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The Mauthner cells are a pair of big and easily identifiable
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...
s (one for each half of the body) located in the rhombomere 4 of the
hindbrain The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephal ...
in fish and
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s that are responsible for a very fast escape
reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
(in the majority of animals – a so-called C-start response). The cells are also notable for their unusual use of both chemical and
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
synapses In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses fr ...
.


Evolutionary history

Mauthner cells first appear in
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like su ...
(being absent in
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
and
lancelet The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochord ...
s), and are present in virtually all
teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...
fish, as well as in
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s (including postmetamorphic
frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
s and
toad Toad is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands. A distinction between frogs and toads is not made in scient ...
s). Some fish, such as lumpsuckers, seem to have lost the Mauthner cells however.


Role in behavior


The C-start

A C-start is a type of a very quick startle or
escape reflex Escape reflex, or escape behavior, is any kind of escape response found in an animal when it is presented with an unwanted stimulus. It is a simple reflectory reaction in response to stimuli indicative of danger, that initiates an escape motion of ...
that is employed by
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
and
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s (including
larval A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
frogs A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...
and toads). There are two sequential stages in the C-start: first, the head rotates about the
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
towards the direction of future escape, and the body of the animal exhibits a curvature that resembles a letter C; then, at the second stage, the animal is propelled forward. The duration of these stages varies from species to species from about 10 to 20 ms for the first stage, and from 20 to 30 ms for the second. In fish this forward propulsion does not require contraction of the antagonistic
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...
, but results from the body stiffness and the
hydrodynamic 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 study of air and other gases in motion) ...
resistance of the
tail The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammal ...
. When an antagonistic muscular contraction does occur during stage 2, the fish rotates in the opposite direction, producing a counter-turn, and a directional change.


The role of the Mauthner cell in the C-start behavior

In cases when an abrupt acoustic,
tactile Tactile may refer to: * Tactile, related to the sense of touch * Haptics (disambiguation) * Tactile (device), a text-to-braille translation device See also * Tangibility, in law * Somatosensory system In physiology, the somatosensory system ...
or
visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
stimulus elicits a single
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
in one M-cell, it always correlates with a
contralateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
C-start escape. An extremely quick mutual
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
inhibitory circuit then assures that only one M-cell reaches spiking threshold—as the C-start has to be
unilateral __NOTOC__ Unilateralism is any doctrine or agenda that supports one-sided action. Such action may be in disregard for other parties, or as an expression of a commitment toward a direction which other parties may find disagreeable. As a word, ''un ...
by definition—and that only one action potential is fired. The Mauthner cell-mediated C-start reflex is very quick, with about 5-10 ms latency between the acoustic/tactile
stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...
and the Mauthner cell discharge, and only about 2 ms between the discharge and the unilateral muscle contraction. Mauthner cells are thus the quickest motor neuron to respond to the stimulus. It makes the C-start response behaviorally important as a way to initiate the escape reflex in an ''all or nothing'' fashion, while the direction and speed of the escape can be corrected later through the activity of smaller motor neurons. In
larval A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
about ~60% of the total population of reticulospinal neurons are also activated by a stimulus that elicits the M-spike and C-start escape. A well-studied group of these reticulospinal neurons are the bilaterally paired M-cell homologues denoted MiD2cm and MiD3cm. These neurons exhibit morphological similarities to the M-cell including a lateral and ventral dendrite. They are located in rhombomeres 5 and 6 of
hindbrain The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephal ...
respectively, and also receive auditory input in parallel with the M-cell from the pVIIIth nerve. In fish, water jet stimuli that activate these neurons elicit non-mauthner initiated C-starts of a longer latency, compared with M-cell associated ones. Although the M-cell is often considered the prototype of a command neuron in
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s, this designation may not be fully warranted. Although electrical stimulation of the M-cell is sufficient for eliciting a C-start, this C-start is normally weaker than the one evoked by a sensory stimulus. Moreover, the C-start can be evoked even with the M-cell
ablated Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for ...
, although in this case the latency of the response increases. The most widely accepted model of the M-cell system, or brainstem escape network, is that the M-cell initiates a fixed action pattern to the left or right by activating a spinal motor circuit initially described by J. Diamond and colleagues, but the precise trajectory of the escape is encoded by population activity in the other classes of reticulospinal neurons functioning in parallel to the M-cell. This notion is supported by studies using ''in vivo'' calcium imaging in larval zebrafish which show that MiD2cm and MiD3cm are activated along with the M-cell when an offending stimulus is directed towards the head but not the tail, and are correlated with C-starts of a larger initial turn angle. Another component of the escape response is mediated by cranial relay neurons that are activated by the Mauthner cell spike. These neurons are electrically coupled with motoneurons which innervate extraocular, jaw and opercular muscles and mediate pectoral fin adduction in hatchetfish. This component of the neural circuit was first described by Michael V.L. Bennett and colleagues.


