A simple cerebellar system: The lateral line lobe of the goldfish

Coping with flow: behavior, neurophysiology and modeling of the fish lateral line system

With the mechanosensory lateral line fish perceive water motions relative to their body surface and local pressure gradients. The lateral line plays an important role in many fish behaviors including the detection and localization of dipole sources and the tracking of prey fish. The sensory units of the lateral line are the neuromasts which are distributed across the surface of the animal. Water motions are received and transduced into neuronal signals by the neuromasts. These signals are conveyed by afferent nerve fibers to the fish brain and processed by lateral line neurons in parts of the brainstem, cerebellum, midbrain, and forebrain. In the cerebellum, midbrain, and forebrain, lateral line information is integrated with sensory information from other modalities. The present review introduces the peripheral morphology of the lateral line, and describes our understanding of lateral line physiology and behavior. It focuses on recent studies that have investigated: how fish behave in unsteady flow; what kind of sensory information is provided by flow; and how fish use and process this information. Finally, it reports new theoretical and biomimetic approaches to understand lateral line function.

Responses of brainstem lateral line units to different stimulus source locations and vibration directions

We recorded responses of lateral line units in the medial octavolateralis nucleus in the brainstem of goldfish, Carassius auratus, to a 50 Hz vibrating sphere and studied how responses were affected by placing the sphere at various locations alongside the fish and by different directions of vibration. In most units (88%), stimulation with the sphere from one or more spatial locations caused an increase and/or decrease in discharge rate. In few units (10%), discharge rate was increased by stimulation from one location and decreased by stimulation from an adjacent location in space. In a minority of the units (2%), changing sphere location did not affect discharge rates but caused a change in phase coupling. Units sensitive to a distinct sphere vibration direction were not found. The data also show that the responses of most brainstem units differ from those of primary afferent nerve fibers. Whereas primary afferents represent the pressure gradient pattern generated by the sphere and thus encode location and vibration direction of a vibrating sphere, most brainstem units do not. This information may be represented in the brainstem by a population code or in higher centers of the ascending lateral line pathway.

Brainstem lateral line responses to sinusoidal wave stimuli in the goldfish, Carassius auratus

Extracellular recordings were made from single lateral line units in the medial octavolateralis nucleus in the brainstem of goldfish, Carassius auratus. Units were defined as receiving lateral line input if they responded to the water motions generated by a stationary, sinusoidally oscillating sphere and/or a moving sphere but not to airborne sound and vibrations. Units which responded to airborne sound or vibrations were assumed to receive input from the inner ear and were not further investigated. Responses of lateral line units were quantified in terms of the number of evoked spikes and the degree of phase-locking to a 50 Hz vibrating sphere presented at various stationary locations along the side of the fish. Receptive fields were characterized based on spike rate, degree of phase-locking and average phase angle as a function of sphere location. Four groups of units were distinguished: 1, units with receptive fields comparable to those of primary afferents; 2, units with receptive fields which consisted of one excitatory and one inhibitory area; 3, units with receptive fields which consisted of more than two excitatory and/or inhibitory areas; 4, units with receptive fields which consisted of a single excitatory or a single inhibitory area. The receptive fields of most units were characterized by adjacent excitatory and inhibitory areas. This organization is reminiscent of excitatory-inhibitory receptive field organizations in other vertebrate sensory systems.

Interneurons of the ganglionic layer in the mormyrid electrosensory lateral line lobe: morphology, immunohistochemistry, and synaptology

