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At least 19 recordsLinked to original sources
PubMed1989

Sensory coding and corollary discharge effects in mormyrid electric fish.

Weakly electric fish use their electrosensory systems for electrocommunication, active electrolocation and low-frequency passive electrolocation. In electric fish of the family Mormyridae, these three purposes are mediated by separate classes of electroreceptors: electrocommunication by Knollenorgan electroreceptors, active electrolocation by Mormyromast electroreceptors and low-frequency passive electrolocation by ampullary electroreceptors. The primary afferent fibres from each class of electroreceptors terminate in a separate central region. Thus, the mormyrid electrosensory system has three anatomically and functionally distinct subsystems. This review describes the sensory coding and initial processing in each of the three subsystems, with an emphasis on the Knollenorgan and Mormyromast subsystems. The Knollenorgan subsystem is specialized for the measurement of temporal information but appears to ignore both intensity and spatial information. In contrast, the Mormyromast subsystem is specialized for the measurement of both intensity and spatial information. The morphological and physiological characteristics of the primary afferents and their central projection regions are quite different for the two subsystems and reflect the type of information which the subsystems preserve. This review also describes the electric organ corollary discharge (EOCD) effects which are present in the central projection regions of each of the three electrosensory subsystems. These EOCD effects are driven by the motor command that drives the electric organ to discharge. The EOCD effects are different in each of the three subsystems and these differences reflect differences in both the pattern and significance of the sensory information that is evoked by the fish's own electric organ discharge. Some of the EOCD effects are invariant, whereas others are plastic and depend on previous afferent input. The mormyrid work is placed within two general contexts: (a) the measurement of time and intensity in sensory systems, and (b) the various roles of motor command (efferent) signals and self-induced sensory (reafferent) signals in sensorimotor systems.

Action Potentials↗
PubMed1990

Different classes of glutamate receptors and GABA mediate distinct modulations of a neuronal oscillator, the medullary pacemaker of a gymnotiform electric fish.

Gymnotiform electric fish generate distinct communicatory signals by modulating the rate of their electric organ discharges (EODs). Each EOD is triggered by a command pulse from the medullary pacemaker nucleus (PN), which contains pacemaker cells and relay cells. The firing rate of this nucleus is modulated by inputs from the diencephalic prepacemaker nucleus (PPN). The NMDA receptor blocker APV and the kainate/quisqualate receptor blocker CNQX, administered to the PN, suppress different types of modulations, indicating that different classes of glutamate receptors mediate the generation of different modulations. A comparison of the 2 genera, Hypopomus and Eigenmannia, reveals that sustained modulations, such as smooth rises in the rate of pacemaker cell firing and the selective silencing of the relay cells (only observed in Hypopomus), are mediated by NMDA receptors, whereas the brief and rapid acceleration, called "chirp" or "decrement burst," is mediated by kainate/quisqualate receptors. Application of the GABA blocker bicuculline reveals that the 2 genera differ in the mechanism by which they slow the firing rate of their pacemaker. Whereas Hypopomus uses GABAergic inhibition to slow down and ultimately silence its pacemaker cells, Eigenmannia reduces tonic, APV-sensitive excitation originating from its PPN and lacks GABAergic inhibition in the PN.

2-Amino-5-phosphonovalerate↗
PubMed1991

Regeneration of electroreceptors in weakly electric fish.

