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The lateral line, also called lateral line system (LLS) or lateral line organ (LLO), is a system of sensory organs found in aquatic jawed vertebrates, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
, known as
hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Be ...
, which respond to displacement caused by motion and transduce these signals into electrical impulses via
excitatory synapses An excitatory synapse is a synapse in which an action potential in a presynaptic neuron increases the probability of an action potential occurring in a postsynaptic cell. Neurons form networks through which nerve impulses travel, each neuron often ...
. Lateral lines serve an important role in schooling behavior, predation, and orientation. Fish can use their lateral line system to follow the
vortices , revealed by colored smoke In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the ...

vortices
produced by fleeing prey. Lateral lines are usually visible as faint lines of pores running lengthwise down each side, from the vicinity of the
gill cover The operculum is a series of bones found in bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrast ...
s to the base of the tail. In some species, the receptive organs of the lateral line have been modified to function as electroreceptors, which are organs used to detect electrical impulses, and as such, these systems remain closely linked. Most
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e and some fully aquatic adult amphibians possess mechanosensitive systems comparable to the lateral line. Due to many overlapping functions and their great similarity in ultrastructure and development, the lateral line system and the inner ear of fish are often grouped together as the octavolateralis system (OLS). Here, the lateral line system detects particle velocities and accelerations with frequencies below 100 Hz. These low frequencies create large wavelengths, which create 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.


Function

The lateral line system allows the detection of movement, vibration, and pressure gradients in the water surrounding an animal, providing spatial awareness and the ability to navigate in the environment. This plays an essential role in orientation, predatory behavior, defense, and social schooling. A related aspect to social schooling is the hypothesis that schooling confuses the lateral line of predatory fishes. In summary, a single prey fish creates a rather simple particle velocity pattern while pressure gradients of many closely swimming (schooling) prey fish will overlap; that creates a complex pattern, and accordingly the predator will be unable to identify the individual fish through lateral line perception. The lateral line system is necessary to detect vibrations made by prey, and to orient towards the source to begin predatory action. Fish are able to detect movement, produced either by prey or a vibrating metal sphere, and orient themselves toward the source before proceeding to make a predatory strike at it. This behavior persists even in blinded fish, but is greatly diminished when lateral line function was inhibited by
CoCl2
CoCl<sub>2</sub>
application. Cobalt chloride treatment results in the release of cobalt ions, disrupting ionic transport and preventing signal transduction in the lateral lines. These behaviors are dependent specifically on
mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the fu ...
located within the canals of the lateral line. The role mechanoreception plays in schooling behavior was demonstrated in a 1976 study. A school of ''
Pollachius virens The saithe ( or ) (''Pollachius virens'') is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...

Pollachius virens
'' was established in a tank and individual fish were removed and subjected to different procedures before their ability to rejoin the school was observed. Fish that were experimentally blinded were able to reintegrate into the school, while fish with severed lateral lines were unable to reintegrate themselves. Therefore, reliance on functional mechanoreception, not vision, is essential for schooling behavior. A study in 2014 suggests that the lateral line system plays an important role in the behavior of Mexican blind cave fish (Astyanax mexicanus). The effectiveness of the lateral line system as a passive sensing system in discriminating between submerged obstacles has been established theoretically. A neural data-processing based on artificial neural networks has been shown to successfully process stationary sensory data to enhance the obstacle shape discrimination capability of the lateral line system.


Anatomy

The major unit of functionality of the lateral line is the neuromast. The neuromast is a
mechanoreceptive A mechanoreceptor, also called mechanoceptor, is a sensory neuron, sensory cell that responds to mechanical pressure or distortion. There are four main types of mechanoreceptors in glabrous, or hairless, mammalian skin: lamellar corpuscles (Pacinian ...
organ which allows the sensing of mechanical changes in water. There are two main varieties of neuromasts located in animals, canal neuromasts and superficial or freestanding neuromasts. Superficial neuromasts are located externally on the surface of the body, while canal neuromasts are located along the lateral lines in subdermal, fluid filled canals. Each neuromast consists of receptive
hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Be ...
whose tips are covered by a flexible and jellylike cupula. Hair cells typically possess both
glutamatergic Glutamatergic means "related to glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) func ...
afferent connections and
cholinergic Cholinergic agents are compounds which mimic the action of and/or . In general, the word "" describes the various containing the . Found in most animal tissues, choline is a primary component of the and functions with as a basic constitu ...

cholinergic
efferent connections. The receptive hair cells are modified
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
and typically possess bundles of 40-50
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption Absorption ma ...

microvilli
"hairs" which function as the
mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the fu ...
. These bundles are organized in rough "staircases" of hairs of increasing length order. The hair cells are stimulated by the deflection of these hair bundles in the direction of the tallest
stereocilia Stereocilia (or stereovilli) are non-motile apical modifications of the cell. They are distinct from cilia The cilium (; the plural is cilia) is an organelle found on eukaryotic cells in the shape of a slender protuberance that projects from the ...

