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BRAIN AND NERVOUS SYSTEM | Physiology of the Mauthner Cell: Discovery and Properties
Physiology of the Mauthner Cell: Discovery and Properties AE Pereda and DS Faber, Albert Einstein College of Medicine, Bronx, NY, USA ª 2011 Elsevier Inc. All rights reserved.
Introduction The Mauthner Cell The Mauthner Cell As Model Vertebrate Neuron
Glossary Antidromic Propagation of an impulse along an axon in a direction that is the reverse of normal. Electrical stimulation either of peripheral nerves or of white matter tracts has long served as a mechanism for electrophysiologically identifying the projections of recorded neurons. Chemical synapse Modality of synaptic transmission whereby transfer of information from presynaptic neurons is mediated by depolarization-induced release of a neurotransmitter, which in turn activates specific receptors located on the postsynaptic membrane. Cholinergic A general term pertaining to processes or structures related to acetylcholine. For example, neurons that release acetylcholine as transmitter. Nicotinic and muscarinic receptors are cholinergic receptors because they bind acetylcholine. Cholinergic synapses are found in both the central and peripheral nervous systems. Electrical synapse Modality of synaptic transmission whereby transfer of information is mediated by intercellular channels known as gap junctions, which provide a pathway of low resistance for the spread of presynaptic electrical currents. Ephaptic Functional interaction between neighboring neurons where the electrical field generated by one neuron influences the excitability of the second.
Introduction During the nineteenth century, comparative anatomical analyses in fish and amphibians led to major break throughs in the understanding of the vertebrate nervous system. As part of this effort, the young Austrian medical student Ludwig Mauthner (Figure 1(a)) stumbled upon the presence of two large, in fact colossal, axons in the fish spinal cord (Figure 1(b)). He was at the time an associate in the laboratory of Ernst Brucke, who, together with Carl Ludwig, Emil Du Bois-Reymond, and Hermann Helmholtz, constituted an influential group of scientists
66
Summary Further Reading
Glycinergic Synapse that uses glycine as its neurotransmitter, one of the major inhibitory neurotransmitters in the spinal cord and the brainstem. Reticulospinal neurons One of the most important descending pathways in the vertebrate nervous system. They integrate sensory inputs and higher motor commands in the brain to ultimately regulate motor functions in the spinal cord. Rhombomeres Distinct segments of the embryonic neural tube in the area that will eventually become the rhombencephalon, or hindbrain. The neural crest cells from each rhombomere do not intermingle and give rise to different ganglia or groups of neurons. Time constant Index that expresses how fast the membrane potential of a neuron changes in response to changes in an applied transmembrane current. The time constant (t) can be calculated as the product between membrane resistance and membrane capacitance. As with other RC circuits, it expresses the time needed to charge the capacitor through the resistor to �63% of full charge, or to discharge it to �37% of its initial voltage (values derived from the mathematical constant e). Thus, due to the presence of the membrane capacitance, the voltage across the membrane usually lags behind current injections. Time constants are usually fast in neurons that require high temporal precision.
that provided the grounds for the modern understanding of the mechanisms underlying physiological processes. In opposition to the then prevalent dogma of vitalism (the life force which gave vitality to all living organisms), they adopted the view that all physiological processes were potentially understandable in terms of ordinary physical and chemical principles (the doctrine of mechanism). Honoring perhaps the context of its discovery, Mauthner’s seminal observation endured over the years by providing the opportunity to examine the physiological properties of a single vertebrate neuron and to correlate them with its structure, leading to seminal discoveries in
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties 67
(a)
(b)
Mauthner axons
(c) Optic lobe
Cerebellum
the understanding of the complexities of mechanisms underlying synaptic transmission. Today, this model ver tebrate neuron provides a catalog of cellular and synaptic specializations that are repeatedly found to be relevant to neurons and their networks in general and for a wide range of species, including mammals. The added attraction of this neuron is that it also has a well-defined function, as its physiological properties are critical to the appropriate expression of a signal behavior, the tail flip or escape response. Yet, the full extent of the functional role of this enigmatic colossal neuron remains undetermined. The physiological properties of the Mauthner cell, emphasizing its use as a model system to investigate cellular and synaptic properties, are reviewed in this article. The physiological role of the Mauthner cell in organizing reflex and voluntary behaviors and the cellular properties of the Mauthner cell in the context of its functional role are discussed elsewhere in this encyclopedia.
Mauthner cells
The Mauthner Cell Vagal lobe
Mauthner axons
Lateral dendrite
Axon Carassius auratus
Soma
Danio rerio Ventral dendrite
100 µm
50 µm
Figure 1 Description of the Mauthner cells. (a) Portrait of Ludwig Mauthner, the discoverer of these giant reticulospinal neurons, which have taken his name. (b) Koestler’s 1898 hand-drawn cross section of pike (Esox lucius) spinal cord, based on microscopic observations, and illustrating the prominent Mauthner axons. (c) Schematic of a dorsal view of the teleost brain, with the paired Mauthner cells superimposed. Note that the axons cross the midline and descend to the spinal cord on the contralateral side. With this perspective, only the lateral dendrites are represented. Below are camera lucida drawings of goldfish (left) and zebrafish (right) Mauthner cells, projected onto the saggital plane and highlighting the two dominant dendrites. Modified from Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556; and Lee RK and Eaton RC (1991) Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio. Journal of Comparative Neurology 304: 34–52.
