The anatomy of the labyrinth of the lamprey ( Lampetra fluviatilis ) is described. The ampullae of the two semicircular canals are each equipped with a complex three-armed sensory crista. They may be considered homologues of the ampullae of the vertical canals of the gnathostomes. However, the complexity of their cristae encourages the assumption that they may cover a spatial range of responses to angular acceleration which includes responses to accelerations in a horizontal plane controlled in the gnathostomes by the horizontal semi-circular canal. The otolith-bearing end organs are found to be located on a common macular structure. This is subdivided into an anterior horizontal, a vertical, and a posterior horizontal macula, each of which portions carries a characteristic arrangement of sensory cells. On the basis of an electronmicroscopic analysis of the orientation of the hair cells in the three main portions of the macula a revision of the homologies found in the older literature appears to be called for. It is suggested to homologize the anterior horizontal macula with the macula utriculi, the vertical macula with the macula sacculi and the posterior horizontal macula with the macula lagenae of the labyrinth of gnathostome animals. A separate sensory ending in the dorsal part of the labyrinth, the dorsal macula, may be the homologue of the macula neglecta. Ultrastructurally the end organs of the lamprey labyrinth conform with those of the gnathostome labyrinth with the exception of the presence of a new type of sensory hair cell which is equipped with a stiff kinoeilium of extraordinary length accompanied by extremely short stereocilia. This cell is found preponderantly in the vertical macula (macula sacculi). A striated organelle in the cytoplasm of the hair cells appears to be uniquely confined to the labyrinth of the lamprey. Morphologically the lamprey labyrinth differs from all other chordate labyrinths including that of its fellow cyclostome Myxine by the presence of large ciliated chambers in its centre in which long and powerful cilia maintain a permanent pattern of four endolymph vortices. The ciliated chambers are in open communication with the ampullae and with the spaces containing the otolith-bearing maculae. The analysis of the functional significance of the anatomical and ultrastructural findings will be described in a separate paper.
Molecular motors are the fundamental agents of movement in living organisms. A prime example is the actomyosin motor that powers muscle contraction. We illustrate the remarkable physics of this motor using a simplified three-state model, in which a myosin cross-bridge attaches to an actin filament, tilts over and then detaches. This `cross-bridge cycle', driven by ATP hydrolysis, is similar to a thermodynamic cycle, except that the molecular system is stochastic. Random transitions in the cycle therefore produce tension fluctuations, which have recently been observed in single-molecule experiments. Furthermore, since the rate constants for attachment and tilting depend on the elastic energy in the cross-bridge spring, the molecular motor is a highly nonlinear mechanical system. A bias tension `stretch activates' the motor, and it then develops the remarkable property of `negative viscosity', which allows it to perform as a self-sustained mechanical oscillator. However, when a series of attachment sites is available, the motor operates instead as a ratchet, pulling the actin filament rapidly forwards against a light load, whilst a heavy load pulls the filament only very slowly in the opposite direction. Similar ideas may apply to the dynein-tubulin motor that powers cilia and flagella and the kinesin-tubulin motor used in intracellular transport.
We have studied the diffraction of a focussed laser beam by single fibres of glycerinated rabbit psoas muscle as a function of the angle of incidence. Diffraction efficiencies as high as 34% were observed at the first-order Bragg angle, indicative of well-ordered striated fibres with a strong periodic modulation of the refractive index. A theory is presented to account for our observations based upon the coupled-wave model developed by Kogelnik (1967) and Magnusson and Gaylord (1977) for the description of thick phase gratings in holography. We have solved the coupled-wave equations on a computer using a realistic index modulation taken from the measurements of Huxley and Hanson (1957). Comparison of theory with experiment shows that coupled-wave effects are indeed present in well-ordered muscle fibres, and the observed diffraction efficiency is in quite good agreement with what would be expected theoretically. Most importantly, the computer model allows us to calculate the diffraction efficiency for curved striations, which are observed for real muscle fibres under a microscope. The sensitivity of the diffraction efficiency to curvature of the striations may have implications for the interpretation of other optical experiments on muscle. We also consider the effects on our measurements of the focussing lens and refraction by the cylindrical fibre.
The development of the labyrinth of Lampetra fluviatilis was investigated on three levels; the development of the gross morphology, the development of the hair cell orientation within the sensory areas, and the subcellular development of the sensory cells. The mode of development of the semicircular canals was found to differ from that found in gnathostomes. The hair cell orientation is derived from a sensory rudiment that has a complex orientation. The adult form is derived from this rudiment by its subdivision and spatial reorganization. The individual sensory cells are autonomous and apparently intrinsically polarized. This polarization occurs at an early stage and is reflected in the orientation of the centriole. In the first phase the sensory cells develop up to the ciliary bud stage. Further development does not take place until neuronal contact is established. This contact initiates the further development of the hair bundle but does not control its orientation. Functional neuronal contact appears to be made later.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.