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.
The propagation of sound waves along relaxed single fibres of glycerinated rabbit psoas muscle has been observed using Brillouin scattering at frequencies up to 1.6 GHz. Two types of waves were observed: one with a velocity of 1508 +/- 7 m s-1, which is attributed to sound waves in intra-cellular saline, the other with a velocity of 912 +/- 25 m s-1, which is attributed to waves propagating along the protein filaments within individual sarcomeres. The latter sound velocity is much higher than that which has been reported by Stienen and Blangé (1985) for 50 microseconds tension transients, and the difference is attributed to the much higher stiffness of the protein filaments compared to the cross-bridges which determine the low-frequency elasticity of muscle fibres.
The anatomy of the labyrinth of Myxine is described. A torus-shaped cartilaginous capsule contains the membranous labyrinth. There are two ampullae, each with a ring-shaped crista, and the ventro-medial wall of the toroidal space is covered by a single macula communis with an otolith consisting of statoconia in a porous matrix. The characteristic ciliated chambers found in the labyrinth of the lamprey are absent. The sensory cells differ strikingly from those of the lamprey and those of all gnathostomes by having a kinocilium lacking the two central filaments and the basal foot. There are also differences in the topographic arrangement of the kinocilium vis-à-vis the stereocilia. The hair-cell map of the macula communis resembles that of the lamprey maculae at either end, the hair cells pointing their kinocilia forward at the anterior and backward at the posterior end. The orientation is rather confused in the middle portion, and an area corresponding to the saccular macula of the lamprey is missing. There is no macula-neglecta. Recordings from parts of the eighth nerve innervating the cristae of the two ampullae show sensitive and characteristic responses to angular accelerations in all planes of space including the horizontal yawing plane. By dint of their orientation within the labyrinth and the uniform topographic arrangement of the hair cells on them, the two cristae are differentially sensitive to roll, pitch, and yaw. Positional responses can be recorded from nerve fibres innervating the macula communis. These resemble those obtained from the labyrinth of the lamprey (Lowenstein 1970) and cover tilts about the longitudinal and transverse body axes. No responses to vibrational stimuli were obtained from any branch of the eighth nerve, and this result may well fit in with the absence of a well-defined sacculus portion of the macula communis and also with the absence of a macula neglecta. Rhythmic discharges as recorded from the labyrinth of the lamprey (Lowenstein 1970) were not encountered.
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