Influence of superior colliculus on cat neck motoneurons

Effect of Neck Muscle Fatigue on Hand Muscle Motor Performance and Early Somatosensory Evoked Potentials

Even on pain free days, recurrent neck pain alters sensorimotor integration (SMI) measured via somatosensory evoked potentials (SEPs). Neck muscle fatigue decreases upper limb proprioception, and thus may interfere with upper limb motor task acquisition and SMI. This study aimed to determine the effect of cervical extensor muscle (CEM) fatigue on upper limb motor acquisition and retention; and SMI, measured via early SEPs. Twenty-four healthy right-handed individuals were randomly assigned to control or CEM fatigue. Baseline SEPs were elicited via median nerve stimulation at the wrist. Participants then lay prone on a padded table. The fatigue group supported a 2 kg weight until they could no longer maintain the position. The control group rested their neck in neutral for 5 min. Participants completed pre- and post-motor skill acquisition while seated, SEPs were again collected. Task retention was measured 24 h later. Accuracy improved post acquisition and at retention for both groups (p < 0.001), with controls outperforming the fatigue group (p < 0.05). The fatigue group had significantly greater increases in the N24 (p = 0.017) and N30 (p = 0.007) SEP peaks. CEM fatigue impaired upper limb motor learning outcomes in conjunction with differential changes in SEP peak amplitudes related to SMI.

Dissecting the Tectal Output Channels for Orienting and Defense Responses

Electrical stimulation and lesion experiments in 1980’s suggested that the crossed descending pathway from the deeper layers of superior colliculus (SCd) controls orienting responses, while the uncrossed pathway mediates defense-like behavior. To overcome the limitation of these classical studies and explicitly dissect the structure and function of these two pathways, we performed selective optogenetic activation of each pathway in male mice with channelrhodopsin 2 (ChR2) expression by Cre driver using double viral vector techniques. Brief photostimulation of the crossed pathway evoked short latency contraversive orienting-like head turns, while extended stimulation induced body turn responses. In contrast, stimulation of the uncrossed pathway induced short-latency upward head movements followed by longer-latency defense-like behaviors including retreat and flight. The novel discovery was that while the evoked orienting responses were stereotyped, the defense-like responses varied considerably depending on the environment, suggesting that uncrossed output can be influenced by top-down modification of the SC or its target areas. This further suggests that the connection of the SCd-defense system with non-motor, affective and cognitive structures. Tracing the whole axonal trajectories of these two pathways revealed existence of both ascending and descending branches targeting different areas in the thalamus, midbrain, pons, medulla, and/or spinal cord, including projections which could not be detected in the classical studies; the crossed pathway has some ipsilaterally descending collaterals and the uncrossed pathway has some contralaterally descending collaterals. Some of the connections might explain the context-dependent modulation of the defense-like responses. Thus, the classical views on the tectal output systems are updated.

Motor action of the frontal eye field on the eyes and neck in the monkey

Focal stimulation in the frontal eye field (FEF) evoked eye movements that were often accompanied by neck movements. Experiments were performed with concurrent recording of both movements in trained monkeys. We recorded neck forces under a head-restrained condition with a force-measuring system. With the system, we measured forces along the x-, y-, and z-axes and torque about the z-axis. Torque about the z-axis that represented yaw rotation of the head was significantly affected by stimulation. We found that stimulation generated two types of motor actions of the eyes and neck. In the first type, contraversive neck forces were evoked by stimulation of the medial part of the FEF, where contraversive saccadic eye movements with large amplitudes were evoked. When the stimulus intensity was increased, saccades were evoked in an all-or-none manner, whereas the amplitude of neck forces increased gradually. In the second type, contraversive neck forces were evoked by stimulation of the medial and caudal part of the FEF, where ipsiversive slow eye movements were evoked. The depth profiles of amplitudes of neck forces were almost parallel to those of eye movements in individual stimulation tracks. The present results suggest that the FEF is involved in the control of motor actions of the neck as well as the eyes. The FEF area associated with contraversive saccades and contraversive neck movements may contribute to a gaze shift process, whereas that associated with ipsiversive slow eye movements and contraversive neck movements may contribute to a visual stabilization process. NEW & NOTEWORTHY Focal stimulation in the frontal eye field (FEF) evoked eye and neck movements. We recorded neck forces under a head-restrained condition with a force-measuring system. Taking advantage of this approach, we could analyze slow eye movements that were dissociated from the vestibuloocular reflex. We found ipsiversive slow eye movements in combination with contraversive neck forces, suggesting that the FEF may be a source of a corollary discharge signal for compensatory eye movements during voluntary neck movements.

Multisensory response modulation in the superficial layers of the superior colliculus

The mammalian superior colliculus (SC) is made up of seven distinct layers. Based on overall differences in neuronal morphology, afferent and efferent projection patterns, physiological properties, and presumptive behavioral role, the upper three layers have been classically grouped together as the superficial layers and the remaining four layers collectively make up the deep layers. Although the superficial layers receive their primary inputs from the retina and primary visual cortex, the deep layers receive inputs from extrastriate visual cortical areas and from auditory, somatosensory, and motor-related structures. In contrast, there is no evidence of monosynaptic nonvisual inputs to the superficial layers. However, more recent studies have revealed anatomical connections between the superficial and deep layers, thus providing the substrate for possible communication between these two functional divisions of the SC. In this study, we provide physiological evidence for auditory influences on visual responses in the superficial layers of the SC. Using extracellular recordings of local field potentials (LFPs) and multiunit activity, we demonstrate multisensory effects in the superficial layers of the cat SC such that subthreshold auditory activity (as seen in the LFP) modulates visual responses (reflected in spiking activity) when the two stimuli are presented together. These results have important implications for our understanding of the functional organization of the SC and for the neural basis of multisensory integration in general.

