Receptive field organization of electrosensory neurons in the paddlefish (Polyodon spathula)

Paddlefish use their electrosense to locate small water fleas (daphnia), their primary prey, in three-dimensional space. High sensitivity and a representation of object location are essential for this task. High sensitivity can be achieved by convergence of information from a large number of receptors and object location is usually represented in the nervous system by topographic maps. However the first electrosensory center in the brain, the dorsal octavolateral nucleus in the hindbrain, is neither topographically organized nor does it show a higher sensitivity than primary afferent fibers. Here, we investigated the response properties of electrosensory neurons in the dorsal octavolateral nucleus (DON), the lateral mesencephalic nucleus (LMN) and the tectum mesencephali (TM). LMN units are characterized by large receptive fields, which suggest a high degree of convergence. TM units have small receptive fields and are topographically arranged, at least in the rostro-caudal axis, the only dimension we could test. Well-defined receptive fields, however, could only be detected in the TM with a moving DC stimulus. The receptive fields of TM units, as determined by slowly scanning the rostrum and head with a 5 Hz stimulus, were very large and frequently two or more receptive fields were present. The receptive fields for LMN units were located in the anterior half of the rostrum whereas TM units had receptive fields predominantly on the head and at the base of the rostrum. A detailed analysis of the prey catching behavior revealed that it consists of two phases that coincide with the location of the receptive fields in LMN and TM, respectively. This suggests that LMN units are responsible for the initial orienting response that occurs when the prey is alongside the anterior first half of the rostrum. TM units, in contrast, had receptive fields at locations where the prey is located when the fish opens its mouth and attempts the final strike.

Response properties of electrosensory units in the midbrain tectum of the paddlefish (Polyodon spathula Walbaum)

Paddlefish use their peculiar rostrum to detect minute electric fields from their main prey, small water fleas. Electroreceptors over the rostrum and head sense these fields and send the information into a single hindbrain area, the dorsal octavolateral nucleus (DON). From there, information is sent to various midbrain structures, including the tectum. The response properties of primary afferent fibers and DON units has been well investigated, but nothing is known about electrosensory units in the midbrain. Here we recorded the responses of single units in the midbrain tectum and DON to uniform electric fields. Tectal units exhibited little spontaneous activity and responded to sine waves with a few, well phase-locked spikes. Phase locking was still significant at amplitudes one order of magnitude lower than in the DON. If stimulated with sinusoidal electric fields of different frequencies, phase locking in DON units decreased proportionally with frequency whereas the response of tectal units depended little on frequency. This is in agreement with behavioral studies showing that relevant frequencies range from DC to ca 20 Hz.

Prey detection in selective plankton feeding by the paddlefish: is the electric sense sufficient?

The long rostrum of the paddlefish Polyodon spathula supports an extensive array of ampullary electroreceptors and has been proposed to function as an antenna for detecting planktonic prey. Evidence in support of this hypothesis is presented in experiments that preclude the use of other sensory mechanisms for plankton detection. Paddlefish swimming in a recirculating observation chamber are shown to feed normally in the dark when prey-related chemical and hydrodynamic sensory cues are masked or attenuated. Specifically, we demonstrate that the spatial distribution of plankton captured by paddlefish is little changed when the plankton are individually encapsulated in agarose, when a high background concentration of plankton extract is added to the chamber, when the nares are plugged and under turbulent water flow conditions. Paddlefish also discriminate between encapsulated plankton and ‘empty’ agarose particles of the same size. Although capture distributions differed somewhat under certain conditions, the general pattern and effectiveness of prey capture were not disrupted by these procedures. These results support the conclusion that paddlefish, as zooplanktivores, rely on their passive electric sense for prey detection.

