Lateral line system

Comparative Account of lateral line system
Dr. P.B.Reddy
M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM
PG DEPARTMENT OF ZOOLOGY
GOVERTNAMENT PG COLLEGE, RATLAM.M.P
reddysirr@gmail.com

Origin of the Lateral Line System
 Lateral line is a canal along the side of a fish containing pores that open into tubes supplied with sense
organs sensitive to low vibrations.
 Robert H. Denison explained the origin of the lateral line system. He explained that early vertebrates had
a pore-canal system in the dermis which functioned as a primitive sensory system in detecting water
movement.
 Through the evidences from fossils, embryology and comparative anatomy, Denison (1966) established
that the inner ear is closely related to the lateral line system. He found a distinct relationship between the
pore canal system and the lateral line in Osteotraci.
 The inner ear and the lateral line are developed from ectodermal thickenings, called dorso-lateral
placodes. These have a number of similarities, including receptors with sensory hairs, and are both
innervated by fibers in the acoustico-lateral area of the brain.
 The pore canal system is present and developed in Osteostraci (ostracoderm).
It is also present in Heterostraci which is another group of ostracoderms and includes early vertebrates
such as lungfishes and crossopterygians.
 As its presence is extensive, it is reasonable to suggest that the pore canal system was a primitive
character in early vertebrates .
 In transverse sections also , it is very difficult to differentiate the pore canal system from a lateral line
canal.

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.
It is absent from all reptiles, birds, and
mammals, even those that are aquatic
(such as turtles, dolphins, and whales).
In the head, the lateral line
canal is separated into three
canals, one passes forward
and above the eye, another
forward and below the eye
and the other downward
and below the jaw. These
three canals have numerous
pores and together with the
lateral line canal, make the
lateral line system.
Structure of the Lateral Line System

Structure of the Lateral Line System
Epidermal structures called neuromasts form the peripheral area
of the lateral line.
 Neuromasts consist of two types of cells, hair cells and
supporting cells.
 Hair cells have an epidermal origin and each hair cell has one
high kynocyle (5-10 Î¼m) and 30 to 150 short stereocilia (2-3 Î¼m).
 The number of hair cells in each neuromast depends on its size,
and they can range from dozens to thousands.
 Hair cells can be oriented in two opposite directions with each
hair cell surrounded by supporting cells.
 At the basal part of each hair cell, there are synaptic contacts
with afferent and efferent nerve fibers. Afferent fibers, transmit
signals to the neural centres of the lateral line and expand at the
neuromast base. The regulation of hair cells is achieved by the
action of efferent fibers.
 Stereocilia and kinocilium of hair cells are immersed into a
cupula and are located above the surface of the sensory
epithelium.
 The cupula is created by a gel-like media, which is secreted by
non-receptor cells of the neuromast.

Figure 2. Lateral line of fish. (a) The figure shows the basic structure of neuromasts and all
its components. (b) Hair cell, depicting the innervations of afferent and efferent fibers.

Superficial and Canal Neuromasts
There are two types of neuromasts, superficial or free neuromasts and canal
neuromasts.
 Superficial neuromasts are located at the surface of the head, body and caudal
fin and are affected by the environment. They have a cylindrical cupula and a
round base with a diameter that can seldom reach 100 Î¼km. The number of hair
cells is small, from several dozens to several hundred.
Can be found in lampreys, teleost fishes and in some bony fishes.
 Superficial neuromasts are categorized into primary or paedomorphic
neuromasts and secondary or neomorphic neuromasts.
 Canal neuromasts are primary neuromasts. These are found inside epidermal
or bony canals and are located on the head or body of the fish.
In canal neuromasts, the sensory area is situated at the bottom of the canal
below the skin.
 Canal neuromasts have a large range in size, shape and orientation within the
canal. Some species have narrow canals and the neuromast can be found in a
local constriction with the long axis running parallel to the canal axis. Some other
fishes have neuromasts which are found in wide canals and have a different
shape.
 Canal neuromasts allow the efficient detection of pressure differentials, which
are created by the current movement across the canal pores.