Arm movements induced by electrical microstimulation in the superior colliculus of the macaque monkey

Neuronal activity in the deep layers of the macaque (Macaca mulatta) superior colliculus (SC) and the underlying reticular formation is correlated with the initiation and execution of arm movements (Werner, 1993). Although the correlation of this activity with EMGs of proximal arm muscles is as strong as in motor cortex (Werner et al., 1997a; Stuphorn et al., 1999), little is known about the influence of electrical microstimulation in the SC on the initiation and trajectories of arm movements. Our experiments on three macaque monkeys clearly show that arm movements can be elicited by electrical microstimulation in the deep layers of the lateral SC and underlying reticular formation. The most extensively trained monkey, M1, extended his arm toward the screen in front of him more or less stereotypically upon electrical SC stimulation. In two other monkeys, M2 and M3, a larger repertoire of arm movements were elicited, categorized into three movement types, and compared before (M3) and after (M2 and M3) training: twitch (56% vs. 62%), lift (6% vs. 5%), and extend (37% vs. 32%), respectively. Therefore, arm movements induced by electrical stimulation in the monkey SC represent a further component of the functional repertoire of the SC using its impact on motoneurons in the spinal cord, probably via premotor neurons in the brainstem, as well as on structures involved in executing more complex movements such as target-directed reaching. Therefore, the macaque SC could be involved directly in the initiation, execution, and amendment of arm and hand movements.

Connections of the superior colliculus to shoulder muscles of the rat: a dual tracing study

Previous investigations indicate that the superior colliculus (SC) is involved in the initiation and execution of forelimb movements. In the present study we investigated the tectofugal, in particular the tecto-reticulo-spinal projections to the shoulder and arm muscles in the rat. We simultaneously retrogradely labeled the premotor neurons in the brainstem by injection of the pseudorabies virus PrV Bartha 614 into the m. rhomboideus minor and m. acromiodeltoideus, and anterogradely visualized the tectofugal projections by intracollicular injection of the tracer FITC dextrane. Our results demonstrate that the connection of the SC to the skeletal muscles of the forelimb is at least trisynaptic. This was confirmed by long survival times after virus injections into the muscles (98-101 h) after which numerous neurons in the deep layers of the SC were labeled. Transsynaptically retrogradely labeled brainstem neurons connected disynaptically to the injected muscles with adjacent tectal terminals were predominantly located in the gigantocellular nuclear complex of the reticular formation. In addition, putative relay neurons were found in the caudal part of the pontine reticular nucleus. Both tectal projections to the nucleus gigantocellularis and the pontine reticular nucleus were bilateral but ipsilaterally biased. We suggest this projection to be involved in more global functions in motivated behavior like general arousal allowing fast voluntary motor activity.

The mesencephalic reticular formation as a conduit for primate collicular gaze control: tectal inputs to neurons targeting the spinal cord and medulla

The superior colliculus (SC), which directs orienting movements of both the eyes and head, is reciprocally connected to the mesencephalic reticular formation (MRF), suggesting the latter is involved in gaze control. The MRF has been provisionally subdivided to include a rostral portion, which subserves vertical gaze, and a caudal portion, which subserves horizontal gaze. Both regions contain cells projecting downstream that may provide a conduit for tectal signals targeting the gaze control centers which direct head movements. We determined the distribution of cells targeting the cervical spinal cord and rostral medullary reticular formation (MdRF), and investigated whether these MRF neurons receive input from the SC by the use of dual tracer techniques in Macaca fascicularis monkeys. Either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin was injected into the SC. Wheat germ agglutinin conjugated horseradish peroxidase was placed into the ipsilateral cervical spinal cord or medial MdRF to retrogradely label MRF neurons. A small number of medially located cells in the rostral and caudal MRF were labeled following spinal cord injections, and greater numbers were labeled in the same region following MdRF injections. In both cases, anterogradely labeled tectoreticular terminals were observed in close association with retrogradely labeled neurons. These close associations between tectoreticular terminals and neurons with descending projections suggest the presence of a trans-MRF pathway that provides a conduit for tectal control over head orienting movements. The medial location of these reticulospinal and reticuloreticular neurons suggests this MRF region may be specialized for head movement control.

The laminar distribution of macaque tectobulbar and tectospinal neurons

The superior colliculus exerts its most direct influence over orienting movements, and saccades in particular, via its descending projections to the brain stem and spinal cord. However, while there is detailed physiological data concerning the generation of saccade-related activity in the primate superior colliculus, there is relatively little data on the detailed connectivity of this structure in primates. Consequently, retrograde transport techniques were utilized to determine the locations of the cells of origin of these descending pathways in macaque monkeys. Tectal cells that projected to the ipsilateral pontine reticular formation were mainly found in the deep gray layer and occasionally in the intermediate gray layer. Tectal cells that projected to the contralateral pontine reticular formation were predominantly located in the intermediate gray layer. The contralaterally projecting population could be subdivided into two groups. The cells in upper sublamina of the intermediate gray layer project primarily to the saccade-related regions of the paramedian reticular formation. Cells in the lower sublamina project primarily to more lateral regions of the pontine reticular formation and to the spinal cord. We conclude that the primate colliculus is provided with at least three descending output channels, which are likely to differ in their connections and functions. Specifically, it seems likely that the lower portion of the intermediate gray layer may be specialized to subserve combined head and eye orienting movements, while the upper sublamina subserves saccades.

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: cortex, diencephalon, midbrain

The caudal forelimb region of right "motor" cortex was repetitively stimulated in normal, conscious rats. Left forelimb movements were produced and (14C) 2-deoxyglucose (2DG) was injected. After sacrifice, regions of increased brain (14C) 2DG uptake were mapped autoradiographically. Uptake of 2DG increased about the stimulating electrode in motor (MI) cortex. Columnar activation of primary (SI) and second (SII) somatosensory neocortex occurred. The rostral or second forelimb (MII) region of motor cortex was activated. Many ipsilateral subcortical structures were also activated during forelimb MI stimulation (FLMIS). Rostral dorsolateral caudate-putamen (CP), central globus pallidus (GP), posterior entopeduncular nucleus (EPN), subthalamic nucleus (STN), zona incerta (ZI), and caudal, ventrolateral substantia nigra pars reticulata (SNr) were activated. Thalamic nuclei that increased (14C) 2DG uptake included anterior dorsolateral reticular (R), ventral and central ventrolateral (VL), lateral ventromedial (VM), ventral ventrobasal (VB), dorsolateral posteromedial (POm), and the parafascicular-centre median (Pf-CM) complex. Activated midbrain regions included ventromedial magnocellular red nucleus (RNm), posterior deep layers of the superior colliculus (SCsgp), lateral deep mesencephalic nucleus (DMN), nucleus tegmenti pedunculopontinus (NTPP), and anterior pretectal nucleus (NCU). Monosynaptic connections from MI or SI to SII, MII, CP, STN, ZI, R, VL, VM, VB, POm, Pf-CM, RNm, SCsgp, SNr, and DMN can account for ipsilateral activation of these structures. GP and EPN must be activated polysynaptically, either from MI stimulation or sensory feedback, since there are no known monosynaptic connections from MI and SI to these structures. Most rat brain motor-sensory structures are somatotopically organized. However, the same regions of R, EPN, CM-Pf, DMN, and ZI are activated during FLMIS compared to VMIS (vibrissae MI stimulation). Since these structures are not somatopically organized, this suggests they are involved in motor-sensory processing independent of which body part is moving. VB, SII, and MII are activated during FLMIS but not during VMIS.

