Lateral line

The lateral line, also called the lateral line organ, is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial cells, known as hair cells, which respond to displacement caused by motion and transduce these signals into electrical impulses via excitatory synapses. Lateral lines play an important role in schooling behavior, predation, and orientation. Early in the evolution of fish, some of the sensory organs of the lateral line were modified to function as the electroreceptors called ampullae of Lorenzini. The lateral line system is ancient and basal to the vertebrate clade, as it is found in fishes that diverged over 400 million years ago.

Function The lateral line system allows the detection of movement, vibration, and pressure gradients in the water surrounding an animal. It plays an essential role in orientation, predation, and fish schooling by providing spatial awareness and the ability to navigate in the environment. Analysis has shown that the lateral line system should be an effective passive sensing system able to discriminate between submerged obstacles by their shape. The lateral line allows fish to navigate and hunt in water with poor visibility. The lateral line system enables predatory fishes to detect vibrations made by their prey, and to orient towards the source to begin predatory action. Blinded predatory fishes remain able to hunt, but not when lateral line function is inhibited by cobalt ions. The lateral line plays a role in fish schooling. Blinded Pollachius virens were able to integrate into a school, whereas fish with severed lateral lines could not. It may have evolved further to allow fish to forage in dark caves. In Mexican blind cave fish, Astyanax mexicanus, neuromasts (organs in the lateral line system) in and around the orbit of the eye are bigger and therefore around twice as sensitive as those of surface-living fish of the same species. One function of schooling may be to confuse the lateral line of predatory fishes. A single prey fish creates a simple particle velocity pattern, whereas the pressure gradients of many closely swimming (schooling) prey fish overlap, creating a complex pattern. This makes it difficult for predatory fishes to identify individual prey through lateral line perception.

Anatomy

Lateral lines are usually visible as faint lines of pores running along each side of a fish's body. The functional units of the lateral line are the neuromasts, discrete mechanoreceptive organs that sense movement in water. There are two main varieties: canal neuromasts and superficial neuromasts. Superficial neuromasts are on the surface of the body, while canal neuromasts are along the lateral lines in subdermal, fluid-filled canals. Each neuromast consists of receptive hair cells whose tips are covered by a flexible jellylike cupula. Hair cells typically possess both glutamatergic afferent connections and cholinergic efferent connections. The receptive hair cells are modified epithelial cells; they typically possess bundles of 40–50 microvilli "hairs" which function as the mechanoreceptors. Within each bundle, the hairs are organized in a rough "staircase" from shortest to longest.

Signal transduction

The hair cells are stimulated by the deflection of their hair bundles in the direction of the tallest "hairs" or stereocilia. The deflection allows cations to enter through a mechanically gated channel, causing depolarization or hyperpolarization of the hair cell. Depolarization opens Cav1.3 calcium channels in the basolateral membrane. Hair cells use a system of transduction with rate coding to transmit the directionality of a stimulus. The hair cells produce a constant, tonic rate of firing. As mechanical motion is transmitted through water to the neuromast, the cupula bends and is displaced according to the strength of the stimulus. This results in a shift in the cell's ionic permeability. Deflection towards the longest hair results in depolarization of the hair cell, increased neurotransmitter release at the excitatory afferent synapse, and a higher rate of signal transduction. Deflection towards the shorter hair has the opposite effect, hyperpolarizing the hair cell and producing a decreased rate of neurotransmitter release. These electrical impulses are then transmitted along afferent lateral neurons to the brain. While both varieties of neuromasts utilize this method of transduction, their specialized organization gives them different mechanoreceptive capacities. Superficial organs are exposed more directly to the external environment. The organization of the bundles within their organs is seemingly haphazard, incorporating various shapes and sizes of microvilli within bundles. This suggests coarse but wide-ranging detection. In contrast, the structure of canal organs allow canal neuromasts more sophisticated mechanoreception, such as of pressure differentials. As current moves across the pores, a pressure differential is created, inducing a flow in the canal fluid. This moves the cupulae of the neuromasts in the canal, resulting in a deflection of the hairs in the direction of the flow.

Electrophysiology The mechanoreceptive hair cells of the lateral line structure are integrated into more complex circuits through their afferent and efferent connections. The synapses that directly participate in the transduction of mechanical information are excitatory afferent connections that utilize glutamate. Species vary in their neuromast and afferent connections, providing differing mechanoreceptive properties. For instance, the superficial neuromasts of the midshipman fish, Porichthys notatus, are sensitive to specific stimulation frequencies. One variety is attuned to collect information about acceleration, at stimulation frequencies between 30 and 200 Hz. The other type obtains information about velocity, and is most receptive to stimulation below 30 Hz.

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