This is the second paper in a series that describes the morphology, immunohistochemistry, and synaptology of the mormyrid electrosensory lateral line lobe (ELL). The ELL is a highly laminated cerebellum-like structure in the rhombencephalon that subserves an active electric sense: Objects in the nearby environment of the fish are detected on the basis of changes in the reafferent electrosensory signals that are generated by the animal's own electric organ discharge. The present paper describes interneurons in the superficial (molecular, ganglionic, and plexiform) layers of the ELL cortex that were analyzed in the light and electron microscopes after Golgi impregnation, intracellular labeling, neuroanatomical tracing, and gamma-aminobutyric acid (GABA) immunohistochemistry. The most numerous interneurons in the ganglionic layer are GABAergic medium-sized ganglionic (MG) cells and small ganglionic (SG) cells. MG cells have 10-20 spiny apical dendrites in the molecular layer, a cell body of 10-12 microns diameter in the ganglionic layer, a single basal dendrite that gives rise to fine, beaded, axon-like branches in either the plexiform layer (MG1 subtype) or the deeper granular layer (MG2 subtype), and an axon that terminates in the plexiform layer. Their apical dendritic tree has 12,000-22,000 spines that are contacted by GABA-negative terminals, and it receives, 1,250-2,500 GABA-positive contacts on the smooth dendritic surface between the spines. The average ratio of GABA-negative to GABA-positive contacts on the interneuron apical dendrites (14:1) is significantly higher than that for the efferent projection cells that have been described previously (Grant et al. [1996] J. Comp. Neurol., this issue). The somata and basal dendrites of MG cells receive a low to moderate density of GABAergic synaptic input, and their axons make GABAergic synaptic contacts with the somata and cell bodies of MG as well as with large ganglionic (LG) cells. SG cells probably represent immature, growing MG cells. Other interneurons in the superficial ELL layers include GABAergic stellate cells in the molecular layer, two types of non-GABAergic cells with smooth dendrites in the deep molecular layer that are named thick-smooth dendrite cells and deep molecular layer cells, and horizontal cells that are encountered particularly in the plexiform layer. Comparison with the ELL of waveform gymnotiform fish, which is another group of active electrolocating teleosts that has been investigated thoroughly, shows striking differences. In these fish, no GABAergic interneurons are found in the ganglionic (pyramidal) layer of the ELL, and GABA-negative interneurons with smooth dendrites in the molecular layer also seem to be lacking. At present, the phylogenetic origin of the described superficial interneurons in the mormyrid ELL is uncertain.

Projection neurons of the mormyrid electrosensory lateral line lobe: morphology, immunohistochemistry, and synaptology

This paper describes the morphological, immunohistochemical, and synaptic properties of projection neurons in the highly laminated medial and dorsolateral zones of the mormyrid electrosensory lateral line lobe (ELL). These structures are involved in active electrolocation, i.e., the detection and localization of objects in the nearby environment of the fish on the basis of changes in the reafferent electrosensory signal generated by the animal's own electric organ discharge. Electrosensory, corollary electromotor command-associated signals (corollary discharges), and a variety of other inputs are integrated within the ELL microcircuit. The organization of ELL projection neurons is analyzed at the light and electron microscopic levels based on Golgi impregnations, intracellular labeling, neuroanatomical tracer techniques, and gamma-aminobutyric acid (GABA), gamma-aminobutyric acid decarboxylase (GAD), and glutamate immunohistochemistry. Two main types of ELL projection neurons have been distinguished in mormyrids: large ganglionic (LG) and large fusiform (LF) cells. LG cells have a multipolar cell body (average diameter 13 microns) in the ganglionic layer, whereas LF cells have a fusiform cell body (on average, about 10 x 20 microns) in the granular layer. Apart from the location and shape of their soma, the morphological properties of these cell types are largely similar. They are glutamaterigic and project to the midbrain torus semicircularis, where their axon terminals make axodendritic synaptic contacts in the lateral nucleus. They have 6-12 apical dendrites in the molecular layer, with about 10,000 spines contacted by GABA-negative terminals and about 3,000 GABA-positive contacts on the smooth dendritic surface between the spines. Their somata and short, smooth basal dendrites, which arborize in the plexiform layer (LG cells) or in the granular layer (LF cells), are densely covered with GABA-positive, inhibitory terminals. Correlation with physiological data suggests that LG cells are I units, which are inhibited by stimulation of the center of their receptive fields, and LF cells are E units, excited by electric stimulation of the receptive field center. Comparison with the projection neurons of the ELL of gymnotiform fish, which constitute another group of active electrolocating teleosts, shows some striking differences, emphasizing the independent development of the ELL in both groups of teleosts.