Weakly electric teleost fish possess two classes of electroreceptors: tuberous and ampullary organs. Ampullary organs are used for detecting prey, while tuberous organs detect the fish's own electric organ discharges (EODs) and those of conspecifics. EOD frequency varies among individuals within a species and a fish's tuberous receptors are sharply tuned to its own EOD frequency. In young, small fish both tuberous and ampullary afferents innervate only single organs. As fish grow new receptor cells are added to each organ and it divides into two daughter organs. This process continues resulting in numerous organs in a cluster; the afferent nerve innervates all the organs in a cluster. When a patch of skin is removed new skin grows back complete with new receptor organs of both classes. From our studies we have shown that: (1) new organs are found only in the presence of nerve fibres; (2) their morphological development during regeneration is similar to their normal development; (3) organs divide rapidly giving rise to daughter organs until each afferent fibre innervates the correct number of organs for a fish of its size; (4) receptor cells are broadly tuned below the EOD frequency of a given fish and they gradually increase their tuned frequency and sharpness of tuning until they become correctly tuned to that EOD frequency; (5) the correct matching of receptor tuning to EOD frequency occurs in fish in which the spinal cord has been severed or with lesions of the medullary pacemaker nucleus, thereby eliminating the EOD and any possible 'calibration' signal; and (6) basal and capsule cells of receptor organs in the intact skin around the wound divide after skin damage and are a possible source of precursor cells for new receptor organs.

Animals↗
PubMed1992

An internal current source yields immunity of electrosensory information processing to unusually strong jamming in electric fish.

The electric organ of a fish represents an internal current source, and the largely isopotential nature of the body interior warrants that the current associated with the fish's electric organ discharges (EODs) recruits all electroreceptors on the fish's body surface evenly. Currents associated with the EODs of a neighbor, however, will not penetrate all portions of the fish's body surface equally and will barely affect regions where the neighbor's current flows tangentially to the skin surface. The computational mechanisms of the jamming avoidance response (JAR) in Eigenmannia exploit the uneven effects of a neighbor's EOD current to calculate the correct frequency difference between the two interfering EOD signals even if the amplitude of a neighbor's signal surpasses that of the fish's own signal by orders of magnitude. The particular geometry of the fish's own EOD current thus yields some immunity against the potentially confusing effects of unusually strong interfering EOD currents of neighbors.

Animals↗
PubMed1992

Development of the jamming avoidance response and its morphological correlates in the gymnotiform electric fish, Eigenmannia.

The electric fish, Eigenmannia, will smoothly shift the frequency of its electric organ discharge away from an interfering electric signal. This shift in frequency is called the jamming avoidance response (JAR). In this article, we analyze the behavioral development of the JAR and the anatomical development of structures critical for the performance of the JAR. The JAR first appears when juvenile Eigenmannia are approximately 1 month old, at a total length of 13-18 mm. We have found that the establishment of much of the sensory periphery and of central connections precedes the onset of the JAR. We describe three aspects of the behavioral development of the JAR: (a) the onset and development of the behavior is closely correlated with size, not age; (b) the magnitude (in Hz) of the JAR increases with size until the juveniles display values within the adult range (10-20 Hz) at a total length of 25-30 mm; and (3) the JAR does not require prior experience or exposure to electrical signals. Raised in total electrical isolation from the egg stage, animals tested at a total length of 25 mm performed a correct JAR when first exposed to the stimulus. We examine the development of anatomical areas important for the performance of the JAR: the peripheral electrosensory system (mechano- and electroreceptors and peripheral nerves); and central electrosensory pathways and nuclei [the electrosensory lateral line lobe (ELL), the lateral lemniscus, the torus semicircularis, and the pace-maker nucleus]. The first recognizable structures in the developing electrosensory system are the peripheral neurites of the anterior lateral line nerve. The afferent nerves are established by day 2, which is prior to the formation of receptors in the epidermis. Thus, the neurites wait for their targets. This sequence of events suggests that receptor formation may be induced by innervation of primordial cells within the epidermis. Mechanoreceptors are first formed between day 3 and 4, while electroreceptors are first formed on day 7. Electroreceptor multiplication is observed for the first time at an age of 25 days and correlates with the onset of the JAR. The somata of the anterior lateral line nerve ganglion project afferents out to peripheral electroreceptors and also send axons centrally into the ELL. The first electroreceptive axons invade the ELL by day 6, and presumably a rough somatotopic organization and segmentation within the ELL may arise as early as day 7. Axonal projections from the ELL to the torus develop after day 18.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Electric fish approach stationary signal sources by following electric current lines.

