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PubMed1997

Phenotypic specification of hindbrain rhombomeres and the origins of rhythmic circuits in vertebrates.

This essay considers the ontogeny and phylogeny of the cranial neural circuitry producing rhythmic behaviors in vertebrates. These behaviors are characterized by predictable temporal patterns established by a neuronal network variously referred to as either a pacemaker, neural oscillator or central pattern generator. Comparative vertebrate studies have demonstrated that the embryonic hindbrain is divided into segmented compartments called rhombomeres, each of which gives rise to a distinct complement of cranial motoneurons and, as yet, unidentified populations of interneurons. We now propose that novel rhythmic circuits were innovations associated with the adoption of cardiac and respiratory pumps during the protochordate-vertebrate transition. We further suggest that the pattern-generating circuits of more recent innovations, such as the vocal, electromotor and extraocular systems, have originated from the same Hox gene-specified compartments of the embryonic hindbrain (rhombomeres 7-8) that gave rise to rhythmically active cardiac and respiratory circuits. Lastly, we propose that the capability for pattern generation by neurons originating from rhombomeres 7 and 8 is due to their electroresponsive properties producing pacemaker oscillations, as best typified by the inferior olive which also has origins from these same hindbrain compartments and has been suggested to establish rhythmic oscillations coupled to sensorimotor function throughout the neuraxis of vertebrates.

Animals↗

PubMed1995

The visual system of the Florida garfish, Lepisosteus platyrhincus (Ginglymodi). IV. Bilateral projections and the binocular visual field.

Infusion of cobaltous-lysine into the optic nerve of juvenile Florida garfish reveals that the preoptic area, pretectum, thalamus and the mediorostral and ventrolateral poles of the optic tectum each receive bilateral, monosynaptic input. The bilateral input to the mediorostral pole of the tectum projects via the dorsal optic tract and terminates in the rostral half of the tectum. The bilateral input to the ventrolateral tectum projects via the ventral optic tract and extends the whole length of the tectum. The incidence of direct ipsilateral input to the tectum suggests binocular vision may play a functional role in the survival of this species. In order to test this hypothesis, we examined the retinal distribution of ipsilateral retinal ganglion cells that project to the mediorostral tectum and the size and location of the binocular visual field. Cell distribution was examined by retrograde labelling with rhodamine-conjugated dextran amine delivered by microinjections into the mediorostral pole of the right optic tectum. In the ipsilateral retina, ganglion cells are distributed in a narrow temporoventral area with a mean of 0.44 +/- 0.14 x 10(3) cells per mm2 apposing the retinal margin. In the contralateral retina, ganglion cells are also distributed within this temporoventral region with a mean peak density of 2.33 +/- 0.47 x 10(3) cells per mm2. Three broad classes of ipsilaterally projecting retinal ganglion cells (orthotopic cells within the ganglion cell layer, displaced cells within the inner nuclear layer, and giant cells within the ganglion and inner nuclear layers) are identified, intermingled in both the ipsilateral and contralateral retinae after retrograde labelling from the mediorostral pole of the tectum. Ophthalmoscopic mapping of the monocular and binocular visual fields reveals two small frontal wedges of binocular overlap. A dorsal wedge (12 degrees wide) extends from approximately 7 degrees above the horizontal axis to 10 degrees beyond the vertical axis, and a ventral wedge (20 degrees wide) extends approximately 10 degrees below the horizontal axis to 15 degrees beyond the vertical axis. The dorsal binocular visual field is subtended by the temporoventral region of the retina possessing both ipsilaterally and contralaterally projecting ganglion cells which terminate in the mediorostral pole of the optic tectum. Therefore, the partial decussation of retinal ganglion cell axons at the optic chiasm brings information from corresponding regions of the binocular visual field into register in the mediorostral pole of the optic tectum.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Descending telencephalic information reaches longitudinal torus and cerebellum via the dorsal preglomerular nucleus in the teleost fish, Pantodon buchholzi: a case of neural preaptation?

