The role of Otx and Otp genes in brain development.
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Over the last ten years, many genes involved in the induction, specification and regionalization of the brain have been identified and characterized at the functional level through a series of animal models. Among these genes, both Otx1 and Otx2, two murine homologues of the Drosophila orthodenticle (otd) gene which encode transcription factors, play a pivotal role in the morphogenesis of the rostral brain. Classical knock-out studies have revealed that Otx2 is fundamental for the early specification and subsequent maintenance of the anterior neural plate, whereas Otx1 is mainly necessary for both normal corticogenesis and sense organ development. A minimal threshold of both gene products is required for correct patterning of the fore-midbrain and positioning of the isthmic organizer. A third gene, Orthopedia (Otp) is a key element of the genetic pathway controlling development of the neuroendocrine hypothalamus. This review deals with a comprehensive analysis of the Otx1, Otx2 and Otp functions, and with the possible evolutionary implications suggested by the models in which the Otx genes are reciprocally replaced or substituted by the Drosophila homologue, otd. (+info)
Hydrodynamic image formation by the peripheral lateral line system of the Lake Michigan mottled sculpin, Cottus bairdi.
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Lake Michigan mottled sculpin (Cottus bairdi) have a lateral-line-mediated prey-capture behaviour that consists of an initial orientation towards the prey, a sequence of approach movements, and a final strike at the prey. This unconditioned behaviour can be elicited from blinded sculpin in the laboratory by both real and artificial (vibrating sphere) prey. In order to visualize what Lake Michigan mottled sculpin might perceive through their lateral line when approaching prey, we have combined anatomical, neurophysiological, behavioural and computational modelling techniques to produce three-dimensional maps of how excitation patterns along the lateral line sensory surface change as sculpin approach a vibrating sphere. Changes in the excitation patterns and the information they contain about source location are consistent with behavioural performance, including the approach pathways taken by sculpin to the sphere, the maximum distances at which approaches can be elicited, distances from which strikes are launched, and strike success. Information content is generally higher for laterally located sources than for frontally located sources and this may explain exceptional performance (e.g. successful strikes from unusually long distances) in response to lateral sources and poor performance (e.g. unsuccessful strikes) to frontal sources. (+info)
The overlapping roles of the inner ear and lateral line: the active space of dipole source detection.
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The problems associated with the detection of sounds and other mechanical disturbances in the aquatic environment differ greatly from those associated with airborne sounds. The differences are primarily due to the incompressibility of water and the corresponding increase in importance of the acoustic near field. The near field, or hydrodynamic field, is characterized by steep spatial gradients in pressure, and detection of the accelerations associated with these gradients is performed by both the inner ear and the lateral line systems of fishes. Acceleration-sensitive otolithic organs are present in all fishes and provide these animals with a form of inertial audition. The detection of pressure gradients, by both the lateral line and inner ear, is the taxonomically most widespread mechanism of sound-source detection amongst vertebrates, and is thus the most likely primitive mode of detecting sound sources. Surprisingly, little is known about the capabilities of either the lateral line or the otolithic endorgan in the detection of vibratory dipole sources. Theoretical considerations for the overlapping roles of the inner ear and lateral line systems in midwater predict that the lateral line will operate over a shorter distance range than the inner ear, although with a much greater spatial resolution. Our empirical results of dipole detection by mottled sculpin, a benthic fish, do not agree with theoretical predictions based on midwater fishes, in that the distance ranges of the two systems appear to be approximately equal. This is almost certainly as a result of physical coupling between the fishes and the substrate. Thus, rather than having a greater active range, the inner ear appears to have a reduced distance range in benthic fishes, and the lateral line distance range may be concomitantly extended. (+info)
The coelotrich: form and function of an unusual sensillum in Lowrya (Ostracoda: Myodocopina: Cypridinidae).
