The earliest vertebrate ears likely subserved a gravistatic function for orientation in the aquatic environment. In extant fishes, afferents from otolithic end organs encode the Rabbit Polyclonal to Caspase 7 (p20, Cleaved-Ala24) axis of particle motion, which is conveyed to the dorsal regions of first-order octaval nuclei. This directional information is further enhanced by bilateral computations in the medulla and the auditory midbrain. We propose that similar direction-sensitive neurons were present in the early aquatic tetrapods and that selection for sound localization in air acted upon pre-existing brain stem circuits like those in fishes. With movement onto land, the early tetrapods may have retained some sensitivity to particle motion, transduced by bone conduction, and later acquired new auditory papillae and tympanic hearing. Tympanic hearing arose in parallel within each of the major tetrapod lineages and would have led to increased sensitivity to a broader frequency range and to modification of the pre-existing circuitry for sound source localization. sp.) and lampreys (and spp.), have very different sensory maculae (fig. 1A), the differences in their ears are discussed here because they may illustrate evolutionary steps in the development of auditory maculae. The hagfish ear has a pair of tubular sacs that are oriented +/? 30 angles with respect to the midline [Lowenstein and Thornhill, 1970]. Within each sac lie two cristae (anterior and posterior) and a VE-821 kinase inhibitor single elongate macula (macula communis) between the cristae patches in the fluid-filled membranous labyrinth (right ear shown in figure 1A). The macula communis in hagfish has sensory hair cells that are oriented in various directions VE-821 kinase inhibitor [fig. 15 in Lowenstein and Thornhill, 1970], and, unlike the end organs in bony fishes, there is no striola where hair cells are in 180 opposition. In contrast, the lamprey macula communis exhibits greater regional specializations than in hagfish: the rostral and caudal ends are oriented horizontally and the central area can be focused vertically (fig. 1A) with a definite type of polarity reversal [Lowenstein et al., 1968]. Regionalization of locks cell orientations might have been an early part of organizing local response characteristics from the macula when encoding stimulus path (e.g., ahead versus backward motion particularly, as opposed to the even more general percept that your body can be shifting) became a significant function from the macula. The almost ubiquitous existence of consistent locks cell orientation patterns for the maculae of different lineages of vertebrates (illustrated in fig. 1B) shows that these patterns are functionally significant [Sienknecht, 2013]. The principal afferents that approached groups of likewise focused locks cells could have provided the benefit of higher directional specificity, which also could have been shown in their insight to the prospective medullary nucleus. Even though the auditory processing features in agnathans never have been evaluated, recordings through the 8th cranial nerve (nVIII) recommend the hearing encodes linear acceleration. Lowenstein and Thornhill  mentioned that nVIII afferents in the hagfish (from the first Carboniferous period [Coates, 1998], you can find three bony semicircular canals (a horizontal canal as well as the anterior and posterior canals in agnathans) and another ventral chamber that enclosed two otolith organs, a saccule and lagena possibly, predicated on the ventral location and the presence of large and small otoliths, all of which indicate functional specializations in the ear. A single ventral chamber housing two otolith organs (saccule and lagena) is present among extant non-teleost fishes [e.g., Meyer et al. 2010]. For a detailed discussion of hypotheses proposed for the evolution of the structures in the vertebrate ear, see Fritzsch and Straka . Extant non-teleost actinopterygians include the Cladistia, bichir and reedfish; the Chondrostei, sturgeons and paddlefish; and the Holostei, commonly the gars (Lepisosteiformes) and the bowfins (Amiiformes). The Holostei are considered to be the living sister taxon of the Teleostei (the most recent radiation of fishes). Although these extant species have evolved along separate paths for millions of years, their shared features are highly relevant to a discussion from the ancestors of modern ray-finned tetrapods and fishes. The non-teleost actinopterygians possess a unique saccule and lagena housed in the same ventral chamber, and both are focused in the vertical aircraft. Both of these maculae possess VE-821 kinase inhibitor identical total hair and areas cell counts generally in most non-teleosts [see Fig. 30 in Popper, 1983], and each can be connected with otoconia (calcareous contaminants embedded inside a matrix) rather than solid, calcareous otolith [Popper, personal conversation]. These saccules are structured with a straightforward bidirectional locks cell orientation design, however the macula can be curved or twisted and could provide directional level of sensitivity in VE-821 kinase inhibitor the rostrocaudal and dorsoventral axes (fig 1B). The comparative part(s) of both maculae in audition and/or gravistatic reactions never have been established. The directional.