Priming of Head Premotor Circuits During Oculomotor Preparation

Large, rapid gaze shifts necessitate intricate coordination of the eyes and head. Brief high-frequency bursts of activity within the intermediate and deeper layers of the superior colliculus (dSC) encode desired gaze shifts regardless of component movements of the eyes and head. However, it remains unclear whether low-frequency activity emitted by oculomotor neurons within the dSC and elsewhere has any role in eye-head gaze shifts. Here we test the hypothesis that such low-frequency activity contributes to eye-head coordination by selectively priming head premotor circuits. We exploited the capacity for short-duration (10 ms, 4 pulses) dSC stimulation to evoke neck muscle responses without compromising ocular stability, stimulating at various intervals of a “gap-saccade” task. Low-frequency neural activity in many oculomotor areas (including the dSC) is known to increase during the progression of the gap-saccade task. Stimulation was passed during either a fixation-interval while a central fixation point was illuminated, a 200-ms gap-interval between fixation point offset and target onset, or a movement-interval following target onset. In the two monkeys studied, the amplitude of evoked responses on multiple neck muscles tracked the known increases in low-frequency oculomotor activity during the gap-saccade task, being greater following stimulation passed at the end of the gap- versus the fixation-interval, and greater still when the location of stimulation during the movement interval coincided with the area of the dSC generating the ensuing saccade. In one of these monkeys, we obtained a more detailed timeline of how these results co-varied with low-frequency oculomotor activity by stimulating, across multiple trials, at different times within the fixation-, gap- and movement-intervals. Importantly, in both monkeys, baseline levels of neck EMG taken immediately prior to stimulation onset did not co-vary with the known pattern of low-frequency oculomotor activity up until the arrival of a transient burst associated with visual target onset. These baseline results demonstrate that any priming of the head premotor circuits occurs without affecting the output of neck muscle motoneurons, We conclude that low-frequency oculomotor activity primes head premotor circuits well in advance of gaze shift initiation, and in a manner distinct from its effects on the eye premotor circuits. Such distinctions presumably aid the temporal coordination of the eyes and head despite fundamentally different biomechanics.

Long descending motor tract axons and their control of neck and axial muscles

It has been tacitly assumed that a long descending motor tract axon consists of a private line connecting the cell of origin to a single muscle, as a motoneuron innervates a single muscle. However, this notion of a long descending motor tract referred to as a private line is no longer tenable, since recent studies have showed that axons of all major long descending motor tracts send their axon collaterals to multiple spinal segments, suggesting that they may exert simultaneous influences on different groups of spinal interneurons and motoneurons of multiple muscles. The long descending motor systems are divided into two groups, the medial and the lateral systems including interneurons and motoneurons. In this chapter, we focus mainly on the medial system (vestibulospinal, reticulospinal and tectospinal systems) in relation to movement control of the neck, describe the intraspinal morphologies of single long descending motor tract axons that are stained with intracellular injection of horseradish peroxidase, and provide evidence that single long motor-tract neurons are implicated in the neural implementation of functional synergies for head movements.

Neck muscle responses to stimulation of monkey superior colliculus. I. Topography and manipulation of stimulation parameters

The role of the primate superior colliculus (SC) in orienting head movements was studied by recording electromyographic (EMG) activity from multiple neck muscles following electrical stimulation of the SC. Combining SC stimulation with neck EMG recordings provides an objective and sensitive measure of the SC drive onto neck muscle motoneurons, particularly in relation to evoked gaze shifts. In this paper, we address how neck EMG responses to SC stimulation in head-restrained monkeys depend on the rostrocaudal, mediolateral, and dorsoventral location of the stimulating electrode within the SC and vary with manipulations of the eye position prior to stimulation onset and changes in stimulation current and duration. Stimulation predominantly evoked EMG responses on the muscles obliquus capitis inferior, rectus capitis posterior major, and splenius capitis. These responses became larger in magnitude and shorter in onset latency for progressively more caudal stimulation locations, consistent with turning the head. However, evoked responses persisted even for more rostral stimulation locations usually not associated with head movements. Manipulating initial eye position revealed that the magnitude of evoked responses became stronger as the eyes attained positions contralateral to the side of stimulation, consistent with a summation between a generic command evoked by SC stimulation and the influence of eye position on tonic neck EMG. Manipulating stimulation current and duration revealed that the relationship between gaze shifts and evoked EMG responses is not obligatory: short-duration (<20 ms) or low-current stimulation evoked neck EMG responses in the absence of gaze shifts. However, long-duration stimulation (>150 ms) occasionally revealed a transient neck EMG response aligned on the onset of sequential gaze shifts. We conclude that the SC drive to neck muscle motoneurons is far more widespread than traditionally supposed and is relayed through intervening elements which may or may not be activated in association with gaze shifts.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

Effects of lesion of pontomedullary reticular formation on visually triggered vertical and oblique head orienting movements in alert cats

The role of the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG) in control of vertical and oblique head orienting movements was investigated in alert cats by lesion of these nuclei with kainic acid. Cats were trained to orient the head vertically or obliquely to various targets. Following unilateral lesion of these nuclei, vertical orienting could be performed correctly with a slight decrease in velocity, while oblique orienting tended to exhibit zigzag course because of severe impairment of horizontal orienting. The horizontal and vertical components became coordinated in the course of experiments due to a significant decrease in vertical component velocity, resulting in smooth oblique trajectories. Results suggest that horizontal and vertical components of head orienting are controlled separately, but impairment of horizontal component causes adaptive change of vertical component velocity in oblique orienting.

Axonal trajectories of the nucleus reticularis gigantocellularis neurons in the C2-C3 segments in cats

Axonal trajectories in the C2-C3 segments of the nucleus reticularis gigantocellularis neurons projecting to the lower cervical cord (C-RSNs) and excited monosynaptically from cortico- and tectofugal fibers were studied by mapping thresholds of antidromic excitation and intra-axonal staining in cats. The C-RSNs descended in various sites of the spinal funiculi, and the projection area of individual cells varied with the funicular location of the stem axon. C-RSNs descending in the ventrolateral funiculus (inRSNs) projected mainly to lamina VIII-IX, those descending in the lateral funiculus (IRSNs) mainly to laminae VI-VIII, and those descending in the contralateral funiculus (coRSNs) chiefly to laminae VIII-IX on that side. It is suggested that inRSNs and coRSNs mediate disynaptic effects from cortico- and tectofugal fibers to dorsal neck motoneurons bilaterally.

