Discharges of superior colliculus neurons during head and eye movements of the alert cat

Summary 452 single neurons from the superior colliculus were recorded in awake and non-paralysed cats. 75 neurons were obtained from cats with unrestrained horizontal head movements.228 neurons remained unaffected by saccadic eye movements. Eye movement related discharge followed the onset of saccades in 156 neurons either only in the presence of a visual pattern (92 neurons) or in darkness, too (64 neurons). The latter reaction type probably depends on eye muscle afferents.In 48 neurons eye movement related activity preceded the onset of eye movements. 12 neurons fired in synchrony with eye movements of any direction (type I). 30 neurons were excited during contralaterally directed eye versions within or into the contralateral head related hemifield. They were inhibited when the eyes moved within or into the ipsilateral head related hemifield (type II). 6 neurons with constant maintained activity during fixation were inhibited by ipsilaterally directed saccades, but remained unaffected by contralateral eye movements.Head movement related discharge followed the onset of head movements in 20 neurons only in presence of a visual pattern and also in darkness in 6 neurons. Ipsilateral head movements or postures strongly suppressed maintained activity and visual responsiveness of some neurons.15 neurons discharged in synchrony with and prior to contralateral head movements. Ipsilateral head movements inhibited these neurons. Activation or inhibition were usually related to movement and to posture, exceptionally to movement or to posture.Electrical stimulation of recording sites of these neurons through the recording microelectrode elicits contralateral head movements.     

Stimulation of the superior colliculus in the alert cat

Summary Electrical stimulation of the cat superior colliculus (SC), in conjunction with the accurate measurement of elicited eye movements and histologically verified electrode positions, has revealed a striking antero-posterior variation in collicular organization. Three zones could be defined in the SC on the basis of eye movement patterns and associated neck muscle EMG activity evoked from the deeper layers. The Anterior zone was coextensive with the central 25 ° of the visual retinotopically coded map contained in the superficial layers. Saccades evoked from this zone were also retinotopically coded, and the latency of EMG activity depended on the position of the eye in the orbit. A similar observation applies to the entire monkey SC. The Intermediate zone was coextensive with the 25 °–70 ° of visual projections. Saccades evoked from this region were goal-directed and were associated with invariant, short latency EMG responses. The Posterior zone was found in the extreme caudo-lateral portion of the SC. Eye movements evoked from this zone were centering saccades associated with constant latency EMG activity. The present results in conjunction with previously demonstrated antero-posterior variations in projections to the SC, suggest that the motor strategies controlling gaze shifts toward visual targets vary depending on the location of the target in the visual field.     

The fixation area of the cat superior colliculus: effects of electrical stimulation and direct connection with brainstem omnipause neurons

The superior colliculus has long been recognized as an important structure in the generation of saccadic displacements of the visual axis. Neurons with presaccadic activity encoding saccade vectors are topographically organized and form a motor map. Recently, neurons with fixation-related activity have been recorded at the collicular rostral pole, at the area centralis representation or fixation area. Another collicular function which deals with the maintenance of fixation behavior by means of active inhibition of orientation commands was then suggested. We tested that hypothesis as it relates to the suppression of gaze saccades (gaze = eye in space = eye in head + head in space) in the head-free cat by increasing the activity of the fixation cells at the rostral pole with electrical microstimulation. Long stimulation trains applied before gaze saccades delayed their initiation. Short stimuli, delivered during the gaze saccades, transiently interrupted both eye and head components. These results provide further support for a role in fixation behavior for collicular fixation neurons. Brainstem omnipause neurons also exhibit fixation-related activity and have been shown to receive a direct excitatory input from the superior colliculus. To determine whether the collicular projection to omnipause neurons arises from the fixation area, the deep layers of the superior colliculus were electrically stimulated either at the rostral pole including the fixation area or in more caudal regions where stimulation evokes orienting responses. Forty-nine neurons were examined in three cats. 61% of the neurons were found to be orthodromically excited by single-pulse stimulation of the rostral pole, whereas only 29% responded to caudal stimulation. In addition, stimuli delivered to the rostral pole activated, on average, omnipause neurons at shorter latencies and with lower currents than those applied in caudal regions. These results suggest that excitatory inputs to omnipause neurons from the superior colliculus are principally provided by the fixation area, via which the superior colliculus could play a role in suppression of gaze shifts.     