While Ludwig Mauthner later became a prominent ophthalmologist, his observation led to the identification by others of the origin of these large axons: a pair of large cells in the medulla of teleost and other fish (Figure 1(c)). Due to their uncommon size and shape, these cells are anatomically identifiable. The cell bodies are large (�100 mm in diameter) and characteristically exhibit, along with a number of much smaller dendritic processes, two large main dendrites: the lateral and the ventral dendrites (Figure 1(c)). Their prominent myelinated axons cross the midline to descend the length of the spinal cord, issuing axon collaterals that massively activate cra nial and spinal motor systems. Such an anatomical arrangement allows a single action potential in this cell to initiate an escape response by producing a tail flip and coordinated movements of the jaw and eyes (see below). These anatomical features seem to be fairly constant among various species of fish, allowing the unequivocal identification of these cells. We know now that the Mauthner cells are two bilat erally paired, cholinergic, reticulospinal neurons located in rhombomere 4. As with most reticulospinal neurons, the Mauthner cells receive multiple sensory inputs (Figure 2). Some of these inputs are clearly segregated: while the lateral dendrite receives information from the octavolateralis system (vestibular, lateral line, and, pro minently, auditory modalities), the ventral dendrite receives visual and somesthetic information (Figure 2). Interestingly, a group of reticulospinal neurons in mam mals is thought to mediate a comparable function, the startle response. The presence of a single, albeit larger,
68
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties
Auditory
Vestibular Lateral dendrite Axon Lateral
line
Soma
techniques now allow direct exploration of cellular prop erties in nearly every vertebrate brain structure, the Mauthner cells still provide the uncommon opportunity of correlating structure with function to study detailed cellular mechanisms. In particular, the use of the Mauthner cells and their identifiable synaptic inputs led to many insights into the mechanisms of synaptic trans mission, including mechanisms of inhibition, transmitter release, and electrical transmission. Inhibitory Mechanisms
Somesthesia Visual
Ventral dendrite
100 µm
Figure 2 The projections of sensory afferents to the Mauthner cell dendrites are highly specified, as revealed by electrophysiological mapping experiments. The lateral dendrite processes stato-acoustic information, with the terminal fields of the auditory, vestibular, and lateral line systems localized to different dendritic regions. In contrast, the ventral dendrite is sensitive to visual and somatosensory stimuli. Goldfish Mauthner cell modified from Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556.
cell in rhombomere 4 seems to be functionally beneficial for the escape responses of fish, enabling a more efficient combination of speed and acceleration under water. This notion of a functional advantage, dependent in part on anatomical specializations, is emphasized by the finding that the amphibian Mauthner cells are identifiable only at the tadpole stage, and are either lost or significantly reduced in size during metamorphosis, when they become terrestrial.
The Mauthner Cell As Model Vertebrate Neuron These uncommonly large cells are not only anatomically but also physiologically identifiable (see below) and have historically constituted a valuable experimental pre paration. The large size of the Mauthner cell, the characteristic morphology of its soma and main dendrites, and its physiological identifiability and accessibility for intracellular microelectrode recordings combined to pro vide one of the first possibilities of exploring cellular and synaptic properties in a vertebrate neuron. While slice
As already noted, the Mauthner cell has served as a model for the discovery and elucidation of basic synaptic mechanisms. This advantage is derived from its in vivo identifiability, linked to a unique property of the axon cap, which is a sophisticated anatomical specialization restricted to the initial segment of the axon and amplifies the extracellular field produced by the action potential of the cell. In fact, this feature also underlies a unique form of inhibition that has made it possible to identify presy naptic inhibitory cells and to study their synapses in exquisite detail. Based on this property, Mauthner cells can be distinguished from thousands of other cells in the brain of fishes. This implies that from fish to fish, record ings can be obtained from the same neuron that was genetically determined to be the Mauthner cell, thereby providing a unique comparative advantage. The special property of the axon cap is that it has a very high extracellular resistance, such that the Mauthner cell action potential, which can be antidromically evoked experimentally by a stimulating electrode on the spinal cord, is as large as 30 mV in the center of the cap (Figure 3(a)). The current associated with this field directly hyperpolarizes the membrane of a special class of interneur ons, called ‘passive hyperpolarizing potential’ (PHP) cells; their impulses in turn electrically inhibit the Mauthner cell. These impulses are associated with a positive extracellular field, and both electrical inhibitions are mediated by field effects. That is, the electrical field produced by one neuron can modify the excitability of a nearby neuron without the existence of cellular contacts. In the case of the Mauthner cell, the positive extracellular electrical field produced by the synchronization of many PHP cells generates a mirror hyperpolarization in the initial segment, the region of the cell in which action potentials originate (Figure 4(b)). While field effects (also known as ephaptic interactions) have been observed to occur in some pathological condi tions (i.e., hippocampus during seizures), the axon cap of the Mauthner cell truly represents a synaptic specialization, and thus this mechanism is considered the best example of electrical inhibition. Similar mechanisms are thought to operate in the cerebellum. The electrical inhibition of the Mauthner cell was discovered first, by Furukawa and Furshpan, as
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties 69
5 mV
(a)
1 ms
Mauthner cells
Extracellular spike
S Spinal cord stimulation
(b)
Electrical inhibition Shunt
(field effect)
Increase in membrane conductance
−
+
Axon cap
+ − +
Chemical PHP cell
(glycine / GABA)
Extra
Shunt
+
Field effect
gCI−
2 ms
Intra
10 mV
10 mV
Intra – Extra
−
2 ms
Figure 3 Hallmarks of Mauthner cell electrophysiology: inhibitory mechanisms. (a) Schematic of in vivo recording arrangement (left panel), with an immobilized and anesthetized fish in air. A bipolar electrode on the exposed vertebral column is used to antidromically activate the Mauthner axon while recording extra- or intracellularly from defined regions of the cell. Right panel shows the Mauthner cell electrophysiological signature that allows this localization, namely an antidromic action potential as large as �40 mV in the center of the axon cap that surrounds the cell’s axon hillock and initial segment. This locus serves as a reference for all the other recording loci. (b) A special class of inhibitory interneurons, the PHP cells, mediates both electrical and chemical (glycinergic) inhibition of the Mauthner cell soma. Electrical inhibition is a consequence of failure of action potentials in PHP cells to actively propagate beyond the border of the axon cap and on the high electrical resistance of this region. It is signaled by an extracellular positivity in the center of the cap. Chemical inhibition, due to release of glycine, and possibly �-aminobutyric acid activated (GABA) receptors, is the consequence of an increase in Cl� conductance that shunts any currents generated in the Mauthner cell. Records below illustrate the electrical (right) and chemical (left) inhibitions evoked experimentally by antidromic stimulation of the Mauthner axon. The former is a frank hyperpolarization that follows the Mauthner spike and is revealed by computing the true transmembrane potential change, that is, the difference between the responses recorded intra- and extracellularly. The time course of the shunt produced by recurrent inhibition is manifested by the reduction in the amplitude of a second antidromic action potential, evoked at different intervals in the lower left panel. Note that the shunt mirrors the underlying increase in Cl� conductance, which appear as a depolarization after Cl� injections (red trace).
