The structural and functional characteristics of tectospinal neurons in the golden hamster

Whisker-related afferents in superior colliculus

Rodents use their whiskers to explore the environment, and the superior colliculus is part of the neural circuits that process this sensorimotor information. Cells in the intermediate layers of the superior colliculus integrate trigeminotectal afferents from trigeminal complex and corticotectal afferents from barrel cortex. Using histological methods in mice, we found that trigeminotectal and corticotectal synapses overlap somewhat as they innervate the lower and upper portions of the intermediate granular layer, respectively. Using electrophysiological recordings and optogenetics in anesthetized mice in vivo, we showed that, similar to rats, whisker deflections produce two successive responses that are driven by trigeminotectal and corticotectal afferents. We then employed in vivo and slice experiments to characterize the response properties of these afferents. In vivo, corticotectal responses triggered by electrical stimulation of the barrel cortex evoke activity in the superior colliculus that increases with stimulus intensity and depresses with increasing frequency. In slices from adult mice, optogenetic activation of channelrhodopsin-expressing trigeminotectal and corticotectal fibers revealed that cells in the intermediate layers receive more efficacious trigeminotectal, than corticotectal, synaptic inputs. Moreover, the efficacy of trigeminotectal inputs depresses more strongly with increasing frequency than that of corticotectal inputs. The intermediate layers of superior colliculus appear to be tuned to process strong but infrequent trigeminal inputs and weak but more persistent cortical inputs, which explains features of sensory responsiveness, such as the robust rapid sensory adaptation of whisker responses in the superior colliculus.

Vibrissa sensation in superior colliculus: wide-field sensitivity and state-dependent cortical feedback

Rodents use their vibrissae (whiskers) to sense and navigate the environment. A main target of this sensory information is the superior colliculus in the midbrain, which rats can use to detect meaningful whisker stimuli in behavioral contexts. Here, we used field potential, single-unit, and intracellular recordings to show that, although cells in the intermediate layers of the superior colliculus respond relatively effectively to single whiskers, the cells respond much more robustly to simultaneous, or nearly simultaneous, wide-field (multiwhisker) stimuli. The enhanced multiwhisker response is temporally stereotyped, consisting of two short latency peaks caused by convergent trigeminal synaptic inputs and cortical feedback, respectively. The cells are highly sensitive to the degree of temporal dispersion and contact order of multiwhisker stimuli, which makes them excellent detectors of initial multiwhisker contact. In addition, their output is most robust during quiescent states because of the dependence of cortical feedback on forebrain activation, and this may serve as an alerting signal to drive orienting responses.

Early sensory pathways for detection of fearful conditioned stimuli: tectal and thalamic relays

Sensory stimuli acquire significance through learning. A neutral sensory stimulus can become a fearful conditioned stimulus (CS) through conditioning. Here we report that the sensory pathways used to detect the CS depend on the conditioning paradigm. Animals trained to detect an electrical somatosensory stimulus delivered to the whisker pad in an active avoidance task were able to detect this CS and perform the task when a reversible or irreversible lesion was placed in either the somatosensory thalamus or the superior colliculus contralateral to the CS. However, simultaneous lesions of the somatosensory thalamus and superior colliculus contralateral to the CS blocked performance in the active avoidance task. In contrast, a lesion only of the somatosensory thalamus contralateral to the same CS, but not of the superior colliculus, blocked performance in a pavlovian fear conditioning task. In conclusion, during pavlovian fear conditioning, which is a situation in which the aversive outcome is not contingent on the behavior of the animal, the sensory thalamus is a critical relay for the detection of the CS. During active avoidance conditioning, a situation in which the aversive outcome is contingent on the behavior of the animal (i.e., the animal can avoid the aversive event), the sensory thalamus and the superior colliculus function as alternative routes for CS detection. Thus, even from early stages of sensory processing, the neural signals representing a CS are highly distributed in parallel and redundant sensory circuits, each of which can accomplish CS detection effectively depending on the conditioned behavior.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Organization of the intermediate gray layer of the superior colliculus. I. Intrinsic vertical connections

A pathway from the superficial visual layers to the intermediate premotor layers of the superior colliculus has been proposed to mediate visually guided orienting movements. In these experiments, we combined photostimulation using "caged" glutamate with in vitro whole cell patch-clamp recording to demonstrate this pathway in the rat. Photostimulation in the superficial gray and optic layers (SGS and SO, respectively) evoked synaptic responses in intermediate gray layer (SGI) cells. The responses comprised individual excitatory postsynaptic currents (EPSCs) or EPSC clusters. Blockade of these EPSCs by TTX confirmed that they were synaptically mediated. Stimulation within a column (approximately 500 microm diam) extending superficially from the recorded cell evoked the largest and most reliable responses, but off-axis stimuli were effective as well. The EPSCs could be evoked by stimuli 1,000 microm off-axis from the postsynaptic neuron. The dimensions of this wider region (approximately 2 mm diam) corresponded to those of the dendrites of superficial layer wide-field neurons. SGI neurons differed in their input from SGS and SO; neurons in the middle of the intermediate layer (SGIb) were less likely to respond to visual layer photostimulation than were those in sublayers just above and below them. However, focal stimulation within SGIa did evoke responses within SGIb, indicating that SGIb neurons may receive input from the visual layers indirectly. These results demonstrate a columnar pathway that may mediate visually guided orienting movements, but the results also reveal spatial attributes of the pathway which imply that it also plays a more complex role in visuomotor integration.

