Dynamics of Excitation and Inhibition in the Light-Adapted Limulus Eye in situ

The dynamics of spike discharge in eccentric cell axons from the in situ lateral eye of Limulus, under small sinusoidal modulation of light to which the eye is adapted, are described over two decades of light intensity and nearly three decades of frequency. Steady-state lateral inhibition coefficients, derived from the very low-frequency response, average 0.04 at three interommatidial spacings. The gain vs. frequency of a singly illuminated ommatidium is described closely from 0.004 to 0.4 cps by the linear transfer function s^0.25; this function also accounts approximately for the measured phase leads, the small signal adaptation following small step inputs, and for Pinter's (1966) earlier low-frequency generator potential data. We suggest that such dynamics could arise from a summation in the generator potential of distributed intensity-dependent relaxation processes along the dendrite and rhabdome. Analysis of the dynamic responses of an eccentric cell with and without simultaneously modulated illumination of particular neighbors indicates an effect equivalent to self-inhibition acting via a first-order low-pass filter with time constant 0.42 sec, and steady-state gain near 4.0. The corresponding filters for lateral inhibition required time constants from 0.35 to 1 sec and effective finite delay of 50–90 msec.     

Fast negative feedback enables mammalian auditory nerve fibers to encode a wide dynamic range of sound intensities

Mammalian auditory nerve fibers (ANF) are remarkable for being able to encode a 40 dB, or hundred fold, range of sound pressure levels into their firing rate. Most of the fibers are very sensitive and raise their quiescent spike rate by a small amount for a faint sound at auditory threshold. Then as the sound intensity is increased, they slowly increase their spike rate, with some fibers going up as high as ～300 Hz. In this way mammals are able to combine sensitivity and wide dynamic range. They are also able to discern sounds embedded within background noise. ANF receive efferent feedback, which suggests that the fibers are readjusted according to the background noise in order to maximize the information content of their auditory spike trains. Inner hair cells activate currents in the unmyelinated distal dendrites of ANF where sound intensity is rate-coded into action potentials. We model this spike generator compartment as an attenuator that employs fast negative feedback. Input current induces rapid and proportional leak currents. This way ANF are able to have a linear frequency to input current (f-I) curve that has a wide dynamic range. The ANF spike generator remains very sensitive to threshold currents, but efferent feedback is able to lower its gain in response to noise.     

Consequences of Converting Graded to Action Potentials upon Neural Information Coding and Energy Efficiency

Information is encoded in neural circuits using both graded and action potentials, converting between them within single neurons and successive processing layers. This conversion is accompanied by information loss and a drop in energy efficiency. We investigate the biophysical causes of this loss of information and efficiency by comparing spiking neuron models, containing stochastic voltage-gated Na⁺ and K⁺ channels, with generator potential and graded potential models lacking voltage-gated Na⁺ channels. We identify three causes of information loss in the generator potential that are the by-product of action potential generation: (1) the voltage-gated Na⁺ channels necessary for action potential generation increase intrinsic noise and (2) introduce non-linearities, and (3) the finite duration of the action potential creates a ‘footprint’ in the generator potential that obscures incoming signals. These three processes reduce information rates by ∼50% in generator potentials, to ∼3 times that of spike trains. Both generator potentials and graded potentials consume almost an order of magnitude less energy per second than spike trains. Because of the lower information rates of generator potentials they are substantially less energy efficient than graded potentials. However, both are an order of magnitude more efficient than spike trains due to the higher energy costs and low information content of spikes, emphasizing that there is a two-fold cost of converting analogue to digital; information loss and cost inflation.     

Repetitive electrical stimulation of X-area and parabrachial lateral nucleus: effects upon ponto-geniculo-occipital activity in the reserpinized cat

The ponto-geniculo-occipital (PGO) activity is a characteristic field potential of paradoxical sleep, that can be continually induced by reserpine administration. It has been postulated that the X area (XA) and parabrachial lateral nucleus (Pbl) contain the generator cells for the PGO activity. In this study, repetitive electrical stimulation in the XA and Pbl was applied, with the aim of inducing progressive plastic changes in PGO activity, which was recorded from the lateral geniculate nucleus (LGN). Reserpinized cats were used; they were curarized and maintained with artificial respiration. We analyzed the PGO spike frequency at one, five and sixty minutes after stimulation, which was given every 30 minutes for at least 8 consecutive hours. Stimulation of the XA did not produce changes, while that of the Pbl induced a relatively poor progressive increment in the PGO spike frequency. The findings obtained with XA stimulation discard the possibility of inducing functional plastic changes in this region. On the other hand, the response to Pbl stimulation indicates an activation of the PGO spike generator system. These differences suggest that these nuclei have different influence on PGO activity, although it is possible that the responses found in the Pbl were indirect effects, given its anatomical relationships.     

