pontine nuclei from face sensory and motor regions of the cerebral cortex in the rat

Orthograde axonal transport tracing methods were used to describe the projections to the basilar pontine nuclei (BPN) which arise within the face representation of motor or somatosensory cerebral cortex. Injections centered in motor face (MF) cortex resulted in the labeling of several corticopontine terminal fields which exhibit a rostrocaudal columnar arrangement within the ipsilateral BPN. The location of such terminal zones is consistent with the somatotopic pattern of termination previously described for limb sensorimotor cortices. In contrast, the projections from somatosensory face (SF) cortical regions largely terminate in BPN areas separate from those receiving either limb sensorimotor or MF inputs. Both MF and SF cortices also give rise to projections to the contralateral BPN; those from SF cortex are less extensive than those of MF origin. In addition to their relationship with limb sensorimotor corticopontine terminations, the MF projections to the BPN also seem to partially overlap the projection zones of the cerebellopontine system, particularly the regions projected upon by the lateral cerebellar nucleus. The SF projections, on the other hand, appear to terminate in BPN areas that also receive input from either the dorsal column nuclei or the spinal trigeminal complex. There is only minimal potential overlap between MF and SF projections in the BPN. With regard to the pontocerebellar system, the projections from MF cortex terminate among BPN neurons which project to the cerebellar hemispheres, particularly lobus simplex, crus I and crus II. The SF projections also overlap BPN neurons which project to the lateral hemispheres in addition to the paraflocculus and vermal lobules VII and IXa,b. Taken together these observations suggest that subsets of BPN neurons might exist such that some receive convergent inputs from systems whose function can generally be regarded as motor (sensorimotor cortex, cerebellopontine) while another population of BPN neurons might integrate signals from systems which transmit somatosensory information (dorsal column nuclei, spinal trigeminal).

Association of the mesencephalic locomotor region with locomotor activity induced by injections of amphetamine into the nucleus accumbens

Injections of amphetamine into the nucleus accumbens increased locomotor activity of rats. Subsequent injections of procaine into the midbrain, in the region of the pedunculopontine nucleus, significantly reduced the amphetamine-induced locomotor activity. Control experiments showed that procaine injections into the contralateral pedunculopontine nucleus had little or no effect, as well as ipsilateral injections dorsal and ventral to the pedunculopontine nucleus. These findings suggest that release of dopamine from amphetamine injections into the accumbens gives rise to ipsilateral descending influences on the region of the pedunculopontine nucleus, a major component of the mesencephalic locomotor region. Descending influences from the nucleus accumbens to mesencephalic locomotor region may serve as a link for limbic-motor integration in behavioral response initiation.

Longitudinal brainstem axons mediating circling: behavioral measurement of conduction velocity distributions

Current applied near the midline brainstem elicits rapid ipsiversive circling. Pontine and midbrain sites were stimulated concurrently with paired pulses and the number of pulse pairs required to produce 3 complete circles in 10 s was measured at various intrapair (C-T) intervals. The same results were obtained when C pulses were presented via the midbrain electrode and T pulses via the pontine electrode, or vice versa: as C-T interval increased from 0.4 to 2.0 ms, the number of pulse pairs required decreased gradually. These decreases occurred at longer C-T intervals than the refractory period decreases observed in single-electrode tests. These results imply that collision occurred in the directly stimulated axons and that a longitudinal bundle of uninterrupted axons mediates the circling behavior. The sites from which collision was obtained overlap with both medial longitudinal fasciculus and crossed tectobulbar and tectospinal tracts. The axons mediating circling appear to have conduction velocities from 2 to roughly 20 m/s. By comparison, superior colliculus units antidromically driven from contralateral electrode sites that produce circling had conduction velocities from 0.7 to 40 m/s.

Limits of neurogenesis in primates

Systematic analysis of autoradiograms prepared from postpubertal rhesus monkeys given single and multiple injections of tritium-labeled thymidine and killed 3 days to 6 years later displayed a slow turnover of glial cells but failed to reveal any radiolabeled neurons. Therefore, unlike neurons of some nonprimate species, all neurons of the rhesus monkey brain are generated during prenatal and early postnatal life. A stable population of neurons in primates, including humans, may be important for the continuity of learning and memory over a lifetime.