The fine organization of nigro-collicular channels with additional observations of their relationships with acetylcholinesterase in the rat

The nigro-collicular pathway that links the basal ganglia to the sensorimotor layers of superior colliculus plays a crucial role in promoting orienting behaviors. This connection originating in the pars reticulata and lateralis of the substantia nigra has been shown in rat and cat to be topographically organized. In rat, a functional compartmentalization of the substantia nigra has also been shown reflecting that of the striatum. In light of this, we reinvestigated the topographical arrangement of the nigro-collicular pathway by examining the innervation of each nigral functional zone. We performed small injections of either biocytin or wheatgerm agglutinin conjugated with horseradish peroxidase restricted to identified somatic, visual and auditory nigral zones. Frontally cut sections showed that innervations provided by the three main nigral zones form a mosaic of complementary domains stratified from the stratum opticum to the ventral part of the intermediate collicular layers, with the somatic afferents sandwiched between the visual and the auditory ones. When reconstructed from semi-horizontal sections, nigral innervations organized in the form of a honeycomb-like array composed of 100 cylindrical modules covering three-quarters of the collicular surface. Such a modular architecture is reminiscent of the acetylcholinesterase lattice we previously described in rat intermediate collicular layers. In the enzyme lattice, the surroundings of the cylindrical modules are composed of a mosaic of dense and diffuse enzyme subdomains. Thus, we compared the distribution of the overall nigral projection and of its constituent channels with the acetylcholinesterase lattice. The procedure combined axonal labelling with histochemistry on single sections for acetylcholinesterase activity. The results demonstrate that the overall nigral projection overlaps the acetylcholinesterase lattice and its constituent channels converge with either the dense or the diffuse enzyme subdomains. The stereometric arrangement of the nigro-collicular pathway is suggestive of an architecture promoting the selection of collicular motor programs for different classes of orienting behavior.

Organization of the neurons of origin of the descending pathways from the ferret superior colliculus

The superior colliculus (SC), through its descending projections to the brainstem and spinal cord, is involved in initiating sensory-driven orienting behaviors. Ferrets are carnivores that hunt both above and below ground using visual (and auditory) cues in the daylight but non-visual cues in darkness and in subterranean environments. The present investigation sought to determine whether the ferret SC shows organizational features similar to those found in other visually dominant animals (e.g. cats), or whether characteristics of colliculi from non-visually dominant animals (e.g. rodents) prevail. Injection of retrograde tracer into the identified targets of the colliculus (cervical spinal cord, the contralateral pontomedullary reticular formation, or the ipsilateral pontine reticular formation) labeled tectospinal, crossed tectoreticular, and ipsilateral tectoreticular neurons, respectively, within the adult ferret SC. Labeled tectospinal and crossed tectoreticular neurons were far outnumbered by neurons with ipsilateral reticular projections. Like those of their visually dominant relatives, ferret tectospinal neurons were well represented throughout the anterior-posterior extent of the SC and crossed tectoreticular neurons tended to be distributed more broadly across the intermediate gray layer than those of rodents. Thus, even though ferrets perform well as subterranean predators where non-visual cues initiate orienting behaviors, these anatomical characteristics indicate that their colliculi are organized similar to that of their visually dominant, carnivorous relatives.

Contribution of superficial layer neurons to premotor bursts in the superior colliculus

In vitro whole-cell patch-clamp methods were used to examine the contribution of one component of intracollicular circuitry, the superficial gray layer, to the generation of bursts of action potentials that occur in the intermediate layer and that command head and eye movements in vivo. Applying a single brief (0.5 ms) pulse of current to the superficial layer of rat collicular slices evoked prolonged bursts of excitatory postsynaptic currents (EPSCs) in the cells of the intermediate layer. The EPSCs were sufficient to elicit bursts of action potentials that lasted as long as 300 ms and resembled presaccadic command bursts. To examine the contribution of neurons within the superficial layer to the production of these bursts, we determined how superficial neurons respond to the same current pulses that evoke bursts in the intermediate layer. Recordings from 61 superficial layer cells revealed 19 neurons that produced multiple action potentials following stimulation. Nine of these 19 neurons were wide- and narrow-field vertical cells, which are known to project to the intermediate layer and could contribute to producing the EPSC bursts. The remaining cells (n = 42) did not generate trains of action potentials and 21 of these showed only subthreshold potential changes in response to the stimulus. Our results indicate that most superficial cells do not directly contribute to production of the EPSC bursts, but a small number do have the properties necessary to provide a prolonged excitatory drive to the premotor neurons.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus, with additional observations in several other mammals: AChE patterning

The aim of the present study was to reinvestigate the stereometric pattern of acetylcholinesterase (AChE) activity staining in the intermediate layers of the superior colliculus in several mammalian species. A pioneering study in the cat and the monkey by Graybiel (1978) stressed the regular arrangement of AChE staining in the deep collicular layers. According to her description, made in the frontal plane, the enzyme was arranged in a mediolateral series of patches, the cores of which tended to line up in the longitudinal axis of the structure, so they formed roughly parallel bands. As exhaustive a description as possible of the AChE distribution was undertaken in the rat by compiling observations in the frontal, sagittal, and tangential planes. It emerged that AChE-positive elements are organized in the form of a conspicuous honeycomb-like network that is divided into about 100 rounded compartments, over virtually the full extent of the intermediate layers. The generality of the rat model was then tested in other rodents such as mouse and hamster and also in cat and monkey. For these species we resorted to a single tangential cutting plane, which proved to be more appropriate for disclosing such a modular arrangement. The data revealed that in all species AChE staining followed the same architectural plan and identified the striking similarity in the number of compartments that compose the various honeycomb-like lattices. In conclusion, the present findings support a unified model of the AChE arrangement within the intermediate layers of the mammalian colliculus; the model comprehensively incorporates the classical description of the patchy and stripy features of the enzyme distribution. We hypothesize here that the modular AChE arrangement might be the anatomical basis for collicular vectorial encoding of orienting movements.