Cytoarchitecture of the medial octavolateralis nucleus in the goldfish, Carassius auratus

The medial octavolateralis nucleus (MON) is the principal first-order medullary lateral line sensory nucleus found in the majority of anamniotic vertebrates. Although its presence has been confirmed in numerous taxa, the cytoarchitecture of this region has not been extensively studied in any species. The purpose of this study was to examine in detail the cytoarchitecture of the MON in the goldfish using Golgi staining and HRP histochemical techniques. The results of this study demonstrated the presence of a number of cell types with distinct cellular morphologies, several of which strongly resemble those described in octavolateralis nuclei dedicated to audition and electroreception. The most prominent of these MON neurons included crest cells of two varieties, either possessing or lacking basilar dendrites. Additionally, we described stellate and cristal interneurons and granule-like cells in the molecular layer, and lateral interneurons and granule-like neurons in deeper MON layers. These morphological similarities together with similarities in functional organization, and the probable close phyletic relationships of this "family" of hair cell sensory systems, argue for parallels in mechanisms of sensory processing and analysis in strongly divergent sensory modalities.

Fine structure of the medullary lateral line area of Chelon labrosus (order perciformes), a nonelectroreceptive teleost

The ultrastructure and synaptic organization of the nucleus medialis and cerebellar crest of the teleost Chelon labrosus have been investigated. The nucleus medialis receives projections from the anterior and posterior lateral line nerves. This nucleus consists of oval neurons and large crest cells ("Purkinje-like" cells) whose apical dendrites branch in the overlying molecular layer, the cerebellar crest. In the dorsal region of the nucleus medialis, the perikarya and smooth primary dendrites of the crest cells are interspersed among myelinated fibers and nerve boutons. The ventral layer of the nucleus medialis contains crest cell perikarya and dendrites as well as oval neurons. The cerebellar crest lacks neuronal bodies, but the apical dendrites of crest cells receive synapses from unmyelinated and myelinated fibers. In the cerebellar crest, two types of terminals are presynaptic to the crest cell dendrites: boutons with spherical vesicles that form asymmetric synapses with dendritic spines and boutons containing pleomorphic vesicles that form symmetric synapses directly on the dendritic shaft. Most axon terminals found on the somata and primary dendrites of crest cells in the nucleus medialis have pleomorphic vesicles and form symmetric contacts, though asymmetric synapses with spherical vesicles and mixed synapses can be observed; these mixed synapses exhibit gap junctions and contain spherical vesicles. Unlike crest cells, the oval neuron perikarya receive three types of contacts (symmetric, asymmetric, and mixed). The origins and functions of these different bouton types in the nucleus medialis are discussed.

Why run parallel fibers parallel? Teleostean Purkinje cells as possible coincidence detectors, in a timing device subserving spatial coding of temporal differences

The present paper explores the possible functional significance of the parallel orientation of parallel fibers in teleostean cerebellar and cerebelloid molecular layers, taking advantage of the restricted width of these molecular layers compared with mammalian ones and several specific configurations of granule cells. These configurations include: (i) a unilateral location, i.e. at only one (lateral) side of the molecular layer, giving rise to parallel fibers without bifurcation in a unidirectional molecular layer, where all parallel fibers conduct signals in the same direction; (ii) a bilateral location at both sides of the molecular layer giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from two discrete sources; and (iii) a basal (or sometimes apical) location underneath (or opposite to) the layer of Purkinje cells, giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from a continuous range of sources. It is argued that molecular layers with a bilateral location of granule cells, exemplified by the mormyrid lobus transitorius, represent an optimal configuration for the analysis of small temporal differences (up to 4 ms) between inputs to the right and left granule cell mass, by means of detection of the site of coincidence of parallel fiber activity running from left to right and vice versa. Morphological aspects that probably optimize such a function include not only the parallel course and bilateral origin of parallel fibers, but also their small diameter, large number and co-extensive location, as well as the sagittal orientation and the presence of many spines of Purkinje cell dendrites and the presence of stellate and other inhibitory interneurons. The only assumption underlying the present coincidence detection hypothesis is that Purkinje cells are supposed to be maximally stimulated by parallel fiber input when all spines are activated in such a way that their excitatory postsynaptic potentials reach the axon hillock simultaneously. For molecular layers with a unilateral location of granule cells, exemplified by the teleostean torus longitudinalis-tectal marginal parallel fiber system, a similar coincidence detecting mechanism is proposed on the basis of the presence of two populations of parallel fibers with slightly different conduction velocities. Such a system might be suitable to adapt the location of coincidence peaks to topographic maps present in deeper layers of nervous tissue. Molecular layers with basally (or apically) located granule cells as encountered in the teleostean corpus cerebelli, are probably involved in the analysis of specific spatio-temporal input waves directed centripetally towards different Purkinje cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiology of lateral line mechanoreceptive regions in the elasmobranch brain