African electric fish of a pulse species, Brienomyrus brachyistius (Mormyridae), housed singly in a large, circular arena, were presented with electrical stimuli which mimicked a conspecific intruder. Stimuli were produced with either dipolar or bipolar electrodes in three different geometries. We tracked the unconditioned approach response paths taken by the fish and compared tracks for each of the geometries. The results suggest that B. brachyistius can determine neither the distance nor the direction of an electric dipole from afar, but that they do manage to find the source by maintaining a precise alignment of their body axis parallel to the direction of the local electric field vector (parallel to current lines) while swimming. This behaviour ultimately leads to the current source. We propose that this behaviour may be a simple mechanism mediating the approach response of one electric fish to another.

Animals↗
PubMed1991

The control of pacemaker modulations for social communication in the weakly electric fish Sternopygus.

Nearly sinusoidal electric organ discharges (EODs) of the weakly electric fish Sternopygus, occur at a regular rate within a range from 50 to 200 Hz and are commanded by a medullary pacemaker nucleus (Pn). During courtship and aggression, the rate of EODs is modulated as smooth EOD-frequency rises or brief EOD-interruptions (Hopkins 1974b). The present study examines the control of such modulations. Rises were elicited by L-glutamate stimulation of the diencephalic prepacemaker nucleus, the only previously known source of input to the Pn. We demonstrate an additional input to the Pn, the sublemniscal prepacemaker nucleus (SPPn). L-glutamate stimulation of this area caused EOD-interruptions. The Pn contains electrotonically coupled 'pacemaker cells' which generate the rhythm of the EODs, as well as 'relay cells' which transmit the command pulse to the spinal motor neurons that innervate the electric organ. Pacemaker cells recorded intracellularly during EOD-interruptions continued firing at their regular frequency but with slightly increased jitter. Relay cells, on the other hand, were strongly depolarized and fired spikelets at a greatly increased frequency during EOD-interruptions. Thus EOD-interruptions were caused by SPPn input to relay cells that caused their massive depolarization, blocking the normal input from pacemaker cells without greatly affecting pacemaker cell firing characteristics. Application to the Pn of an antagonist to NMDA-type glutamate receptors blocked EOD-frequency rises and EOD-interruptions. Antagonists to quisqualate/kainate receptor-types were ineffective.

Animal Communication↗
PubMed1987

Dynamics and stimulus-dependence of pacemaker control during behavioral modulations in the weakly electric fish, Apteronotus.

1. Weakly electric fish generate around their bodies low-amplitude, AC electric fields which are used both for the detection of objects and intraspecific communication. The types of modulation in this signal of which the high-frequency wave-type gymnotiform, Apteronotus, is capable are relatively few and stereotyped. Chief among these is the chirp, a signal used in courtship and agonistic displays. Chirps are brief and rapid accelerations in the normally highly regular electric organ discharge (EOD) frequency. 2. Chirping can be elicited artificially in these animals by the use of a stimulus regime identical to that typically used to elicit another behavior, the jamming avoidance response (JAR). The neuronal basis for the JAR, a much slower and lesser alteration in EOD frequency, is well understood. Examination of the stimulus features which induce chirping show that, like the JAR, there is a region of frequency differences between the fish's EOD and the interfering signal that maximally elicits the response. Moreover, the response is sex-specific with regard to the sign of the frequency difference, with females chirping preferentially on the positive and most males on the negative Df. These features imply that the sensory mechanisms involved in the triggering of these communicatory behaviors are fundamentally similar to those explicated for the JAR. 3. Additionally, two other modulatory behaviors of unknown significance are described. The first is a non-selective rise in EOD frequency associated with a JAR stimulus, occurring regardless of the sign of the Df. This modulation shares many characteristics with the JAR. The second behavior, which we have termed a 'yodel', is distinct from and kinetically intermediate to chirping and the JAR. Moreover, unlike the other studied electromotor behaviors it is generally produced only after the termination of the eliciting stimulus.

