Lateral line

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 in and around the orbit of the eye are bigger and around twice as sensitive as those of surface-living fish. 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 ==

with stained neuromasts Lateral lines are usually visible as faint lines of pores running along each side of a fish's body. 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. == 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. Some efferent projections to lateral line hair cells use dopamine as a transmitter, likely enhancing the activity of hair cell presynaptic calcium channels and thereby increasing neurotransmission. Signals from the hair cells are transmitted along lateral neurons to the brain. The area where these signals most often terminate is the medial octavolateralis nucleus (MON), which probably processes and integrates mechanoreceptive information. The deep MON contains distinct layers of basilar and non-basilar crest cells, suggesting computational pathways analogous to the electrosensory lateral line lobe of electric fish. The MON is likely involved in the integration of excitatory and inhibitory parallel circuits to interpret mechanoreceptive information. == Evolution ==

organs called ampullae of Lorenzini (red dots), illustrated here on the head of a shark, evolved from the mechanosensory lateral line organs (gray lines) of the last common ancestor of vertebrates. Here, the lateral line system detects particle velocities and accelerations with frequencies below 100 Hz. These low frequencies create large wavelengths, which induce strong particle accelerations in the near field of swimming fish that do not radiate into the far field as acoustic waves due to an acoustic short circuit. The auditory system detects pressure fluctuations with frequencies above 100 Hz that propagate to the far field as waves. The lateral line system is ancient and basal to the vertebrate clade; it is found in groups of fishes that diverged over 400 million years ago, including the lampreys, cartilaginous fishes, and bony fishes. The terrestrial tetrapods have secondarily lost their lateral line organs, which are ineffective when not submerged. The electroreceptive organs, called ampullae of Lorenzini, appearing as pits in the skin of sharks and some other fishes, evolved from the lateral line organ. Passive electroreception using ampullae is an ancestral trait in the vertebrates, meaning that it was present in their last common ancestor. }} == References ==