Rostrocaudal and lateromedial density distributions of superior colliculus neurons projecting in the predorsal bundle and to the spinal cord: a retrograde HRP study in the cat

Summary Efferent neurons of the cat superior colliculus (SC) which project in the predorsal bundle (PDB) and to the spinal cord (PDB neurons) form a major pathway by which the SC controls the changes of the direction of gaze in response to stimuli of visual and other modalities. Knowledge of rostrocaudal and lateromedial density distributions of different groups of PDB neurons within the SC is necessary to analyse their relationships with the topography of sensory and motor maps. Density gradients may also bear on the efficacy of connections originating from topographically different collicular regions. In the present study, large injections of HRP/ WGA-HRP were made in the C1 segment of the spinal cord and in the pontobulbar tegmentum. Judged by several morphological criteria, axons of passage, including those not subjected to a direct mechanical damage, were participating in the uptake of tracers. Therefore, labeled SC neurons corresponded to the nearly total populations of contralaterally projecting tectospinal neurons (TSNs) and neurons projecting in the PDB, respectively. Subtraction of the TSN density map from that of the whole PDB population was used to infer the distribution of tectal neurons terminating in the rhombencephalic tegmentum (TRhN). This subtotal labeling method proved useful in resolving the contradictions between the earlier HRP studies on the TSN and TRhN topography. The following density distributions were obtained for different groups of PDB neurons: 1) The mean TSN density is more than two times higher in the lateral half of the SC, representing the lower visual field. In this region the density remains constant from rostral to caudal, i.e., from the representation of vertical meridian to large contralateral azimuths. In the medial half, the average density decreases from rostral to caudal. Consequently, TSNs do not show the caudalward increment predicted by the higher efficacy of caudal stimulation points in eliciting head movements. 2) The distribution of PDB neurons is symmetrical with respect to the representation of the horizontal meridian. It is close to homogeneous at all azimuths of the retinotopic map and within the zone limited by small (10–15°) upward and downward elevations. There are clear density decrements towards the representation of greater elevations. We conclude that the assumption of homogeneous distribution of efferent neurons, made in the models of the primate SC, is valid in the cat, but only for the whole population of neurons projecting in the PDB and within the area representing the proximity of the horizontal meridian. The same restrictions apply to the validity of the translation invariance principle. 3) The distribution of TRhNs, obtained by subtraction, shows a tendency to higher densities in the caudal half of the SC, and a clear bias towards its medial zone, including the representation of the horizontal meridian and upper visual field. The different types of asymmetry in TRhN and TSN populations may be related to different types of head movements depending, respectively, on distant and near stimuli in the upper and lower visual fields.Abbreviations NRTP nucl. reticularis tegmenti pontis - PDB predorsal bundle - PPRF paramedian pontine reticular formation - RGc nucl. reticularis gigantocellularis - Rmc nucl. reticularis magnocellularis - RPc nucl. reticularis pontis caudalis - RPo nucl. reticularis pontis oralis - SGI stratum griseum intermediale - SGP deeper layers, beginning from stratum album intermediale - SC superior colliculus - TRhN tectorhombencephalic neuron with crossed projection - TSN tectospinal neuron On leave from: Laboratoire de Neurophysiologie, Faculté de Médecine, Université de Louvain, Belgium     

Properties of saccade-related neurons in the cat superior colliculus: patterns of movement fields and discharge timing