70
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties
(a)
Electrical (C�35) Large myelinated club endings
Chemical (glutamate) 5 mV
Mixed synapse 1 ms
V
Hair cell
VIII nerve
(b)
Mauthner cell
Paired-pulse facilitation
Long-term potentiation
(c)
3 mV
Facilitated chemical component
Amplitude (%con)
250 200 150 100 Electrical Chemical
HFS
50 0
1 ms
0
10
20
30
40 50 Time (min)
60
70
80
Figure 4 Club endings mediate mixed synaptic transmission. (a) Experimental arrangement showing VIII nerve auditory primary afferents (from saccular hair cells) terminating as club endings on the Mauthner cell lateral dendrite. (Inset) Both mechanisms of synaptic transmission, electrical (gap junction), and chemical (glutamatergic), coexist. VIII nerve stimulation evokes graded, mixed electrical and chemical synaptic potential. (b) Glutamatergic synapses at club endings exhibit paired-pulse facilitation. (c) High-frequency stimulation (HFS) of club endings ( VIII nerve stimulation) leads to long-term potentiation of both the electrical and chemical components of the mixed synaptic response.
antidromic stimulation triggers a feedback activation of about 40 PHP cells, and their synchronous activity hyper polarizes the Mauthner cell by about 10–15 mV. However, a major advance came when these cells were identified by the field effect from Mauthner cell activa tion, and it became apparent that their impulses also mediated glycinergic inhibition of the Mauthner cell. Thus, as shown in Figure 3(b), action potentials in term inals of PHP cells produce a two-component inhibition of the Mauthner cell: a fast and brief electrical component followed by a slower and longer-lasting chemical compo nent. The latter is due to the opening of Cl– channels, and because the Cl� equilibrium potential is close to the Mauthner cell resting potential, this inhibition does not
itself produce a significant membrane potential change. Rather, the inhibition works by producing a shunting effect (i.e., synaptic conductance short-circuits the cur rents that are generated at adjacent excitatory synapses), which can be clearly appreciated by comparing the time course of the synaptic potential with that of the under lying conductance change (Figure 3(b)), evidenced in this case by a reduction in the amplitude of consecutive antidromic spikes obtained at different intervals. The Mauthner cell action potential, generated in the initial segment, is a convenient indicator of the cell’s conduc tance. Because it propagates passively through the somatodendritic membrane, changes in the conductance of the cell alter its amplitude. As a footnote, the two
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties 71
electrical inhibitions are associated with fields of oppos ing polarities. This observation has driven home the point that the sign of a field effect can only be determined by computing the true transmembrane potential change, which is the difference between the potentials recorded intracellularly and extracellularly (Figure 3(b)). Mechanisms of Transmitter Release There followed a decade of research in which the proper ties of central inhibitory synapses were studied with paired presynaptic and postsynaptic intracellular record ings, as well as dye injections to expose fundamental properties of synaptic transmission and their morpholo gical correlates. This breakthrough was achieved by combining physiological and ultrastructural data that were possible to collect due to the anatomical and phy siological identifiability of the Mauthner cell and the afferent synapses of the PHP cells. Some basic properties discovered in this system include: (1) the quantal and probabilistic nature of transmitter release in the CNS, with the realization that at most one or two synaptic vesicles undergo exocytosis at a single synaptic site; (2) the first support for the concept that there is crosstalk between neighboring central synapses due to the lateral diffusion of transmitter; and (3) the evidence for activitydependent plasticity of inhibition. Mixed Electrical and Chemical Synapses at Primary Auditory Afferents Because of their large size, characteristic myelinization, and dendritic localization, a group of auditory afferents constitutes the most recognizable synaptic input to the Mauthner cells. Such anatomical characteristics support high-speed impulse conduction and suggest that they likely provide the Mauthner cells with critical auditory information for the initiation of escape responses. First described by Bartelmez in 1915 in the catfish (Ameiurus), (Figure 4(a)), this population of �90 large afferents (8–15 mm in diameter) originates in the rostral portion of the saccular macula (the main auditory com ponent of fish ear) and runs in the posterior branch of the VIII nerve. Although each afferent issues a few substan tially thinner branches, likely targeting other neurons in the hindbrain, the primary axon characteristically termi nates as a large single terminal (about the same diameter as that of the parent axon) on the lateral dendrite of the Mauthner cell. These endings are known as large myeli nated club endings or simply club endings. A wealth of anatomical and electrophysiological data shows that club endings support both chemical and elec trical modalities of transmission (Figure 4(a)). In fact, these contacts provided one of the first demonstrations of gap junction plaques and electrical transmission in the