Activity of cells in the deeper layers of the superior colliculus of the rhesus monkey: evidence for a gaze displacement command

When the head is free to move, microstimulation of the primate superior colliculus (SC) evokes coordinated movements of the eyes and head. The similarity between these stimulation-induced movements and visually guided movements indicates that the SC of the primate is involved in redirecting the line of sight (gaze). To determine how movement commands are represented by individual collicular neurons, we recorded the activity of single cells in the deeper layers of the superior colliculus of the rhesus monkey during coordinated eye-head gaze shifts. Two alternative hypotheses were tested. The "separate channel" hypothesis states that two displacement commands are generated by the SC: one signal specifying the amplitude and direction of eye movements and a second signal specifying the amplitude and direction of head movements. Alternatively, a single gaze displacement command could be generated by the SC ("gaze displacement" hypothesis). The activity of collicular neurons was examined during three behavioral dissociations of gaze, eye, and head movement amplitude and direction (metrics). Subsets of trials were selected in which the amplitude and direction of either gaze shifts or eye movements or head movements were relatively constant but the metrics of the other two varied over wide ranges. Under these conditions, the separate channel and gaze displacement hypotheses make differential predictions about the patterns of SC activity. We tested these differential predictions by comparing observed patterns with predicted patterns of neuronal activity. We obtained data consistent with the predictions of the gaze displacement hypothesis. The predictions of the separate channel hypothesis were not confirmed. Thus microstimulation data, single-unit recording data, and behavioral data are all consistent with the gaze displacement hypothesis of collicular function--the hypothesis that a gaze displacement signal is derived from the locus of activity within the motor map of the SC and subsequently is decomposed into separate eye and head displacement signals downstream from the colliculus.

Axonal branching and termination of cervical reticulospinal neurons in the cat brachial segments

Axonal branching patterns in the brachial segments of cervical reticulospinal neurons (C-RSNs) were examined in cats using intraaxonal injection of horseradish peroxidase (HRP). Axons of these neurons were electrophysiologically identified by their projection to the lower cervical but not to the lumbar segments and monosynaptic activation after tectal and pyramidal stimulation. Six axons were stained up to terminals. Their stem axons descended in the ventral funiculus near the boundary of the spinal gray. The majority of collateral axons crossed lamina VIII and distributed terminals in the whole area of lamina VIII, the middle part of lamina VII, the lateral 2/3 of lamina VI, the ventral part of lamina V, and sparsely in the limb motor nuclei. Their possible functional role in head orienting movements is discussed.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Morphology of single axons of tectospinal neurons in the upper cervical spinal cord

Morphology of single axons of tectospinal (TS) neurons was investigated by intraaxonal injection of horseradish peroxidase (HRP) at the upper cervical spinal cord of the cat. TS axons were electrophysiologically identified by their direct responses to stimulation of the contralateral superior colliculus (SC). None of these axons responded to thoracic stimulation at Th2. Three-dimensional reconstructions of the axonal trajectories were made from 20 well-stained TS axons at C1-C3. Cell bodies of these axons were located in the intermediate or deep layers of the caudal two-thirds of the SC. Usually, TS axons had multiple axon collaterals, and up to seven collaterals were given off per stem axon [2.7 +/- 1.6 (mean +/- S.D.); n = 20]. Collaterals had simple structures and ramified a few times mainly in the transverse plane. The number of terminals for each collateral was small. These collaterals terminated in the lateral parts of laminae V-IX, mainly in laminae VI, VII, and VIII. There were usually gaps free from terminal arborizations between adjacent collaterals, because the rostrocaudal spread of each collateral (mean = 700 microns) was narrower than the intercollateral interval (mean = 2,500 microns). Seven of the 19 TS axons had terminals in the lateral parts of laminae V-VIII, with little projection to lamina IX, and the other 12 axons had terminals in lamina IX besides the projection to the lateral parts of laminae V-VIII. Axon terminals in lamina IX did not appear to make contacts with the somata or proximal dendrites of retrogradely labeled motoneurons, but contacts were found with the somata of counterstained interneurons in the lateral parts of laminae V-VIII. Three spinal interneurons (two in lamina VIII and one in lamina V at C1) that received monosynaptic excitation from the SC were stained, and their axonal trajectories were reconstructed. They had multiple axon collaterals at C1-C2 and mainly projected to laminae VIII and IX, with smaller projections to lamina VII. Many axon terminals of the interneurons were found in multiple neck motor nuclei, where some of them made contacts with retrogradely labeled motoneurons. The present finding provides evidence that the direct TS projection to the spinal cord may influence activities of multiple neck muscles, mainly via spinal interneurons, and may play an important role in control of head movement in parallel with the tectoreticulospinal system.

Morphology of single axons of tectospinal and reticulospinal neurons in the upper cervical spinal cord

Single axons of tectospinal (TS) and reticulospinal (RS) neurons were stained with intraaxonal injection of HRP after electrophysiological identification, and their axonal trajectory was reconstructed at C1-C3 of the cat. TS neurons were located in the intermediate or deep layers of the caudal two-thirds of the superior colliculus (SC) and had multiple axon collaterals (up to seven collaterals) per stem axon). Collaterals had a simple structure, ramified several times mainly in the transverse plane, and terminated in the lateral parts of laminae V-VIII. More than half also had terminals in lamina IX. Terminals of TS neurons did not appear to make contacts with either the somas or proximal dendrites of retrogradely-labeled motoneurons in lamina IX, but clear contacts were found on counterstained interneurons in the lateral part of laminae V-VIII. Here, we examined three stained spinal interneurons receiving monosynaptic excitation from the SC. These interneurons had multiple axon collaterals mainly in laminae VII-IX, and made extensive contacts with retrogradely-labeled motoneurons of multiple neck muscles. Stem axons of single RS neurons receiving input from the contralateral SC ran in the ventromedial funiculus and gave off multiple axon collaterals to laminae VII-IX over at least several cervical segments. Their terminal boutons appeared to make contact with both the somas and proximal dendrites of retrogradely-labeled neck motoneurons. Single RS neurons made contacts with motoneurons of different neck muscles. These results provide evidence for functional synergies at the level of single RS neurons and spinal interneurons for neck movements. The present finding indicates that the direct TS projection to the spinal cord may influence the activity of multiple neck muscles mainly via spinal interneurons, and plays an important role in control of head movement in parallel with the tecto-reticulospinal system.