Recurrent inhibitory circuitry in the deep layers of the rabbit superior colliculus

1. Local inhibition in the deep layers of the superior colliculus plays a crucial role in sensorimotor integration. Using intracellular and extracellular recording techniques, we studied the organization of inhibitory circuits in the deep layers of the superior colliculus in anaesthetized rabbits. 2. We identified a new cell type in the deep superior colliculus that showed a characteristic burst response to stimulation of both the predorsal bundle and optic chiasm. The response had a jittering latency and failed to follow high frequency stimuli, indicating trans-synaptic (orthodromic) events. Moreover, the predorsal bundle stimulation-evoked orthodromic response could be made to collide with the response to a preceding stimulation of the optic chiasm, suggesting that burst-firing cells received excitatory inputs from the axonal collaterals of predorsal bundle-projecting cells. 3. Stimulation of the predorsal bundle could evoke an IPSP in predorsal bundle-projecting cells. The latency of the IPSP was 0.5-1.0 ms longer than the orthodromic response in burst-firing cells. Simultaneous recordings showed that the IPSP in predorsal bundle-projecting cells was preceded by a burst of extracellular spikes from burst-firing cells with short latency ( approximately 0.9 ms), indicating an inhibitory monosynaptic connection from burst-firing cells to predorsal bundle-projecting cells. 4. Burst-firing cells exhibited a prolonged depression after the predorsal bundle or optic chiasm stimulation due to an inhibitory postsynaptic potential. Latency analysis implies that burst-firing cells may form mutual inhibitory connections. 5. Together our results suggest that burst-firing cells and predorsal bundle-projecting cells form reciprocal excitatory and inhibitory connections and burst-firing cells may function as the recurrent inhibitory interneurons in the deep layers of the rabbit superior colliculus.

Cortical somatosensory and trigeminal inputs to the cat superior colliculus: light and electron microscopic analyses

Two different axonal transport tracers were used in single animals to test the hypothesis that the expansive intermediate gray layer of the cat superior colliculus (stratum griseum intermediale, SGI) is composed of sensorimotor domains. The results show that two sensory pathways, the trigeminotectal and the corticotectal arising from the fourth somatosensory area, commingle in patches across the middle tier of the SGI. Furthermore, the data reveal that tectospinal cells are distributed within these patches. Taken together, these results show a commingling of functionally related afferents and a consistent spatial relationship between these afferents and tectospinal neurons. These relationships indicate that the SGI consists of domains that can be distinguished by their unique combinations of afferent and efferent connections. The ultrastructural characteristics and synaptic relationships of these somatosensory afferent pathways suggest that they have distinct roles within the sensorimotor domain of the SGI. The trigeminotectal terminals are relatively small, contain round vesicles and make asymmetrical synapses on small, presumably distal, dendrites. We submit that these trigeminal terminals bestow the basic receptive field properties upon SGI neurons. In contrast, the somatosensory corticotectal terminals are relatively large, contain round vesicles, make asymmetrical synapses, participate in triads, and are presynaptic to proximal dendrites. We suggest that these cortical terminals bestow integrative abilities on SGI neurons.

Nociceptive neurones in rat superior colliculus. I. Antidromic activation from the contralateral predorsal bundle

Accumulating evidence suggests that the rodent superior colliculus (SC) plays as important a role in avoidance and defensive behaviours as it does in orientation and approach. These two complementary behaviours are associated with two anatomically segregated tectofugal output pathways, such that orientation and approach are mediated by the crossed descending projection, whereas avoidance and defence are subserved via the uncrossed projection. Because nociceptive neurones in the SC have been presumed to participate in withdrawal or defensive behaviours, it has been proposed that they have direct access only to the uncrossed efferent pathway. However, in certain behavioural situations, the most adaptive response to injury, or to a painful object in prolonged contact with the skin, is to orient towards the source of discomfort so that the skin can be licked and/or the offending object removed. Presumably then, nociceptive as well as low-threshold neurones would have access to the crossed descending pathway in order to initiate such behaviours. Determining whether or not this is the case was the objective of the present study. Both nociceptive-specific (82%) and wide-dynamic-range (18%) SC neurones were identified using long-duration (up to 6 s), frankly noxious mechanical and thermal stimuli in urethane-anaesthetised Long-Evans hooded rats. The majority (85.7%) of the nociceptive neurones encountered were located within the intermediate layers, which corresponds with the location of the cells-of-origin of the crossed descending projection. Nearly half (44.9%) were activated antidromically from electrical stimulation of the crossed descending pathway at a site in the brainstem below its decussation. The mean conduction velocity of these nociceptive output neurones was 9.02 m/s, which corresponds well to previous estimates of conduction velocity in the crossed tecto-reticulo-spinal tract. These data demonstrate that a significant proportion of nociceptive neurones in the rat SC have axons that project to the contralateral brainstem via the crossed descending projection. Nociceptive neurones could, therefore, effect orientation responses to noxious stimuli via similar output pathways that low-threshold neurones utilize to initiate orientation to innocuous stimuli.

Interlaminar connections of the superior colliculus in the tree shrew. II: Projections from the superficial gray to the optic layer