Animal Communication↗
PubMed1987

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fish Apteronotus.

Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genus Apteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined. Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17 beta, rather than androgens, induces discharge frequency decreases. This result can account for the 'reversed' discharge frequency dimorphism found in Apteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning in Apteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity in Apteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.

Androgens↗
PubMed1987

A microprocessor system for the digital synthesis of pulsed and continuous discharges of electric fish (or animal vocalizations).

Traditionally, the sensitivity of electric fish to electric stimuli has been investigated using a variety of electronically generated signal functions, for example sine, square, and sawtooth waves (both continuous and pulsed). None of these resemble the species- or sex-specific electric organ discharges (EODs) of fish. Therefore, EODs recorded on magnetic tape were used in open-loop playback experiments. However, as shown in the present paper, the waveforms of EODs reproduced from magnetic tape are distorted, and cannot be manipulated with speed and accuracy as to frequency or pulse repetition rate by feedback from the experiment. Good EOD imitations are generated by our microprocessor-based system for the digital synthesis of EODs which is controlled by a small 'personal computer'. Digitized EOD samples edited by computer are output by the fast digital-to-analogue converter of the microprocessor system. This system was devised for the synthesis of pulse-type (discontinuous) and wave type (continuous) EODs with continuous, on-line frequency and amplitude control. The rate of digital synthesis is 500 kHz in pulse-type EODs, and 250-500 kHz in wave type EODs at 0.06-0.24 Hz frequency resolution, depending on the repetition frequency of the synthesized wave EOD. The present paper describes the steps in EOD synthesis, and indicates some applications which benefit from the playback of high-fidelity imitations of natural EODs at amplitudes and frequencies selected by the experimenter, or automatically controlled by feedback from the experiment in closed-loop applications.

Analog-Digital Conversion↗
PubMed1986

Jamming avoidance in electric fish and frogs: strategies of signal oscillator timing.

Repetitive spontaneous calling in frogs and electric signalling in pulse-type electric fishes are driven by neural pacemakers. Minimization of signal overlap between neighbors is achieved in frogs and certain electric fishes by mutual entrainment, requiring cycle-by-cycle adjustment of pacemaker interval. Other fishes, which require a regular electric organ discharge, are more constrained in their ability to avoid jamming. Entrainment may also serve to reciprocally place signals in a temporal 'blindspot' of the receiver, the period of elevated sensory threshold following signal production, to reduce aggressive interactions.

Animal Communication↗
PubMed1976

Electric signals and schooling behavior in a weakly electric fish, Marcusenius cyprinoides L. (Mormyriformes).

Field recordings of electric organ discharges and catches of Marcusenius cyprinoides showed that these electric fish form groups and move about in schools. The role electric organ discharges in group cohesion was investigated by comparing interactions in groups of intact and operated, electrically silent fish. The absence of electric organ dischares reduced locomotor activity and resulted in the disappearance of two behaviors: parallel lineup and single file swimming. Electric signals are considered part of a schooling mechanism that aids the fish in maintaining group cohesion in their turbid enviornment and during migration at night.

Animals↗
PubMed1990

Functional analysis of sexual dimorphism in an electric fish, Hypopomus pinnicaudatus, order Gymnotiformes.

Hypopomus pinnicaudatus, an electric fish, has a marked sexual dimorphism in its tail filament. Sexually mature males have long, 'feathered' tails as compared with females. The sexual dimorphism emerges when a fish reaches about 110 mm total length. Mature males have larger electrocytes which are more widely spaced and more numerous than those in mature females. The biphasic electric organ discharge (EOD) is longer in males than in females. The peak-to-peak amplitude of the male's EOD is weaker than a female's of the same total length. The weaker discharge is unexpected given the increase in size and number of electrocytes. It is suggested that the reduction in EOD amplitude is a consequence of the increase in EOD duration among males. Further, female choice probably played a role in the evolution of long duration EODs among males, and males may have secondarily grown long tails to compensate for the loss in active space that would otherwise accompany a weaker EOD.