The weakly electric mormyrids are known to have an ascending neuronal pathway that reaches the diencephalon and carries information concerned with electrolocation. The recipient diencephalic center, the dorsal preglomerular nucleus, receives a massive telencephalic input and projects to the corpus and valvula cerebelli. This circuitry has been interpreted as a uniquely derived (autapomorphic) feature for mormyrids. In the present study, we demonstrate with the fluorescent neuronal tracer DiI that the closely related, but non-electroreceptive, teleost Pantodon buchholzi possesses a dorsal preglomerular nucleus with similar telencephalo-cerebellar circuitry. The projection to the cerebellum only reaches the corpus, however, not the valvula cerebelli. Further, the dorsal preglomerular nucleus of Pantodon displays a descending pathway via the torus longitudinalis. Two phylogenetic interpretations for the presence of telencephalo-cerebellar pathways in both mormyrids and Pantodon are possible: if such a pathway existed as a preaptation in the common ancestor of mormyrids and Pantodon, it must be an exaptation for electroreception in mormyrids, since this sensory modality evolved anew in this teleost group; alternatively, the pathway evolved in parallel homoplasy, once in Pantodon, as part of a descending premotor pathway, and independently in mormyrids, where the system gains access to ascending electrosensory information.

Animals↗

PubMed1994

Central topography of anterior lateral line nerve projections in the channel catfish, Ictalurus punctatus.

Electroreception evolved in the catfishes probably as a specialization of the mechanosensory lateral line system. Fibers of the anterior lateral line nerve in catfishes innervate electrosensory ampullary organs and mechanosensory neuromasts of the head lateral line system. The purpose of this study is to determine the projection patterns of the major principal branches of the ALLN and to investigate the topography within the various nuclei of the terminal fields of these different branches. Fibers of the superficial ophthalmic, buccal and hyomandibular branches of the anterior lateral line nerve terminate in a somatotopic fashion within medullary and cerebellar nuclei. These fibers project to, and terminate within, several discrete nuclei in the medulla and cerebellum, notably the electrosensory lateral line lobe, the medial and caudal octavolateralis nuclei, and portions of a nuclear complex in the cerebellum called the eminentia granularis. Furthermore, the dorsoventral somatotopy in the medullary electrosensory nucleus is a reversed or 'mirror' image of that in the mechanosensory nucleus. This reversed map is similar to that observed in other electrosensory systems and suggests that there may be a common mechanism for the copying and preservation of spatial information as new systems are evolved from primitive sensory pathways.

Afferent Pathways↗

PubMed1994

Cytoarchitecture and staining for acetylcholinesterase and zinc in the visual cortex of the Parma wallaby (Macropus parma).

Cytoarchitecture and distributions of acetylcholinesterase-containing fibers and zinc-containing neuropil are described in the primary visual cortex of a metatherian, the Parma wallaby (Macropus parma). Although some cytoarchitectural features of layer III suggest that a subdivision and a corresponding layer IIIa-c terminology may be employed, neurochemistry is more concisely described using a simple layer I-VI terminology. Horizontal acetylcholinesterase-containing fibers in layer I take on a vertical course as they pass through layer II to form a reticulate pattern in upper layer III. Fibers gather into vertical groups in lower layer III and, markedly, in layer IV. No dominant orientation is seen in layer V, whereas an horizontal course is often seen in layer VI. Zinc-containing neuropil is very dense layers I, II and upper III, where particles are homogeneously distributed. Particles begin to form strands in lower layer III and form conspicuous vertical clusters in an otherwise almost unstained layer IV. There is a sharp transition to a densely and homogeneously stained layer V, a decrease in staining intensity in layer VI, and a further decrease within layer VI towards the white matter. The distribution of acetylcholinesterase-containing fibers in M. parma, a metatherian representative, adds to the number of species-specific patterns observed in eutherians. The distribution of zinc-containing neuropil in M. parma resembles the consistent pattern seen in eutherian species. Thus, pathways incorporating zinc, as a putative neuromodulator of excitatory amino acid receptor responses, may be organized similarly in the cortex of meta- and eutherians.