This study of the tree shrew, Tupaia belangeri, provides evidence for an intracollicular pathway that arises in the superficial gray layer and terminates in the optic layer. As a first step, Nissl, myelin, and cytochrome oxidase stains were used to identify the layers of the superior colliculus in the tree shrew. Second, anterograde and retrograde axonal transport methods were used to determine relationships between laminar borders and patterns of connections. Intraocular injections of wheat germ agglutinin conjugated to horseradish peroxidase showed that the border between the superficial gray and optic layers in the tree shrew is marked by a sharp decrease in the density of retinotectal projections. The optic layer also could be distinguished from the subjacent intermediate gray layer by differences in connections. Of the two layers, only the intermediate gray layer received projections following injections of wheat germ agglutinin conjugated to horseradish peroxidase within substantia nigra pars reticulata. Similarly, following injections of horseradish peroxidase or biocytin in the paramedian pons, the intermediate gray but not the optic layer contained labeled cells of origin for the main premotor pathway from the tectum, the predorsal bundle. Next, cells in the superficial gray layer were intracellularly injected with biocytin in living brain slices. Axons were traced from narrow and wide field vertical cells in the deep part of the superficial gray layer to the gray matter surrounding the fiber fascicles of the optic layer. Small extracellular injections of biocytin in brain slices showed that the optic layer gray matter contains a population of stellate cells that are in position to receive the input from the superficial layer. Finally, small extracellular injections of biocytin in the intermediate gray layer filled cells that sent prominent apical dendrites into the optic layer, where they may be directly contacted by the superficial gray layer cells. Taken together, the results support the hypothesis that the optic layer is functionally distinct from its adjacent layers, and may provide a link in the transfer of information from the superficial, retinal recipient, to the intermediate, premotor, layer of the superior colliculus.

Physiological properties of the output neurons in the deep layers of the superior colliculus of the rabbit

Using antidromic and orthodromic stimulation techniques, we studied physiological properties of the output neurons in the deep layers of the superior colliculus (SC) of 34 Now Zealand rabbits. SC cells antidromically activated from the contralateral predorsal bundle (PDB) could also be activated by stimulation of the contralateral SC and ipsilateral central lateral nucleus of the thalamus (CL). The majority of these output neurons responded predominantly to the stimulation of the optic nerve, and only a small proportion of the output neurons were responsive to the stimulation of somatosensory and auditory (and/or vestibular) nerves. These results suggest that the orienting reflex might be elicited mainly by visual afferents in the rabbit. The output SC neurons were subject to a 70 ms inhibition after antidromic stimulation of the PDB and a 40 ms inhibition after transsynaptic (orthodromic) stimulation of the optic chiasm (OX), indicating that the output neurons in the deep layers of the SC might be subject to at least two inhibitory circuits. These results are discussed in the context of a putative saccadic suppression circuitry model.

Correlations between the receptive field properties and morphology of neurons in the deep layers of the hamster's superior colliculus

Extracellular and intracellular recording, receptive field mapping, and intracellular HRP injection techniques were used to define the morphological classes of cells in the deep laminae of the hamster's superior colliculus and to determine whether there are any correlations between the structural and functional characteristics of these neurons. A total of 110 neurons were characterized and reconstructed. Of these, 23.6% (N = 26) were visual, 60% (N = 66) were somatosensory, 0.9% (N = 1) were bimodal (visual-somatosensory), and 15.4% (N = 17) were unresponsive. Of the somatosensory neurons, 72.7% (N = 48) were low threshold, 4.5% (N = 3) had a wide dynamic range, 9.1% (N = 6) responded only to noxious stimulation, and 13.6% (N = 9) had complex somatosensory receptive fields. Deep layer cells were divided into eight morphological classes. These classes were multipolar cells (26.4%, N = 29), bipolar cells (9.1%, N = 10), widefield vertical cells (7.3%, N = 8), horizontal cells (13.6%, N = 15), stellate cells (10.9%, N = 12), ventrally directed cells (5.5%, N = 6), sparse radial cells (17.3%, N = 19), and small sparse radial cells (6.4%, N = 7). Four cells (3.6%) did not fit into this classification scheme. Univariate and multivariate analyses of variance of properties such as soma area, number of branch points, total dendritic length, and volume and orientation of dendritic arbor indicated that these classes were significantly different. However, chi 2 analysis and multivariate analysis of variance indicated no significant relationships between morphological class and either laminar location or receptive field type. There was a significant positive relationship between the possession of dendrites that extended into the superficial laminae and visual responsivity.

Topographical organization of the nigrotectal projection in rat: evidence for segregated channels

Recent evidence suggests that projections from the superior colliculus to the brainstem in rat are organized into a series of anatomically segregated output channels. To understand how collicular function may be modified by the basal ganglia it is important to know whether particular output modules of the superior colliculus can be selectively influenced by input from substantia nigra. The purpose of the present study was, therefore, to examine in more detail topography within the nigrotectal system in the rat. Small injections (10-50 nl) of a 1% solution of wheatgerm agglutinin conjugated with horseradish peroxidase were made at different locations within substantia nigra and surrounding structures. A discontinuous puff-like pattern of anterogradely transported label was found in medial and caudal parts of the ipsilateral intermediate layers of the superior colliculus. In contrast, the rostrolateral enlargement of the intermediate layers contained a greater density of more evenly distributed terminal label. Injection sites associated with this dense pattern of laterally located label were concentrated in lateral pars reticulata, while the puff-like pattern was produced by injections into ventromedial pars reticulata. Retrograde tracing experiments with the fluorescent dyes True Blue and Fast Blue revealed that injections involving the rostrolateral intermediate layers were consistently associated with a restricted column of labelled cells in the dorsolateral part of ipsilateral pars reticulata. Comparable injections into medial and caudal regions of the superior colliculus produced retrograde labelling in ventral and medial parts of the rostral two-thirds of pars reticulata. Both anterograde and retrograde tracing data indicated that contralateral nigrotectal projections arise from cells located in ventral and medial pars reticulata. The present results suggest that the main ipsilateral projection from substantia nigra pars reticulata to the superior colliculus comprises two main components characterized by regionally segregated populations of output cells and spatially separated zones of termination. Of particular interest is the apparent close alignment between terminal zones of the nigrotectal channels and previously defined populations of crossed descending output cells in the superior colliculus. Thus, the rostrolateral intermediate layers contain a concentration of terminals specifically from dorsolateral pars reticulata and output cells which project to the contralateral caudal medulla and spinal cord. Conversely, the medial and caudal intermediate layers receive terminals from ventral and medial pars reticulata and contain cells which project specifically to contralateral regions of the paramedian pontine and medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

