Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

Contrast sensitivity of neurons in pigeon optic tectum

An analysis of contrast sensitivity of neurons in the optic tectum of intact and wulst-ablated pigeons was carried out using moving light bars.

(1) Changes of contrast from 0.1% to 89%, obtained with different bar luminances, induced increases of response amplitude linearly related to log contrast.

(2) The units differed in their contrast threshold but 90.7% had a contrast threshold below 2.5%.

(3) Both contrast threshold and threshold-saturation range values were independent of the location of the units in the tectal layers.

(4) Contrast threshold remarkably increased on passing from low photopic to scotopic adaptation levels.

(5) The relationship between unit responses and contrast values, as well as the contrast threshold value, did not differ between intact and wulst-ablated animals.

Periventricular efferent neurons in the optic tectum of rainbow trout

The efferent connections and axonal and dendritic morphologies of periventricular neurons were examined in the optic tectum of rainbow trout to classify periventricular efferent neurons in salmonids. Among the target nuclei of tectal efferents, tracer injections to the following four structures labeled periventricular neurons: the area pretectalis pars dorsalis (APd), nucleus pretectalis superficialis pars magnocellularis (PSm), nucleus ventrolateralis of torus semicircularis (TS), and nucleus isthmi (NI). Two types of periventricular neurons were labeled by injections to the APd. One of them had an apical dendrite ramifying at the stratum fibrosum et griseum superficiale (SFGS), with an axon that bifurcated into two branches at the stratum griseum centrale (SGC), and the other had an apical dendrite ramifying at the SGC. Two types of periventricular neurons were labeled after injections to the TS. One of them had an apical dendrite ramifying at the boundary between the stratum opticum (SO) and the SFGS, and the other had dendritic branches restricted to the stratum album centrale or stratum periventriculare. Injections to the PSm and NI labeled periventricular neurons of the same type with an apical dendrite ramifying at the SO and a characteristic axon that split into superficial and deep branches projecting to the PSm and NI, respectively. This cell type also possessed axonal branches that terminated within the tectum. These results indicate that periventricular efferent neurons can be classified into at least five types that possess type-specific axonal and dendritic morphologies. We also describe other tectal neurons labeled by the present injections.     

Postsynaptic phenomena in optic tectum neurons following optic nerve stimulation in fish

In the field potential evoked in optic tectum by electrical stimulation of the optic nerve, the first postsynaptic deflection (wave 4) is shown to represent (a) a graded excitatory phenomenon which gives rise to (b) a spike followed by refractory period. The second postsynaptic deflection (wave 5) probably also represents 2 such phenomena. Both the graded and spike potentials, sustained by vertically elongated neurons, originate in the tectal strata where the bulk of retino-tectal afferents terminate,i.e., the stratum fibrosum et griseum superficiale. The positive-negative (or purely positive) configuration of unitary spikes recorded at deep tectal strata, presumably from the neuronal perisomatic region, suggests that the monosynaptic excitatory phenomena are sustained by the fusiform and large pyriform neurons, whose somata are deeply located. Also neurons whose somata lie in superficial tectal strata are excited monosynaptically, and they most likely also contribute to the field potential. Among these are large neurons which always fire in association with wave 5. Inhibitory phenomena, evidenced by depression of neuronal excitability, set in some 10 msec after the stimulus and lasted for 40–200 msec. For the first 40 msec or so, a slight facilitatory state can be shown to overlap with this depression.     

Spatial-tuning properties of auditory neurons in the optic tectum of the pigeon

We studied the auditory neurons in the optic tectum of the unanesthetized pigeon, using single-unit recordings and acoustic free-field stimulation. Most units showed spatial tuning, with best areas located in the contralateral hemifield. All units responded also to visual stimuli, the auditory best areas being in rough alignment with visual receptive fields.     

Postsynaptic potentials in neurons of the pigeon''s optic tectum in response to afferent stimulation from the retina and other visual structures: an intracellular study

The responses of the cells in the pigeon's optic tectum to electrical stimulation of the contralateral optic nerve, the ipsilateral visual Wulst and the opposite optic tectum were intracellularly recorded. Optic nerve or visual Wulst stimulation elicited 3 types of responses: (1) a pure EPSP which gave rise to one or two action potentials; (2) an EPSP which sometimes gave rise to a spike, followed by an IPSP; and (3) a pure IPSP. Opposite tectum stimulation evoked in the tectal cells either a pure IPSP or a pure EPSP. The mono- or polysynaptic nature of the pathways involved in the excitatory and inhibitory responses of the tectal cells was assessed by increasing the frequency of the optic nerve stimulation. At low stimulus rates (2-6 Hz), all the excitatory events showing latencies longer than 5 ms were blocked suggesting that they were polysynaptic. Excitatory events having latencies shorter than 5 ms were generally able to follow high rate frequencies of optic nerve stimulation (40, 50 or 90 Hz) and we considered them to be monosynaptic. All but 3 IPSPs evoked by optic nerve stimulation, were blocked by stimulus rates beyond 5 Hz. Thus, although most IPSPs are generated through polysynaptic paths, direct retino-tectal inhibitory paths may also exist. The latency of the responses of individual cells to optic nerve, visual Wulst and opposite tectum stimulation show that the polysynaptic IPSPs to optic nerve stimulation did not involve relays in the visual Wulst or the opposite tectum.     