The discharge characteristics of saccade-related neurons in the superior colliculi (SC) of trained cats were investigated in order to understand the involvement of the SC in dynamic control of saccades. In particular, two aspects of saccade-related collicular activity in relation to visually guided saccades were quantitatively analyzed: the bounded pattern of amplitude tuning and the timing of neural activity in relation to saccade offset. In order to determine whether the amplitude tuning of SC cells is bounded or unbounded, we characterized it with a quantitative index, the unbounded-field (UF) index. Analysis of the characteristics of the distribution of this index, including the absence of unbounded fields for cells within the central 10 degrees, suggested that all cells form a single class and that apparent unbounded amplitude tuning is an artifact occurring when the range of gaze shift is truncated by restraining the head. This interpretation is supported by the results of a computer simulation based on truncation of gaze-shift range that closely replicated the experimental data. Analyses of relative location of peak motor activity in the spatial and temporal dimensions revealed that the peak discharge was more closely linked to the midpoint of the saccade than to the offset of saccade, regardless of the pattern of amplitude tuning. These results do not support the proposal advanced by the moving-hill hypothesis: the bounded pattern of amplitude tuning is inconsistent with rostral migration of the activity locus during execution of a saccade, and the relative location of the peak motor activity is inconsistent with the idea that the SC activity encodes dynamic motor error (the difference between desired and current gaze directions).     

Neurons of the superficial tectal gray an intracellular HRP-study on the kitten superior colliculus in vitro

Summary An en bloc preparation of the mammalian superior colliculus in vitro has been used to study neurons of the superficial gray layer (SGS) with intracellular recording and HRP-technics. Electrophysiological data from kittens at 4–19 days of age suggest that at this stage SGS-neurons possess multiple spike trigger zones which can be activated by synaptic depolarization and are probably located on dendrites. In response to intratectal stimulation SGS-neurons generate EPSP-IPSP sequences or IPSPs. IPSPs are found in all penetrated cells as early as the 4th postnatal day. Ascending projection cells (APCs) and inter-layer cells (ILCs) have been identified based on antidromic activation and/or intracellular labeling with HRP. The extended dendritic arbor of APCs and ILNs (dorsal spread up to 10–20 m below surface, horizontal spread up to 1100–1500 m) enables these cells to sample visual information from a wide area of the visual field. Recurrent collaterals, in conjunction with potent inhibitory mechanisms, could contribute to the formation of receptive field properties of superficial tectal neurons. ILCs establish collateral connections with the intermediate gray layer.     

Gaze-related activity of brainstem omnipause neurons during combined eye-head gaze shifts in the alert cat

Summary Studies undertaken in head-restrained animals have long implicated the omnipause neurons (OPNs) in the initiation of saccadic eye movements. These inhibitory neurons discharge tonically but cease firing just before and during saccades in all directions. By recording from OPNs in alert behaving head-unrestrained cats, we have demonstrated that the activity of these cells is related to the displacement of the visual axis in space (gaze), which is the sum of the eye movement relative to the head and head movement relative to space. OPNs were found to exhibit a complete cessation of discharge for a period equivalent to the duration of the gaze shift, and not to the duration of either the rapid eye movement or the head movement components. In large gaze shifts, OPNs were silent even when the eye was immobile in the orbit, as long as the gaze shift was not completed. The results of this study show that OPNs are controlled by neural elements that take into account the actual position of the visual axis relative to its final desired position, irrespective of the trajectory of the eye in the orbit or of whether the head is moving or not.     

Burst activity of identified tecto-reticulo-spinal neurons in the alert cat

Summary Activity of tecto-reticulo-spinal neurons (TRSN), identified electrophysiologically and/or by intra-axonal HRP injections, was studied in alert cats during presentation of moving visual stimuli. A majority of TRSNs showed complex visuomotor properties: directionally selective visual responses in the absence of motor counterparts of orienting, enhanced bursting when stimuli triggered saccades, and no activation for spontaneous saccades. Highest intraburst frequencies were observed during active orienting towards novel, interesting objects. The more vigorous bursts usually contained repetitive grouped discharges attaining instantaneous frequencies up to 700 imp/s but average firing rates remained in the range of 120–300 imp/s. Intra-axonal HRP injections confirmed terminations of TRSN collaterals in the premotor areas of the lower brain stem, including the abducens nucleus, but also disclosed differences in the details of collateralization between neurons showing different types of visuo-motor activity.     

