Mauthner cell: Difference between revisions
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The Mauthner cell of teleost fish and it's associated neural circuit mediates a escape response called the C-start that is directed away from a predator (Moulton and Dixon 1967; Blaxter et al. 1981; Eaton and Emberly, 1991; Canfield and Rose, 1996; Preuss and Faber, 2003). This neural circuit is composed of a distributed network of reticulospinal neurons (Gahtan et al., 2002). In cyclostomes, teleosts, amphibians, and birds the reticulospinal system is the major descending motor pathway. It exhibits a segmental organization with each region separated by glial boundaries (Fig. 4, Kimmel et al., 1982; Lee et al., 1993 Metcalfe et al., 1986). |
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The bilaterally paired M-cells, in rhombomere 4, are the most prominent members of the reticulospinal population in fish. The two primary aspiny dendrites of the M-cell, the lateral and ventral dendrites, receive segregated inputs from statoacoustic endorgans (see Eaton and Popper, 1995) and the optic tectum, and spinal cord (Zottoli et al., 1987; Chang et al., 1987), respectively. One of the primary sources of synaptic input to the M-cell are the ~100 large diameter ipsilateral posterior VIIIth nerve fibers that originate from the sacculus (Bartelmez, 1915; Furukawa and Ishii, 1967; Lin et al., 1983) and to a lesser extent the lagena (Szabo et al., 2007). These fibers terminate as mixed electrical and chemical synapses on the lateral dendrite (Furshpan, 1963; Faber et al., 1980; Tuttle et al., 1986; Pereda et al., 2003; Pereda et al., 2004). Other inputs to the M-cell include the ipsilateral fibers of the anterior VIIIth nerve, originating from the utricle (Zottoli and Faber, 1979; Szabo et al., 2007), and inputs from the lateral line (Korn and Faber, 1975a). Inner ear afferents also terminate with electrical synapses on a population of identifiable inhibitory interneurons, the PHP cells, that produce a bilateral feed-forward electrical, i.e. field effect, and chemical inhibition of the M-cell (Fig. 3C, Fig. 5, Zottoli and Faber, 1980; Triller and Korn, 1981). The mechanisms underlying field effects, and electrical inhibition in particular, will be discussed further (see Part III). |
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When an abrupt acoustic or mechanosensory stimulus elicits a single M-spike in one M-cell it always correlates with a contralateral C-start escape (Wilson, 1959; Zottoli, 1977; Eaton et al., 1981; O’Malley et al, 1996) and mutual inhibition assures that only one M-cell reaches threshold (Fig. 2B1, Furukawa and Furshpan, 1963). There are two sequential stages to the C-start (Domenici and Blake, 1997; Eaton et al., 2001): stage 1, in which the head rotates about the center of mass and the fish's body exhibits a curvature that resembles a C, and stage 2, during which the fish is propelled forward. Notably, forward propulsion does not require contraction of the antagonistic muscle, but probably 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 counterturn or directional change (Foreman and Eaton, 1993). |
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In larval zebrafish Danio rerio ~60% of the total population of reticulospinal neurons ~100-130, also are activated by a mechanosensory stimulus that elicits the C-start escape (Gahtan et al., 2002). This is most likely the case in other teleosts. 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 respectively, and they also receive auditory input in parallel with the M-cell from the pVIIIth nerve (Fig. 2A1, Nakayama and Oda, 2004). Notably, the auditory afferents that synapse onto the M-cell homologues have a slower conduction velocity (see also Casagrand and Eaton, 1999) than those that synapse on the M-cell, suggesting the M-cell homologues play a secondary role in initiating the short latency behavior. |
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Evidence indicates that the M-cell only partially meets the criteria for a command neuron. Electrical stimulation of the M-cell is sufficient for eliciting a C-start, albeit with a smaller turn angle than normal (Nissanov et al., 1990). Yet when the M-cell is ablated the C-start can still be elicited and thus the M-cell is not necessary for the behavior (DiDomenico et al., 1988). However, ablating the M-cell does increase the latency of the escape in goldfish Carassius auratus. |
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The prevailing model of the M-cell command system, proposed by Eaton and colleagues, is that activation of the M-cell initiates a fixed action pattern to the left or right by activating a defined spinal motor circuit (Fetcho and Faber, 1988; Fetcho 1990, 1992; Fetcho and O’Malley 1995; Ritter et al., 2001; Bhatt et al., 2007) , and 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 (Fig. 2A1, Foreman and Eaton, 1993; Eaton et al., 2001). This model is supported by evidence that the direction of a stimulus determines the direction of stage 1 of the C-start with a high degree of precision (Eaton and Emberley, 1991), requiring that the motor commands generated by the escape system provide more information than could be encoded by a single spike in one of the two M-fibers alone (Eaton et al., 1988). |
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In support of this notion, as well as the command system model presented by Eaton (Foreman and Eaton, 1993), studies using in vivo calcium imaging in larval zebrafish show that MiD2cm and MiD3cm, are activated along with the M-cell specifically when an offending stimulus is directed towards the head. Furthermore, activation of the homologues appears to be correlated with C-starts with a larger initial turn angle, indicating that a population code involving the homologues encodes the trajectory of the escape (O’ Malley and Fetcho, 1996). |
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However, it has also been demonstrated in larval zebrafish that when the M-cell is ablated there is no change in the latency of the head directed stimulus evoked C-start, but when the M-cell is ablated and MiD2cm and MiD3cm are ablated there is a significant increase in latency (Liu and Fetcho, 1999) . Hence, the M-cell homologues appear to substitute functionally for the M-cell. This is contrary to Eaton’s model in which the M-cells alone initiate the short latency behavior. One alternative model that could account for this result is that the M-cell system is arranged as a mixed parallel-series circuit (Fig. 2A2). |
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The lesion studies also call in question whether other identifiable reticulospinal neurons can be designated command neurons. This would potentially be the case if they could also initiate escape behaviors. It remains an open possibility that certain stimuli may evoke a C-start, or a C-start like behavior, in the absence of an M-spike (see Kohashi and Oda, 2007), although this has not been observed in adult animals. Furthermore, in certain fish species, such as the roach (Karlsen et al., 2004), infrasound stimuli can elicit non C-start escape behaviors such as the S-start and J-start that are named with respect to the shape of the fish’s initial body bend. The neural basis of these behaviors is unclear (Hale, 2002), but they could potentially be elicited by a single reticulospinal neuron besides the M-cell. |
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