Paddlefish strike at artificial dipoles simulating the weak electric fields of planktonic prey

The freshwater paddlefish Polyodon spathula (Polyodontidae) feeds primarily on the water flea (Daphnia sp.), and previous studies suggest that these fish detect their planktonic prey using their rostral electrosensory system. Zooplankton produce direct-current and oscillating alternating-current electric fields containing multiple frequencies and amplitudes. We asked whether an inanimate electric field is sufficient to elicit paddlefish strikes equivalent to their feeding behavior. Juvenile paddlefish respond to artificial dipole stimuli by investigating the electric field and striking at the dipole electrode tips. These behavioral responses, scored as strikes, exhibit a bandpass characteristic with a maximum response between 5 and 15 Hz. Responses were less frequent at higher (20, 30, 40, 50 Hz) and lower (0.1, 0.5, 1 Hz) test frequencies, with a steep drop-off below 5 Hz. Strike rates also varied with stimulus intensity. Response frequency was greatest at 0.25 microA peak-to-peak amplitude, with reduced responses at lower and higher amplitudes (0.125 and 1.25 microA). Striking behavior was also influenced by water conductivity: strike rate was reduced at higher water conductivity. Dipole-elicited strikes exhibit behavioral plasticity. Fish habituate to repetitive dipole stimuli that are not reinforced by prey capture, and they dishabituate after food reinforcement. These experiments characterize paddlefish feeding strikes towards dipole electrodes at signal frequencies and intensities simulating the electric fields of zooplankton, their natural prey, and demonstrate that electric fields are sufficient to elicit feeding behavior. The results support the conclusion that paddlefish use their passive electrosensory system for planktivorous feeding.

Use of behavioural stochastic resonance by paddle fish for feeding

Stochastic resonance is the phenomenon whereby the addition of an optimal level of noise to a weak information-carrying input to certain nonlinear systems can enhance the information content at their outputs. Computer analysis of spike trains has been needed to reveal stochastic resonance in the responses of sensory receptors except for one study on human psychophysics. But is an animal aware of, and can it make use of, the enhanced sensory information from stochastic resonance? Here, we show that stochastic resonance enhances the normal feeding behaviour of paddlefish (Polyodon spathula), which use passive electroreceptors to detect electrical signals from planktonic prey. We demonstrate significant broadening of the spatial range for the detection of plankton when a noisy electric field of optimal amplitude is applied in the water. We also show that swarms of Daphnia plankton are a natural source of electrical noise. Our demonstration of stochastic resonance at the level of a vital animal behaviour, feeding, which has probably evolved for functional success, provides evidence that stochastic resonance in sensory nervous systems is an evolutionary adaptation.

Light enhances hydrodynamic signaling in the multimodal caudal photoreceptor interneurons of the crayfish

1. The caudal photoreceptor (CPR) interneurons in the sixth abdominal ganglion of the crayfish are complex, multi-modal interneurons. These cells respond directly to light with tonic spike discharges, and they integrate synaptic input from an array of fili-form mechanoreceptors on the tailfan. They also provide input to rostral command centers, inducing backward walking at high firing frequencies, and thus directly influence behavior. 2. We recorded CPR activity in response to weak hydrodynamic stimulation of the tailfan mechanoreceptors while under varying intensities of light shined on the sixth ganglion. Spike trains were characterized according to the mean discharge rate (MDR) and the power spectrum from which the signal-to-noise ratio (SNR) was calculated. 3. Illumination of the CPR enhances the efficiency of transmitting mechanosensory signals. It does so by increasing the SNR of mechanosensory input received from tailfan receptors. A sevenfold, nonlinear increase in the SNRs was observed with increasing light intensity, an effect especially pronounced for weak hydrodynamic stimuli. In comparison with the dark, illumination of the ganglion lowered the hydrodynamic threshold and heightened the response to suprathreshold stimulation. Unlike the SNR, the MDR is little affected by mechanosensory input. 4. These results are compared with simulated electronic activity from an analogue threshold model and are discussed with respect to the mechanism of stochastic resonance.