Origin,course and termination of dopaminergic substantia nigra neurons projecting to the amygdaloid complex in the cat

The organization of dopaminergic neurons projecting to the amygdala was examined using retrograde (horseradish peroxidase histochemistry) and anterograde ([³H]leucine autoradiography) transport methods and Falck-Hillarp histofluorescence techniques combined with microspectrofluorometry and radiofrequency lesions. Cell bodies located within the pars lateralis and pars compacta of the substantia nigra were found to project to the lateral and central amygdaloid nuclei, respectively. Both of these areas within the substantia nigra contained dopaminergic perikarya, while the central and lateral amygdaloid nuclei contained fluorescent varicosities with features indicative of dopaminergic neurons. Lesions restricted to the pars lateralis of the substantia nigra resulted in a loss of fluorescence in the lateral amygdaloid nucleus. Autoradiographic experiments revealed that the projections from the pars lateralis did not run with fibers originating from the pars compacta in the nigrostriatal tract but rather had their own course occupying a lateral position adjacent to the cerebral peduncle and joining the ventral amygdalo-fugal bundle.The data indicate that, in the cat, there are two separate dopaminergic projections to the amygdala arising from the substantia nigra.     

Axonal projections and conduction properties of olfactory peduncle neurons in the rat

Summary Electrophysiological methods were employed to study the axonal properties of the neurons of anterior olfactory nucleus (AON), transition zone (TZ), and rostral prepyriform cortex (RPPC) and their projections towards the ipsilateral and contralateral olfactory bulb (IOB, COB) in the rat.Of 91 antidromically driven cells, 39 (43%) and 32 (35%) responded to IOB and COB stimulation, respectively; 20 (22%) were discharged from both bulbs. Collision tests performed on the latter group indicated that these neurons have a short main axon which divides near the soma, projecting one branch to the COB and a thinner one toward the IOB. Mean conduction velocities of axons projecting to the IOB and the COB were 0.4 m/s and 0.7 m/s, respectively, the faster conducting axons having shorter refractory periods.Of the 38 neurons tested, 92% showed decreases in threshold and latency (up to 20% of control antidromic latency) after a test volley that was preceded by a conditioning pulse at intervals of 20–215 ms. Latency decreases were greater for slowly conducting axons than for the faster ones. These after-effects of impulse activity in OB afferent axons were attributed to the presence of a supernormal period of increased conduction velocity and excitability similar to that found in the olfactory nerve (Bliss and Rosenberg, 1974).This work was supported by a grant, Convenio de Cooperatión Científico-Tecnológica, Argentino-Norteamericano, No. 7363/ 75, provided by the Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina     

Assessment of brightness and color differences by neurons in the superior colliculus of the rabbit

The responses of 83 neurons in the rabbit superior colliculus to substitution of color stimuli of different brightnesses and black-and-white stimuli of different intensities were studied. Superior colliculus neurons were found to respond with initial and late phasic discharges (over the periods 50–90 msec and 120–300 msec from the moment of stimulus substitution respectively), along with prolonged tonic discharges whose spike frequencies depended on the intensity of the stimulus. Analysis of the phasic responses of the neurons allowed three groups of cells to be identified. One group of cells (25 of the cells studied, 30%), identified on the basis of early neuron responses, were specialized for detecting brightness differences between black-and-white and color stimuli of different intensities. The sensory spaces reconstructed on the basis of spike discharge frequencies in the early discharges of these neurons were achromatic and two-dimensional. Another group of neurons (16 of the cells studied, 19%) were mainly identified on analysis of late phasic discharges and had four-dimensional spaces with two color and two achromatic axes. The third group of cells (four neurons, 5%) had early discharges with two-dimensional achromatic sensory spaces and late discharges with four-dimensional spaces. It is suggested that reconstruction of the four-dimensional space requires processing of information from the visual cortex on color and intensity differences between stimuli. The sensory spaces of superior colliculus neurons reconstructed on the basis of phasic discharges essentially coincided with the sensory spaces of neurons in the visual cortex and lateral geniculate body and spaces obtained by analysis of the N85 component of visual evoked potentials in rabbits recorded using similar stimulation. This may support the vector coding principle in the visual analyzer. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 57, No. 5, pp. 576–590, September–October, 2007.     

