Publications by authors named "Hofmann Michael H"

Introduction: Comparative studies of brain anatomy between closely related species have been very useful in demonstrating selective changes in brain structure. Within-species comparisons can be particularly useful for identifying changes in brain structure caused by contrasting environmental selection pressures. Here, we aimed to understand whether differences within and between species in habitat use and foraging behaviour influence brain morphology, on both ecological and evolutionary time scales.

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The vertebrate eye allows to capture an enormous amount of detail about the surrounding world which can only be exploited with sophisticated central information processing. Furthermore, vision is an active process due to head and eye movements that enables the animal to change the gaze and actively select objects to investigate in detail. The entire system requires a coordinated coevolution of its parts to work properly.

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Dopamine is present in all vertebrates and the functional roles of the subsystems are assumed to be similar. Whereas the effect of dopaminergic modulation is well investigated in different target systems, less is known about the factors that are causing the modulation of dopaminergic cells. Using the zebra mbuna, Pseudotropheus zebra, a cichlid fish from Lake Malawi as a model system, we investigated the activation of specific dopaminergic cell populations detected by double-labeling with TH and pS6 antibodies while the animals were solving different learning tasks.

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Brains are very plastic, both in response to phenotypic diversity and to larger evolutionary trends. Differences between taxa cannot be easily attributed to either factors. Comparative morphological data on higher taxonomic levels are scarce, especially in ray-finned fishes.

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Comparing the relative volumes of body parts is a useful tool in morphology, but it is not trivial to do this in animals that differ in overall size. To account for scaling differences, a "reference size" has to be determined and the original absolute volumes have to be "corrected for" by this scaling reference. However, the outcome of a statistical analysis is greatly affected by this "reference size," and it is practically impossible to determine the "overall size" of a structure independent of the changes in the relative size of the parts of it.

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The optic tectum of the ocellated dragonet (Synchiropus ocellatus) was studied with immunohistochemistry. Antibodies raised against the calcium binding protein calretinin (CR) revealed a lamination similar to that already reported for other ray finned fish. Most immunoreactive fibers could be observed in those layers receiving retinal afferents and most immunoreactive cells occur in the stratum periventriculare.

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This study is a re-examination of the direct primary sensory input to the valvula cerebelli in spiny eel. The valvula in Macrognathus zebrinus receives a primary afferent projection from the trigeminal nerve as revealed by injections of biotinylated dextran amines into the rostrum. The descending trigeminal nucleus and nucleus of the tractus solitarius are innervated as well.

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It has long been known that many elasmobranch fishes have relatively large brains. The telencephalon, in particular, has increased in size in several groups, and as a percent of total brain weight, it is as large as in some mammals. Little is known, however, about the organization, connections, and functions of the telencephalon in elasmobranchs.

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It is known that steady skin potentials are present in fishes due to chloride pumps in the gills and in the skin. We have found previously that these skin potentials can fluctuate and oscillate in the electrosensory paddlefish. Here we show that larger, discharge like potentials can be triggered by applying external electric fields in the water surrounding the fish.

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Paddlefish are uniquely adapted for the detection of their prey, small water fleas, by primarily using their passive electrosensory system. In a recent anatomical study, we found two populations of secondary neurons in the electrosensory hind brain area (dorsal octavolateral nucleus, DON). Cells in the anterior DON project to the contralateral tectum, whereas cells in the posterior DON project bilaterally to the torus semicircularis and lateral mesencephalic nucleus.

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In vertebrates, almost all motor neurons innervating skeletal muscles are located in the hindbrain and spinal cord, and all brain centers that control behavior have descending projections into these parts of the central nervous system. With tracer injections into the spinal cord and hindbrain, we have studied cell groups with descending projections in the paddlefish. Spinal cord injections reveal retrogradely labeled cells in all reticular and raphe nuclei, as well as the nucleus of the medial longitudinal fascicle.

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In many fishes, prey capture is guided primarily by vision. In the paddlefish, the electrosense can completely substitute for the visual system to detect tiny daphnia, their primary prey. Electroreceptors are distributed over the entire rostrum, head, and gill covers, and there are no accessory structures like a lens to form an image.