stereocilia
. The deflection allows
cations An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
to enter through a theoretical mechanically gated channel, causing depolarization of the hair cell. This depolarization opens Cav1.3 channels in the
basolateral membrane Cell polarity is a fundamental feature of many types of cells. Epithelial cells are one example of a polarized cell type, featuring distinct 'apical', 'lateral' and 'basal' plasma membrane The cell membrane (also known as the plasma membrane ( ...
. This use of mechanosensitive hairs is homologous to the functioning of hair cells in the auditory and
vestibular system The vestibular system, in vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, ...
s, indicating a close link between these systems.FLOCK, A. (1967).
Hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Be ...
utilize a system of transduction that uses
rate coding Neural coding (or Neural representation) is a neuroscience Neuroscience (or neurobiology) is the scientific study of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coor ...
in order to transmit the directionality of a stimulus. Hair cells of the lateral line system produce a constant, tonic rate of firing. As mechanical motion is transmitted through water to the neuromast, the cupula bends and is displaced. Varying in magnitude with the strength of the stimulus, shearing movement and deflection of the hairs is produced, either toward the longest hair or away from it. This results in a shift in the cell's ionic permeability, resulting from changes to open
ion channels s (typically four per channel), 2 - outer vestibule, 3 - selectivity filter, 4 - diameter of selectivity filter, 5 - phosphorylation site, 6 - cell membrane. Ion channels are pore-forming membrane protein Membrane proteins are common proteins ...

ion channels
caused by the deflection of the hairs. Deflection towards the longest hair results in
depolarization In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
of the hair cell, increased neurotransmitter release at the excitatory afferent synapse, and a higher rate of
signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation residue Protein phosphorylation is a reversible post-translational ...
. 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, the specialized organization of superficial and canal neuromasts allow them different
mechanoreceptive A mechanoreceptor, also called mechanoceptor, is a sensory neuron, sensory cell that responds to mechanical pressure or distortion. There are four main types of mechanoreceptors in glabrous, or hairless, mammalian skin: lamellar corpuscles (Pacinian ...
capacities. Located at the surface of an animal's skin, superficial organs are exposed more directly to the external environment. Though these organs possess the standard "staircase" shaped hair bundles, overall the organization of the bundles within the organs is seemingly haphazard, incorporating various shapes and sizes of
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption Absorption ma ...

microvilli
within bundles. This suggests a wide range of detection, potentially indicating a function of broad detection to determine the presence and magnitude of deflection caused by motion in the surrounding water. In contrast, the structure of canal organs allow canal neuromasts to be organized into a network system that allows more sophisticated mechanoreception, such as the detection of pressure differentials. As current moves across the pores of a canal, a pressure differential is created over the pores. As pressure on one pore exceeds that of another pore, the differential pushes down on the canal and causes flow in the canal fluid. This moves the cupula of the hair cells in the canal, resulting in a directional deflection of the hairs corresponding to the direction of the flow. This method allows the translation of pressure information into directional deflections which can be received and transduced by hair cells.


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 Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, sid ...
. However, a variety of different neuromast and afferent connections are possible, resulting in variation in mechanoreceptive properties. For instance, a series of experiments on the superficial neuromasts of revealed that neuromasts can exhibit a receptive specificity for particular frequencies of stimulation. Using an immobilized fish to prevent extraneous stimulation, a metal ball was vibrated at different frequencies. Utilizing single cell measurements with a microelectrode, responses were recorded and used to construct tuning curves, which revealed frequency preferences and two main afferent nerve types. One variety is attuned to collect mechanoreceptive information about acceleration, responding to stimulation frequencies between 30–200 Hz. The other type is sensitive to velocity information and is most receptive to stimulation below <30 Hz. This suggests a more intricate model of reception than was previously considered. The efferent synapses to hair cells are inhibitory and utilize
acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells, such as neurons, musc ...

acetylcholine
as a transmitter. They are crucial participants in a
corollary discharge In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physic ...
system designed to limit self-generated interference. When a fish moves, it creates disturbances in the water that could be detected by the lateral line system, potentially interfering with the detection of other biologically relevant signals. To prevent this, an efferent signal is sent to the hair cell upon motor action, resulting in inhibition which counteracts the excitation resulting from reception of the self-generated stimulation. This allows the fish to retain perception of motion stimuli without interference created by its own movements. After signals transduced from the hair cells are transmitted along lateral neurons, they eventually reach the brain. Visualization methods have revealed that the area where these signals most often terminate is the medial octavolateralis nucleus (MON). It is likely that the MON plays an important role in the processing and integration of mechanoreceptive information. This has been supported through other experiments, such as the use of Golgi staining and microscopy by New & Coombs to demonstrate the presence of distinct cell layers within the MON. Distinct layers of basilar and non-basilar crest cells were identified within the deep MON. Drawing a comparison to similar cells in the closely related electrosensory lateral line lobe of electric fish, it seems to suggest possible computational pathways of the MON. The MON is likely involved in the integration of sophisticated excitatory and inhibitory parallel circuits in order to interpret mechanoreceptive information.


Notes


References

* Coombs S and van Netten S (2006
"The Hydrodynamics and Structural Mechanics of the Lateral Line System"
In: R E Shadwick and G V Lauder (Eds) ''Fish Physiology: Fish Biomechanics'', 23: 103–140, Academic Press. . * *


See also

* Artificial lateral line {{diversity of fish Ethology Fish nervous system Sensory organs in animals