The lateral line system is a collection of small mechanoreceptive patches or neuromasts located superficially on the skin or just under the skin in fluid-filled canals on the head and body of all fishes. The mechanoreceptive component of the neuromast is the hair cell - the same sensory cell found in all vertebrate ears, including the human ear. These cells transduce mechanical energy into electrical energy when their apical hairs or "cilia" are displaced. The nerves contacting these receptors enter the brain in close association with the auditory processing areas of the fish nervous system. Although auditory and lateral line pathways in the central nervous system are separate, they are largely parallel and share many of the same organizational features, suggesting that the two systems have developed and evolved in close association with each other and may share common principles of operation.
Lateral line physiological experiments are designed to investigate the ways in which sources of water disturbance are detected by the lateral line system of fish and how flow patterns arising from these sources are represented in the peripheral and central nervous system. Current experiments involve recording the responses of posterior lateral line nerve fibers innervating neuromasts on the trunk and first-order brainstem cells to a dipole source (vibrating sphere) that slowly changes its location along the length of the fish. These experiments have been designed in tandem with behavioral experiments on source localization and modeled predictions of how lateral line excitation patterns change with source location. Recent work in this area has demonstrated that changes in the pressure gradient pattern, as modeled and measured under experimental conditions, are rather faithfully encoded by peripheral lateral line fibers in both the goldfish (Coombs, Hastings & Finneran 1995) and mottled sculpin (Coombs & Conley 1995).
Anatomical studies have involved three separate approaches and goals, one of which has been to use histological staining and cross-sectioning techniques to identify the location of electrodes used in neurophysiological experiments on brain cells. The second approach employs relatively gross anatomical techniques (e.g. dissection, light and scanning electron microscopy) to describe the distribution and morphology of lateral line neuromasts and associated structures (e.g. canals). Descriptive anatomical data can be used to model the biomechanical properties of peripheral endorgans, which in turn can be compared to the physiological response properties of peripheral lateral line fibers. This type of approach recently revealed that there could be considerable morphological slop in the peripheral anatomy of the lateral line system in a monophyletic radiation of antarctic fish without altering basic function (Coombs & Montgomery 1992; Montgomery, Coombs & Janssen 1994). A third approach involves histological staining and tract-tracing techniques to describe the cytoarchitecture and interconnections of processing centers (nuclei) in the brain. Recent studies using this approach have shown that there are striking similarities in the cell types and organization of the first-order brainstem nucleus in the lateral line system relative to first-order brainstem nuclei in other octavolateralis systems, including the dorsal cochlear nucleus (DCN) of the mammalian auditory system (New, Coombs, McCormick & Oshel 1996). The functional significance of these similarities and possible clues to the function of the somewhat enigmatic DCN are summarized by Montgomery, Coombs, Conley & Bodznick (1995).