Tectoreticular pathways in the turtle, Pseudemys scripta. I. Morphology of tectoreticular axons

Tectoreticular projections in turtles were examined by reconstructing from serial sections axons that were anterogradely filled with horseradish peroxidase after tectal injections. Three tectoreticular pathways each contain extensively collateralized axons. The crossed dorsal pathway (TBd) contains large and small caliber axons. After leaving the tectum, TBd axons emit collaterals into the ipsilateral profundus mesencephali rostralis and then give off a main rostral branch that bears secondary collaterals in the ipsilateral interstitial nucleus of the medial longitudinal fasciculus and the suprapeduncular nucleus. The main trunks cross the midline and descend in the predorsal bundle, generating collaterals at regular intervals. These terminate mostly in the medial half of the reticular core from the midbrain to the caudal medulla. Axons in the uncrossed intermediate pathway also emit collaterals into a midbrain reticular nucleus (profundus mesencephali caudalis) and often have a thick rostral branch. The main caudal trunks, however, remain ipsilateral and travel in a diffuse, laterally placed tract, where each emits a long series of collaterals into the lateral half of the reticular core. The uncrossed ventral pathway (TBv) contains medium and small caliber axons. TBv axons often have collaterals within the tectum and apparently lack main rostral branches. Their caudal trunks run in the tegmental neuropile below the TBi where they collateralize less exuberantly than do TBd and TBi axons. The morphology of axons in all three pathways suggests that projections from disjunct tectal loci converge at many rostrocaudal levels within the reticular formation. This point was examined explicitly in experiments in which two disjunct injections were placed in one tectal lobe. Intermediate pathway axons traced from the two loci initially formed two distinct bundles but then intermingled in the reticular formation.

Cortical and tectal control of visual orientation in the gerbil: evidence for parallel channels

Two experiments were carried out with Mongolian gerbils to determine the roles of optic tectum and visual cortex in the mediation of visually guided head turns and locomotion elicited and controlled by discrete visual targets. In Experiment 1, the behavior of animals with either a sham operation, a bilateral lesion of optic tectum, or a bilateral ablation of areas 17, 18a, and 18b was recorded on videotape as they ran from the center of a circular arena toward a small visual target projected in different locations around the perimeter of the arena. The amplitude and direction of the head turns and the accuracy of their locomotor responses were reconstructed from a frame by frame analysis of the videotapes. Sham-operate gerbils made a series of head turns before running accurately and efficiently toward the target. The gerbils with lesions of areas 17, 18a, and 18b rarely made more than one head turn before running toward the perimeter of the arena. Although the single head turn they did make was often well-correlated with the position of the target in their visual field, the direction of their locomotor response was largely determined by the direction and amplitude of that head turn. As a consequence, these animals undershot the target more often than did the sham-operate animals, and even ran into the visual half field opposite the target if their head turn had also been made into that half field. Unlike the sham operates, these animals were unable to make further adjustments in their orientation toward the stimulus after their initial head turn. The head turns and locomotor behavior of the gerbils with lesions of optic tectum were even more disorganized and inaccurate than those of the posterior decorticates. Nevertheless, when the target was presented within 45 degrees from their visual midline, their head turns and locomotor responses showed a systematic relationship with the eccentricity of the target. Their behavior to stimuli outside this central wedge of their visual field was completely disorganized and showed no relationship to the location of the target. In Experiment 2, unilateral lesions of area 17 were performed in the gerbils that had already received bilateral tectal lesions to determine whether such lesions would affect the "residual" ability of these animals to orient toward stimuli located within the central portion of their visual field. During retesting, these animals were able to respond to targets only if they were located in the central portion of the field ipsilateral to the cortical lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiologic identification of projections from the midbrain to the paraflocculus and midvermis in the rat

Regions of the midbrain in the rat were stimulated electrically with bipolar electrodes to identify responsive, single neurons in the parafloccular lobule of the cerebellum. Eighty four percent (44/52) of the cells recorded in the paraflocculus showed evidence of a modulation in simple spike discharge activity (mossy fiber activations) following stimulation with a bipolar electrode whose tip was placed in the ventral layers of the contralateral superior colliculus. Mossy fiber (MF) evoked responses were indicated by the presence of an excitation followed by an inhibition of simple spike frequency at latencies of 5-16 msec and by the demonstration of responsiveness to stimulus frequencies up to 50 Hz. Ten percent (4/41) of identified Purkinje cells in the paraflocculus demonstrated activation of complex spike potentials following stimulation of regions in the ventral superior colliculus. Experiments involving stimulation of the midbrain and visual cortices indicated that 70% of the parafloccular neurons are responsive to inputs from both the cortex and deep regions of the colliculus. Electrophysiologic evidence also is presented that demonstrates the existence of a midbrain projection to midvermal lobules VI and VII of the cerebellum.