Post-spike facilitation of neck EMG by cat tectoreticulospinal neurones during orienting movements

1. The activity of fourteen tectoreticulospinal neurones (TRSNs) was recorded intraaxonally in the caudal pons of alert cats during orienting movements towards visual stimuli. TRSN spikes were used to compute the spike-triggered average (STA) of rectified EMG of dorsal neck muscles. 2. Eight TRSNs for which 400-2532 spikes were available were analysed with the STA technique. When the STA was computed from all spikes, significant post-spike facilitation (PSF) was obtained for six of eighteen cell-muscle pairs investigated (5 TRSNs). The mean relative amplitude of PSFs was 7.4% (S.D. 3.7). The onset latencies ranged from 1.1 to 5.0 ms and mean duration was 11.4 +/- 3.1 ms (mean +/- S.D.). 3. Interspike interval distributions were unimodal, with modes between 2.7 and 12.7 ms. Spike trains of TRSNs that produced significant PSFs contained 5-13% of the interspike intervals < or = 5 ms and 22-37% of the intervals < or = 10 ms. To evaluate the contribution of short intervals to PSF, STAs were computed separately for spikes preceded by 'short' (< or = 5 or < or = 10 ms) and 'long' (> 5 or > 10 ms) intervals. 4. When computed from spikes preceded by 'long' intervals, PSF amplitudes were small (mean +/- S.D., 5.3 +/- 2.7%) and onset latencies measured by cusum ranged between 2.4 and 5.4 ms. This is longer than the estimated minimal latency of monosynaptic facilitatory effect on neck EMG (1.9-2.1 ms). 5. Relative amplitudes of PSF obtained with spikes preceded by 'short' intervals were much larger (mean +/- S.D., 14.8 +/- 7.4%), but cusums indicated negative latencies for four of six PSFs. The unrealistically short onset latencies could be accounted for by the summation of facilitation from the trigger spike with that of the preceding spikes. In four of five TRSNs a large increase of PSF amplitude (from 3.2 to 7.2 times the amplitude obtained from 'long' intervals) suggests the presence of frequency-dependent potentiation of synaptic transmission. 6. This study unequivocally demonstrates that some TRSNs produce significant post-spike facilitation of neck motoneurones. This facilitation could be mediated by monosynaptic tectomotoneuronal connections although a contribution by disynaptic connections cannot be definitively ruled out. The high instantaneous firing rates of TRSNs produce a potentiation of the otherwise weak facilitatory action of TRSNs that presumably contributes to a rapid recruitment of motoneurones during initiation of head orienting movements.

Innervation of multiple neck motor nuclei by single reticulospinal tract axons receiving tectal input in the upper cervical spinal cord

Axons of reticulospinal neurons (RSNs) activated monosynaptically by stimulation of the contralateral superior colliculus (SC) were stained with intraaxonal injection of horseradish peroxidase in the cat upper cervical spinal cord. Stem axons of single RSNs gave rise to multiple axon collaterals to laminae IX, VIII and VII over a few cervical segments. Single RSNs made contacts with retrogradely labeled neck motoneurons of different neck muscles. Therefore, RSNs were regarded as mediating output of the SC to functionally different groups of neck muscles simultaneously. The result gave evidence of neural implementation of a functional synergy for a neck movement at a single neuron level.

Tracing premotor brain stem networks of orienting movements

Methods allowing a direct matching of movement-related firing patterns and connectivity of individual neurons have been used in the analysis of premotor networks controlling orienting movements. Advances have been made in the description of coding properties of orienting-related tectal output neurons, as well as in specifying their distributed connections in the brain stem and possible modes of coupling to saccadic pattern generators in the reticular formation. New data on the properties of signals and connectivity patterns have also been obtained for the tecto-recipient reticulo-spinal neurons. At least a small portion of the network performing the spatio-temporal transformations of orienting-related tectal efferent signals can now be described both in functional and in morphological terms.

Retinal projections to the pretectum, accessory optic system and superior colliculus in pigmented and albino ferrets

Retinal projections to the pretectal nuclei, accessory optic system and superior colliculus in pigmented and albino ferrets were studied using anterograde tracing techniques. Both Nissl- and myelin-stained material was used to identify the pretectal nuclei, nuclei of the accessory optic system and the layers of the superior colliculus. Following monocular injection of either horseradish peroxidase or rhodamine-B-isothiocyanate, four pretectal nuclei, including the nucleus of the optic tract, posterior pretectal nucleus, anterior pretectal nucleus and the olivary pretectal nucleus, could be identified to receive direct retinal input in both pigmented and albino strains. In the accessory optic system, retinal terminals were observed in the dorsal, lateral and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. The retinal projection to the superior colliculus was found to innervate the three superficial layers. The retinal projections to the pretectal nuclei and nuclei of the accessory optic system in the pigmented animals were bilateral, although the label was most dense contralateral to the injected eye. Ipsilateral retinal projections to the pretectal nuclei and nuclei of the accessory optic system appeared to be absent in albino ferrets, i.e. they were invisible with our methods. In both pigmented and albino ferrets retinal terminals in the contralateral superior colliculus densely innervated the three superficial layers. In both strains the ipsilateral projection appeared as clusters which were absent in rostral and caudal poles. In pigmented animals the ipsilateral projection was much denser and more extensive than in albinos. Following injection of retrograde tracers into the brainstem at the level of the dorsal cap of the inferior olive, retrogradely labelled neurons in the pretectum were found in the ipsilateral nucleus of the optic tract. Their somata overlapped mainly with scattered retinal terminals close to the pretectal surface and rarely or not all with the deeper prominent terminal clusters. In the accessory optic system, inferior olive projecting neurons were observed in all four ipsilateral nuclei and fully coincided with the retino-recipient zones. In the superior colliculus, retrogradely labelled neurons were found contralateral to the injection site in the deep layers.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: facilitatory interactions