The responses of central octavolateralis cells to moving sources

Mechanosensory lateral line units recorded from the medulla (medial octavolateralis nucleus) and midbrain (torus semicircularis) of the bottom dwelling catfish Ancistrus sp. responded to water movements caused by an object that passed the fish laterally. In terms of peak spike rate or total number of spikes elicited responses increased with object speed and sometimes showed saturation (Figs. 7, 14). At sequentially greater distances the responses of most medullary lateral line units decayed with object distance (Fig. 11). Units tuned to a certain object speed or distance were not found. The signed directionality index of most lateral line units was between –50 and +50, i.e. these units were not or only slightly sensitive to the direction of object motion (Figs. 10, 17). However, some units were highly directionally sensitive in that the main features of the response histograms and/or peak spike rates clearly depended on the direction of object movement (e.g. Fig. 9C, D and Fig. 16). Midbrain lateral line units of Ancistrus may receive input from more than one sensory modality. All bimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. Units which receive bimodal input may show an extended speed range (e.g. Fig. 18).Abbreviations MON medial octavolateralis nucleus - MSR mean spike rate - PSR peak spike rate - p-p peak-to-peak - SDI signed directionality index     

Hair Cell Interactions in the Statocyst of Hermissenda

Hair cells in the statocyst of Hermissenda crassicornis respond to mechanical stimulation with a short latency (<2 ms) depolarizing generator potential that is followed by hyperpolarization and inhibition of spike activity. Mechanically evoked hyperpolarization and spike inhibition were abolished by cutting the static nerve, repetitive mechanical stimulation, tetrodotoxin (TTX), and Co⁺⁺. Since none of these procedures markedly altered the generator potential it was concluded that the hyperpolarization is an inhibitory synaptic potential and not a component of the mechanotransduction process. Intracellular recordings from pairs of hair cells in the same statocyst and in statocysts on opposite sides of the brain revealed that hair cells are connected by chemical and/or electrical synapses. All chemical interactions were inhibitory. Hyperpolarization and spike inhibition result from inhibitory interactions between hair cells in the same and in opposite statocysts.     

Auditory Nerve Spike Generator Modeled as a Variable Attenuator Based on a Saddle Node on Invariant Circle Bifurcation

Mammalian inner hair cells transduce the sound waves amplified by the cochlear amplifier (CA) into a graded neurotransmitter release that activates channels on auditory nerve fibers (ANF). These synaptic channels then charge its dendritic spike generator. While the outer hair cells of the CA employ positive feedback, poising on Andronov-Hopf type instabilities which make them extremely sensitive to faint sounds and make CA output strongly nonlinear, the ANF appears to be based on different principles and a different type of dynamical instability. Its spike generator “digitizes” CA output into trains of action potentials and behaves as a linear filter, rate-coding sound intensity across a wide dynamic range. Here we model the spike generator as a 3 dimensional version of a saddle node on invariant circle (SNIC) bifurcation. The generic 2d SNIC increases its spike rate as the square root of the input current above its spiking threshold. We add negative feedback in the form of a low voltage-threshold potassium conductance that slows down the generator’s rate of increase of its spike rate. A Poisson random source simulates an inner hair cell, outputting a series of noisy periodic current pulses to the model ANF whose spikes phase lock to these pulses and have a linear frequency to current relation with a wide dynamic range. Also, the spike generator compartment has a cholinergic feedback connection from the olive and experiments show that such feedback is able to alter the amount of H conductance inside the generator compartment. We show that an olive able to decrease H would be able to shift the spike generator’s dynamic range to higher sound intensities. In a quiet environment by increasing H the olive would be able to make spike trains similar to those caused by synaptic input.     