The pontine substrate of circling behavior

Single or twin movable stimulating electrodes were implanted in 9 rats in order to investigate the pontine substrate of circling behavior. The region located between the caudal part of the interpeduncular nucleus and the fourth ventricle was examined. The electrodes were implanted 6 mm below the surface of the skull and subsequently moved down by steps of 0.13 or 0.16 mm. The stimulating current consisted of trains of cathodal rectangular pulses of constant intensity and width (100 microA and 0.1 ms respectively) and of variable frequency. The effectiveness of the stimulation in eliciting a circling reaction was inferred from a psychophysical determination of the pulse period required at each site in order for the animal to maintain a criterion rotation speed. In the average, 48 brain sites were investigated per animal. Stimulation of 166 out of a total of 387 sites elicited ipsiversive rotation. Depending on the coronal plane of implantation, the dorsal boundary of the circling substrate was located within the pedunculus cerebellaris superior or the floor of the substantia grisea centralis. In addition, the positive region extended 1.7-2 mm ventrally and 1.9 mm from the midline. The distribution of the positive sites seems to suggest that the circling substrate is a large bundle which runs sagittally through the medial part of the reticular formation.

Mapping midbrain sites for circling using current-frequency trade off data

Electrical stimulation applied near the medial longitudinal fasciculus (MLF) produces ipsiversive circling. In this experiment we examined the spatial and temporal properties of this substrate in rats by determining the current required to produce a constant amount of circling at frequencies from 15 to 500 Hz. In general, when frequency was high, the required current was low and vice versa, but there were deviations from a reciprocal relationship. Frequencies above 200 Hz failed to reduce the current needed to reach criterion. Electrodes placed distant from the MLF required more current, and deviated from reciprocity in a systematic way. A geometric model of the overlap between substrate and stimulation field is proposed that allows estimation of spatial properties of the two regions: (1) the location of the center of the substrate for circling (0.7 mm lateral, 7.0 mm below dura in the coronal plane 1.0 mm posterior to ear bars); (2) the radius of the substrate (0.77 mm); and (3) the radius of excitation at each current. Current-frequency trade off data can provide an in vivo estimate of the behavioral effectiveness of a given electrode, and hence its placement relative to a behavioral substrate.

The inferior colliculopontine neurons of the cat in relation to other collicular descending neurons

Cells of origin of the pontine afferents from the inferior colliculus (IC) of cats have been identified by means of retrograde axonal transport of horseradish peroxidase (HRP). Following injections of HRP in the pontine nuclei, many labeled neurons were found ipsilaterally in the caudal parts of the external and pericentral nuclei, while a few cells were found in the central nucleus. Additional neurons occurred in the nuclei of the middle and rostral parts. This organization contrasts with that of other collicular descending systems. Thus, neurons projecting to the superior olivary complex and cochlear nuclei were found predominantly in the central nucleus bilaterally. In the external nucleus labeled cells tended to be distributed in the middle to rostral regions, but they were few in number in the caudal part. Since the locations of IC-pontine neurons are found to be different from those of other IC descending neurons, it may be assumed that the IC-pontine system does not share common functional properties with the other collicular descending neurons. Functional aspects of the IC-pontine neurons are discussed, with a comment on a pathway for the transmission of auditory impulses to the midvermal area of the cerebellum.