Facilitatory interactions between disynaptic EPSPs evoked from the contralateral tectum, ipsilateral tegmentum and contra- and/or ipsilateral pyramid have been investigated in dorsal neck motoneurones of the cat. Monosynaptic convergence on common intercalated neurones was found from ipsi- and contralateral pyramidal, contralateral tectal and ipsilateral tegmental fibres. In addition, disynaptic facilitation was observed from ipsilateral pyramidal fibres on disynaptic contralateral pyramidal EPSPs. Transection of cortico-fugal fibres in the pyramid showed that the location of the interactions occurred in the lower brain stem, suggesting that reticulospinal neurones are mediating the effects.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: brain stem relay

The location of intercalated neurones mediating disynaptic excitation from tectum, tegmentum and pyramids to dorsal neck motoneurones has been investigated by: (a) recording field potentials in the lower brain stem evoked from the above systems, (b) systematic stimulation in the brain stem during intracellular recording from motoneurones innervating the splenius, biventer cervicis and complexus muscles, and (c) comparing the effects of lesions of the brain stem with kainic acid on the disynaptic EPSPs elicited from the above three systems. Electrical stimulation of the contralateral superior colliculus evoked monosynaptic field potentials which were largest in the caudal pontine reticular formation rostral to the abducens nucleus and in the rostral part of the medullary reticular formation caudal to the abducens nucleus. Likewise, stimulation of the ipsilateral tegmentum (the cuneiform and subcuneiform nucleus) evoked field potentials which were large in the caudal medulla and small in the pons. In contrast, stimulation of the contralateral tegmentum was ineffective in evoking field potentials. Stimulation of the pyramid 2-3 mm rostral to the obex elicited monosynaptic field potentials in the reticular formation of the lower brain stem that were only about 25% of those from the superior colliculus. In contrast to the field potentials from the superior colliculus, the pyramidal ones were large in the medulla and small in the pons. Lesions of the reticular formation in the lower brain stem by unilateral kainic acid injection caused disappearance of disynaptic EPSPs in motoneurones from the above three systems. These results strongly suggest that the intercalated neurones mediating pyramidal, tectal and tegmental EPSPs are reticulospinal neurones in the lower brain stem. Systematic stimulation in various locations of the lower brain stem showed that monosynaptic EPSPs were evoked from the regions of the reticular formation which received projection from the above three descending systems. The effective regions for evoking the EPSPs in splenius (SPL) were located somewhat more dorsally than for biventer cervicis and complexus (BCC) motoneurones. The descending axons of presumed reticulospinal neurones were stimulated with electrodes placed in medial, middle and lateral positions at the spinomedullary junction. Monosynaptic EPSPs in SPL and BCC motoneurones were evoked from the medial and middle electrodes but not from the lateral electrode.

Tectal and tegmental excitation in dorsal neck motoneurones of the cat

1. Intracellular recordings were made from 116 splenius (SPL) and 103 biventer cervicis and complexus (BCC) alpha-motoneurones in nineteen cats anaesthetized with alpha-chloralose. 2. Electrical stimulation in the contralateral tectum evoked disynaptic excitatory postsynaptic potentials (EPSPs) in the motoneurones when a train of stimuli was applied in the ventral layers throughout the superior colliculus. In the rostral half of the superior colliculus, these EPSPs were due to stimulation of ascending collaterals of tectofugal neurones. EPSPs of a presumed trisynaptic linkage could only be evoked from the dorsal and intermediate tectal layers in the caudal half of the superior colliculus. It is concluded that the tectofugal neurones which evoked the disynaptic EPSPs are mainly located in the caudal half of the superior colliculus. 3. Disynaptic EPSPs were evoked in the motoneurones by a train of stimuli in the contralateral fields of Forel and Zona incerta, which were due to stimulation of ascending collaterals from the tectofugal neurones. 4. Spatial facilitation experiments revealed that tectal disynaptic EPSPs in the neck motoneurones were mediated via reticulospinal neurones with convergent input from cortico-reticular neurones. 5. A train of stimuli in the ipsilateral tectum evoked EPSPs with latencies compatible with a trisynaptic linkage, while disynaptic EPSPs at low threshold could be elicited from the underlying tegmentum. Similar disynaptic EPSPs could be evoked from the ipsilateral fields of Forel. It is suggested that some of the disynaptic tegmental EPSPs in SPL and BCC motoneurones can be mediated via a tegmento-reticulospinal pathway which originates in the cuneiform nucleus.

Dorsal mesencephalic projections to pons, medulla, and spinal cord in the cat: limbic and non-limbic components

The vertebrate dorsal mesencephalon consists of the superior colliculus, the dorsal portion of the periaqueductal gray, and the mesencephalic trigeminal neurons in between. These structures, via their descending pathways, take part in various behavioral responses to environmental stimuli. This study was undertaken to compare the origins and trajectories of these pathways in the cat. Injections of horseradish peroxidase into the cervical spinal cord and upper medullary medial tegmentum retrogradely labeled cells mainly in the contralateral intermediate and deep superior colliculus, and in the ipsilateral dorsal and lateral periaqueductal gray and adjacent tegmentum. Only injections in the medullary lateral tegmental field labeled mesencephalic trigeminal neurons ipsilaterally. Autoradiographic tracing results, based on injections across the dorsal mesencephalon, revealed three efferent fiberstreams. A massive first fiberstream (limbic pathway), consisting of thin fibers, descended ipsilaterally from the dorsal and lateral periaqueductal gray and adjacent superior colliculus through the mesencephalic and pontine lateral tegmentum, terminating in these areas as well as in the ventral third of the caudal pontine and medullary medial tegmentum. A few fibers from the dorsal periaqueductal gray matter (PAG) were distributed bilaterally to the dorsal vagal, solitary, and retroambiguus nuclei. The second fiberstream (the predorsal bundle) descended contralaterally from the superior colliculus (SC) and consisted of both thick and thin labeled fibers. The thin fibers terminated bilaterally in the dorsomedial nucleus reticularis tegmenti pontis and the medial half of the caudal medial accessory inferior olive. The thick fibers targeted the contralateral dorsal two thirds of the caudal pontine and medullary medial tegmental fields, and the facial, abducens, lateral reticular, subtrigeminal, and prepositus hypoglossi nuclei. A few fibers recrossed the midline to terminate in the ipsilateral medial tegmentum. Caudal to the obex, fibers terminated laterally in the tegmentum and upper cervical intermediate zone. From the lateral SC, fibers terminated bilaterally in the lateral tegmental fields of the pons and medulla and lateral facial subnuclei. The third fiberstream (mesencephalic trigeminal or Probst tract) terminated in the supratrigeminal and motor trigeminal nuclei, and laterally in the tegmentum and upper cervical intermediate zone. In summary, neurons in the PAG and in the deep layers of the SC give rise to a massive ipsilateral descending pathway, in which a medial-to-lateral organization exists. A similar topographical pattern occurs in the crossed SC projections. The possibility that these completely different descending systems cooperate in producing specific defensive behaviors is discussed.