vertebrate central nervous system and, because of their accessibility, constitute a valuable model for exploring the properties of this modality of transmission. These gap junctions are formed by connexin 35, the fish ortholog of the widely expressed mammalian connexin 36, which is responsible for electrical coupling between many cell types, including neocortical inhibitory interneurons and inferior olivary cells. From the electrophysiological point of view, a presy naptic impulse generates a mixed excitatory response in the Mauthner cell. The stimulation of the posterior branch of the VIII nerve, where these fibers run, evokes a gap junction-mediated electrical synaptic potential fol lowed by a chemically mediated excitatory postsynaptic potential (Figure 4(a)). Due to the fast time constant of the Mauthner cell (�400 ms), the two components can be easily distinguished. Chemical transmission is mediated by the release of glutamate, which activates both N-methyl-D-aspartate (NMDA)-activated glutamate receptors and non-NMDA receptors. Even at its relatively hyperpolarized resting potential (��80 mV), the chemical component of the mixed synaptic response of the club endings on the Mauthner cell is normally mediated by the activation of both NMDA and non-NMDA receptors and, surprisingly, the NMDA component is almost as fast as the non-NMDA component. In contrast to most primary auditory afferent synapses, which are known to depress in response to multiple stimuli, glutamatergic synapses at club endings characteristically exhibit frequency-dependent facilitation (Figure 4(b)), suggesting the existence of unusual synaptic specializations in these teleost afferents. While mixed synapses are often found in lower ver tebrates, the combination of these two forms of transmission matches the functional role of club ending afferents in the Mauthner cell system. Electrical trans mission provides speed and reliability of transmission (i.e., absence of synaptic delay and associated probabil istic mechanisms), and the presence of chemical transmission, with a relatively longer duration, allows temporal summation during repetitive responses in a cell in which the membrane time constant is unusually brief (Figure 4(b)). In fact, electrical component of the response predominates when naturalistic acoustic sti muli are used, suggesting chemical transmission might also have another functional role. Indeed, the advantage gained by combining electrical and chemical synapses is emphasized by the existence of important functional interactions between these two modalities of transmis sion. The most remarkable of these interactions is the induction by glutamatergic synapses of long-term activity-dependent changes in the efficacy of both chemical and electrical synaptic transmission. That is, not only the glutamatergic component but also the gap junction-mediated component of the mixed synaptic
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Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties
response undergo activity-dependent potentiation of their synaptic strengths in response to high-frequency stimula tion of the VIII nerve (Figure 4(c)). This phenomenon provided the first example of activity-dependent plasticity of electrical synapses and the opportunity to explore its mechanisms in an accessible experimental model. The induction of long-term activity-dependent synaptic plasti city requires stimulation with brief trains, a protocol inspired by the natural bursting properties of these affer ents. Induction also requires activation of NMDA receptors, leading in turn to a localized increase in the intracellular concentration of Ca2+ that activates the kinase Ca2+-calmodulin-dependent kinase II (CaM-KII), which is necessary for the potentiations. Interestingly, another activity-dependent protocol has also been shown to lead to potentiation of the mixed synaptic response, suggesting that multiple mechanisms converge for the regulation of the strength of these important synapses. In this second case, the mechanism involves the production of endocan nabinoids by the Mauthner cell and is indirectly mediated via the release of dopamine from nearby varicosities that in turn leads to potentiation of the synaptic response via a cyclic adenosine monophosphate (cAMP)-dependent pro tein kinase-mediated (PKA) postsynaptic mechanism. The increased synaptic gain of these auditory nerve synapses should sensitize a vital escape response, lowering its threshold to acoustic stimuli, and it likely represents a mechanism for modulating sensory-motor processing.
Summary Because of their experimental accessibility, the Mauthner cells have provided important insights into basic mechan isms of cellular excitability and synaptic transmission. Despite the availability of modern techniques, which now allow direct exploration of cellular properties in slices of brain tissue, these cells will still provide the uncommon
opportunity of directly correlating structure with function to study detailed cellular mechanisms. Moreover, the rich and singular physiological properties of the Mauthner cells will continue to provide a unique, fascinating window for the discovery of basic mechanisms of brain function. See also: Brain and Nervous System: Physiology of the Mauthner Cell: Function. Design and Physiology of the Heart: Action Potential of the Fish Heart; Intracardiac Neurons and Neurotransmitters in Fish.
Further Reading Cachope R, Mackie K, Triller A, O’Brien J, and Pereda A (2007) Potentiation of electrical and glutamatergic synaptic transmission mediated by endocannabinoids. Neuron 56: 1034–1047. Curti S and Pereda AE (2009) Functional specializations of primary auditory afferents on the Mauthner cells: Interactions between membrane and synaptic properties. Journal of Physiology – Paris 104: 203–214. Faber DS, Fetcho JR, and Korn H (1989) Neuronal networks underlying the escape response in goldfish: General implications for motor control. Annals of the New York Academy of Sciences 563: 11–33. Faber DS and Korn H (1978) Electrophysiology of the Mauthner cell, basic properties, synaptic mechanisms, and associated networks. In: Faber DS and Korn H (eds.) Neurobiology of the Mauthner Cell, pp. 47–131. New York: Raven. Furshpan EJ and Furukawa T (1962) Intracellular and extracellular responses of the several regions of the Mauthner cell of the goldfish. Journal of Neurophysiology 25: 732–71. Korn H and Faber DS (2005) The Mauthner cell half a century later: A neurobiological model for decision-making?. Neuron 47: 13–28. Lee RK and Eaton RC (1991) Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio. Journal of Comparative Neurology 304: 34–52. Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556. Pereda AE, Rash JE, Nagy JI, and Bennett MVL (2004) Dynamics of electrical transmission at club endings on the Mauthner cells. Brain Research Reviews 47: 227–244. Seyfarth EA and Zottoli SJ (1991) Ludwig Mauthner (1840–1894): Neuroanatomist and noted ophthalmologist in Fin-de-Sie`cle Vienna. Brain Behavior and Evolution 37: 252–259.