Intra- and interhemispheric collateral branching in the rat pontocerebellar system, a fluorescence double-label study

A double-labeling method which employed the injection and subsequent retrograde transport of two different fluorescent dyes was utilized to investigate the possibility that some pontocerebellar neurons might give rise to collateral branches which distributed to more than one lobule of the contralateral lateral cerebellar hemisphere (intrahemispheric branching) or bilaterally to homotopic or heterotopic lobules in both lateral hemispheres (interhemispheric branching). With regard to intrahemispheric branching, these studies revealed the largest number of double-labeled neurons when the dye injections involved a combination of crus I and the paraflocculus. A considerable number of double-labeled cells were also observed in simplex-crus II cases while a modest number were noted in crus I-paramedian and simplex-paramedian combinations. Evidence for interhemispheric branching was also apparent but the number of double-labeled cells was generally less than that observed in the intrahemispheric experiments. Following bilateral injections, a modest number of double-labeled cells was noted with simplex-simplex and crus I-Crus I homotopic injections and crus II-paramedian heterotopic combinations. In contrast to those cases with unilateral injections, not all bilateral injection combinations produced double-labeling, the most conspicuous in this regard being homotopic and heterotopic injections involving the paraflocculus. Overall, the population of double-labeled cells included many varieties of pontine neurons ranging from small, spindle-shaped to large multipolar. No clear topographic patterns emerged when the location or distribution of either intrahemispheric or interhemispheric double-labeled cells was compared. It was noted, however, that most double-labeled neurons were situated in zones of overlap that occurred between single-labeled neurons projecting to one or the other of the injected lobules. Conversely, in some situations, sharp borders were maintained between adjacent groups of single-labeled neurons. These studies have demonstrated the existence of both intrahemispheric and interhemispheric branching in the pontocerebellar system. The interhemispheric category included pontine neurons which distributed bilaterally to either homotopic or heterotopic lobules in each hemisphere. The overall distribution pattern of double-labeled pontine neurons, those whose axons distribute to more than one hemispheral lobule, is extremely complex and, although some trends were noted, no system-wide topographic organization was apparent. The category of double-labeled neurons included a wide variety of shapes and sizes ranging from the smallest to the largest types of pontine neurons. Most double-labeled neurons were intermixed with single-labeled pontine cells which projected to one or the other of the two injected lobules.(ABSTRACT TRUNCATED AT 400 WORDS)

The cerebellopontine system in the rat. I. Autoradiographic studies

This study utilized light microscopic autoradiographic procedures to describe the projections from the three major subdivisions of the deep cerebellar nuclei (DCN) to the basilar pontine nuclei (BPN). Although the vast majority of cerebellopontine axons reached the BPN via the descending limb of the brachium conjunctivum (BC) after crossing the midline within the midbrain, a relatively small number of ipsilaterally projecting fibers was also observed. Fascicles of cerebellopontine axons left the main bundle of descending limb fibers throughout much of the rostrocaudal length of the BPN and passed around and through the medial lemniscus and cerebral peduncle to enter the pontine gray. The lateral cerebellar nucleus gave rise to the largest number of cerebellopontine fibers, whose terminal fields exhibited both diffuse and patchlike labeling patterns within each of the major subdivisions of the BPN including medial, ventral, lateral, and dorsal areas. Projections from the interpositus complex exclusive of its posterior division were fewer and less widely distributed than those from the lateral nucleus. Interpositopontine fibers terminated primarily in the caudal one-half of the BPN in medial, ventral, and lateral regions and overlapped somewhat with projections from the lateral cerebellar nucleus. Pontine projections emanating from the medial cerebellar nucleus were the fewest and most restricted in distribution relative to the other two cerebellar efferent systems. Such fibers formed a patchlike network of terminal fields which extended throughout much of the rostrocaudal length of the BPN in medial and dorsomedial regions. A relatively small but considerable number of ipsilateral cerebellopontine fibers terminated in pontine regions, which often mirrored the typical contralateral projection fields. Although it proved difficult to determine the precise origin of the ipsilateral fiber systems, it appeared that each of the three major DCN subdivisions made some contribution. Also it was apparent that considerable overlap existed between cerebellopontine projection zones and those of other pontine afferents including sensorimotor, visual, and auditory cortices, the superior colliculus, and the mammillary nuclei of the hypothalamus. Moreover, cerebellopontine terminal fields were congruent in some instances with discrete clusters of BPN neurons which serve as the source of pontocerebellar fiber systems, reaching portions of the lateral cerebellar hemispheres, posterior vermis, and the paraflocculus.