Agonistic Behavior↗
PubMed1988

Electrosensory maps form a substrate for the distributed and parallel control of behavioral responses in weakly electric fish.

Electroreceptors, distributed over the body surface of weakly electric fish, code the local amplitude and phase, or timing of zerocrossing, of the animal's electric signals. These signals are generated by rhythmic discharges of the electric organ and form a dipole-like field around the animal. This field is perturbed by interference with electric fields of other fish as well as by the appearance of objects electrically different from water. The spatial and temporal structure of such perturbations can be interpreted as the electric image of interfering fields and moving objects. This strategy of assessing the environment is called 'electrolocation', a form of 'seeing' with the body surface. Electric images are analyzed in somatotopically ordered strata of neurons within the central nervous system. Primary electrosensory afferents project to somatotopically ordered layers of higher-order neurons in the electrosensory lateral line lobe (ELL) of the hindbrain. Phase and amplitude information are processed in separate layers of the ELL. The phase of the signal in a given region of the body surface is coded by the timing of spikes of spherical cells marking the zerocrossings of the electric signal. This phase information is relayed to lamina 6 of the torus semicircularis of the midbrain. Rises and falls in local amplitude are coded by the activity of different pyramidal cell types, E- and I-units, which project to various laminae of the torus above and below lamina 6. The somatotopic organization of the torus allows for computations of spatial patterns in electrosensory information. Within lamina 6, differences in the phase of signals from different parts of the body surface are computed. Differential-phase information is then relayed to deeper laminae of the torus and remains in topographic register with amplitude information. This organization allows for joint evaluation of spatially related patterns of amplitude and phase modulations on the animal's body surface within local neuronal circuits of the torus. A topographic projection of the torus relays amplitude and differential-phase information to the optic tectum where a further joint evaluation of amplitude and phase serves to control behavioral responses. The control of a particular behavioral performance, the 'jamming avoidance response', is of a distributed nature in that the representations of individual sites on the body surface contribute cumulatively to shift the electric organ pacemaker frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1990

Time course of structural changes in regenerating electroreceptors of a weakly electric fish.

We examined the regenerating electroreceptors of the weakly electric fish Sternopygus by light and electron microscopy to search for possible structural correlates of known physiological changes that occur during regeneration (Zakon: J. Neurosci. 6(11):3297-3308, 1986) and to compare them with developing electroreceptors in larval fish (Vischer: Brain Behav. Evol. 33:223-236). Nine days after removal of a patch of cheek skin, new skin had filled the wound and undifferentiated precursor cell clusters were located in the epidermis just above the dermis. Nerve fibers were present near most, but not all, cell clusters. A few recognizable tuberous and ampullary precursor organs were seen at this time. Tuberous organs were composed of a group of large cells surrounded by smaller cells without a lumen and showed the beginning of a cellular plug. Ampullary organs appeared as a ball of cells with a small lumen opening into a nascent canal. Degenerating cells were found within organs, and sometimes entire organs degenerated. These were not innervated. By 2 weeks the large cells of the tuberous organ were developing into sensory cells, while the smaller cells were forming the capsule wall and the underlying basal cells. The characteristic tuberous organ canal filled with loosely packed epidermal cells was evident. The sensory cells of the ampullary organs were visible within the epithelial layer at the base of the lumen, and the large synaptic discs were beginning to form. The sensory cells and postsynaptic terminals contained numerous vesicles. The presynaptic vesicles, which appear in normal receptor cells, remained throughout regeneration and presumably underlie transmitter release. The postsynaptic vesicles appeared transiently in large numbers but declined to adult values by 4 weeks. We presume that these may serve a trophic role. By 3 weeks, organs generally appeared mature and began dividing into daughter organs. The formation of individual receptor organs during regeneration is similar to that observed in development. Receptor organs continued dividing until the appropriate number of organs per afferent was reached for the size of the fish. Although the organization of the receptors appeared generally normal, there were a few anomalies. Some afferents sent sprouts into the epidermis, and, as a result of such sprouting, some of these afferents innervated multiple organs over a greater distance than normal. This was first seen early in regeneration and persisted for as long as 5 months.(ABSTRACT TRUNCATED AT 400 WORDS)

Age Factors↗
PubMed1989

From distributed sensory processing to discrete motor representations in the diencephalon of the electric fish, Eigenmannia.