Introduction The Mauthner Cell The Mauthner Cell As Model Vertebrate Neuron
Glossary Antidromic Propagation of an impulse along an axon in a direction that is the reverse of normal. Electrical stimulation either of peripheral nerves or of white matter tracts has long served as a mechanism for electrophysiologically identifying the projections of recorded neurons. Chemical synapse Modality of synaptic transmission whereby transfer of information from presynaptic neurons is mediated by depolarization-induced release of a neurotransmitter, which in turn activates specific receptors located on the postsynaptic membrane. Cholinergic A general term pertaining to processes or structures related to acetylcholine. For example, neurons that release acetylcholine as transmitter. Nicotinic and muscarinic receptors are cholinergic receptors because they bind acetylcholine. Cholinergic synapses are found in both the central and peripheral nervous systems. Electrical synapse Modality of synaptic transmission whereby transfer of information is mediated by intercellular channels known as gap junctions, which provide a pathway of low resistance for the spread of presynaptic electrical currents. Ephaptic Functional interaction between neighboring neurons where the electrical field generated by one neuron influences the excitability of the second.
Introduction During the nineteenth century, comparative anatomical analyses in fish and amphibians led to major break throughs in the understanding of the vertebrate nervous system. As part of this effort, the young Austrian medical student Ludwig Mauthner (Figure 1(a)) stumbled upon the presence of two large, in fact colossal, axons in the fish spinal cord (Figure 1(b)). He was at the time an associate in the laboratory of Ernst Brucke, who, together with Carl Ludwig, Emil Du Bois-Reymond, and Hermann Helmholtz, constituted an influential group of scientists
66
Summary Further Reading
Glycinergic Synapse that uses glycine as its neurotransmitter, one of the major inhibitory neurotransmitters in the spinal cord and the brainstem. Reticulospinal neurons One of the most important descending pathways in the vertebrate nervous system. They integrate sensory inputs and higher motor commands in the brain to ultimately regulate motor functions in the spinal cord. Rhombomeres Distinct segments of the embryonic neural tube in the area that will eventually become the rhombencephalon, or hindbrain. The neural crest cells from each rhombomere do not intermingle and give rise to different ganglia or groups of neurons. Time constant Index that expresses how fast the membrane potential of a neuron changes in response to changes in an applied transmembrane current. The time constant (t) can be calculated as the product between membrane resistance and membrane capacitance. As with other RC circuits, it expresses the time needed to charge the capacitor through the resistor to �63% of full charge, or to discharge it to �37% of its initial voltage (values derived from the mathematical constant e). Thus, due to the presence of the membrane capacitance, the voltage across the membrane usually lags behind current injections. Time constants are usually fast in neurons that require high temporal precision.
that provided the grounds for the modern understanding of the mechanisms underlying physiological processes. In opposition to the then prevalent dogma of vitalism (the life force which gave vitality to all living organisms), they adopted the view that all physiological processes were potentially understandable in terms of ordinary physical and chemical principles (the doctrine of mechanism). Honoring perhaps the context of its discovery, Mauthner’s seminal observation endured over the years by providing the opportunity to examine the physiological properties of a single vertebrate neuron and to correlate them with its structure, leading to seminal discoveries in
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties 67
(a)
(b)
Mauthner axons
(c) Optic lobe
Cerebellum
the understanding of the complexities of mechanisms underlying synaptic transmission. Today, this model ver tebrate neuron provides a catalog of cellular and synaptic specializations that are repeatedly found to be relevant to neurons and their networks in general and for a wide range of species, including mammals. The added attraction of this neuron is that it also has a well-defined function, as its physiological properties are critical to the appropriate expression of a signal behavior, the tail flip or escape response. Yet, the full extent of the functional role of this enigmatic colossal neuron remains undetermined. The physiological properties of the Mauthner cell, emphasizing its use as a model system to investigate cellular and synaptic properties, are reviewed in this article. The physiological role of the Mauthner cell in organizing reflex and voluntary behaviors and the cellular properties of the Mauthner cell in the context of its functional role are discussed elsewhere in this encyclopedia.
Mauthner cells
The Mauthner Cell Vagal lobe
Mauthner axons
Lateral dendrite
Axon Carassius auratus
Soma
Danio rerio Ventral dendrite
100 µm
50 µm
Figure 1 Description of the Mauthner cells. (a) Portrait of Ludwig Mauthner, the discoverer of these giant reticulospinal neurons, which have taken his name. (b) Koestler’s 1898 hand-drawn cross section of pike (Esox lucius) spinal cord, based on microscopic observations, and illustrating the prominent Mauthner axons. (c) Schematic of a dorsal view of the teleost brain, with the paired Mauthner cells superimposed. Note that the axons cross the midline and descend to the spinal cord on the contralateral side. With this perspective, only the lateral dendrites are represented. Below are camera lucida drawings of goldfish (left) and zebrafish (right) Mauthner cells, projected onto the saggital plane and highlighting the two dominant dendrites. Modified from Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556; and Lee RK and Eaton RC (1991) Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio. Journal of Comparative Neurology 304: 34–52.