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The paddlefish is a passive electrosensory ray-finned fish with a special rostral appendage that is covered with thousands of electroreceptors, which makes the fish extremely sensitive to electric fields produced by its primary prey, small water fleas. We reexamined the electrosensory pathways from the periphery to the midbrain by injecting the neuronal tracer BDA into different branches of the lateral line nerve and into different parts of the dorsal octavolateral nucleus (DON) and the tectum. Primary afferents from the anterior to posterior body axis terminate in different areas in the mediolateral axis of the DON, the first electrosensory processing station.

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The forebrain of elasmobranchs is well developed, and in some species the relative brain/body weight is comparable to that in mammals. However, little is known about the organization of major telencephalic pathways. We injected biotinylated dextran amines into the olfactory bulb, lateral pallium, dorsomedial pallium, and the forebrain bundles of the thornback ray, Platyrhinoidis triseriata.

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If exposed to bulk water flow, fish lateral line afferents respond only to flow fluctuations (AC) and not to the steady (DC) component of the flow. Consequently, a single lateral line afferent can encode neither bulk flow direction nor velocity. It is possible, however, for a fish to obtain bulk flow information using multiple afferents that respond only to flow fluctuations.

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The discharges of anterior and posterior lateral line nerve afferents were recorded while stimulating goldfish, Carassius auratus, with bulk water flow. With increasing flow velocity lateral line afferents increased their discharge rates. However, an increased response to flow rates occurred even if flow direction was reversed.

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Many fishes and amphibians are able to sense weak electric fields from prey animals or other sources. The response properties of primary afferent fibers innervating the electroreceptors and information processing at the level of the hindbrain is well investigated in a number of taxa. However, there are only a few studies in higher brain areas.

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Introduction: The paddlefish electrosensory system consists of receptor cells in the skin that sense minute electric fields from their prey, small water fleas. The receptors thereby measure the difference of the voltage at the skin surface against the voltage inside the animal. Due to a high skin impedance, this internal voltage is considered to be relatively fixed.

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Fish use the lateral line system for prey detection, predator avoidance, schooling behavior, intraspecific communication and spatial orientation. In addition the lateral line may be important for station holding and for the detection of the hydrodynamic trails (vortex streets) generated by swimming fish. We investigated the responses of anterior lateral line nerve fibers of goldfish, Carassius auratus, to unidirectional water flow (10 cm s(-1)) and to running water that contained a Kármán vortex street.

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We investigated how fibres in the anterior lateral line nerve of goldfish, Carassius auratus, respond to sinusoidal water motions in a background of still or running water. Two types of fibres were distinguished: type I fibres, which most likely innervate superficial neuromasts, were stimulated by running water (10 cm s(-1)) while type II fibres, which most likely innervate canal neuromasts, were not stimulated by running water. The responses of type I fibres to sinusoidal water motions were masked in running water whereas responses of type II fibres were not masked.

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We studied the responses to sensory stimulation of three diencephalic areas, the central posterior nucleus of the dorsal thalamus, the anterior tuberal nucleus of the hypothalamus, and the preglomerular complex. Units sensitive to acoustic (500 Hz tone burst), hydrodynamic (25 Hz dipole stimulus) and visual (640 nm light flash) stimuli were found in both the central posterior and anterior tuberal nucleus. In contrast, unit responses or large robust evoked potentials confined to the preglomerular complex were not found.

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The passive electrosense is used by many aquatic animals to detect weak electric fields from other animals or from geoelectric sources. In contrast to the active electrosense, ;passive' means that there are no electric organs, and only external fields are measured. Electroreceptors are distributed in the skin, but are different from other skin senses because they can detect and localize sources a considerable distance away.

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Many aquatic vertebrates can sense the weak electric fields generated by other animals and may also sense geoelectric or electromagnetic phenomena for use in orientation. All these sources generate stationary (dc) fields. In addition, fields from animals are modulated by respiration and other body movements.

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Behavioral and electrophysiological experiments have shown that the elongated paddlefish rostrum, with its extensive population of ampullae of Lorenzini, constitutes a passive electrosensory antenna of great sensitivity and spatial resolution. As demonstrated in juvenile paddlefish, the passive electrosense serves a novel function in feeding serving as the primary, if not exclusive sensory modality for the detection and capture of zooplanktonic prey. Ampullary receptors are sensitive to the weak electrical fields of plankton from distances up to 9 cm, and juvenile paddlefish capture plankton individually with great swimming dexterity in the absence of vision or other stimulus signals.

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