The physiology of mechanoreceptive lateral line areas was investigated in the thornback guitarfish, Platyrhinoidis triseriata, from medulla to telecephalon, using averaged evoked potentials (AEPs) and unit responses as windows to brain functions. Responses were analysed with respect to frequency sensitivity, intensity functions, influence of stimulus repetition rate, response latency, receptive field (RF) organization and multimodal interaction. 1. Following a quasi-natural vibrating sphere stimulus, neural responses were recorded in the medullary medial octavolateralis nucleus (MON), the dorsal (DMN) and anterior (AN) nucleus of the mesencephalic nuclear complex, the diencephalic lateral tuberal nucleus (LTN), and a telencephalic area which may correspond to the medial pallium (Figs. 2, 3, 13, 14, 15, 16). 2. Within the test range of 6.5-200 Hz all lateral line areas investigated responded to minute water vibrations. Best frequencies (in terms of displacement) were between 75 and 200 Hz with threshold values for AEPs as low as 0.005 microns peak-to-peak (p-p) water displacement calculated at the skin surface (Fig. 6). 3. AEP-responses to a vibrating sphere stimulus recorded in the MON are tonic or phasic-tonic, i.e., responses are strongest at stimulus onset but last for the whole stimulus duration in form of a frequency following response (Fig. 3). DMN and AN responses are phasic or phasic-tonic. Units recorded in the MON are phase coupled to the stimulus, those recorded in the DMN, AN or LTN are usually not (Figs. 5, 8, 9). Diencephalic LTN and telencephalic lateral line responses (AEPs) often are purely phasic. However, in the diencephalic LTN tonic and/or off-responses can be recorded (Fig. 11). 4. For the frequencies 25, 50, and 100 Hz, the dynamic intensity range of lateral line areas varies from 12.8 to at least 91.6 dB (AEP) respectively 8.9 and 92 dB (few unit and single unit recordings) (Fig. 7). 5. Mesencephalic, diencephalic, and telecephalic RFs, based on the evaluation of AEPs or multiunit activity (MUA), are usually contralateral (AN and LTN) or ipsi- and contralateral (telencephalon) and often complex (Figs. 10, 12, 16). 6. In many cases no obvious interactions between different modalities (vibrating sphere, electric field stimulus, and/or a light flash) were seen. However, some recording sites in the mesencephalic AN and the diencephalic LTN showed bimodal interactions in that an electric field stimulus decreased or increased the amplitude of a lateral line response and vice versa (Fig. 13 B).

Distribution of afferent fibers in the brainstem from end organs in the ear and lateral line in the European eel

Sensory nerve fibers from the lateral line system and labyrinth of Anguilla anguilla were labeled with horseradish peroxidase and traced to various targets in the ipsilateral brainstem. The three rami of the anterior lateral line nerve and the supratemporal ramus of the posterior lateral line nerve form overlapping terminal zones in the ventral portion of nucleus medialis. The posterior lateral line nerve on the body is represented exclusively in the dorsal half of the nucleus medialis. Eighth nerve fibers from the otolithic end organs in the inner ear send fibers into dorsal portions of three octavus nuclei: anterior, magnocellular, and descending, and saccular fibers lie most medial and utricular fibers most lateral. Fibers from vestibular organs, especially the semicircular canals and utricle, end densely in ventral portions of these nuclei and in the tangential nucleus. All labyrinthine sense organs send fibers into the region of a Mauthner-like neuron, and all except the saccule terminate in the reticular formation, tangential nucleus, and eminentia granularis of the cerebellum. Primary sensory input to the octavolateralis efferent nucleus comes only from the labyrinth, and fibers from the saccule alone penetrate the region of efferent neuronal somata. Fibers from labyrinthine end organs except the saccule project to the reticular formation where they may contact the dendrites of efferent somata. Fibers from the lateral line and the eighth nerve overlap most extensively at the rostral pole of the nucleus medialis and in the eminentia granularis of the cerebellum.