Mauthner cells in other types of behavior

Mauthner cells may be involved into behavioral patterns other than the C-start, if these types of behavior also require extremely quick bending movement of the body. Thus in
goldfish The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the wild have bec ...
Mauthner cells are activated during prey capture near the surface of the water, as this type of hunting is dangerous for the fish, and it would benefit from leaving the surface as soon as possible after the prey is captured. In adult postmetamorphic
anurans A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...
(frogs and toads) that do not have a tail, M-cells are nevertheless preserved and their discharges are associated with rapid movement of
leg A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ca ...
s during an escape. In addition, larval
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like su ...
(eel-like jawless fish of superclass Cyclostomata) exhibit rapid withdrawal behavior that is correlated with Mauthner cell activity and involves bilateral, posture-dependent muscular contractions along the length of the body. Larval lampreys (ammocoetes) are filter feeders that occupy crescent-shaped burrows in the silt or mud bottoms of freshwater stream beds, with their mouths positioned at, or just above the surface of the mud. Sudden vibration activates both Mauthner neurons in the lamprey brainstem, which causes an accordion-like muscular contraction in the trunk and tail and pulls the head down into the burrow.


Morphology and connections


Inputs to the M-cell: excitation and feed forward inhibition

The M-cell has two primary aspiny (lacking dendritic spines)
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the ...
s which receive segregated inputs from various parts of the neural system. One dendrite projects laterally and the other projects either in the ventral or medial direction, depending on the species. The ventral dendrite receives information from the optic tectum and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
while the lateral dendrite receives inputs from the octovolateralis systems (the
lateral line The lateral line, also called the lateral line organ (LLO), 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 ...
, acoustic inputs from the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
, and inertial information from the statoliths brought by the cranial nerve VIII). The fibers from the
ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
cranial nerve VIII terminate in excitatory mixed
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
and
glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can synt ...
rgic
synapses In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses fr ...
on the M-cell. They also electrically activate
glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
rgic inhibitory interneurons that terminate on the M-cells. Despite the inhibitory input having one more synapse in its pathway, there is no delay between the excitation and inhibition because the intervening synapse is electrical. It was shown that for weak stimuli the inhibition wins over the excitation, preventing the M-cell from a discharge, while for stronger stimuli excitation becomes dominant. The
Inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
afferents also terminate with electrical synapses on a population PHP inhibitory interneurons (see below) to provide an additional level of feed forward inhibition. The Mauthner cell also has GABA-,
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% o ...
-,
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and va ...
- and
somatostatin Somatostatin, also known as growth hormone-inhibiting hormone (GHIH) or by several other names, is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G protein-cou ...
ergic inputs, each restricted to certain dendritic region. Inputs from the optic tectum and the lateral line help control which way the C-startle bends by biasing the mauthner cells when there are obstacles in the vicinity. In cases where movement away from the stimulus is blocked, the fish may bend towards the disturbance.