Axonal projections from Bötzinger expiratory neurons to contralateral ventral and dorsal respiratory groups in the cat

Summary We studied projection patterns of the augmenting expiratory neurons of the Bötzinger complex (BÖT) in the contralateral brainstem. Three experimental approaches were used: 1) electrophysiological analysis using antidromic microstimulation, and morphological analyses using 2) intraaxonal injection of HRP, and 3) application of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). Taken together, the three methods revealed morphological details of the axonal arborizations of the expiratory neurons in the BÖT and the ventral respiratory group (VRG). The majority of augmenting expiratory neurons of the BÖT had axonal collaterals in the contralateral brainstem. The stem axons to the contralateral side crossed the midline almost at the level of the cell somata. They descended dorsomedial to the ventral spinocerebellar tract and gave off collateral branches directed dorsomedially. Terminal boutons were distributed abundantly in the caudal part of the BÖT and in the more caudally situated VRG. Axon collaterals sometimes ran to the dorsal respiratory group (DRG) and distributed terminal boutons there. Together with the fact of extensive ipsilateral arborizations shown previously, the present results indicate that the augmenting expiratory neurons of the BÖT have wide bilateral influence on the BÖT, VRG, DRG, and spinal cord.Abbreviations VII facial nucleus - XII hypoglossal nucleus - AMB nucleus ambiguus - AP area postrema - CX external cuneate nucleus - D descending vestibular nucleus - DX dorsal-motor nucleus of the vagus - M medial vestibular nucleus - NTS nucleus of the solitary tract - R nucleus of Roller - S solitary tract - RFN retrofacial nucleus This paper is dedicated to Professor Hajime Mannen on the occasion of his 65th birthdaySupported by grants-in-aid for Scientific Research nos. 60304044, 62570068, 62770043, and 63570027 from the Japan Ministry of Education, Science and Culture     

The effect of body tilt upon the transfer- and output function of neurons of the cat's superior colliculus

Summary The influence of body tilt upon the reactions to a visual stimulus was tested in 54 single neurons of the cat's colliculus superior. The statistical analysis did not reveal modification of neuronal reactions by body tilt. In a second series of experiments we studied the influence of body position on eye position, which had been induced by focal electrical stimulation of the colliculus superior. Again, the body tilt did not modify the electrical stimulation effect. Our results suggest, that tonic vestibular stimuli do not influence the excitability of neurons of the superior colliculus.This study was supported by the Deutsche Forschungsgemeinschaft as a research project in the Sonderforschungsbereich Kybernetik.     

The cerebellar corticonuclear projection from lobule Vb/c of the cat anterior lobe: a combined electrophysiological and autoradiographic study

Summary The activity of 24 reticulo-spinal neurons (RSN) identified by antidromic stimulation at the C1-C2 level has been recorded intra-axonally in the pons of alert head-fixed cats during spontaneous gaze shifts and orienting towards novel targets. Relationship of neuronal discharge to saccadic eye movements, positions of fixation and EMG of dorsal neck muscles were analysed. The present report describes behavioral properties of a group of 14 RSN showing similar types of correlations with motor parameters during eye-head synergies. These eye-neck RSN (EN-RSN) generate bursts in synchrony with phasic components of ipsilateral neck EMG and leading ipsiversive saccades by a variable lead time. Bursts are followed by a prolonged discharge whose frequency decays even when eccentric eye position is maintained constant and accompanied by sustained neck muscle activity. The firing rate of EN-RSNs depends on eye position: they are silent with saccades in their ON-direction when the eyes are deviated towards the contralateral half of the oculomotor range and the ipsilateral neck muscles are relaxed. When the eyes cross the vertical meridian, the frequency of phasic and tonic components related to eye-head synergies increase proportionally to ipsilateral eye position. Ten of the 14 EN-RSNs, located in the pontine reticular formation, received monosynaptic input from the contralateral superior colliculus. Two were labeled by intra-axonal injection of HRP which revealed extensive branching in the abducens, facial, medial and lateral vestibular, prepositus and intercalatus nuclei and in the caudal pontine and bulbar reticular formation. It is concluded that the caudal pontine tegmentum, including the region just anterior to the abducens nucleus, contains RSNs whose signals seem appropriate to control phasic neck muscle activity and which also project to structures related to ocular and facial movements. Comparisons with the perisaccadic activity of tectal neurons projecting in the predorsal bundle reveals a profound transformation of the descending signal at the level of EN-RSNs which represent first order relay neurons of the tecto-reticulo-spinal pathway.     