While Ludwig Mauthner later became a prominent ophthalmologist, his observation led to the identification by others of the origin of these large axons: a pair of large cells in the medulla of teleost and other fish (Figure 1(c)). Due to their uncommon size and shape, these cells are anatomically identifiable. The cell bodies are large (�100 mm in diameter) and characteristically exhibit, along with a number of much smaller dendritic processes, two large main dendrites: the lateral and the ventral dendrites (Figure 1(c)). Their prominent myelinated axons cross the midline to descend the length of the spinal cord, issuing axon collaterals that massively activate cra nial and spinal motor systems. Such an anatomical arrangement allows a single action potential in this cell to initiate an escape response by producing a tail flip and coordinated movements of the jaw and eyes (see below). These anatomical features seem to be fairly constant among various species of fish, allowing the unequivocal identification of these cells. We know now that the Mauthner cells are two bilat erally paired, cholinergic, reticulospinal neurons located in rhombomere 4. As with most reticulospinal neurons, the Mauthner cells receive multiple sensory inputs (Figure 2). Some of these inputs are clearly segregated: while the lateral dendrite receives information from the octavolateralis system (vestibular, lateral line, and, pro minently, auditory modalities), the ventral dendrite receives visual and somesthetic information (Figure 2). Interestingly, a group of reticulospinal neurons in mam mals is thought to mediate a comparable function, the startle response. The presence of a single, albeit larger,
68
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties
Auditory
Vestibular Lateral dendrite Axon Lateral
line
Soma
techniques now allow direct exploration of cellular prop erties in nearly every vertebrate brain structure, the Mauthner cells still provide the uncommon opportunity of correlating structure with function to study detailed cellular mechanisms. In particular, the use of the Mauthner cells and their identifiable synaptic inputs led to many insights into the mechanisms of synaptic trans mission, including mechanisms of inhibition, transmitter release, and electrical transmission. Inhibitory Mechanisms
Somesthesia Visual
Ventral dendrite
100 µm
Figure 2 The projections of sensory afferents to the Mauthner cell dendrites are highly specified, as revealed by electrophysiological mapping experiments. The lateral dendrite processes stato-acoustic information, with the terminal fields of the auditory, vestibular, and lateral line systems localized to different dendritic regions. In contrast, the ventral dendrite is sensitive to visual and somatosensory stimuli. Goldfish Mauthner cell modified from Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556.
cell in rhombomere 4 seems to be functionally beneficial for the escape responses of fish, enabling a more efficient combination of speed and acceleration under water. This notion of a functional advantage, dependent in part on anatomical specializations, is emphasized by the finding that the amphibian Mauthner cells are identifiable only at the tadpole stage, and are either lost or significantly reduced in size during metamorphosis, when they become terrestrial.
The Mauthner Cell As Model Vertebrate Neuron These uncommonly large cells are not only anatomically but also physiologically identifiable (see below) and have historically constituted a valuable experimental pre paration. The large size of the Mauthner cell, the characteristic morphology of its soma and main dendrites, and its physiological identifiability and accessibility for intracellular microelectrode recordings combined to pro vide one of the first possibilities of exploring cellular and synaptic properties in a vertebrate neuron. While slice
As already noted, the Mauthner cell has served as a model for the discovery and elucidation of basic synaptic mechanisms. This advantage is derived from its in vivo identifiability, linked to a unique property of the axon cap, which is a sophisticated anatomical specialization restricted to the initial segment of the axon and amplifies the extracellular field produced by the action potential of the cell. In fact, this feature also underlies a unique form of inhibition that has made it possible to identify presy naptic inhibitory cells and to study their synapses in exquisite detail. Based on this property, Mauthner cells can be distinguished from thousands of other cells in the brain of fishes. This implies that from fish to fish, record ings can be obtained from the same neuron that was genetically determined to be the Mauthner cell, thereby providing a unique comparative advantage. The special property of the axon cap is that it has a very high extracellular resistance, such that the Mauthner cell action potential, which can be antidromically evoked experimentally by a stimulating electrode on the spinal cord, is as large as 30 mV in the center of the cap (Figure 3(a)). The current associated with this field directly hyperpolarizes the membrane of a special class of interneur ons, called ‘passive hyperpolarizing potential’ (PHP) cells; their impulses in turn electrically inhibit the Mauthner cell. These impulses are associated with a positive extracellular field, and both electrical inhibitions are mediated by field effects. That is, the electrical field produced by one neuron can modify the excitability of a nearby neuron without the existence of cellular contacts. In the case of the Mauthner cell, the positive extracellular electrical field produced by the synchronization of many PHP cells generates a mirror hyperpolarization in the initial segment, the region of the cell in which action potentials originate (Figure 4(b)). While field effects (also known as ephaptic interactions) have been observed to occur in some pathological condi tions (i.e., hippocampus during seizures), the axon cap of the Mauthner cell truly represents a synaptic specialization, and thus this mechanism is considered the best example of electrical inhibition. Similar mechanisms are thought to operate in the cerebellum. The electrical inhibition of the Mauthner cell was discovered first, by Furukawa and Furshpan, as
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Figure 3 Hallmarks of Mauthner cell electrophysiology: inhibitory mechanisms. (a) Schematic of in vivo recording arrangement (left panel), with an immobilized and anesthetized fish in air. A bipolar electrode on the exposed vertebral column is used to antidromically activate the Mauthner axon while recording extra- or intracellularly from defined regions of the cell. Right panel shows the Mauthner cell electrophysiological signature that allows this localization, namely an antidromic action potential as large as �40 mV in the center of the axon cap that surrounds the cell’s axon hillock and initial segment. This locus serves as a reference for all the other recording loci. (b) A special class of inhibitory interneurons, the PHP cells, mediates both electrical and chemical (glycinergic) inhibition of the Mauthner cell soma. Electrical inhibition is a consequence of failure of action potentials in PHP cells to actively propagate beyond the border of the axon cap and on the high electrical resistance of this region. It is signaled by an extracellular positivity in the center of the cap. Chemical inhibition, due to release of glycine, and possibly �-aminobutyric acid activated (GABA) receptors, is the consequence of an increase in Cl� conductance that shunts any currents generated in the Mauthner cell. Records below illustrate the electrical (right) and chemical (left) inhibitions evoked experimentally by antidromic stimulation of the Mauthner axon. The former is a frank hyperpolarization that follows the Mauthner spike and is revealed by computing the true transmembrane potential change, that is, the difference between the responses recorded intra- and extracellularly. The time course of the shunt produced by recurrent inhibition is manifested by the reduction in the amplitude of a second antidromic action potential, evoked at different intervals in the lower left panel. Note that the shunt mirrors the underlying increase in Cl� conductance, which appear as a depolarization after Cl� injections (red trace).