Axon cap

The Mauthner cell
axon hillock The axon hillock is a specialized part of the cell body (or soma) of a neuron that connects to the axon. It can be identified using light microscopy from its appearance and location in a neuron and from its sparse distribution of Nissl substance. ...
is surrounded by a dense formation of neuropil, called the axon cap. The high resistance of this axon cap contributes to the typical shape of the Mauthner cell field potential (see below). In its most advanced form the axon cap consists of a core, immediately adjacent to the Mauthner cell axon, and containing a network of very thin
unmyelinated Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
fibers, and a peripheral part. This peripheral part contains the large unmyelinated fibers of the PHP neurons (see below) that mediate the inhibitory feedback to the Mauthner cell; the Mauthner cell itself also sends small dendrites from its axon hill to the peripheral part of the axon cap. Finally, the surface of the axon cap is covered with a ''cap wall'' composed of several layers of
astrocyte Astrocytes (from Ancient Greek , , "star" + , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of e ...
-like
glia Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form myel ...
l cells. Both glial cells and the unmyelinated fibers are coupled with each other by means of
gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...
s. Evolutionarily, the axon cap is a more recent development than the Mauthner cell itself, so some animals, such as
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s and eels, while having functional Mauthner cells, don't have axon cap at all, while some other animals, such as
amphibia Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
and
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, do have a very simplified version of it.


Feedback network

The main part of the Mauthner cell-associated network is the negative
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
network, which assures that only one of the two Mauthner cells fires in response to the stimulus and that, whichever Mauthner cell fires, it does so only once. Both these requirements are quite natural considering that the consequences of a single Mauthner cell discharge are so strong; a failure to comply with these two rules would not only prevent the animal from escaping, but could even physically damage it. The fastest part of this negative feedback network, which is also the one closest to the Mauthner cell, is that of the so-called ''passive hyperpolarizing field potential'' or PHP neurons. The fibers of these neurons are located in the axon cap, and they receive inputs from both
ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and
contralateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
Mauthner cells. The
field potential Local field potentials (LFP) are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning ...
s of PHP neurons are strongly positive, and form a part of the 'Signature field potential' of the Mauthner cell (see below), with the early (ipsilaterally initiated) component being called the Extracellular Hyperpolarizing Potential (EHP), and the later (contralaterally initiated) component being sometimes addressed in the literature as the Late Collateral Inhibition (LCI). The action of PHP neurons onto the Mauthner cells is mediated by electrical, and not chemical effects: the outward currents generated by the
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
s in axon cap fibers flow inward across the Mauthner cell
axon hillock The axon hillock is a specialized part of the cell body (or soma) of a neuron that connects to the axon. It can be identified using light microscopy from its appearance and location in a neuron and from its sparse distribution of Nissl substance. ...
and hyperpolarize it.


Outputs

The only
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
of the Mauthner cell reaches from the cell to the midline of the
hindbrain The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephal ...
, promptly crosses it to the contralateral side, and then descends caudally along the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
. A single discharge of the M-cell achieves a whole set of parallel effects onto the spinal motor networks: 1) it monosynaptically excites large primary motoneurons at one side of the body; 2) disinaptically excites smaller motoneurons at the same side of the body; 3) initiates action potentials in inhibitory
interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. ...
s electrically coupled to the M-cell axon, and by their means inhibits a) inhibitory interneurons still at the same side of the body (to prevent them from interfering with the C-start), as well as b) motoneurons at the other side of the body. As a result of this pattern of activation the quick
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...
s at one side of the body contract simultaneously, while the muscles at the other side of the body relax.


Electrophysiology


Ephaptic properties

200px, Ephaptic inhibition at the mauthner axon cap by PHP cells The inhibition of the M-cell by the PHP cells occurs by ephaptic interactions. The inhibition is brought about without a
chemical synapse Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous syste ...
s or electrical synaptic coupling having low resistance
gap junctions Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...
joining the cells. When the region of the PHP cell axon outside the axon cap depolarizes, the influx of positive charge into the cell through voltage gated sodium channels is accompanied by a passive outflow of current from the PHP cell axon into the region bound by the axon cap. Due to the high resistance of the surrounding glial cells, the charge does not dissipate and the potential across the M-cell membrane is increased, hyperpolarizing it.