During their jamming avoidance response (JAR), weakly electric fish of the genus Eigenmannia shift their electric organ discharge (EOD) frequency away from a similar EOD frequency of a neighboring fish. The behavioral rules and neural substrates for stimulus recognition and motor control of the JAR have been extensively studied (see review by Heiligenberg 1986). The diencephalic nucleus electrosensorius (nE) links sensory processing within the torus semicircularis and optic tectum with the mesencephalic prepacemaker nucleus which, in turn, modulates the medullary pacemaker nucleus and hence the EOD frequency. Two separate areas within the nE responsible for JAR-related EOD frequency rises and frequency falls, respectively, were identified by iontophoresis of the excitatory amino acid L-glutamate. Bilateral lesion of the areas causing EOD frequency rises resulted in elimination of JAR-related frequency rises above a baseline frequency obtained in the absence of a jamming stimulus. Similarly, bilateral lesion of the areas causing frequency falls resulted in a loss of JAR-related frequency falls below the baseline frequency. Whether these areas are also responsible for non-JAR-related frequency shifts is not known. The strength of response and spatial extent of the areas causing frequency shifts varied among fish and also varied in individual fish, reflecting the strength of JAR-related frequency shifts and the balance of activities in frequency-rise and frequency-fall areas. Local application of bicuculline-methiodide or GABA demonstrated a tonic inhibitory input to each area and suggests a reciprocal inhibitory interaction between the two ipsilateral areas, possibly accounting for much of the individual plasticity. The nE thus is a site for neuronal transformation from distributed, topographically organized processing within the laminated structures of the torus and tectum to discrete cell clusters which control antagonistic motor responses.

Action Potentials↗
PubMed1992

Electric organ morphology of Sternopygus macrurus, a wave-type, weakly electric fish with a sexually dimorphic EOD.

In several species of electric fish with a sex difference in their pulse-type electric organ discharge (EOD), the action potential-generating cells of the electric organ (electrocytes) of males are larger and more invaginated compared to females. Androgen treatment of females and juveniles produces a longer-duration EOD pulse that mimics the mature male EOD, with a concurrent increase in electrocyte size and/or membrane infolding. In Sternopygus macrurus, which generates a wave-type EOD, androgen also increases EOD pulse duration. To investigate possible morphological correlates of hormone-dependent changes in EOD in Sternopygus, we examined electric organs from both fish collected in the field, and untreated and androgen-treated specimens in the laboratory. The electrocytes are cigar shaped, with prominent papillae on the posterior, innervated end. Electrocytes of field-caught specimens were significantly larger in all parameters than were electrocytes of specimens maintained in the laboratory. EOD pulse duration and frequency were highly correlated, and were significantly different between the sexes in sexually mature fish. Nevertheless, no sex difference in electrocyte morphology was observed, nor did any parameters of electrocyte morphology correlate with EOD pulse duration or frequency. Further, whereas androgen treatment significantly lowered EOD frequency and broadened EOD pulse duration, there was no difference in electrocyte morphology between hormone-treated and control groups. Thus, in contrast to results from studies on both mormyrid and gymnotiform pulse fish, electrocyte morphology is not correlated with EOD waveform characteristics in the gymnotiform wave-type fish Sternopygus. The data, therefore, suggest that sex differences in EOD are dependent on changes in active electrical properties of electrocyte membranes.

Analysis of Variance↗