Acetylcholinesterase↗

PubMed1994

Laminar distribution of the cells of origin of ascending and descending tectofugal pathways in turtles: implications for the evolution of tectal lamination.

The major outputs of the tectum appear fundamentally similar in all vertebrate species, suggesting that the major types of tectal output neurons have been evolutionarily conserved across a wide variety of vertebrate species. Nonetheless, tectal lamination patterns vary dramatically among vertebrates. To explore the basis of this variation in lamination at a neuronal level, the laminar distribution of the cells of origin of the major ascending and descending tectofugal pathways was determined in turtles using retrograde HRP labeling and the results compared to those from similar studies in diverse other species. Four major tectal outputs were studied: 1) the ipsilateral ascending projection to nucleus rotundus of thalamus; 2) the ipsilateral ascending projection to the dorsal and ventral lateral geniculate nuclei of the thalamus; 3) the crossed descending projection to paramedian regions of the pons and medulla; and 4) the ipsilateral descending projection to cell groups of the ventrolateral pons and medulla. The projection to nucleus rotundus was found to arise exclusively from multipolar neurons of the stratum griseum centrale, while the projection to the geniculate nuclei was found to arise from radial cells with long ascending dendrites in the stratum griseum periventriculare. The crossed descending projection to the paramedian hindbrain was found to arise almost exclusively from large multipolar neurons, the majority of which were located in the stratum griseum centrale and some of which were located in the stratum griseum periventriculare. This pathway was also found to give rise to a collateral projection that ascends in the ipsilateral hypothalamus. The ipsilateral descending projection was found to arise from multipolar neurons in the stratum griseum centrale and stratum griseum periventriculare, from radial and pyramidal neurons in the stratum griseum periventriculare, and from radial neurons in stratum griseum et fibrosum superficiale. These results show that the laminar distribution of the cells of origin of tectofugal pathways in turtles does not fit the simple notion that the midbrain roof is organized into superficial layers containing neurons with ascending projections and deep layers containing neurons with descending projections. Rather, individual layers in turtles appear to give rise to both ascending and descending projections, with there being evidence that each projection arises largely from a separate set of neurons. The pattern of distribution of the cells of origin of these various pathways is different in turtles than in either birds, mammals, frogs, salamanders or teleosts. Such results suggest that considerable interspecific variation occurs in the extent to which individual homologous neuronal populations migrate superficially during tectal development.

Animals↗

PubMed1993

The role of the cerebellum in motor control and perception.

The cerebellum has an important role in control and coordination of movements, but in some species, notably weakly electric fish of the family Mormyridae, anatomical, electrophysiological and behavioural evidence indicates that parts of cerebellar cortex are concerned with tracking movements of objects around the animal, rather than with controlling movements of the animal itself. The existence of such anomalies suggests that the cerebellum may not be exclusively, or even primarily, a structure for motor control. Evidence reviewed in this paper shows that the cerebellum is associated with sensory systems used for tracking movements of targets in the environment, as well as movements made by the animal itself, in all vertebrates, not just in a few isolated cases. The evidence indicates that the standard theory that the function of the cerebellum is control and coordination of movements only partially characterises cerebellar function. The cerebellum may be better characterised as a tracking system, with an important role in control and coordination of movements which arises because of an animal's need to track moving objects, to track its own movements, and to analyse the sensory consequences of movements in order to control movements. This theory not only predicts the known motor consequences of cerebellar dysfunction, it also predicts a specific kind of perceptual deficit caused by cerebellar dysfunction, namely an inability to accurately follow and predict trajectories of objects moving in the environment. A variety of behavioural and perceptual tasks in addition to motor control and movement tracking may require dynamical state estimation, and therefore may involve the cerebellum.