The anterior and posterior lateral line cranial nerves collect sensory
input from lateral line mechanoreceptors (afferent) on the head and
trunk, respectively.
 Cranial nerve VIII (the octaval or stato-acoustic nerve) is an aggregate
of nerve branches ( afferent) that collect input from each of the inner
ear sensory end organs.
 Cranial nerve VIII enters the brain (medial octavo lateralis nucleus
(MON) at the level of the medulla oblongata between the entry points of
the anterior and posterior lateral line nerves.
 The lateral line and octaval nerves also contain the axons of central
octavolateral efferent neurons that innervate the sensory hair cells.
The final pathway for information processing is the relay of
information from the midbrain to different diencephalic nuclei.
 Bending of the cupula caused by the movement of water particles
results in bending of the cilia on the hair cell and the sending of signals
to the neurons which take signals to the lateral line region of the brain.

Hair cells utilize a system of transduction that uses rate coding 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 caused by the deflection of the hairs. 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.

Lateral Line Modifications
The lateral line system of elasmobranchs is different to that of teleost fish.
 Elasmobranchs have superficial neuromasts and two morphological classes
of sub-epidermal canals.
 Elasmobranch canals have skin pores that allow direct contact with the
surrounding water. They may also have absent skin pores which prevent the
contact of canal fluid with the external environment.
In teleost fish, hydrodynamic pressure differences at the skin pores cause
fluid motion. This results in pored canal neuromasts being able to detect the
acceleration of external water flow and induce behaviours such as
hydrodynamic imaging, detection of prey and schooling.
Sharks and batoids have non-pored canals which are located on the ventral
body surface, rostrum and around the mouth. The absence of skin pores
demonstrates that localized weak hydrodynamic flow which causes pressure
differences will not produce canal fluid motion directly, as it occurs in the pored
canal systems.
 Ventral non-pored canals are sensitive to small movements of the skin, and
primary afferents encode the velocity of fluid induced in the canal by these
stimuli.

Conclusion
 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.
The lateral line system which is a sensory system in fish and
amphibians has various functions in schooling, navigation, and prey
detection.
 Through paleontology, comparative anatomy and embryology it
was demonstrated that there is a phylogenetic connection between
the pore canal system in the dermis of early vertebrates and the
lateral line.
 Moreover, through the action of neuromasts and hydrodynamic
imaging, the fish is able to detect its surrounding environment.

 The lateral line is a sensory system in fish and amphibians. It is made up of
mechanoreceptors called neuromasts which are sensitive to water movement. The
lateral line system has an important role in the detection of stationary objects,
navigation, prey detection, capture and in swimming in schools.
 It is 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 serve an important role in schooling behavior, predation, and
orientation.
 Fish can use their lateral line system to follow 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 covers 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 larvae and some fully aquatic adult amphibians possess
mechanosensitive systems comparable to the lateral line.

Due to many overlapping functions and their great similarity in ultra structure 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
 It allows the detection of movement, vibration, and pressure gradients in the water.
 Provides spatial awareness and the ability to navigate the animal in the environment.
 Plays an essential role in orientation, predatory behavior, defense, and social schooling.
 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 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 located within the canals of the lateral line.

Lateral Line System Function
 The lateral line system function similar to the senses of touch and
hearing.
 The earliest hypothesis about the function of the lateral line was that it
secretes mucus to cover the body. Several years later, it was determined
that the lateral line is used to detect water current and stimuli from
moving objects.
 Fish can sense water movements ranging from large-scale currents to
small disturbances caused by plankton. This is due to the superficial
neuromasts which are able to respond to very weak water currents, with
speeds from 0.03 mm/s and higher. Canal neuromasts can respond to
current speeds from 0.3 to 20 mm/s.
 The lateral line has functions in schooling, prey detection, spawning,
rheotaxis (which is a form of taxis when fish face an ongoing current),
courtship and station holding.
 It is thought that the lateral line system can create hydrodynamic
images of the surrounding area. This can be achieved by detecting moving
and stationary objects in active and passive ways.

Anatomy
 The receptor organ of the lateral line system is the neuromast.
 The neuromast is a mechanoreceptive 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 sub dermal, fluid filled
canals.
 Each neuromast consists of receptive hair cells whose tips are covered by a
flexible and jellylike cupula. Hair cells typically possess both glutamatergic
afferent connections and cholinergic efferent connections.
 The receptive hair cells are modified epithelial cells and typically possess
bundles of 40-50 microvilli "hairs" which function as the mechanoreceptors.
 These bundles are organized in rough "staircases" of hairs of increasing
length order.
 This use of mechanosensitive hairs is homologous to the functioning of hair
cells in the auditory and vestibular systems, indicating a close link between
these systems.

Lateral line system

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