The nigral projection to predorsal bundle cells in the superior colliculus of the rat

Predorsal bundle cells give rise to the major efferent pathway from the superior colliculus to the premotor centers of the brainstem and spinal cord responsible for initiating orienting movements. The activity of predorsal bundle cells is profoundly influenced by an inhibitory pathway from substantia nigra pars reticulata that uses gamma aminobutyric acid (GABA) as a neurotransmitter. The present study examines the morphological basis for this influence of substantia nigra on predorsal bundle cells in the rat. In the first experiments, the laminar distributions of the nigrotectal tract terminals and the predorsal bundle cells were compared. The predorsal bundle cells were labeled by the retrograde axonal transport of horseradish peroxidase from either the decussation of the predorsal bundle or the cervical spinal cord, while the terminations of the pathway from substantia nigra pars reticulata were labeled by anterograde axonal transport from the substantia nigra. Either horseradish peroxidase, wheat germ agglutinin conjugated to horseradish peroxidase, or Phaseolus vulgaris leucoagglutinin were used as anterograde tracers. The results showed that the distributions of both the predorsal bundle cells and the nigrotectal terminals are restricted almost entirely to the intermediate grey layer and that they overlap extensively. Predorsal bundle cells varied in size. Within the areas of maximum overlap, the majority, regardless of size, was closely apposed by nigrotectal terminals. In a second series of experiments, the synaptic contacts between nigrotectal terminals and the tectospinal component of the predorsal bundle were examined in tissue in which both the terminals and the tectospinal cells were labeled for electron microscopy. In the final experiments, the distribution and fine structure of the nigrotectal terminals were compared to those of terminals that had been labeled immunocytochemically with an antibody to glutamic acid decarboxylase, the synthesizing enzyme for GABA. The results showed that nigrotectal terminals contain large numbers of mitochondria and pleomorphic vesicles, and form synaptic contacts with the somas and proximal dendrites of tectospinal cells. These synapses have modest postsynaptic densities. In both their distribution and fine structure, these terminations resemble the glutamic acid decarboxylase immunoreactive terminals that contact tectospinal cells. Taken together, these results support the view that the nigrotectal tract is an important source of GABAergic input to most, if not all, predorsal bundle cells.

Structural and functional consequences of neonatal deafferentation in the superficial layers of the hamster's superior colliculus

Intracellular recording and horseradish peroxidase (HRP) injection techniques were used to evaluate the effects of neonatal enucleation upon the structural and functional properties of cells in the superficial retinorecipient laminae of the hamster's superior colliculus (SC). The physiological recordings confirmed previous results that normally visual superficial layer neurons develop somatosensory receptive fields in the enucleated animals. This study further showed that all of the physiological subclasses of somatosensory neurons normally encountered in the deep layers were present in the superficial laminae. With the exception of marginal cells, all of the morphological classes of neurons in the superficial SC laminae of sighted hamsters (narrowfield vertical cells, widefield vertical cells, stellate cells, horizontal cells, and giant stellate cells) were recovered from the blinded animals. Quantitative comparison of neurons within a given morphological class demonstrated only slight differences between cells from blind and sighted hamsters. However, there was a significant reduction in the percentage of neurons with dorsally directed dendrites in the neonatally enucleated animals. Additional experiments with the Golgi technique also demonstrated that neonatal enucleation altered the distribution of morphological cell types in the superficial SC laminae. These results suggest that enucleation in the hamster may result in relative reductions in specific cell types in the superficial SC laminae rather than dendritic changes in all of the cell classes present in these layers.

Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection

A select population of superior colliculus (SC) neurons receives and integrates information from the visual, auditory and somatosensory systems. Determining which SC neurons comprise this population and where they send their multisensory messages is important in understanding the functional impact of the SC on attentive and orientation behavior. One of the major routes by which the SC influences these behaviors is the tecto-reticulo-spinal tract, a descending pathway that plays an integral role in the orientation of the eyes, ears and head. Of the 182 tecto-reticulo-spinal neurons (TRSNs) encountered in the present study, almost all (94%) responded to sensory stimuli and the overwhelming majority (84%) were multisensory. The present results demonstrate that the TRSN serves as an important link among the different sensory systems and provides a substrate through which they may gain access to the circuitry mediating orientation behavior.

Sensory responses of intralaminar thalamic neurons activated by the superior colliculus

The intralaminar thalamus of anesthetized rats was explored for neurons activated by stimulation of the superior colliculus and responsive to sensory inputs. Neurons activated by stimulation of the intermediate and deep collicular layers were distributed throughout the intralaminar thalamus. Approximately one half of them responded to tectal as well as sensory inputs. The majority were nociceptive or had a more complex response pattern including responses to auditory stimulation. A small population of low threshold units had contralateral orofacial receptive fields and responded to light taps; these units were preferentially localized anteriorly in the central lateral and paracentral nuclei. Neurons responsive to tectal and sensory stimulation were randomly intermingled with other neurons which had no detectable sensory input. The results indicate that ascending projection neurons of the intermediate and deep layers of the superior colliculus provide an input to functionally diverse subpopulations of intralaminar thalamic neurons. In view of its projections to motor cortex and basal ganglia, the intralaminar thalamus appears directly implicated in basal ganglia and superior colliculus related mechanisms of attention, arousal and postural orienting.

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.

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.