Putative synaptic mechanisms of inhibition in Limulus lateral eye

Serotonin (5-HT) perfusion of a thin section of Limulus lateral eye hyperpolarizes retinular and eccentric cell membrane potential, and blocks spike action potentials fired by the eccenteric cell. The indoleamine does not directly affect retinular cell receptor potential or eccenteric cell generator potential in response to light stimuli. LSD perfusion blocks both this inhibitory action of 5-HT and light-evoked, synaptically mediated, lateral inhibition. Iontophoretic application of 5-HT to the synaptic neuropil produces shorter latency and duration and larger amplitude of inhibition than does the perfusion technique. This inhibition is dose dependent; the accompanying inhibitory postsynaptic potential (IPSP) appears to have an equilibrium potential more hyperpolarized than normal resting potential levels of ca. -50 mV. IPSP amplitude is sensitive to extracellular potassium ion concentration: it increases with decreased [K+]0 and decreases with increased [K+]0. LSD blocks the inhibition produced by iontophoretic application of 5-HT. Interaction between light-evoked, natural synaptic transmitter-mediated IPSP's and 5-HT IPSP's suggests a common postsynaptic receptor or transmitter-receptor-permeability change mechanism.     

Morphological and electrophysiological properties of a novel in vitro preparation: the electrosensory lateral line lobe brain slice

Summary An in vitro brain slice preparation of the electrosensory lateral line lobe (ELL) of weakly electric fish was developed. The morphology of this slice was studied and revealed that most ELL neurons and synapses retained their normal appearance for at least 10 h in vitro. The electrophysiological characteristics of the main ELL output neurons, the pyramidal cells, were measured. Extracellular electrode recordings demonstrated that pyramidal cells are capable of spontaneous, rhythmic spike activity. Intracellular recordings showed that intrinsic oscillations in membrane potential underlie the bursting behavior. The majority of pyramidal cells respond to depolarizing current pulses with an initial lag in spike firing followed by a non-accommodating, higher frequency spike train.Time and voltage-dependent properties of pyramidal cell responsiveness, as well as the effects of pharmacological blocking agents indicated that rhythmic activity and repetitive firing are dominated by a persistent, subthreshold sodium conductance (gNa) which activates at depolarized levels and is the driving force behind the membrane potential oscillations and the sustained (non-accommodating) spike firing. In addition, a transient, outward potassium conductance (gA) is responsible for the lag in spike firing by acting as a brake during the initial 50–200 ms of a depolarizing stimulus.Calcium currents and calcium-dependent potassium conductance add to the interval between spontaneous bursts but appear insufficient for spike frequency accommodation.The in vitro behaviour of pyramidal cells differs substantially from the behaviour of the same cell type in vivo. These observations raise possibilities that intrinsic membrane properties together with local synaptic interactions may regulate pyramidal cell responsiveness.Abbreviations ACSF artificial cerebrospinal fluid - 4-AP 4-amino-pyridine - BP basilar pyramidal cell - DML dorsal molecular layer - EGTA ethyleneglycol-bis(B-aminoethylether)-N,Ntetraacetic acid - ELL electrosensory lateral line lobe - EOD electric organ discharge - EPSP excitatory postsynaptic potential - FPP fast prepotential - IPSP inhibitory postsynaptic potential - LF Lucifer Yellow - NBP non-basilar pyramidal cell - Rin input resistance - SP slow potential - TEA tetraethyl ammonium - tsf tractus stratum fibrosum - TTX tetrodotoxin - Vm membrane potential - VML ventral molecular layer     

Continuous detection of weak sensory signals in afferent spike trains: the role of anti-correlated interspike intervals in detection performance

An important problem in sensory processing is deciding whether fluctuating neural activity encodes a stimulus or is due to variability in baseline activity. Neurons that subserve detection must examine incoming spike trains continuously, and quickly and reliably differentiate signals from baseline activity. Here we demonstrate that a neural integrator can perform continuous signal detection, with performance exceeding that of trial-based procedures, where spike counts in signal- and baseline windows are compared. The procedure was applied to data from electrosensory afferents of weakly electric fish (Apteronotus leptorhynchus), where weak perturbations generated by small prey add ~1 spike to a baseline of ~300 spikes s^–1. The hypothetical postsynaptic neuron, modeling an electrosensory lateral line lobe cell, could detect an added spike within 10–15 ms, achieving near ideal detection performance (80–95%) at false alarm rates of 1–2 Hz, while trial-based testing resulted in only 30–35% correct detections at that false alarm rate. The performance improvement was due to anti-correlations in the afferent spike train, which reduced both the amplitude and duration of fluctuations in postsynaptic membrane activity, and so decreased the number of false alarms. Anti-correlations can be exploited to improve detection performance only if there is memory of prior decisions.Abbreviations B binomial - CV coefficient of variation - EOD electric organ discharge - ELL electrosensory lateral line lobe - EPSP excitatory postsynaptic potential - ISI interspike interval - M₀ Markov order zero - M₁ Markov order one - N noise - OC operating characteristic - PDF probability density function - ROC receiver operating characteristic - S signal - SNR signal-to-noise ratio - S+N signal in noise     