The reciprocal connections of the suprageniculate nucleus and the superior colliculus in the rat

Reciprocally bilateral connections between the superior colliculus and the suprageniculate nucleus have been studied in the rat, using the anterograde and retrograde transport techniques of HRP. In those cases where the HRP deposit was restricted to the superficial layers of the colliculus, anterogradely labeled fibers and retrogradely labeled neurons were observed in the ipsilateral suprageniculate nucleus. However, upon HRP injection extended into the intermediate layers of the colliculus, the number of labeled fibers and neurons was not only increased ipsilaterally but were also observed in the contralateral suprageniculate nucleus. The density of the labeled fibers and the number of labeled neurons was always greater in the ipsilateral side. These results show that the suprageniculate nucleus and the superior colliculus are connected reciprocally and bilaterally, with an ipsilateral dominance.     

Cytochemical investigation of binaurally converging connections in nuclei of the cat superior olive

Binaurally converging connections at the level of the superior olivary nuclei were investigated in anesthetized cats by the method of quantitative cytospectrophotometry. As a result of monaural stimulation for 2 h with burst of rhythmic noise a marked increase was observed in the content of cytoplasmic RNA in neurons of the ipsilateral and contralateral medial and lateral nuclei. The volume of functioning neurons of these nuclei either showed no significant change or was increased. Some idea of the character of the functional organization of binaurally convergent connections can be deduced from these results.Laboratory of Physiology of Hearing and Laboratory of Functional Neurchemistry, I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. (Presented by Academician V. N. Chernigovskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 81, No. 2, pp. 242–244, February, 1976.     

Cluster-and-sheet pattern of enkephalin-like immunoreactivity in the superior colliculus of the cat

The distribution of enkephalin-like immunoreactivity in the superior colliculus has been studied in the cat with the peroxidase-antiperoxidase method. Two striking patterns of immunoreactivity were observed. In the superficial layers there is a thin, dense horizontal band of immunoreactivity in the neuropil of the most dorsal tier of the superficial gray layer (sublamina 1). Because this sublayer corresponds to the zone of densest contralateral retinotectal projection, an intraocular injection of horseradish peroxidase was made in one cat to allow direct comparison of the distributions of opiate-like immunoreactivity and transported tracer in the contralateral superior colliculus. There was a detailed similarity between the two, including the presence of a gap in both at the presumptive site of the optic disc representation. The presence of enkephalin-like immunoreactivity in neural perikarya in and near sublamina 1 of the superficial gray layer, however, raised the possibility that the immunoreactive band is part of an intrinsic opiate system. Deeper in the superficial gray layer there was appreciable but weaker immunoreactivity in the neuropil and fewer immunoreactive neurons. In the intermediate gray layer and, especially medially, even deeper in the superior colliculus, enkephalin-like immunoreactivity was organized into small (100-300 micron wide) patches. In the intermediate gray layer these tended to be arranged periodically, five-seven patches being spaced at 200-600 micron intervals in caudal transverse sections. In some sections adjoining patches appeared to be fused. The patches were absent or difficult to detect in rostral sections. Caudally, they sometimes were adjacent to blood vessels penetrating the intermediate gray layer, but other times were not. Serial section reconstructions suggested that the patches observed in individual sections are part of larger arrays which have the form of anastomotic bands running in longitudinal directions somewhat oblique to the sagittal plane. It is concluded that an opiate mechanism may play a part in controlling the effects of incoming retinal information in the superficial gray layer, directly or indirectly, and that opiate peptides may also act in modulating one or more afferent or efferent systems of the deep collicular layers. Accordingly, from the functional standpoint, enkephalin-like peptides may influence both visual and sensory motor processing in the superior colliculus.     