Animals↗

PubMed1993

The diencephalon and optic tectum of the longnose gar, Lepisosteus osseus (L.): cytoarchitectonics and distribution of acetylcholinesterase.

The cytoarchitecture of nuclei in the diencephalon and the distribution of acetylcholinesterase (AChE) in the diencephalon and optic tectum were analyzed in the longnose gar, Lepisosteus osseus, a non-teleost actinopterygian fish. Nuclei were identified in the preoptic area, thalamus, posterior tubercle, hypothalamus, synencephalon, and pretectum which are homologous to like-named nuclei in teleosts and other non-teleost actinopterygians. Of particular note, a nucleus in the rostral diencephalon, nucleus rostrolateralis, which has previously been identified only in the osteoglossomorph Pantodon, is present in the long-nose gar. The posterior pretectal nucleus, previously identified in teleosts and in the bowfin Amia, is also present in gars. The small size of the posterior pretectal nucleus in gars supports the hypothesis that this nucleus was small plesiomorphically. The distribution of AChE in the diencephalon and optic tectum corresponds in most respects to that found in teleosts. The superficial pretectal nuclei, including the posterior pretectal nucleus, are strongly positive for AChE. In contrast, most of the nuclei within the preglomerular complex are negative for AChE. Acetylcholinesterase is present in some of the fibers in the optic tracts and in most retinorecipient nuclei, as well as in some other nuclei and tracts.

Acetylcholinesterase↗

PubMed1993

Distribution of types of neurons in the optic tectum of the small-spotted dogfish, Scyliorhinus canicula L. A Golgi study.

The distribution of neuronal types in the six layers of the optic tectum of the small-spotted dogfish, Scyliorhinus canicula, was studied using Golgi methods. The eight types of neurons have a variable distribution, some of them represented in all six tectal layers. Most neurons of the optic tectum have a radial orientation, and a small portion are disposed horizontally. The relative ratios of the different neuronal types in the six layers indicate that about 80% of the tectal neurons are intermingled with the optic nerve axon terminals in the superficial layers of the tectum. These results corroborate previous reports that the tectum of this selachian species is more specialized than formerly supposed, in spite of the differences in degree of neuropil lamination with other non-mammalian vertebrates.

Animals↗

PubMed1993

Auditory projections to the anterior telencephalon in the budgerigar (Melopsittacus undulatus).

The connections of a higher order auditory area in the neostriatum intermedium pars ventrolateralis (NIVL) were mapped with pathway tracing techniques in order to elucidate possible pathways by which auditory feedback may influence vocal learning in the budgerigar (Melopsittacus undulatus). Previous research has shown that NIVL receives projections from Field 'L' as well as adjacent portions of the dorsolateral neostriatum intermedium (NIDL) and hyperstriatum ventrale (HV) and, therefore, may be homologous to previously described auditory centers in the dorsal and lateral portions of the auditory neostriatum of songbirds. The efferent connections of NIVL terminate within a small portion of the rostromedial archistriatum as well as a more rostrally situated area within the medial neostriatum intermedium (NI) and HV. Near by (but not overlapping) fields in NI and HV receive input from the nucleus dorsomedialis posterior thalami (DMP), the archistriatum and ectostriatum. Interestingly, only the DMP projection field overlaps a neural field known to be related to the vocal motor system. The DMP projection field corresponds to that previously described as the magnocellular nucleus of the anterior neostriatum; this nucleus is known to project to the higher vocal center in the budgerigar. In addition to projections from NIVL to medial NI and HIV, auditory information is relayed to the anterolateral telencephalon directly from the brainstem via the ventrolateral nucleus of the lateral lemniscus (VLV). This latter pathway appears comparable to that described in pigeons derived from the intermediate nucleus of the lateral lemniscus. The projection field of VLV overlaps a restricted portion of the caudal and medial aspect of nucleus basalis. These results support the notion that many aspects of telencephalic auditory pathways in birds are primitive characters, although a direct connection between auditory and vocal motor circuits was not found in the present study.