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Figure 4 Club endings mediate mixed synaptic transmission. (a) Experimental arrangement showing VIII nerve auditory primary afferents (from saccular hair cells) terminating as club endings on the Mauthner cell lateral dendrite. (Inset) Both mechanisms of synaptic transmission, electrical (gap junction), and chemical (glutamatergic), coexist. VIII nerve stimulation evokes graded, mixed electrical and chemical synaptic potential. (b) Glutamatergic synapses at club endings exhibit paired-pulse facilitation. (c) High-frequency stimulation (HFS) of club endings ( VIII nerve stimulation) leads to long-term potentiation of both the electrical and chemical components of the mixed synaptic response.
antidromic stimulation triggers a feedback activation of about 40 PHP cells, and their synchronous activity hyper polarizes the Mauthner cell by about 10–15 mV. However, a major advance came when these cells were identified by the field effect from Mauthner cell activa tion, and it became apparent that their impulses also mediated glycinergic inhibition of the Mauthner cell. Thus, as shown in Figure 3(b), action potentials in term inals of PHP cells produce a two-component inhibition of the Mauthner cell: a fast and brief electrical component followed by a slower and longer-lasting chemical compo nent. The latter is due to the opening of Cl– channels, and because the Cl� equilibrium potential is close to the Mauthner cell resting potential, this inhibition does not
itself produce a significant membrane potential change. Rather, the inhibition works by producing a shunting effect (i.e., synaptic conductance short-circuits the cur rents that are generated at adjacent excitatory synapses), which can be clearly appreciated by comparing the time course of the synaptic potential with that of the under lying conductance change (Figure 3(b)), evidenced in this case by a reduction in the amplitude of consecutive antidromic spikes obtained at different intervals. The Mauthner cell action potential, generated in the initial segment, is a convenient indicator of the cell’s conduc tance. Because it propagates passively through the somatodendritic membrane, changes in the conductance of the cell alter its amplitude. As a footnote, the two
Brain and Nervous System | Physiology of the Mauthner Cell: Discovery and Properties 71
electrical inhibitions are associated with fields of oppos ing polarities. This observation has driven home the point that the sign of a field effect can only be determined by computing the true transmembrane potential change, which is the difference between the potentials recorded intracellularly and extracellularly (Figure 3(b)). Mechanisms of Transmitter Release There followed a decade of research in which the proper ties of central inhibitory synapses were studied with paired presynaptic and postsynaptic intracellular record ings, as well as dye injections to expose fundamental properties of synaptic transmission and their morpholo gical correlates. This breakthrough was achieved by combining physiological and ultrastructural data that were possible to collect due to the anatomical and phy siological identifiability of the Mauthner cell and the afferent synapses of the PHP cells. Some basic properties discovered in this system include: (1) the quantal and probabilistic nature of transmitter release in the CNS, with the realization that at most one or two synaptic vesicles undergo exocytosis at a single synaptic site; (2) the first support for the concept that there is crosstalk between neighboring central synapses due to the lateral diffusion of transmitter; and (3) the evidence for activitydependent plasticity of inhibition. Mixed Electrical and Chemical Synapses at Primary Auditory Afferents Because of their large size, characteristic myelinization, and dendritic localization, a group of auditory afferents constitutes the most recognizable synaptic input to the Mauthner cells. Such anatomical characteristics support high-speed impulse conduction and suggest that they likely provide the Mauthner cells with critical auditory information for the initiation of escape responses. First described by Bartelmez in 1915 in the catfish (Ameiurus), (Figure 4(a)), this population of �90 large afferents (8–15 mm in diameter) originates in the rostral portion of the saccular macula (the main auditory com ponent of fish ear) and runs in the posterior branch of the VIII nerve. Although each afferent issues a few substan tially thinner branches, likely targeting other neurons in the hindbrain, the primary axon characteristically termi nates as a large single terminal (about the same diameter as that of the parent axon) on the lateral dendrite of the Mauthner cell. These endings are known as large myeli nated club endings or simply club endings. A wealth of anatomical and electrophysiological data shows that club endings support both chemical and elec trical modalities of transmission (Figure 4(a)). In fact, these contacts provided one of the first demonstrations of gap junction plaques and electrical transmission in the
vertebrate central nervous system and, because of their accessibility, constitute a valuable model for exploring the properties of this modality of transmission. These gap junctions are formed by connexin 35, the fish ortholog of the widely expressed mammalian connexin 36, which is responsible for electrical coupling between many cell types, including neocortical inhibitory interneurons and inferior olivary cells. From the electrophysiological point of view, a presy naptic impulse generates a mixed excitatory response in the Mauthner cell. The stimulation of the posterior branch of the VIII nerve, where these fibers run, evokes a gap junction-mediated electrical synaptic potential fol lowed by a chemically mediated excitatory postsynaptic potential (Figure 4(a)). Due to the fast time constant of the Mauthner cell (�400 ms), the two components can be easily distinguished. Chemical transmission is mediated by the release of glutamate, which activates both N-methyl-D-aspartate (NMDA)-activated