Lateral line system


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Lateral line system

A primitive vertebrate sensory system that is present in all larval and adult fishes, in larval amphibians (such as tadpoles), and in some adult amphibians that retain an aquatic lifestyle. The lateral line system consists of 100 or more sensory organs (neuromasts) that are typically arranged in lines on or just under the skin of the head and body. Neuromasts are composed of sensory hair cells, which are also found in the auditory system of all vertebrates. The lateral line system responds to water flowing past the skin surface and uses different flow patterns over the body to form hydrodynamic images of the animal's nearby surroundings, just as the visual system forms visual images of the environment using different light patterns on the retina.

Neuromasts are found just under the skin in fluid-filled canals that communicate with the skin surface through a series of pores (canal neuromasts, found in fishes only), or on the skin surface (superficial neuromasts, found in fishes and amphibians). A prominent canal that often forms a visible line along the trunk of most fishes is probably the origin of the term “lateral line,” but in reality the lateral line system includes neuromasts that are distributed all over the head and body of the animal.

A neuromast contains up to hundreds of mechanosensory hair cells that are surrounded by support cells. Hair cells function as directional sensors that convey information to the brain about both the strength and direction of water currents. The ciliary bundle of each hair cell contains one long cilium (kinocilium) and a cluster of shorter cilia (stereocilia). The ciliary bundles of the hair cells are embedded in a gelatinous cupula. Hair cells are activated when water flows past the skin surface, causing the cupula to move, thus causing the cilia to bend. The neural response of each hair cell is proportional to both the degree of cilia displacement and the direction in which the stereocilia are displaced relative to the eccentrically placed kinocilium of each hair cell.

Information from neuromasts is transmitted to the brain by sensory (afferent) nerve fibers, which form five cranial nerves (the lateral line nerves) that terminate in distinct medullary regions of the brainstem (medulla oblongata). Distinct regions and pathways in the brain are dedicated to processing information from the lateral line system. These are similar in overall organization, and in proximity to regions of the brain dedicated to processing information from two other closely allied sensory systems, the auditory system and the electrosensory system. Information is also carried from the brain to the sense organs by efferent nerve fibers, which can modulate the sensitivity of the organs to certain stimuli (for example, to reduce sensitivity when water flows are produced by the animal's own movements). See Amphibia

The lateral line system is thought to have a function that is intermediate between touch and hearing, and is best described as a sense of touch-at-a-distance. In general, large-scale water movements such as oceanic currents, tides, and river flows that are strong enough to carry a fish with them are not by themselves very effective lateral line stimuli. Smaller-scale movements, such as those produced by a slowly moving (less than 8 cm or 0.25 ft/s) stream or by nearby animals (less than one or two body lengths away), can be effective lateral line stimuli.

Fishes can use different types of water flows to form hydrodynamic images of their surroundings. They can form images passively by remaining still and simply detecting the water currents created by other moving animals, or by detecting current distortions or turbulent wakes created by a stationary obstacle in moving water. Alternatively, fishes can actively form images by swimming past a stationary obstacle and then detecting the distortions in their own self-generated flows due to the presence of the obstacle.

Fishes and amphibians can also use their lateral line system to orient themselves relative to a water current (rheotaxis), hold a stationary position in a stream, capture prey, avoid predators, and communicate with intraspecifics. Many stream-dwelling fishes (such as trout and salmon) show rheotactic and station-holding behaviors by orienting their bodies upstream and holding positions behind stationary rocks or boulders. These behaviors are important for the upstream spawning migrations of these fishes and for capturing prey that are being carried downstream. See Elasmobranchii, Nervous system (vertebrate)