The lateral line mechanoreceptive mesencephalic, diencephalic, and telencephalic regions in the thornback ray, Platyrhinoidis triseriata (Elasmobranchii)

Central lateral line pathways were mapped in the thronback ray, Platyrhinoidis triseriata, by analyzing depth profiles of averaged evoked potentials (AEPs), multiunit activity (MUA), and single unit recordings. Neural activity evoked by contra- or ipsilateral posterior lateral line nerve (pLLN) shock is restricted to the tectum mesencephali, the dorsomedial nucleus (DMN) and anterior nucleus (AN) of the mesencephalic nuclear complex, the posterior central thalamic nucleus (PCT), the lateral tuberal nucleus of the hypothalamus, and the deep medial pallium of the telencephalon (Figs. 2, 3, 4, 6, 7). Neural responses (AEPs and MUA) recorded in different lateral line areas differ with respect to shape, dynamic response properties, and/or latencies (Figs. 9, 10 and Table 1). Ipsilaterally recorded mesencephalic and diencephalic AEPs are less pronounced and of longer latency than their contralateral counterpart (Fig. 9 and Table 1). In contrast, AEP recorded in the telencephalon show a weak ipsilateral preference. If stimulated with a low amplitude water wave most DMN, AN, and tectal lateral line units respond in the frequency range 6.5 Hz to 200 Hz. Best frequencies (in terms of least displacement) are 75-150 Hz with a peak-to-peak water displacement of 0.04 micron sufficient to evoke a response in the most sensitive units (Fig. 11A, B, C). DMN and AN lateral line units have small excitatory receptive fields (RFs). Anterior, middle, and posterior body surfaces map onto the rostral, middle, and posterior brain surfaces of the contralateral DMN (Fig. 12). Some units recorded in the PCT are bimodal; they respond to a hydrodynamic flow field--generated with a ruler approaching the fish--only if the light is on and the eye facing the ruler is left uncovered (Fig. 13).

A monoclonal antibody to mammalian neurofilament protein stains somata and dendrites in gymnotid fish

Monoclonal antibody N210 (mabN210) recognizes the 210 kdalton neurofilament protein in mammals and gives characteristic immunocytochemical staining of neurofilament-rich processes. For example, in the cerebellum it recognizes myelinated axons and the calyx formed by basket cell axon collaterals. The distribution of mabN210 immunoreactivity was studied in the gymnotid brain (Apteronotus albifrons). In contrast to the mammalian distribution, mabN210 immunoreactivity was not found in most axons of the gymnotid brain. Instead, deposits of reaction product were present in the somata and dendrites of most neurons and were especially dense in those neurons with extensive dendritic trees, the Purkinje cells, pyramidal cells of the electrosensory lateral line lobe, the crest cells of the nucleus medialis and the pyramidal cells of the tectum. Electrosensory lateral line lobe pyramidal cells are known to contain few, if any, neurofilaments in their dendrites. Western blots of whole gymnotid brain proteins demonstrated that mabN210 recognizes two polypeptides apparent molecular weights 60 and 19 kdaltons. These proteins are thus antigenically similar to neurofilament protein and their expression in the gymnotid brain may be related to the peculiar dendritic branching pattern of Purkinje cells and similar cell types.

Connections of the auditory midbrain in a teleost fish, Cyprinus carpio

Central auditory pathways were traced in Japanese carp, Cyprinus carpio, using electrophysiological mapping and HRP tract-tracing methods. Multiunit recordings made from the carp torus semicircularis, the major midbrain area for processing octavolateralis information, revealed a mediolateral segregation of auditory and lateral line sensory modalities. Iontophoretic injections of HRP were made into the medial torus to trace afferent and efferent projections of the carp auditory midbrain. Following unilateral HRP injections into the medial torus, retrogradely labeled neurons were observed within six nuclei of the carp medulla. Two octaval nuclei, the anterior octavus nucleus and descending octavus nucleus, contained HRP-filled neurons. Labeled neurons were also observed within the ipsilateral superior olive, scattered among fibers of both lateral lemnisci, and bilaterally within the medullary reticular formation. In addition, bilateral retrograde cell labeling was found within a group of Purkinje-like cells located adjacent to the IVth ventricle, just rostral to the level of the VIIIth nerve. Few labeled neurons were found within the nucleus medialis, a principal target for lateral line afferents within the medulla. At midbrain levels, retrogradely labeled neurons were observed within the contralateral torus semicircularis and the ipsilateral optic tectum. Three forebrain nuclei project to the carp auditory midbrain. Within the diencephalon, descending projections originate from the anterior tuberal nucleus, bilaterally, and from the ipsilateral central posterior thalamic nucleus. The ipsilateral caudal telencephalon also projects to the carp auditory midbrain via large multipolar neurons within area dorsalis pars centralis. Anterograde labeling of fibers and terminals revealed efferent projections of the carp auditory midbrain to the following targets: the ipsilateral superior olive, the ipsilateral medullary reticular formation, the deep layers of the optic tectum, the contralateral torus semicircularis, the anterior tuberal nucleus, and the central posterior thalamic nucleus. These results, together with recent studies of lateral line pathways in teleosts (Finger, '80, '82a), demonstrate that central auditory and lateral line pathways are anatomically distinct in the carp, at least from medullary to diencephalic levels. Furthermore, there are striking similarities in the organization of the central auditory pathways of the carp and those of amphibians and land vertebrates.