Effects of d-amphetamine on arousal

This article is a review of studies conducted on the effects of amphetamine on behavior in animals by the late Walter Isaac and his associates. The principle findings and hypotheses developed over the 20 or more years of research are outlined and discussed. d-Amphetamine is thought to affect an organism's behavior by influencing the arousal level, mainly via the visual system.

Projections of the tectospinal tract to the upper cervical spinal cord of the cat: a study with the anterograde tracer PHA-L

The goal of the present experiments was to re-examine the spinal projections of neurons in the superior colliculus (SC) of the cat by taking advantage of the high sensitivity of the anterograde tracer, phaseolus vulgaris leucoagglutinin (PHA-L). In seven experiments, multiple injections of PHA-L into different regions of the SC labelled a total of 172 axons in the predorsal bundle; yet only 11 tectospinal tract (TST) axons were found in the upper cervical spinal cord. Collaterals emerging from these axons were rare and arose exclusively from TST axons with a diameter of less than 1 micron. Individual collaterals had different termination zones: some terminated in the lateral part of lamina V and VI after taking a dorsolateral course through lamina VII and VIII; others terminated in the medial part of lamina VII. One collateral terminated within lamina IX and the ventral part of lamina VIII. The combined termination of all collaterals was densest in lamina VII and dorsal lamina VIII. A small number of boutons were also found in the lateral parts of laminae V and VI, and in lamina IX and immediately adjacent regions in lamina VIII. Compared to axons belonging to other spinal descending systems, individual TST axons give rise to much simpler intraspinal collaterals with relatively few boutons. This feature, together with the relative paucity of TST axons, suggests that direct connections from the SC to neurons in the upper cervical spinal cord are sparse. Furthermore, our results are consistent with electrophysiological studies that show that few, if any, neck motoneurons receive monosynaptic connections from TST neurons. Projections to neck motoneurons must therefore involve a relay, either through other descending pathways, such as the reticulospinal system, or via local segmental interneurons.

The effects of d-amphetamine on the electrophysiological activity of the superior colliculus in the rat

The superior colliculus (SC) is thought to have an inhibitory effect on arousal (and hence activity) in rats when light is present. d-Amphetamine is believed to suppress this inhibition of the SC on arousal. The present study investigated the electrophysiological activity of the SC of the rat under the influence of d-amphetamine in both light and dark conditions. A single numerical arousal index value (AI) for the electrophysiological data obtained by dividing the frequency of the waves by the voltage over a specified interval was used. Results showed that light conditions decreased the AI of the SC. The AI was, however, not affected by d-amphetamine. Also, the electrophysiological activity of the SC showed spontaneous emissions of high-voltage (600 microV) alpha waves which generally occurred in two- to four-second bursts. Occurrence of this high-voltage activity was reduced by increasing doses of d-amphetamine.

Tectospinal neurons in hamster contain glutamate-like immunoreactivity

Immunocytochemistry with a monoclonal antibody directed against glutamate revealed numerous immunoreactive cells in the hamster's superior colliculus (SC). A large number of these neurons were located in the deep layers and many were in the stratum album intermedium (SAI). These neurons appeared similar to the large multipolar cells that have been shown to project to the cervical spinal cord in this species. The combination of retrograde tracing using either Fluoro-Gold- or FITC-labelled latex beads with immunocytochemistry for glutamate revealed that many of the immunoreactive cells did, in fact, project into the predorsal bundle and extend axons as far as the cervical spinal cord.

Input-output organization of reticulospinal neurones, with special reference to connexions with dorsal neck motoneurones in the cat

Dorsal neck motoneurones receive disynaptic tectal and pyramidal EPSPs via common reticulospinal neurones (RSNs). This study was aimed at identification of the RSNs projecting directly to neck motoneurones and mediating these EPSPs. 1. Stimulation of the tectum and the cerebral peduncle evoked monosynaptic descending volleys in the spinal cord, which were chiefly mediated by reticulospinal neurones in the pons and the medulla. Systematic tracking of the C3 and C7 segments was made to locate descending volleys in the spinal funiculi. The tectal monosynaptic volley was largest in the medial part of the ventral funiculus and decreased gradually as the recording electrode was moved to the lateral part of the ventral funiculus and the lateral funiculus. In contrast, the peduncle-evoked monosynaptic volley was distributed rather evenly in the ventral funiculus and the ventral half of the lateral funiculus. 2. Differences in funicular distribution of the two descending volleys suggest the existence of subgroups of RSNs which differed in strength of inputs from the two descending fibre systems and in the funicular location of descending axons. 3. The RSNs were classified into the following four groups; (1) mRSNs which descended in the medial part of the ventral funiculus, (2) in RSNs which descended in the ventrolateral funiculus, (3) 1RSNs which descended in the dorsal 2/3 of the lateral funiculus and (4) coRSNs which descended in the contralateral funiculi. The mRSNs were located in a fairly localized region corresponding to the nucleus reticularis pontis caudalis (N.r.p.c), while inRSNs, 1RSNs and coRSNs were mainly in the nucleus reticularis gigantocellularis (N.r.g.), in the nucleus reticularis magnocellularis (N.r.m.) and in the nucleus reticularis ventralis (N.r.v.). RSNs were further divided into three types depending on the levels of projection. L-RSNs projected to the lumbar spinal segments. C-RSNs descended to the C6-C7 spinal segment but not to the lumbar segments. N-RSNs projected to the C3 but not to the C6-C7 segments. 4. Stimulation of the tectum and the cerebral peduncle produced monosynaptic negative field potentials in the medial two thirds of the reticular formation in the pons and medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Head and body movements evoked electrically from the caudal superior colliculus of rats: pulse frequency effects

The effects of pulse frequency and current intensity on circling elicited from the caudal superior colliculus (SC) of rats were studied. The displacement of the head with respect to the body were measured for different levels of frequency (20, 29, and 50 Hz) and current (200 or 500 microA) at a pulse duration of 0.1 ms. The rate of circling increased monotonically with frequency and current. The rate at which the head was displaced laterally varied as a function of frequency. It is postulated that lateral head and body movements are affected by the firing frequency of SC output neurons.