Sources of subcortical GABAergic projections to the superior colliculus in the cat

The goal of this study was to identify GABAergic input to the cat superior colliculus from neurons located in the caudal diencephalon, mesencephalon, pons and medulla. Cells efferent to the superior colliculus were labeled retrogradely with the tracer horseradish peroxidase, and an antibody to gamma-aminobutyric acid was used to label GABAergic neurons in the same sections. The results indicate that neurons in several distinct areas of the caudal diencephalon and brainstem are both immunocytochemically labeled for GABA and retrogradely labeled with horseradish peroxidase. These areas include zona incerta, nucleus of the posterior commissure, anterior and posterior pretectal nuclei, nucleus of the optic tract, superior colliculus, cuneiform nucleus, subcuneiform area, substantia nigra pars reticulata and pars lateralis, periparabigeminal area, external nucleus of the inferior colliculus, the area ventral to the external nucleus of the inferior colliculus, mesencephalic reticular formation, dorsal and ventral nuclei of the lateral lemniscus, and the perihypoglossal nucleus. The role that such diverse inhibitory input to the superior colliculus might play, particularly in influencing eye movements, is discussed.

Output pathways from the rat superior colliculus mediating approach and avoidance have different sensory properties

Neuroanatomical studies have demonstrated that the two major descending pathways from the superior colliculus arise from regionally segregated, distinct, cells of origin. Stimulation and lesion studies have implicated the crossed descending tecto-reticulo-spinal projection in approach movements towards novel stimuli whereas the ipsilateral pathway appears to be involved in the control of avoidance and escape-like behaviours. The present electrophysiological study attempted to characterise the sensory properties of antidromically identified cells of origin of these pathways in anaesthetised rats. We found that the contralaterally projecting predorsal bundle (PDB) efferents were primarily somatosensory while the ipsilateral cuneiform (CNF) projection was primarily visual. PDB cells, mainly found in the intermediate layers, responded principally to vibrissal stimulation with their overlying visual fields optimally stimulated by small dark moving objects in the lower rostral and lateral field. In contrast, most CNF cells were located rostromedially, with the greatest contribution from visual cells responsive to stimuli in the upper rostral field. A significant proportion of these showed no response to small moving dark discs but fired vigorously to 'looming' stimuli. Ethological considerations suggest that these are appropriate stimulus characteristics for a system controlling approach and avoidance behaviour in an animal such as the rat where predators generally appear from above and prey is found on the ground.

Somatosensory neurons projecting from the superior colliculus to the intralaminar thalamus in the rat

Neurons of the rat superior colliculus projecting to the intralaminar thalamus were tested for their responses to somatosensory stimulation. They were identified by antidromic stimulation of the parafascicular nucleus and central lateral nucleus. To establish the existence of descending as well as ascending axon collaterals antidromic stimulation was applied to the upper cervical spinal cord in some cases. Somatosensory receptive fields were delineated and their laminar location in the superior colliculus was noted. Units were distributed throughout the intermediate and deep tectal layers, none were located in the superficial layers. Units with somatosensory receptive fields could be classified as low threshold, high threshold, wide dynamic range or complex. The majority of the peripherally responsive units (52%) were low threshold somatosensory units with contralateral receptive fields. All units were distributed throughout the intermediate and deep layers. Their distribution reflected the typical somatotopic organization of the superior colliculus. These results indicate that the intralaminar thalamus receives some sensory information by way of the tectum. In turn, the basal ganglia may gain direct access to this information by way of the thalamoneostriatal projection.

A light- and electron-microscopic investigation of the optic tectum of the frog, Rana pipiens, II: The neurons that give rise to the crossed tecto-bulbar pathway

The superficial layers of the frog's optic tectum, Potter's (1969) layers A-G, comprise a complex neuropil made up of many afferent axons, the somata of a few neurons, and many dendrites from the neurons located in the deeper layers. Different types of retinal axons are believed to terminate in different layers (Maturana et al., 1960; Kuljis & Karten, 1988; Sargent et al., 1989), but little is known about the relationships between each type of input and the dendrites of the deep tectal neurons that extend into these superficial layers. The present study used the method of retrograde transport of horseradish peroxidase to study the synaptic contacts on the dendrites of the neurons that give rise to the crossed tecto-bulbar pathway. These cells have apical dendrites that ascend through the superficial retino-recipient layers. The somata of the cells that give rise to the crossed tecto-bulbar pathway are located in the superficial half of layer 6, preferentially clustered along the caudal, lateral, and rostral margins of the tectum. The somata of these cells range from 8-30 microns in diameter. Their axons are large (2-4 microns in diameter) myelinated fibers that arise from either their somata or proximal dendrites. Their axons travel within the deep medullary layer to leave the tectum at the lateral margin. Their dendritic arbors extend obliquely through the superficial layers to reach layer B where they turn and extend within the layer for up to 0.5 mm. The somata of these cells receive only a scant synaptic input. In contrast, their dendrites receive input in every layer, but the nature of this input varies from layer to layer. Synaptic terminals that resemble retinal ganglion cell boutons contact the labeled dendrites in layers B, F, and G. This indicates that the dendrites may receive monosynaptic input from several types of retinal ganglion cells. Terminals with small, flattened vesicles also contact the dendrites of these cells in each layer. In layer F and below, the terminals with flattened vesicles constitute 15% of the contacts; above layer F they constitute only 5-8% of the contacts. Terminals with medium-sized, flattened vesicles also contact the dendrites of these cells in every layer and constitute a large proportion of their input (33-95%). The latter terminals resemble those that are often postsynaptic to retinal terminals.