Influence of rubrospinal tract and the adjacent mesencephalic reticular formation on the activity of medullary respiratory neurons and the phrenic nerve discharge in the rabbit

Suprapontine brain sites acting on the central respiratory system have been demonstrated to give rise to inspiratory as well as expiratory facilitatory effects. In the present study the inspiratory inhibitory effect which has been reported in the cat to be elicited consistently by electrical stimulation of the rubrospinal tract and the adjacent mesencephalic reticular formation was examined in the urethane-anaesthetized rabbit. Stimulation of these sites with single electrical shocks of moderate intensity induced a short latency (onset after 3.0 ms) transient (duration: 29 ms) inhibition of the phrenic nerve activity (PHR). Short volleys of stimuli applied in mid- to late-inspiration led to a premature off-switch of inspiration. The extracellularly recorded discharge activity of the different types of medullary respiration-related units (RRU) reflected these alterations, accordingly. Axonal connections of RRU with mesencephalic structures were evaluated. Examination of orthodromic responses of medullary RRU to stimulation of this pathway revealed that most bulbospinal inspiratory neurons (10 out of 13) were paucisynaptically inhibited after short latency (at least 1.2 ms). The conduction time from bulbospinal inspiratory neurons to the recording site of PHR was 1.6 ms. Thus, a disynaptic pathway — including bulbospinal inspiratory neurons — is suggested inducing inspiratory inhibition 3.0 ms after single shock midbrain stimulation. This inhibition results in disfacilitation of phrenic motoneurons. The fact that extensive electrolytic lesions of the pneumotaxic center in rostral pons did not abolish the observed inspiratory inhibitions excludes these structures from being involved. A direct pathway from the red nucleus and the adjacent reticular formation to phrenic nuclei of the spinal cord, however, can not be excluded from being involved in the demonstrated inspiratory inhibition. The described effects may play a role in behavioral or voluntary control of respiration.     

The cerebellar projections to the superior colliculus and pretectum in the cat: An autoradiographic and horseradish peroxidase study

Efferent projections from the cerebellar nuclei to the superior colliculus and the pretectum have been studied using both retrograde and orthograde labeling techniques in the cat. In order to identify what parts of the cerebellar nuclei project to the superior colliculus and the pretectum, the retrograde horseradish labeling technique was employed. In another set of experiments, tritiated amino acids were injected into each of the cerebellar regions from which the cerebello-tectal and cerebellopretectal projections arise, and the laminar and spatial distributions of orthograde labeling in the superior colliculus and the pretectum were compared.The results showed that the cerebello-tectal projections arise from two different regions of the cerebellar nuclei: the caudal half of the medial nucleus and the ventrolateral part of the posterior interposed nucleus. Fibers arising from the medial nucleus distribute bilaterally in the superficial zone of the intermediate gray layer in the superior colliculus, while those originating from the posterior interposed nucleus terminate contralaterally in the deeper aspect of the intermediate gray layer and in the deep gray and white layers. Although the lateral nucleus does not contribute to the cerebello-tectal projection, it projects profusely to the pretectum contralaterally. The origin of the cerebello-pretectal projection lies in the parvicellular part of the lateral nucleus. Among several pretectal nuclei, the posterior pretectal, the medial pretectal nucleus and the reticular part of the anterior pretectal nucleus receive the cerebellar afferents.The findings of the differential projections from the cerebellum to the superior colliculus and the pretectum suggest that the cerebellum exerts a regulatory influence on visuo-motor and somato-motor transfer in these midbrain structures by differential circuits.     

Anatomy and physiology of intracellularly labelled omnipause neurons in the cat and squirrel monkey

Summary Saccadic omnipause neurons (OPNs) were intracellularly labelled with horseradish peroxidase (HRP) in alert cats and squirrel monkeys. The somas of OPNs were located on or near the midline in the caudal pons and their axons projected to regions of the pontomedullary reticular formation that contain the excitatory and inhibitory burst neurons.