Animals↗

PubMed1993

Ampullary sense organs, peripheral, central and behavioral electroreception in chimeras (Hydrolagus, Holocephali, Chondrichthyes).

Ampullary sense organs are distributed in groups over the head of Hydrolagus colliei with their pores in clusters and innervated by the buccal, hyomandibular and superficial ophthalmic branches of the anterior lateral line nerve. The ampullae contain ciliated sense cells in an alveolate-shaped epithelium, which communicates to the surface through a jelly-filled tube. The sense cells synapse at their bases with the afferent nerve fibers that terminate in the dorsal nucleus of the anterior lateral line lobe of the medulla. The anatomy and ultrastructure support the homology with the ampullae of Lorenzini of elasmobranchs. Single units recorded from the buccal branch of the anterior lateral line nerve are either lateral line or ampullary in character, the former being sensitive only to mechanical stimuli, the latter to both mechanical and to weak electric stimuli. They are also distinguished by the positions of their receptive fields. The electroreceptive units are spontaneously active and are excited by a cathode placed near the opening of their pore and inhibited by an anode. Compound evoked potentials are recorded from beneath the lateral aspect of the tectum in response to weak electric fields in the bath. Each recording locus has a best position and orientation of the electric field. The electric fields are effective if their duration is longer than ca. 2 ms; longer than 10 ms makes no difference until an OFF effect becomes distinct at ca. 50 ms. The reception is tuned to low frequencies but is not sensitive to maintained current (DC). Evoked potentials summating moderate numbers of responses are clear at < 1 microV/cm. Ratfish were conditioned in a ring-shaped tank to reverse the direction of swimming when an electric field was switched ON. The stimulus was a 5 Hz square wave or the onset of a DC of 1-10 microA between a pair of electrodes on the floor of the tank. The fish responded to fields as weak as 0.2 microV/cm. A specialized sense modality for electroreception, similar to that in elasmobranchs and most other groups of nonteleost fishes, except for Myxini and Neopterygii (holosteans), is present in the subclass Holocephali. The notion is supported that this modality and its central as well as peripheral apparatus arose early in the evolution of vertebrates. Only two losses of the whole system need be hypothesized, on this idea, once in the ancestors of the hagfishes and once in the ancestors of the neopterygians, which include the teleosts. Some orders of teleosts then evolved a new system of electroreception independently. The ciliary receptor cells are probably primitive; microvillar sense cells evolved independently.

Animals↗

PubMed1993

Organization of catecholaminergic systems in the hypothalamus of two elasmobranch species, Raja undulata and Scyliorhinus canicula. A histofluorescence and immunohistochemical study.

We examined the organization of catecholaminergic neurons in the hypothalamus of the painted ray, Raja undulata, and the small-spotted dogfish, Scyliorhinus canicula, with the use of formaldehyde-induced fluorescence (FIF) methods and tyrosine hydroxylase (TH) immunohistochemistry. In both species we identified distinct populations of catecholamine-containing neurons differing in a) their immunoreactivity to antibodies against the enzyme tyrosine hydroxylase (TH), b) their fluorescence in response to FIF methods for the detection of catecholamines, and c) their relationship with the third ventricle. One population is made up of FIF-positive and TH-negative neurons (most of which are CSF [cerebrospinal fluid]-contacting) and located in two circumventricular organs, the preoptic recess organ and the organon vasculosum hypothalami. Another population comprises TH-immunoreactive (TH-IR), FIF negative neurons that are located in the suprachiasmatic nucleus and the posterior tuberculum and are not related to the third ventricle recesses. A third population of TH-IR, CSF-contacting neurons is also present in the organon vasculosum hypothalami. The existence of three catecholaminergic populations suggests differences in the metabolism of catecholamines and/or different functions. The circumventricular neurons are not associated with the hypophysis and appear to accumulate catecholamine (dopamine) obtained from exogenous sources. In both Raja and Scyliorhinus the neurointermediate lobe is innervated by TH-IR fibres originating from dopamine-synthesizing neurons of the second catecholaminergic population.