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This study reports a previously undescribed and unique sensillum. Some species of Lowrya Parker 1998 (Ostracoda: Cypridinidae) possess sensillae of an unusual type arranged in a single row parallel to, and near, the anterior, ventral and posterior margins of the external surface of each carapace valve. These sensilla are here termed coelotrichs, and are each centred within an almost spherical cavity in the carapace. The cavity forms at the base of a depression, and the depression in turn lies within a sunken channel on the exterior carapace surface. The opening of the coelotrich cavity bears a 'plug', through which only the widened, divided base of a seta passes. The seta is very fine and stiff throughout most of its length, and exhibits no pores. For most of its length the seta is orientated parallel to the carapace surface, shielded within the sunken channel. The coelotrich is probably a velocity detector, acted upon by the viscous drag of the surrounding fluid. Its function is probably to detect vibrations in the substrate, such as acoustic motion in the surrounding liquid, or steady fluid drainage motion. (+info)
The laterophysic connection in chaetodontid butterflyfish: morphological variation and speculations on sensory function.
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The laterophysic connection is a novel specialization in chaetodontid butterflyfish, in which paired diverticula of the swim-bladder ('horns') extend anteriorly and approach or directly contact a medial fossa in the lateral line canal contained within the supracleithrum. This study examined the morphology of the laterophysic connection in eight ecologically diverse species belonging to five subgenera within Chaetodon. Two types of laterophysic connections, indirect and direct, were found among Chaetodon species. Intraspecific variation (including sexual dimorphism) in the morphology of the laterophysic connection was not found. The type of laterophysic connection is not correlated with ecological characteristics among Chaetodon species, but appears to be correlated with subgeneric affinities of Chaetodon species. The presence of swim-bladder horns probably increases pressure sensitivity to the inner ear. It is suggested that the presence of a direct laterophysic connection, and possibly an indirect laterophysic connection, imparts pressure sensitivity to the lateral line canal system as well. (+info)
The pit organs of elasmobranchs: a review.
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Elasmobranchs have hundreds of tiny sensory organs, called pit organs, scattered over the skin surface. The pit organs were noted in many early studies of the lateral line, but their exact nature has long remained a mystery. Although pit organs were known to be innervated by the lateral line nerves, and light micrographs suggested that they were free neuromasts, speculation that they may be external taste buds or chemoreceptors has persisted until recently. Electron micrographs have now revealed that the pit organs are indeed free neuromasts. Their functional and behavioural role(s), however, are yet to be investigated. (+info)
Multimodal sensory integration in the strike-feeding behaviour of predatory fishes.
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The search for useful model systems for the study of sensory processing in vertebrate nervous systems has resulted in many neuroethological studies investigating the roles played by a single sensory modality in a given behaviour. However, behaviours relying solely upon information from one sensory modality are relatively rare. Animals behaving in a complex, three-dimensional environment receive a large amount of information from external and internal receptor arrays. Clearly, the integration of sensory afference arising from different modalities into a coherent 'gestalt' of the world is essential to the behaviours of most animals. In the last several years our laboratory team has examined the roles played by the visual and lateral line sensory systems in organizing the feeding behaviour of two species of predatory teleost fishes, the largemouth bass, Micropterus salmoides, and the muskellunge, Esox masquinongy. The free-field feeding behaviours of these fishes were studied quantitatively in intact animals and compared to animals in which the lateral line and visual systems had been selectively suppressed. All groups of animals continued to feed successfully, but significant differences were observed between each experimental group, providing strong clues as to the relative role played by each sensory system in the organization of the behaviour. Furthermore, significant differences exist between the two species. The differences in behaviour resulting when an animal is deprived of a given sensory modality reflect the nature of central integrative sensory processes, and these behavioural studies provide a foundation for further neuroanatomical and physiological studies of sensory integration in the vertebrate central nervous system. (+info)
Sensory processing of water currents by fishes.
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Water currents are extremely important in the aquatic environment and play a very significant role in the lives of fishes. Sensory processing of water currents involves a number of sensory modalities including the inner ear, vision, tactile sense and the mechanosensory lateral line. The inner ear will detect whole-body accelerations generated by changes in flow, or by turbulence, whereas visual and tactile inputs will signal translational movement with respect to an external visual or tactile reference frame. The superficial neuromasts of the mechanosensory lateral line detect flow over the surface of the body and have the appropriate anatomical distribution and physiological properties to signal the strength and the direction of flow and, hence, contribute to the detection of regional differences in flow over different parts of the body. (+info)