The optic tectum of the carp: pyramidal neurons of the SFGS

We have studied the stratum fibrosum et griseum superficiale (SFGS) of the carp optic tectum with optic and electron microscopy. This stratum is a dense neuropil with disordered appearance, in which numerous neuronal bodies of different characteristics and variable distribution according to the tectal regions are intercalated, more abundant in the dorsomedial zones of the tectum. Within these neuronal types, the most characteristic of SFGS are the large pyramidals of vertical development. Such neurons shows an ascendant dendritic shaft, very developed in the stratum marginale, a thinner dendritic shaft in the basal pole and a descending axon that reaches the internal zones of the stratum griseum centrale. Glial elements are highly associated to the pyramidal neuron bodies. The synaptic contacts are abundant and of various types, specially on the spinous dendritic branches which lie in the stratum marginale.     

Development of retino-recipient projection neurons in the optic tectum of the chicken.

The dendritic development of a well-characterized retino-recipient neuronal type in the chicken optic tectum has been traced with intracellular labeling. Normal dendritic development can be divided into three phases: extension, differentiation and pruning. During the first phase, cells extend their dendrites, generate large dendritic fields and position their distal endings in a certain retino-recipient tectal layer. In the second phase, these dendritic endings arborize into characteristic bottlebrush-like structures, while the overall morphology of the neurons remains unaltered. After hatching, the number and width of the bottlebrush endings are reduced. The findings are discussed with respect to the innervation of the optic tectum by retinal afferents and possible guidance mechanisms for synapse formation in this system.     

Visual receptive thalamopetal neurons in the optic tectum of teleosts (holocentridae)

Tectal neurons previously known to receive retinofugal input were herein shown to project to the nucleus prethalamicus. Following HRP injections into the nucleus prethalamicus, pyriform neurons in the stratum periventiculare and stratum album centrale, and fusiform neurons in the stratum griseum centrale, were retrogradely labeled. Because the labeled types of neurons have been characterized as the main visual receptive neurons of the optic tectum, and because the nucleus prethalamicus of teleosts projects to the telencephalon, this nucleus can now be considered homologous to the nucleus rotondus of reptiles and birds and to the nucleus lateralis postterior-pulvinar complex of mammals, that is, it provides a relay for retinotectal visual input to the telencephalon. Orthogradely labeled terminals as well as retrogradely labeled neurons were also found in the dorsal area of the telencephalon. The tecto-prethalamotelencephalic projections are only ipsilateral.     

After-effects of moving textured background in motion-sensitive neurons of anuran optic tectum

Motion-sensitive neurons in anuran optic tectum were shown to respond to a stationary object centered in the excitatory receptive field, if a textured background moved for a while and then stopped ('motion after-response'). This motion after-response was attributed to a post-inhibitory rebound activation derived from effects of masking the excitatory receptive field center surrounded by an antagonistic inhibitory region. It was suggested that a similar rebound activation mechanism may also be involved in a certain type of perceptual motion after-effects in humans.     

Distribution within the barn owl's inferior colliculus of neurons projecting to the optic tectum and thalamus

Behavioral studies in barn owls indicate that both the optic tectum (OT) and the auditory arcopallium (AAr) mediate sound localization through the presence of neurons that respond only when sound comes from a circumscribed direction in space. The early stages of the computations leading to these so-called space-specific neurons are shared in a common brainstem pathway, which then splits at the level of the inferior colliculus (IC) such that the last computational stage is thought to be duplicated. The study presented here addresses whether the space-specific neurons in OT and AAr are indeed partially independent of each other by using anatomical methods more precise than those used in previous studies. Specifically, projection neurons in IC were retrogradely labelled with injections of fluorescein- and rhodamine-conjugated dextran amines into OT and nucleus ovoidalis (OV), the thalamic nucleus leading to AAr. By labelling the OT-projecting and OV-projecting neurons in the same owl, it was confirmed that neurons in IC project to either OV or OT but not both. However, although a segregation was generally observed between the medially positioned OV-projecting neurons and the laterally positioned OT-projecting neurons, there was also a slight overlap between the two populations. Moreover, electrolytic lesions demarcating physiological tuning properties indicate that many OV-projecting neurons are within the area containing space-specific neurons. These results highlight the need for more detailed studies elucidating the microcircuitry and corresponding physiology of IC, such as have been done in the cortices of the mammalian cerebellum and cerebrum.     