Animals↗

PubMed1992

Afferent and efferent connections of the bullfrog medial pallium.

Horseradish peroxidase or tritiated proline was unilaterally injected into the medial pallium in bullfrogs in order to determine the sources of afferent projections to the medial pallium and the targets of pallial efferent projections. Some cells in all telencephalic centers, except the corpus striatum and the pars lateralis of the amygdala, project to the ipsilateral medial pallium. The medial pallium receives projections from fewer centers in the contralateral hemisphere, which include the medial septal nucleus, the pars medialis of the amygdala, the bed nucleus of the pallial commissure and the medial pallium. The raphe nucleus and the anterior thalamic nuclei appear to be the only sources of afferents to the medial pallium from outside the telencephalon. Efferents of the medial pallium are far more extensive than reported in earlier studies. The medial pallium projects ipsilaterally to all telencephalic nuclei, with the exception of a large part of the corpus striatum, and contralaterally to the medial septal nucleus, the olfactory tubercle, amygdala, medial pallium and bed nucleus of the pallial commissure. Extensive efferent projections also terminate in preoptic and hypothalamic regions, as well as in most thalamic relay nuclei, the pretectum and, possibly, the optic tectum. Similarities to the medial pallium in other tetrapods and to that in mammals suggest that the medial pallium in anurans is homologous to the subicular and CA fields and, possibly, the dentate gyrus in mammals. However, the extensive projections of the medial pallium to the dorsal thalamus and pretectum in anurans may be primitive features of the medial pallium retained in anurans, or uniquely derived features in anurans.

Afferent Pathways↗

PubMed1992

The role of heterarchical control in the evolution of central pattern generators.

The acceptance of the concept of central pattern generators (CPGs) led to the perception that descending inputs initiate stereotyped movements, such as locomotion, but play relatively minor roles after the movement begins. Sensory input could entrain the CPG, and the CPG was responsive to the proper inputs for switching, etc. Evidence is here presented that the influences of both descending and sensory inputs are two-way. Descending inputs are shown to be involved in an ongoing manner during locomotion, as it has been found that CPGs are phasically driving the same descending systems that themselves activate the CPGs. Similarly, sensory inputs are being actively processed by the CPG and, here again, produce a two-way interaction between sensory input and CPGs. Finally, mechanical factors are shown to be major contributors to the form of the movement. Thus, overall the CPG can only be considered as one of several contributors to any movement; all concurrently process the flow of information. Control is viewed as distributed, that is, as heterarchical as opposed to hierarchical. Because of the complexity of interaction between the levels of the system, it is argued that any change in the body will propagate through the system and effect the final output regardless of where the change originates. Thus, ontogenetic and phylogenetic changes must have influences felt throughout the entire system. Examples are presented to demonstrate that this is, indeed, the case. The effect of these changes will primarily be manifest in the interface between the layers, that is, in the interface between the descending and sensory inputs and the CPG, so that the changes can be adaptively accommodated on a moment by moment basis when necessary.

Animals↗

PubMed1992

Physiological characterization of lateral line function in the Antarctic fish Trematomus bernacchii.

The Antarctic notothenioids are a monophyletic radiation of fishes that have evolved under conditions of low light and cold, where non-visual sensory systems, such as the mechanosensory lateral line system, would be of importance. As part of a study of the structure and function of the mechanosensory lateral line system in these fishes we systematically characterized the function of the anterior lateral line system in one of the common benthic species, Trematomus bernacchii. Frequency-response and threshold-tuning curve methods yield similar functional characterizations of single afferent fibre responses to vibrational stimulation. Curve fitting of generalized transfer function to frequency-response curves allows an objective splitting of responses into velocity and acceleration sensitive populations thought to correspond to superficial and canal neuromasts, respectively. Both response types are characterized by a low-pass frequency-response curve, with a relatively low upper frequency cut-off when compared with data from temperate species. The sensitivity of the lateral line system of T. bernacchii is comparable to that of the mottled sculpin Cottus bairdi.