Involvement of nigrotecto-reticulospinal pathways in the iminodipropionitrile (IDPN) model of spasmodic dyskinesias: a 2-deoxy-D-[1-14C]glucose study in the rat

Chronic administration of iminodipropionitrile (IDPN) to rats causes a persistent behavioral syndrome characterized by lateral and vertical head twitches, random circling, and increased tactile and acoustic startle responses. In order to identify brain areas which are affected in rats manifesting this syndrome, we used the autoradiographic 2-deoxy-D-[1-14C]glucose ([14C]DG) method to map cerebral glucose utilization in IDPN-treated rats. One day after the development of the dyskinetic syndrome, there were significant decreases in local glucose utilization in the substantia nigra pars reticulata (SNr) and compacta (SNc), the dorsal raphe, the superficial and deep layers of the superior colliculus, the inferior colliculi, the interpeduncular nucleus, the medial and dorsolateral geniculate nuclei, and the superior and lateral vestibular nuclei. There were also significant decreases in layer 2 of the cingulate cortex and in the temporal and occipital cortices. In contrast, there were no changes in the motor cortex, the caudate-putamen, the nucleus accumbens, or the median raphe. These findings suggest that deleterious effects of IDPN on the nigrotectal pathways which affect head and neck movements and circling behaviors via the brainstem reticulospinal tracts may play an important role in the IDPN-induced persistent spasmodic dyskinetic syndrome in rats.

How visual inputs to the ponto-bulbar reticular formation are used in the synthesis of premotor signals during orienting

The primate superior colliculus (SC) is known as a structure subserving the transformation of visual information into "commands" for orienting eye movements. Collicular burst neurons discharging with short lead times in relation to visually triggered or spontaneous saccades are supposed to be the output elements linking the SC to immediately premotor pattern generators. In this paper we summarize some data available for the cat's SC neurones, identified as tecto-reticulo-spinal projection cells (TRSN), and reticulospinal neurones (RSN), identified as receiving excitatory collicular input. Some TRSNs respond to visual stimuli in the absence of orienting movements and, hence, their signals cannot be regarded as motor "commands", in spite of their proven connections with premotor pools in the brain stem and with the spinal cord. Moreover, a small fraction of RSNs belonging to polysynaptic descending collicular pathways also displays visual responses dissociated from movement, in addition to discharges related to the performance of orienting eye-head synergies. The processes of visual to motor transformation, assumed by current models as being definitively accomplished in the SC, appear thus to be partially performed in the reticular network incorporating the overlapping collaterals of tectal projection cells and their target neurons in the reticular core. It is concluded that, at least as for visuomotor transformations underlying orienting movements in the cat, the deep division of the SC and the brain stem reticular formation represent an ensemble, rather than a sequence of hierarchically arranged levels of processing.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Ultrastructural study of large efferent neurons in the superior colliculus of the cat after retrograde labeling with horseradish peroxidase

The ultrastructure of large neurons in the stratum griseum intermedium of the cat superior colliculus was examined following injections of horseradish peroxidase (HRP) into the dorsal tegmental decussation. Four HRP-labeled cells were selected, and the synaptology of their cell bodies and selected regions of proximal and distal dendrites was examined. The four neurons represent four morphologically distinct cell types: multipolar radiating, tufted, large vertical, and medium-sized trapezoid radiating. These four neurons correspond with cell types X1, X2, X3, and T1 respectively, according to the recent classification of neurons in the superior colliculus of the cat by Moschovakis and Karabelas (J. Comp Neurol. 239:276-308, '85). The three X type neurons are similar in having 83% of their somata and over 74% of their proximal dendrites contacted by synaptic profiles. Distal dendrites of the X type neurons, however, receive fewer synaptic contacts. In contrast, in the T1 cell, only 69% of the soma membrane is contacted by synaptic profiles, and the synaptic coverage on proximal and distal dendrites does not vary much from this. Of the eight types of synaptic terminals described in the stratum griseum intermedium of the cat superior colliculus by Norita (J. Comp. Neurol. 190:29-48, '80), only five are found in contact with the X and T type efferent neurons described here. There are some regional differences in terminal distribution, although each terminal is represented on each cell. Type III terminals (small, contain mostly pleomorphic vesicles, and make symmetrical contacts) are the most abundant on cell bodies and dendrites of all four cell types. Terminal types II (medium-sized, containing round and flattened vesicles, and making asymmetrical contacts), and IV (medium to large in size, containing flattened vesicles, and making symmetrical contacts) are well represented. In general, terminal types I (small, containing densely packed round vesicles, and making asymmetrical contacts) and VI (small and irregular in shape, containing flattened vesicles and making symmetrical contacts) are found infrequently. The identity of different types of synaptic terminal is discussed.

Visual cells in the inferior colliculus of the cat

Extracellular recordings from 91 cells of the pericentral nucleus (ICP) of the inferior colliculus of the cat revealed that 83 of them were responsive to auditory stimuli and the other 8 to visual stimuli. All visually driven cells were binocular and showed large receptive fields located in the contralateral hemifield. The best stimulus was either a spot or a bar moved in any direction across the receptive field. No directional selectivity was found. It is suggested that the visual input to the ICP participates in an integrated reflex-orienting behavior, in which the visual information is important for the localization of the sound source.

Organization of the nigrotectospinal pathway in the cat: a light and electron microscopic study

The relationships between nigrotectal terminals and tectospinal neurons in the deep layers of the superior colliculus have been examined light and electron microscopically by using antero- and retrograde transport of tracer, and anterograde degeneration. For light microscopy, horseradish peroxidase (HRP) was injected into the cervical spinal cord, and HRP conjugated to wheat germ agglutinin (WGA-HRP) into the substantia nigra at the same time. It appeared that about 22% of the retrogradely labeled tectospinal neurons were totally or partially in the anterogradely labeled nigral terminal clusters. This coexistence was particularly manifest in the dorso-lateral part of the intermediate gray layer. For electron microscopy, nigral lesions and HRP injections into the cervical spinal cord were combined in the same animals. Degenerating nigrotectal terminals were found in both axosomatic and axodendritic symmetrical synaptic contacts with retrogradely labeled tectospinal neurons in the intermediate gray layer. Present experiments show tectospinal neurons receiving monosynaptic input from the substantia nigra do exist in the intermediate gray layer of the superior colliculus, especially in its dorsolateral portion.