Intermediate and deep layers of the macaque superior colliculus: a Golgi study

We studied the intermediate and deep layers of the macaque superior colliculus by means of the Golgi technique in an attempt to better understand the structural features of this important oculomotor center. For this study, we examined the optic (stratum opticum, SO), intermediate gray (stratum griseum intermedium, SGI), intermediate white (stratum album intermedium, SAI), and deep gray (stratum griseum profundum, SGP) layers. These are the four layers in which neurons having saccade-related activity are localized. We identified eight neuronal types on the basis of differences in somatic and dendritic morphologies: large multipolar neurons (Type I); large pyramidal neurons (Type II); large fusiform neurons (Type III); medium fusiform neurons with spiny, radially oriented dendrites (Type IV); medium round neurons with fan-shaped dendritic trees (Type V); medium stellate neurons with varicose dendrites (Type VI); medium multipolar neurons with robust, spiny dendrites (Type VII); and local interneurons (Type VIII). Most neuronal types possessed features that are homologous to presynaptic dendritic features in other brain centers. With the exception of the medium stellate neurons (Type VI), which are aspinous, and the local interneurons (Type VIII), which are sparsely spinous, all other types had a moderate number of spines on their dendrites. Dendrites that terminated in the optic layer had specializations not observed elsewhere, suggesting that these tips may sample a tectal afferent that is not present in the more ventral layers. These eight types comprise all the neuronal morphologies observed in a large number of Golgi-impregnated macaque brains (n = 50). We suggest that they represent the full range of neuron types in the saccade-related layers of the macaque tectum.

Vibrissa-responsive neurons of the superior colliculus that project to the intralaminar thalamus of the rat

Responses to deflection of vibrissae were studied in neurons of the superior colliculus projecting to the intralaminar thalamus. Forty-two percent were activated by deflection of several vibrissae. Some units showed habituation and directional sensitivity but no other feature extraction was noted. All responsive units were located in the intermediate and deep layers in a roughly somatotopic distribution and intermingled with other projecting and non-projecting units.

Organization of the crossed tecto-reticulo-spinal projection in rat--II. Electrophysiological evidence for separate output channels to the periabducens area and caudal medulla

The previous paper (Redgrave et al., Neuroscience 37, 571-584, 1990) presented anatomical evidence indicating there are at least two largely segregated components of the crossed tecto-reticulo-spinal pathway which project to the periabducens area and caudal medulla. An immediate question arising from this finding is whether tectal cells which project either to the periabducens area or to the caudal medulla have different electrophysiological response properties. An answer to this question would be relevant to the issue of whether different components of the tecto-reticulo-spinal system are specialized for the production of different classes of orienting movement. Accordingly, extracellularly recorded units in the superior colliculus of urethane anaesthetized rats were tested for antidromic activity following electrical stimulation of the periabducens area or the caudal medulla. When antidromic potentials were successfully recorded the sensory properties of the units were tested with a range of unimodal visual, somatosensory and auditory stimuli. The following results were obtained. (i) Tectal cells antidromically activated by stimulation of the caudal medulla were preferentially sensitive to somatosensory stimuli from the perioral region, while cells activated from the periabducens area were more frequently responsive to auditory stimuli. (ii) Tectal fibres activated by stimulation of the caudal medulla had significantly higher conduction velocities than the fibres activated by electrodes in the periabducens region. (iii) More than 90% of antidromically activated cells were located in stratum album intermediale or dorsal stratum profundum. These electrophysiological findings confirm and extend previous anatomical observations which indicate that components of the crossed descending projection of the colliculus may be functionally specialized for the production of different classes of orienting movements.

Organization of the crossed tecto-reticulo-spinal projection in rat--I. Anatomical evidence for separate output channels to the periabducens area and caudal medulla

The superior colliculus has been used to study principles of sensorimotor transformation underlying the guidance of orienting movements by multimodal sensory stimuli. We have previously suggested that there may be two different classes of mechanism which can produce orienting-like movements towards a novel event; one that locates a stimulus on the basis of remembered position, and another which uses continuous feedback relating to target velocity. The crossed descending pathway of the superior colliculus is widely considered the projection most likely to relay signals associated with the production of orienting movements. However, if different neural mechanisms are used to produce functionally distinct types of orienting, we might expect this pathway to have separate anatomical components related to function. The purpose of the present experiment was to see if collicular fibres innervating two important pre-motor targets of the crossed descending pathway, the periabducens area and the caudal medulla-spinal cord, come from the same population of tectal cells. One of the retrogradely transported fluorescent tracers (Diamidino Yellow) was injected into the periabducens area, and another (True Blue or Fast Blue) was injected into tectospinal fibres at the level of the ventromedial caudal medulla. Under these conditions we found: (i) less than 10% of labelled cells within the superior colliculus contained both tracers; (ii) the bulk of singly labelled cells projecting to the periabducens area or the caudal medulla were concentrated at different locations within the colliculus, (iii) in regions of the superior colliculus where there was overlap of singly labelled cells, neurons projecting to the periabducens area or the caudal medulla could be distinguished morphologically. These data provide three classes of evidence which indicate that the crossed descending projection in rat can be subdivided into at least two relatively independent anatomical components. This conclusion may, in part, provide an anatomical substrate for the functional dissociations proposed for orienting movements.

Organization of the projections from the trigeminal brainstem complex to the superior colliculus in the rat and hamster: anterograde tracing with Phaseolus vulgaris leucoagglutinin and intra-axonal injection

Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and intra-axonal recording and injection techniques were employed to describe the projection from the trigeminal (V) brainstem complex to the deep laminae of the superior colliculus (SC) in the hamster and the rat. The organization of these projections was the same in the two species. Deposits of PHA-L into V nucleus principalis (PrV) produced labelled axons and boutonlike swellings in the lower stratum griseum intermediale (SGI) and upper stratum album intermedium (SAI) in the SC bilaterally. Plots of boutonlike swellings indicated that the terminals of this projection were arrayed in clusters. Nucleus principalis also projected to the stratum griseum profundum (SGP) and stratum album profundum (SAP). This deeper projection did not terminate in clusters and it was most prominent in the lateral SC. The ipsilateral PrV-SC projection appeared to arise mainly from axons that recrossed the midline at the level of the SC commissure. Reconstruction of individual PHA-L labelled fibers demonstrated that single axons gave rise to terminals on both sides of the midline. Deposits of PHA-L into V subnucleus interpolaris (SpI) yielded results that were identical to those obtained with PrV injections with one exception: none of these deposits produced any labelled terminals in the ipsilateral SC. Deposits of PHA-L into V subnucleus caudalis (SpC) produced only sparse labelling in SC. Most labelled swellings were located in the SGP and SAP and they were visible only in the SC contralateral to the PHA-L injection site. Single axons arising from cells in SpI were recorded and injected with horseradish peroxidase (HRP) in the hamster's SC. These fibers all responded to stimulation of multiple mystacial vibrissae and gave rise to 2-5 clusters of bouton-like swellings in the lower SGI and upper SAI.