Acceleration↗

PubMed1992

Variation of tectal morphology in teleost fishes.

The morphological organization of the optic tectum was analyzed in a number of divergent teleost species. Particular attention was focused on the organization of the periventricular gray zone and of afferent and efferent fiber systems within or adjacent to it. A dorsal part of the periventricular gray zone is recognized which is in register with the distribution of tangentially and radially oriented fascicles. A lateral part of the periventricular gray can be identified as being lateral to the lateral-most of the tangentially and radially oriented fascicles and being in register with a large number of circular fascicles in the deep white zone. Circular fascicles pass between the lateral part of the periventricular gray and a more ventral part. There are few if any circular fascicles in the part of the deep white zone that is in register with the ventral part of the gray. These parts of the periventricular gray are most clearly apparent in clupeomorph fishes, due to an exaggerated separation related to the presence of two tectal lobes within the hemisphere. It is hypothesized that the dorsal part of the gray, as can be recognized cytoarchitectonically, is in register with the projection of the dorsal optic tract and that the lateral and ventral parts of the gray are in register with the projection of the ventral optic tract.

Animals↗

PubMed1992

Central projections of the vagus nerve in Chelon labrosus Risso (Teleostei, O. Perciformes).

The primary projections of the vagus nerve and its nuclei in a perciform teleost, Chelon labrosus, are described after application of hoseradish peroxidase (HRP) labeling and Nissl and silver staining methods. The vagus nerve has three peripheral roots, rostral, medial and caudal, which are formed from viscerosensory and visceromotor rootlets. In addition to viscerosensory and visceromotor rootlets, the caudal root has a cutaneous root and a 'descending' root. Dorsally in the caudal root, a fiber bundle enters the medulla from the retroocular and opercular skin region; this bundle joins the descending trigeminal root and courses with it rostrally and caudally. Fibers from another rootlet, which enters dorsolaterally, course caudally towards the spinal cord. This rootlet has not been described in other fishes. The vagal viscerosensory nucleus, where most of the viscerosensory fibers terminate, forms a column in which two types of neurons can be distinguished. The most caudal part of the nucleus is continuous with the commissural nucleus, into which vagal nerve projections also enter. No vagal projections were found in the area postrema. In the visceromotor column, two types of neurons are seen lateroventral to the IV ventricle.

Animals↗

PubMed1992

Retinal projections in the house musk shrew, Suncus murinus, as determined by anterograde transport of WGA-HRP.

Retinal projections in the house musk shrew (Suncus murinus) were determined by the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Unilateral injection of WGA-HRP into the vitreous body resulted in the terminal labeling of the optic projections in the suprachiasmatic nucleus (SCH), the ventral (CGLv) and dorsal (CGLd) lateral geniculate nuclei, the intergeniculate leaflet (IGL), the pretectum, the superficial layers of the superior colliculus (CS), and the dorsal terminal nucleus (DTN) of the accessory optic system (AOS). Labeling of the SCH was bilateral, with ipsilateral predominance, and covered the whole dorsoventral extent of the nucleus. Immunohistochemical studies revealed that VIP-like immunoreactive neurons and fibers were present in almost all parts of the SCH. No hypothalamic regions other than the SCH received the optic fibers. The ipsilateral projections to the CGLv, CGLd, and IGL were sparse, and a considerable number of uncrossed retinal fibers were found in the pretectal olivary nucleus. No retinal projections to the lateral posterior thalamic nucleus (LP) were found. Ipsilateral optic fibers projected sparsely to the medial part of the CS. The AOS consisted of a small DTN with a very few crossed retinal projections but no lateral and medial terminal nuclei. In addition, the AOS had no inferior fascicle.

Animals↗