Signature field potential

Because of its size, presence of a quick feedback network, and abundance of
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
and quasi-electrical ( ephaptic) synapses, the Mauthner cell has a strong
field potential Local field potentials (LFP) are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning ...
of a very characteristic shape. This field potential starts with a high-amplitude potential sink up to tens of millivolts in amplitude that originates from the Mauthner cell discharge, and which is closely followed by a positive potential, called Extrinsic Hyperpolarizing Potential or EHP, which is associated with the activity of the recurrent feedback network. Due to its high amplitude, in some animals the negative part of Mauthner cell field potential can be detected up to several hundred micrometres away from the cell itself. The positive components of the field potential are strongest in the axon cap, reaching amplitudes of 45 mV in adult goldfish. With a knowledge of these properties of the field potential, it is possible to use field potential monitoring as a way to find the Mauthner cell body
in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and p ...
, or
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology a ...
in a whole brain preparation, moving the recording electrode in the
hindbrain The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephal ...
, while at the same time stimulating the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, thus evoking antidromic action potentials in the Mauthner cell axon.


Plasticity

Application of
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and va ...
was shown to increase inhibitory inputs to the M-cell, while application of
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% o ...
– to increase the amplitude of both chemical and electrical components of the VIIIth nerve responses via a
G protein G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their a ...
-mediated activation of postsynaptic
D2 receptor Dopamine receptor D2, also known as D2R, is a protein that, in humans, is encoded by the ''DRD2'' gene. After work from Paul Greengard's lab had suggested that dopamine receptors were the site of action of antipsychotic drugs, several groups, i ...
. An activity-dependent
LTP LTP may refer to: Biology and medicine * Lateral tibial plateau, part of a leg bone * Lipid transfer proteins, proteins found in plant tissues * Long-term potentiation (neurophysiology), a long-lasting enhancement in signal transmission between ...
can be evoked in M-cells by a high-frequency stimulation of the VIIIth nerve. Surprisingly, this LTP is
electrical synapse Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
-mediated, and is presumed to involve modification of the
gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...
channels. A possibility of LTP induction by sensory stimuli
in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and p ...
, and the evidence for the LTP of inhibitory inputs to M-cells were also demonstrated. Spontaneous preference in turn direction in young goldfish is correlated with one of the Mauthner cells being bigger than the other one. It is possible to change the preference of fish by raising them in conditions facilitating turns in a specific direction; this shift is accompanied by a correspondent change in M-cell sizes.


History of research

The Mauthner cell was first identified by the Viennese ophthalmologist Ludwig Mauthner in the
teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...
fish for its associated neural circuit which mediates an escape response called the C-start or C- startle to direct the fish away from a predator. The M-cell is a model system in the field of
Neuroethology Neuroethology is the evolutionary and comparative approach to the study of animal behavior and its underlying mechanistic control by the nervous system. It is an interdisciplinary science that combines both neuroscience (study of the nervous syste ...
. The M-cell system has served for detailed neurophysiological and
histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
investigations of
synaptic transmission Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron ...
and
synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuits ...
. Studies by Donald Faber and Henri Korn helped to establish the one
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
hypothesis of
synaptic transmission Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron ...
in the CNS. Other important research topics that have been investigated in the M-cell system include studies by Yoichi Oda and colleagues on inhibitory
long-term potentiation In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons ...
and auditory
conditioning Conditioning may refer to: Science, computing, and technology * Air conditioning, the removal of heat from indoor air for thermal comfort ** Automobile air conditioning, air conditioning in a vehicle ** Ice storage air conditioning, air conditio ...
of the startle response, and studies by Alberto Pereda and colleagues on plasticity of
electrical synapse Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
s. Other research topics investigated in the M-cell system include studies of spinal neural networks and neural regeneration by Joe Fetcho and colleagues, as well as underwater
sound localization Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance. The sound localization mechanisms of the mammalian auditory system have been extensively studied. The auditory system us ...
, and the biophysics of computation in single neurons.


References


Further reading

* * * * * * * * * {{Neuroethology Neuroethology Fish nervous system