Peripheral configuration and central projections of the lateral line system in Astronotus ocellatus (Cichlidae): a nonelectroreceptive teleost

The lateral line system of Astronotus ocellatus comprises one trunk canal, one tail canal, and three head canals. The sensory receptors on the head are innervated by rami of the dorsal anterior, ventral anterior, and posterior lateral line nerves, and those along the trunk and tail by rami of the posterior lateral line nerve. The peripheral configuration of lateral line canals and nerves was examined in whole mount preparations, the central connections of restricted groups of endorgans studied using HRP and degeneration methods, and the neuronal morphology and cytoarchitecture of the lateralis region investigated with Nissl, silver, and Golgi methods. Neurons of the lateralis cell column are diffusely arrayed and of variable morphology. They are oriented primarily in the transverse plane and, with the exception of a dorsal lamina of large multipolar cells, are not organized into zones. Lateralis fibers bifurcate on entering the brainstem, course in lateral tracts, and give off medially directed collaterals to terminate in the ipsilateral nucleus medialis and nucleus caudalis. In addition, fibers terminate in the eminentia granularis of the cerebellum, but only fibers supplying endorgans in the head canals penetrate the granule cell zone of the cerebellar corpus. Fibers supplying sense organs in adjacent canals overlap in their central endings, whereas fibers of distantly separated receptors do not overlap. The rami supplying trunk and tail canal organs do not project as far rostrally in the central neuropil as do the other rami. Endings of posterior lateral line fibers lie dorsal to those of the anterior lateral line nerves, and some lateralis fibers terminate within the confines of the magnocellular, descending, and posterior nuclei of the octavus column. Although there is spatial order to the lateralis projections, there is no clear somatotopic organization in the lateralis region.

Localization of vitamin D-dependent calcium binding protein in the electrosensory and electromotor system of high frequency gymnotid fish

Vitamin D-dependent calcium binding protein (D-CaBP) was localized in the brains of high frequency gymnotid fish. In birds and mammals this protein is seen in a variety of cell types including Purkinje cells, inferior olivary cells and CA1 pyramids of the hippocampus. This distribution has led us to speculate that D-CaBP may be important in buffering intracellular calcium, perhaps more specifically that calcium which enters the cell during dendritic calcium spikes. In the gymnotid fish D-CaBP was found in many of the same cell types in which it is also seen in birds and mammals. In addition, D-CaBP is specifically present in neurons which drive the electric organ (pacemaker and relay cells) and neurons within the electrosensory system which are phase-locked to the electric organ discharge (spherical and giant cells). Relay cells and giant cells have exceptionally high concentrations of D-CaBP. These cells do not exhibit calcium spikes and the role of their D-CaBP may be to regulate calcium released from intracellular stores.

Central projections of the octavolateralis nerves of the clearnose skate, Raja eglanteria

