The organs of the vertebrate inner ear respond to a variety of mechanical stimuli: semicircular canals are sensitive to angular velocity, the saccule and utricle respond to linear acceleration (including gravity), and the cochlea is sensitive to airborne vibration, or sound. The ontogenically related lateral line organs, spaced along the sides of aquatic vertebrates, sense water movement. All these organs have a common receptor cell type, which is called the hair cell, for the bundle of enlarged microvilli protruding from its apical surface. In different organs, specialized accessory structures serve to collect, filter, and then deliver these physical stimuli to the hair bundles. The proximal stimulus for all hair cells is deflection of the mechanosensitive hair bundle. Hair cells convert mechanical information contained within the temporal pattern of hair bundle deflections into electrical signals, which they transmit to the brain for interpretation.
Jeffrey R. Holt and Gwenaëlle S.G. Géléoc
All fish have a mechanosensory lateral line system for the detection of hydrodynamic stimuli. It is thus not surprising that the lateral line system is involved in numerous behaviors, including obstacle avoidance, localization of predators and prey, social communication, and orientation in laminar and turbulent flows. The sensory units of the lateral line system are the neuromasts, which occur freestanding on the skin (superficial neuromasts) and within subdermal canals (canal neuromasts). The canals are in contact with the surrounding water through a series of canal pores. Neuromasts consist of a patch of sensory hair cells covered by a gelatinous cupula. Water flow causes cupula motion, which in turn leads to a change in the hair cells’ receptor potentials and a subsequent change in the firing rate of the innervating afferent nerve fibers. These fibers encode velocity, direction, and vorticity of water motions by means of spike trains. They project predominantly to lateral line neurons in the brainstem for further processing of the received hydrodynamic signals. From the brainstem, lateral line information is transferred to the cerebellum and to midbrain and forebrain nuclei, where lateral line information is integrated with information from other sensory modalities to create a three-dimensional image of the hydrodynamic world surrounding the animal. For fish to determine spatial location and identity of a wave source as well as direction and velocity of water movements, the lateral line system must analyze the various types of hydrodynamic stimuli that fish are exposed to in their natural habitat. Natural hydrodynamic stimuli include oscillatory water motions generated by stationary vibratory sources, such as by small crustaceans; complex water motions produced by animate or inanimate moving objects, such as by swimming fish; bulk water flow in rivers and streams; and water flow containing vortices generated at the edges of objects in a water flow. To uncover the mechanisms that underlie the coding of hydrodynamic information by the lateral line system, neurophysiological experiments have been performed at the level of the primary afferent nerve fibers, but also in the central nervous system, predominantly in the brainstem and midbrain, using sinusoidally vibrating spheres, moving objects, vortex rings, bulk water flow, and Kármán vortex streets as wave sources. Unravelling these mechanisms is fundamental to understanding how the fish brain uses hydrodynamic information to adequately guide behavior.