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Mauthner cell
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Mauthner cell
The Mauthner cells are a pair of big and easily identifiable neurons (one for each half of the body) located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex (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 synapses.
Mauthner cells first appear in lampreys (being absent in hagfish and lancelets), and are present in virtually all teleost fish, as well as in amphibians (including postmetamorphic frogs and toads). Some fish, such as lumpsuckers, seem to have lost the Mauthner cells however.
A C-start is a type of a very quick startle or escape reflex that is employed by fish and amphibians (including larval frogs and toads). There are two sequential stages in the C-start: first, the head rotates about the center of mass 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, but results from the body stiffness and the hydrodynamic resistance of the tail. 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.
In cases when an abrupt acoustic, tactile or visual stimulus elicits a single action potential in one M-cell, it always correlates with a contralateral C-start escape. An extremely quick mutual feedback inhibitory circuit then assures that only one M-cell reaches spiking threshold—as the C-start has to be unilateral 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 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 zebrafish 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 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 vertebrates, 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, 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.
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Mauthner cell
The Mauthner cells are a pair of big and easily identifiable neurons (one for each half of the body) located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex (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 synapses.
Mauthner cells first appear in lampreys (being absent in hagfish and lancelets), and are present in virtually all teleost fish, as well as in amphibians (including postmetamorphic frogs and toads). Some fish, such as lumpsuckers, seem to have lost the Mauthner cells however.
A C-start is a type of a very quick startle or escape reflex that is employed by fish and amphibians (including larval frogs and toads). There are two sequential stages in the C-start: first, the head rotates about the center of mass 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, but results from the body stiffness and the hydrodynamic resistance of the tail. 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.
In cases when an abrupt acoustic, tactile or visual stimulus elicits a single action potential in one M-cell, it always correlates with a contralateral C-start escape. An extremely quick mutual feedback inhibitory circuit then assures that only one M-cell reaches spiking threshold—as the C-start has to be unilateral 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 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 zebrafish 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 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 vertebrates, 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, 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.