Collateral projections of predorsal bundle cells of the superior colliculus in the rat

The deep layers of the superior colliculus contain cells which are premotor in the sense that they respond prior to the onset of shifts in gaze and send axons, by way of a pathway called the predorsal bundle, to the contralateral brainstem gaze centers and cervical spinal cord. Previous studies have suggested that these cells also contribute to other efferent pathways which arise in the deep layers. The present study examines the contributions of the cells of origin of the predorsal bundle to these additional pathways as a step toward understanding their roles in gaze mechanisms. In one series of experiments, retrograde tracers were used to compare the laminar distribution of predorsal bundle cells with the distributions of the cells of origin of three other pathways: those that project to the intralaminar region of the dorsal thalamus, those that project to the contralateral superior colliculus, and those that project to the ipsilateral brainstem tegmentum. Predorsal bundle cells were found primarily in stratum griseum intermedium sublayer b. This distribution overlaps extensively with the distribution of colliculus cells that project to the intralaminar region of the thalamus. In contrast, the majority of the colliculus cells that project to either the contralateral superior colliculus or the ipsilateral brainstem tegmentum do not overlap extensively with the predorsal bundle cells; instead, they are primarily located dorsal or ventral to sublayer b of stratum griseum intermedium. In a second series of experiments, two regions were injected with different retrograde fluorescent traces in single animals in order to study the collateral projections of the cells of origin of these pathways. The results indicate that many predorsal bundle cells project to the intralaminar region of the dorsal thalamus but that only a few contribute to the tectotectal pathway. The results also indicate that few tectotectal cells contribute to the ipsilateral tectobulbar pathway.

Event or emergency? Two response systems in the mammalian superior colliculus

Recent studies of the effects of stimulating the superior colliculus (SC) in rodents suggest that this structure mediates at least two classes of response to novel sensory stimuli. One class contains the familiar orienting response, together with movements resembling tracking or pursuit, and appears appropriate for undefined sensory 'events'. The second class contains defensive movements such as avoidance or flight, together with cardiovascular changes, that would be appropriate for a sudden emergency such as the appearance of a predator, or of an object on collision course. The two response systems appear to depend on separate output projections, and are probably subject to different sensory and forebrain influences. These findings (1) suggest an explanation for the complex anatomical organization of the SC, with multiple output pathways differentially accessed by a very wide variety of inputs, (2) emphasize the similarities between the SC and the optic tectum in non-mammalian species, and (3) suggest that the SC may be useful as a model for studying both the sensory control of defensive responses, and how intelligent decisions can be taken about relatively simple sensory inputs.

Laminar and segregated distribution of immunoreactivities for some neuropeptides and adenosine deaminase in the superior colliculus of the rat

The distribution and morphology of adenosine deaminase, substance P, leucine-enkephalin, corticotropin-releasing factor, and calcitonin gene-related peptidelike immunoreactive cells and fibers throughout the superior colliculus of the rat were examined by means of the unlabelled-antibody peroxidase-antiperoxidase method. Adenosine deaminase immunoreactive cells were found in the stratum opticum and lower stratum griseum superficiale; substance P immunoreactive cells were localized to the upper stratum griseum superficiale, and calcitonin gene-related peptide immunolabelled neurons were situated in deeper strata. Substance P, leucine-enkephalin, and calcitonin gene-related peptide immunoreactive fibers were distributed similarly in their lamination and in their patchlike organization. Corticotropin-releasing factor immunoreactive fibers were observed evenly throughout all the strata and were fewer in the stratum griseum superficiale. These findings suggest that, as in afferent modules and segregated efferents of the mammalian superior colliculus, the cells and fibers containing neuroactive substances and neuroactive substance-related enzymes also show a segregated and laminar distribution.

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.

The spinal and commissural projections from the superior colliculus in rat and hamster arise from distinct neuronal populations

Multiple retrograde labelling techniques were used to determine whether the superior collicular (SC) cells that project to the spinal cord in hamster and rat also innervate the contralateral colliculus. In 25 rats and 13 hamsters, various combinations of the tracers horseradish peroxidase, True blue, Diamidino yellow, fluorogold and rhodamine-labelled latex microspheres were used to label tectospinal and SC commissural neurons. No double-labelled cells were observed in any of these experiments. Analysis of neurons that were retrogradely labelled with horseradish peroxidase showed further that SC commissural neurons were much smaller than tectospinal cells. The average soma area for tectospinal cells in hamster was 225.9 micron 2 and that for such neurons in rat was 214.4 micron 2. The mean soma areas for SC commissural neurons in hamster and rat were 85.4 micron 2, respectively. In additional experiments (6 rats and 6 hamsters), True blue was injected into the left predorsal bundle and Diamidino yellow was deposited into the left SC. In both hamsters and rats, these injections invariably produced a small number of double-labelled cells in the deep layers of the right SC. In a final group of animals (7 rats and 2 hamsters), large thalamic deposits of Diamidino yellow were combined with bilateral injections of True blue into the spinal cord. This combination also produced small numbers of double-labelled neurons in both colliculi. These results indicate that the tectospinal and SC commissural pathways arise from distinct neuronal populations, but that a small number of cells that send axons into the predorsal bundle also have commissural collaterals. They demonstrate further that some tectospinal cells also send axon collaterals to the thalamus.