Antennal Responses to Hydrodynamic and Tactile Stimuli in the Spiny Lobster Panulirus argus

The responses of the long, spiny, antennal flagella of the spiny lobster Panulirus argus to hydrodynamic and tactile stimuli were investigated. Experiments were performed in the dark and included videographic laboratory studies of small tethered lobsters (<20 mm carapace length) and nighttime field observations of larger, subadult, foraging animals. The antennae are held laterally in both tethered and free-ranging animals. Water jets trigger bilateral antennal responses in which both flagella are swept forward for rostrally directed stimuli, backward for caudal stimuli, and in an intermediate backward direction when stimulated laterally. Mean response angles are greater for caudal stimuli (17°-48°) than for rostral stimuli (10°-16°), and lobsters exhibit lateralized sensitivity when jets are directed from the caudal sector, as indicated by larger ipsilateral responses--up to twice the amplitude of contralateral responses in field experiments. Untethered lobsters frequently turn the body in the direction of the water jet and tailflip away or tailflip without first turning. Tactile stimuli to the lateral edges of the antenna, carapace, walking leg, abdomen, and tailfan also trigger primarily backward sweeps of the antennae. Only the antennule and medial antennal receptive fields yield forward movements, and these elicit smaller responses (mean response ≤ 5°) than in the backward direction (mean responses up to 15°). Threshold tactile stimuli trigger exclusively ipsilateral responses; thus, lateralization is absolute. These results demonstrate that spiny lobsters accurately localize mechanosensory stimuli and direct their antennal flagella in the perceived direction, a response consistent with a defensive function of the antennae in these nonchelate decapods. Overall sensitivity is greatest for hydrodynamic stimuli, a result interpreted as being important for the detection of and defense against large predatory fish whose nearby movements would generate broad, directional, water-current pulses.

Hyperpolarizing photoreceptors in the eyes of the giant clam Tridacna: physiological evidence for both spiking and nonspiking cell types

Intracellular studies on photoreceptors in the eyes of the giant clam Tridacna give evidence for two types of light-sensitive cells, both of which are hyperpolarized by light. These cells are distinguished by the presence or absence of spikes and corresponding characteristics of the receptor potential. In non-spiking (NS) receptors, the average resting potential in the dark is low (-15 mV) and peak receptor potentials are large (to 100 mV) and adapt rapidly to light. Spiking (S) receptors have higher average resting potentials (-45 mV), but receptor potentials do not exceed 20 mV and also do not adapt to light. The spikes in S-receptors are small (3-8 mV), occur spontaneously at low levels of illumination and are inhibited by light. Bursts of spikes arise on the repolarizing off-component of the receptor potential. Light adaptation increases the excitability of S-receptors in terms of a higher frequency and shorter latency of the off response burst. The receptor potential in both cells is due to a light-activated increase in membrane conductance to potassium ions. Membrane conductance decreases in NS-receptors in relation to light adaptation. Unlike the scallop eye, no depolarizing photoreceptors are present.

Mechanosensory interneurons (MSIs) in the crayfish 6th abdominal ganglion are inhibited by activation of other MSIs

1. Identified mechanosensory interneurons (MSIs) in the 6th abdominal ganglion of the crayfish Procambarus clarkii have been shown to inhibit other projecting MSIs. 2. Interneurons sensitive to water-current stimulation of the tailfan, and which inhibited the tactile response of other MSIs when activated by depolarizing currents, were identified by iontophoresis of fluorescent dye. 3. Ten inhibitory interneurons have been identified, including both non-adapting, directional cells and phasic "touch" cells. 4. Inhibition triggered by activation of the identified cells was not widespread among fibers in the connectives. 5. Inhibition recorded intracellularly was mediated by compound inhibitory postsynaptic potentials of long duration (300-400 msec) and latencies of 13-15 msec, and therefore was apparently polysynaptic. 6. Depolarization and/or activity in MSIs, which modulates the stimulus response characteristics of related cells is a possible mechanism for contrast enhancement among directional or frequency-selective interneurons.