The central projections of first-order lateral line and octavus nerve afferents of the clearnose skate, Raja eglanteria, were determined by nerve degeneration and horseradish peroxidase techniques. The octavolateralis area of the medulla, which receives these afferents, is organized into dorsal, intermediate, and ventral longitudinal columns of cells and neuropil. Fibers that innervate the electroreceptive sense organs enter the dorsal longitudinal column via the dorsal root of the anterior lateral line nerve and terminate within the dorsal nucleus. Mechanoreceptive fibers from neuromasts of the head and trunk are carried by the ventral root of the anterior lateral line nerve and posterior lateral line nerve, respectively. Both nerves enter the intermediate longitudinal column and terminate throughout the rostrocaudal extent of the intermediate nucleus. Fibers of the ventral root of the anterior lateral line nerve are confined to the medial portion of the intermediate nucleus and posterior lateral line nerve fibers to the lateral portion. In addition, ascending mechanoreceptive fibers from both head and trunk neuromasts project to the vestibulolateral lobe of the cerebellum. Octavus nerve afferents enter the medulla and terminate primarily within the four octaval nuclei that comprise the ventral longitudinal column. Rostrocaudally, these nuclei are the anterior, magnocellular, descending, and posterior octaval nuclei. A few ascending axons continue beyond the anterior octaval nucleus and course to the vestibulolateral lobe of the cerebellum. Some descending axons emanate from the descending octaval nucleus and course to the reticular formation and intermediate nucleus. Therefore, electroreceptive lateral line, mechanoreceptive lateral line, and octavus nerve afferents project ipsilaterally and terminate predominantly within separate medullary nuclei. The significance of octavus nerve projections to the intermediate nucleus and overlap of mechanoreceptive and octavus afferents within the vestibulolateral lobe of the cerebellum cannot be determined until it is known which fibers of the inner ear sense organs project to these areas. Retrograde transport of horseradish peroxidase results in the labeling of large multipolar cells, both ipsilaterally and contralaterally, within a column of gray that lies dorsolateral to the reticular formation. These cells are interpreted as the cell of origin of the efferent components of the anterior and posterior lateral line nerves.

Octavolateral projections to the torus semicircularis of the trout, Salmo gairdneri

Microiontophoretic delivery of horseradish peroxidase in the torus semicircularis of the trout resulted in heavy labeling of somata in the rhombencephalic nucleus intermedius octavolateralis and nucleus octavus magnocellularis. In addition some labeled somata were found closely to the fasciculus longitudinalis lateralis and in the diencephalon. Efferents leave the torus to the diencephalon, the tegmentum and the tectum and, via the fasciculus longitudinalis lateralis, to the rhombencephalon and the spinal cord. It is concluded that in the trout the torus receives octaval and lateral line input mainly directly from the octavolateral area in the hindbrain, without involvement of a superior olive.

The mormyrid mesencephalon. II. The medio-dorsal nucleus of the torus semicircularis: afferent and efferent connections studied with the HRP method

The connections of the medio-dorsal nucleus (nMD) were established with the HRP tracing method in two mormyrid species, Brienomyrus niger and Gnathonemus petersii. The medio-dorsal nucleus (nMD) is the second largest nucleus of the mormyrid torus semicircularis which comprises 7 nuclei. According to our histological observations, the nMD is composed of three regions: anterior (a), medialis (m) and posterior (p), each has distinct connections. The nMD receives medullary, rhombencephalic inputs, and mesencephalic inputs. The rhombencephalic afferents arising from the acoustico-lateral area constitute the most extensive projection: they arise from nucleus octavius, nuclear anterior and a 'crest cell layer'. The largest part of these rhombo-mesencephalic connections is bilateral. The bifurcated axons are gathered in the pars medialis of the lemnisci laterales and end, respectively, in the three subdivisions--pars posterior, pars medialis, pars anterior--of the nMD. The contralateral projections are larger than the ipsilateral ones except for the octavo-mesencephalic projection, where the ipsilateral is predominant. Collaterals of these axons end bilaterally in the mesencephalic paralemniscal nuclei (nPL). The mesencephalic afferents originate in the contralateral nPL, while medullary afferents arise in the contralateral nucleus subfunicularis (nSF). None of these projections show topological specificity within the nMD. Concerning the efferent connections, the nMD projects to the ipsilateral tectum opticum and valvula cerebelli.

Choline acetyltransferase activity in the cerebellum and in centers of lateral line system of teleosts

Using choline acetyltransferase as a marker of cholinergic activity, different cerebellar areas and brainstem nuclei were assayed in the goldfish and the catfish. Enzyme activity resulted remarkedly higher in archicerebellum than in paleocerebellum. In addition three brainstem nuclei related to lateral line system, showed high or very high levels of choline acetyltransferase. The results suggest that cholinergic transmission would play an important role in central circuits of lateral line system, including the projection towards archicerebellar areas.