glutamate receptors and non-NMDA receptors. Even at its relatively hyperpolarized resting potential (��80 mV), the chemical component of the mixed synaptic response of the club endings on the Mauthner cell is normally mediated by the activation of both NMDA and non-NMDA receptors and, surprisingly, the NMDA component is almost as fast as the non-NMDA component. In contrast to most primary auditory afferent synapses, which are known to depress in response to multiple stimuli, glutamatergic synapses at club endings characteristically exhibit frequency-dependent facilitation (Figure 4(b)), suggesting the existence of unusual synaptic specializations in these teleost afferents. While mixed synapses are often found in lower ver tebrates, the combination of these two forms of transmission matches the functional role of club ending afferents in the Mauthner cell system. Electrical trans mission provides speed and reliability of transmission (i.e., absence of synaptic delay and associated probabil istic mechanisms), and the presence of chemical transmission, with a relatively longer duration, allows temporal summation during repetitive responses in a cell in which the membrane time constant is unusually brief (Figure 4(b)). In fact, electrical component of the response predominates when naturalistic acoustic sti muli are used, suggesting chemical transmission might also have another functional role. Indeed, the advantage gained by combining electrical and chemical synapses is emphasized by the existence of important functional interactions between these two modalities of transmis sion. The most remarkable of these interactions is the induction by glutamatergic synapses of long-term activity-dependent changes in the efficacy of both chemical and electrical synaptic transmission. That is, not only the glutamatergic component but also the gap junction-mediated component of the mixed synaptic
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response undergo activity-dependent potentiation of their synaptic strengths in response to high-frequency stimula tion of the VIII nerve (Figure 4(c)). This phenomenon provided the first example of activity-dependent plasticity of electrical synapses and the opportunity to explore its mechanisms in an accessible experimental model. The induction of long-term activity-dependent synaptic plasti city requires stimulation with brief trains, a protocol inspired by the natural bursting properties of these affer ents. Induction also requires activation of NMDA receptors, leading in turn to a localized increase in the intracellular concentration of Ca2+ that activates the kinase Ca2+-calmodulin-dependent kinase II (CaM-KII), which is necessary for the potentiations. Interestingly, another activity-dependent protocol has also been shown to lead to potentiation of the mixed synaptic response, suggesting that multiple mechanisms converge for the regulation of the strength of these important synapses. In this second case, the mechanism involves the production of endocan nabinoids by the Mauthner cell and is indirectly mediated via the release of dopamine from nearby varicosities that in turn leads to potentiation of the synaptic response via a cyclic adenosine monophosphate (cAMP)-dependent pro tein kinase-mediated (PKA) postsynaptic mechanism. The increased synaptic gain of these auditory nerve synapses should sensitize a vital escape response, lowering its threshold to acoustic stimuli, and it likely represents a mechanism for modulating sensory-motor processing.
Summary Because of their experimental accessibility, the Mauthner cells have provided important insights into basic mechan isms of cellular excitability and synaptic transmission. Despite the availability of modern techniques, which now allow direct exploration of cellular properties in slices of brain tissue, these cells will still provide the uncommon
opportunity of directly correlating structure with function to study detailed cellular mechanisms. Moreover, the rich and singular physiological properties of the Mauthner cells will continue to provide a unique, fascinating window for the discovery of basic mechanisms of brain function. See also: Brain and Nervous System: Physiology of the Mauthner Cell: Function. Design and Physiology of the Heart: Action Potential of the Fish Heart; Intracardiac Neurons and Neurotransmitters in Fish.
Further Reading Cachope R, Mackie K, Triller A, O’Brien J, and Pereda A (2007) Potentiation of electrical and glutamatergic synaptic transmission mediated by endocannabinoids. Neuron 56: 1034–1047. Curti S and Pereda AE (2009) Functional specializations of primary auditory afferents on the Mauthner cells: Interactions between membrane and synaptic properties. Journal of Physiology – Paris 104: 203–214. Faber DS, Fetcho JR, and Korn H (1989) Neuronal networks underlying the escape response in goldfish: General implications for motor control. Annals of the New York Academy of Sciences 563: 11–33. Faber DS and Korn H (1978) Electrophysiology of the Mauthner cell, basic properties, synaptic mechanisms, and associated networks. In: Faber DS and Korn H (eds.) Neurobiology of the Mauthner Cell, pp. 47–131. New York: Raven. Furshpan EJ and Furukawa T (1962) Intracellular and extracellular responses of the several regions of the Mauthner cell of the goldfish. Journal of Neurophysiology 25: 732–71. Korn H and Faber DS (2005) The Mauthner cell half a century later: A neurobiological model for decision-making?. Neuron 47: 13–28. Lee RK and Eaton RC (1991) Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio. Journal of Comparative Neurology 304: 34–52. Lee RK, Eaton RC, and Zottoli SJ (1993) Segmental arrangement of reticulospinal neurons in the goldfish hindbrain. Journal of Comparative Neurology 329: 539–556. Pereda AE, Rash JE, Nagy JI, and Bennett MVL (2004) Dynamics of electrical transmission at club endings on the Mauthner cells. Brain Research Reviews 47: 227–244. Seyfarth EA and Zottoli SJ (1991) Ludwig Mauthner (1840–1894): Neuroanatomist and noted ophthalmologist in Fin-de-Sie`cle Vienna. Brain Behavior and Evolution 37: 252–259.