Localization of efferent sympathetic neurons and afferent neurons in dorsal root ganglia innervating the heart

The retrograde transport of horseradish peroxidase (HRP) has been used to study the localization and the number of neurons innervating the heart in the right stellate ganglion and accessory cervical ganglion, spinal cord and dorsal root ganglia of the cat. HRP was applied to the central cuts of anastomose of the stellate ganglion with the vagal nerve, of the vagosympathetic trunk caudal to anastomose and of the inferior cardiac nerve. HRP-labelled neurons were detected in the stellate ganglion in the regions which give off nerves, whereas in the accessory cervical ganglion labelled neurons were distributed throughout the whole ganglion. HRP-stained cells were found in the anastomose. In the spinal cord labelled neurons were detected in the lateral horn of T1-T5 segments. In the dorsal root ganglion the greatest number of neurons was observed in T2-T4 segments.     

Responses of the optic tectum to telencephalic stimulation in catfish

In order to test physiologically for cerebrotectal connections in a fish, averaged evoked potentials and unit responses were recorded from the optic tectum following electrical stimulation applied to the telencephalon in the siluroid teleost Ictalurus nebulosus. A single shock applied to the area dorsalis centralis (Dc) of the telencephalon, and only to this area, elicits a sequence of deflections in the ipsilateral optic tectum: an initial negative peak at about 8 ms, (= N8), a larger N25 and a slow P50-N95. The configurations, depth profiles, latencies and susceptibility to repetitive stimulation, together with the known tectal anatomy, suggest that the first wave is due to the afferent fibers from the telencephalon and that N25 is due to deep tectal neurons. Telencephalic input exerts a conditioning effect on the field potentials and unit responses evoked by direct optic nerve shock. Such a shock elicits, in the contralateral tectum, small negative, optic tract axon peaks followed by a large N6, believed to be postsynaptic, and a still later P12. As a first approximation it is argued that the telencephalic input and the retinal input are activating different sets of neuronal elements in the optic tectum, since the configuration and depth profile of the telencephalic and optic nerve shock-elicited potentials are different. A conditioning Dc stimulus has a long-lasting effect on the form of the optic nerve field potential, maximally when the pallial shock precedes the optic by about 90 ms. The effect, observed by subtracting the conditioned from the unconditioned tectal response to optic nerve shock, is a difference wave with N11 and P20. The unit activity from deep tectal laminae is either activated or accelerated following Dc stimulation, while superficially located neurons are not affected. In another group of tectal units, the optic nerve shock-induced response is depressed by a preceding pallial dorsalis centralis stimulus. The evidence is compatible with the assumption of direct projections from Dc to the deep layers of the tectum, but the timing could also permit indirect pathways. In any case, the influence is not simple or identical for different tectal cell classes.     

Developmental pattern of NADPH-diaphorase positive neurons in chick optic tectum is sensitive to changes in visual stimulation

The chick retinotectal system is a suitable model to investigate the mechanisms involved in the establishment of synaptic connections in whose refinement nitric oxide was implicated. The purpose of this work was to describe the developmental pattern of the nitric oxide synthase (NOS)-positive neurons as well as to determine if it is sensitive to changes in visual stimulation. The NADPH-diaphorase histochemical method was used to describe and quantify NOS neurons in normally stimulated and subnormally stimulated chickens. Nine types of NOS neurons were identified; seven of them express NOS until adulthood, while two of them show only a transient expression. The developmental pattern of NOS neurons follows the process of laminar segregation. It can be divided into three phases. The first includes the onset of NOS expression in periventricular neurons and the formation of a deep network of NOS fibers during early development. These neurons do not show any significant change in subnormally stimulated animals. The second phase includes the appearance of two transient NOS populations of bipolar neurons that occupy the intermediate layers during the optic fibers ingrowth. One of them significantly changes in subnormally stimulated chicks. The third phase occurs when the transitory expression of bipolar neurons decreases. It includes NOS expression in six neuronal populations that innervate the superficial retinorecipient layers. Most of these cells suffer plastic changes in subnormally stimulated chicks. The diversity of neuronal types with regard to their morphology, location, and sensitivity to visual stimulation strongly suggests that they serve different functions.     

Long-term potentiation in the goldfish optic tectum

Field potential recordings were made in the primary retinal synaptic area, the stratum fibrosum et griseum superficiale (SFGS), of an in vitro goldfish optic tectum preparation. Stimulation of the optic nerve at frequencies of 1 and 5 Hz produced a long-term potentiation (LTP) of the synaptic response which developed gradually. No potentiation was seen with lower or higher frequencies. These results demonstrate a significant LTP with a slow time course and restricted low-frequency dependence.