A mathematical model for the motion of mechanoreceptor hairs in fluid environments

A differential equation has been derived for the motion of the mechanosensory hairs of animals when they are stimulated by the motion of their fluid environment. Specific solutions of the equation are obtained for three states of fluid flow including steady-state sinusoidal oscillations. The model is specifically applied to crayfish sensilla in an aqueous medium, but the assumptions of the model are also shown to be valid in air for the sensory hairs of insects. The calculations are consistent with available experimental data.

Central inhibition of an identified mechanosensory interneuron in the crayfish

Inhibitory input from crayfish mechanoreceptors is mediated polysynaptically to sensory interneurons. An identifiable sensory interneuron, the caudal photoreceptor (CPR), has been used as a model system to characterize inhibitory intermediate cells. A survey of the abdominal connectives, by antidromic stimulation, has identified eleven inhibitory cells, some of which also function as ascending sensory interneurons. These results indicate that lateral interactions within networks of mechanosensory interneurons form an integral part of the information processing mechanisms.

Electrophysiological Studies on the Heart of the Bivalve Mollusc, Modiolus Demissus

1. The membrane potential in the heart of Modiolus demissus is dependent primarily on the unequal distribution of potassium ions across the cell.

2. The membrane potential is low, however, and the reduced slope of the potassium equilibrium potential is not predictable by the familiar Nernst equation.

3. The membrane undergoes a sustained depolarization in Cl-free SW which reduces the magnitude of subsequent potassium depolarization. Chloride ions contribute, perhaps independently, up to 15-30% of the total potential.

4. The high degree of variability in membrane potential is due, in part, to a sodium leakage current. Membrane hyperpolarization in Na-free SW is greatest in hearts with a low initial potential.

5. An electrogenic sodium pump is described which contributes to the membrane potential in the form of a rhythmical after-hyperpolarization (positive after-potential) following each spike.

6. Inhibition of the pump (reduction of the after-potential) can be achieved using ouabain and cyanide, by substituting lithium for sodium, by removing extracellular potassium and by cooling the heart.

7. The ionic basis of the membrane potential is discussed in relation to the diffuse pacemaker properties of the heart.

Electrophysiological Studies on the Heart of the Bivalve Mollusc, Modiolus Demissus

1. Electrical activity from the heart of Modiolus demissus was recorded simultaneously by sucrose-gap and microelectrode techniques, confirming the validity of the extracellular method of assessing the shape of the action potential.

2. Hearts beat in Na-free SW (Tris) but with loss of the plateau and a reduced rise time. Mechanical contractions did not persist, however, for more than 2 h in the absence of sodium, and the viability of the preparation declined rapidly after 3 h.

3. Lithium substitution (for sodium) also failed to support the plateau and tetrodotoxin had little effect on the spike.

4. Hearts also beat in Ca-free SW. Here, the spike component and contractile activity were lost at nearly the same time although slowly rising plateau-like potentials persisted for over 6 h.

5. EGTA potentiated the effects of Ca-free SW, and manganese had calcium-antagonizing effects producing longer plateaus.

6. Spikes and plateau phases could be selectively initiated from hearts made quiescent in Na-free and Ca-free SW by readmitting either ion individually.

7. In Na-free SW spike amplitude and rate of rise were increased by raising calcium concentration. This effect was self-limiting at high levels of calcium, however, and the spike was reduced.

8. In divalent-free solutions (Ca- and Mg-free SW) the membrane was depolarized to near zero potential. In this situation the membrane permeability to sodium exceeded that to potassium, and the membrane behaved as a sodium electrode.

9. It is concluded that the action potential in Modiolus heart is dependent primarily on calcium for the spike component and on sodium for the amplitude of the plateau.

10. Calcium is also important in the control of plateau duration and membrane permeability by reason of its stabilizing properties.

11. A possible correlation exists between the occurrence of the plateau, calcium sensitivity and habitat of three bivalve species. The hearts of Modiolus and of the oyster have plateaus and will beat in Ca-free SW, and these bivalves live in estuarine habitats, whereas Mytilus lives in a subtidal environment and its heart is sensitive to low calcium and has no plateau component.