Electron microscopic identification of superior colliculo-pontine axon terminals

Synaptic boutons emanating from axons of superior colliculus origin were identified by electron microscopy in the neuropil of the basilar pontine nuclei. Such boutons were relatively small (0.6-2.0 microns) and exhibited electron-dense degeneration within a 1-2 day period following electrolytic lesions which involved much of the superior colliculus. Degenerating boutons were observed in synaptic contact with dendritic shafts and spines as well as neuronal somata. The reactive boutons were rapidly engulfed by phagocytic elements and were no longer visible in the neuropil after 6 days of survival.

An autoradiographic study of the cerebellopontine projections in the rat. I. Projections from the medial cerebellar nucleus

The medial cerebellar nucleus of the rat is shown by the autoradiographic technique to project to both the contralateral nucleus reticularis tegmenti pontis and the pontine nuclei proper. The former projection is more concentrated in the medial--parvocellular --region. In the pontine gray, the bulk of the projection concerns the dorsal aspect of the medial nucleus. Rostral parts of the medial cerebellar nucleus project to caudal pontine levels whereas caudal parts seem to project throughout the rostrocaudal extent of the basilar pons.

The tecto-olivo-cerebellar pathway in the rat

The olivo-cerebellar projection to the vermis of cerebellar lobule VII, and the projection of the midbrain tectum onto the inferior olive (IO), were examined in the rat employing, respectively, retrograde and anterograde transport of horseradish peroxidase (HRP). Olivary neurons afferent to lobule VII vermis are located in a restricted region of the contralateral, caudomedial, medial accessory olive (MAO). The termination of the olivary projection demonstrated after HRP placement in the superior colliculus (SC), corresponds closely with this region.

Regional (14C) 2-deoxyglucose uptake during vibrissae movements evoked by rat motor cortex stimulation

Repetitive left mystacial vibrissae movements were produced by electrical stimulation of right motor cortex (MI) were a bipolar electrode in the alert, unanesthetized rat. Regional increases of (14C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically during these left vibrissae movements. Uptake of 2DG increased in a 2-4-mm-diameter area about the stimulating electrode in right MI and in a smaller region in left MI cortex. Columnar increases of 2DG uptake occurred bilaterally in somatosensory cortex in the face region of somatosensory cortex (SI). Bilateral increases of 2DG uptake occurred subcortically in dorsolateral caudate-putamen (CP) and subthalamic nucleus. Primary right-sided increases of 2DG uptake occurred in other basal ganglia structures including dorsal globus pallidus (GP), posterior, entopeduncular nucleus (EPN), ventrolateral substantia nigra pars reticulata (SNr), and anterolateral substantia nigra pars compacta (SNc). Uptake of 2DG increased on the right side of the following thalamic regions: much of the ventrolateral (VL) nucleus, particularly dorsally; the anterodorsal reticular nucleus; dorsolateral posteromedial (POm) nucleus; the ventromedial nucleus; and dorsolateral parafasicular nucleus. The anterior and ventral posterior portions of VL were not activated. Caudal to thalamus right-sided 2DG uptake increased in the medial, ventral, and lateral pontine nuclei, deep layers of superior colliculus, lateral deep mesencephalic nucleus (DMN), and nucleus cuneiformis (NCU). UPtake of 2DG increased in right rostral parvocellular and red nucleus in a few animals. Discrete portions of the right internal capsule and right medial pyramidal tract increased 2DG uptake during MI stimulation. Uptake of 2DG increased on the left side of the brain during right MI stimulation in the left lateral nucleus (NL) of cerebellum and in several discrete regions of left cerebellar hemisphere granule cells including anterior paravermis, lobulus simplex, crus II, and the paramedian lobule. Uptake of 2DG increased in left nucleus of the spinal tract of the trigeminal nerve (ntV) ventrally in subnuclei interpolaris and oralis. Left lateral portions of the facial nucleus were activated in a few animals. The lateral portions of the facial nucleus are known to project to vibrissae musculature. All of the above structures may be involved in the motor-sensory processing responsible for vibrissae movements. Regions not previously suggested to play a major role in vibrissae movements include DMN and NCU. Though NCU has been called the "locomotor center" it may play a role in facial movements as well. Polysynaptic activation of GP, EPN, NL, and cerebellar hemisphere occurred since no connections between MI and these regions exist. A pathway from ntV to POm to MI and SI is suggested to provide proprioceptive input to motor-sensory cortex from the moving vibrissae since neither the principal trigeminal sensory nucleus nor the ventrobasal nucleus of the thalamus increased 2DG uptake during MI stimulation.