Patterns of synaptic inhibition in motoneurons and interneurons during fictive swimming in the lamprey,as revealed by Cl− injections

Summary During fictive swimming in the isolated spinal cord of the lamprey (Ichthyomyzon unicuspis andPetromyzon marinus) the membrane potentials of motoneurons (MNs), lateral interneurons (L INs), and CC interneurons (CC INs) oscillate between a depolarised and a relatively hyperpolarised phase. After intracellular Cl^– injections (usually combined with a DC hyperpolarisation) IPSP's became depolarising, and in cells which were phasically inhibited, phases of relative hyperpolarisation became phases of relative depolarisation. The peak depolarisation and/or spike burst mid point in MNs after Cl^– injection occurred at a phase of 0.65 ± 0.12 (mean ±S.D.) in the cycle, with zero being the start of the ipsilateral ventral root burst. In CC INs the peak depolarisation and/or spike burst mid point after Cl^– occurred significantly earlier, at a phase of 0.40 ± 0.18. L INs were also phasically inhibited with peak depolarisation and/or spike burst mid point after Cl^– injection at an intermediate phase of 0.52 ± 0.21. It is concluded that the central pattern generator for fictive swimming has at least three synaptic outputs: an early excitation, and inhibition at a range of phases, which could be combinations of an early and a late inhibition.Abbreviations CC IN interneuron with contralateral caudal axon - MN motoneuron - L IN lateral interneuron - VR ventral root     

Balanced synaptic input shapes the correlation between neural spike trains

Stimulus properties, attention, and behavioral context influence correlations between the spike times produced by a pair of neurons. However, the biophysical mechanisms that modulate these correlations are poorly understood. With a combined theoretical and experimental approach, we show that the rate of balanced excitatory and inhibitory synaptic input modulates the magnitude and timescale of pairwise spike train correlation. High rate synaptic inputs promote spike time synchrony rather than long timescale spike rate correlations, while low rate synaptic inputs produce opposite results. This correlation shaping is due to a combination of enhanced high frequency input transfer and reduced firing rate gain in the high input rate state compared to the low state. Our study extends neural modulation from single neuron responses to population activity, a necessary step in understanding how the dynamics and processing of neural activity change across distinct brain states.     

Structure of the flower and inflorescence ofHouttuynia cordata thunb

In the spike ofHouttuynia cordata with many large bracts all flowers in a spike are regularly arranged in a secondary pseudo-opposite order. Arrangment of flowers in the normal spike also follows the same order. A pistillate organ is usually borne at the terminal of the spike. Vascular anatomy of the flower, inflorescence and the arrangement of flowers suggest that this terminal organ should be interpreted as a compound structure composed of four units, each of which represents one flower with a single carpel and a single dorsal stamen. The terminal pistillate organ of a lateral spike is considered to have a structure intermediate between that of the normal hermaphoriditic flower and that of the terminal pistillate organ of the main spike.     

The responses of peripheral and central mechanosensory lateral line units of weakly electric fish to moving objects

Mechanosensory lateral line afferents of weakly electric fish (Eigenmannia) responded to an object which moved parallel to the long axis of the fish with phases of increased spike activity separated by phases of below spontaneous activity. Responses increased with object speed but finally may show saturation. At increasingly greater distances the responses decayed as a power function of distance. For different object velocities the exponents (mean±SD) describing this response falloff were -0.71±0.4 (20 cm/s object velocity) and-1.9±1.25 (10 cm/s). Opposite directions of object movement may cause an inversion of the main features of the response histograms. In terms of peak spike rate or total number of spikes elicited, however, primary lateral line afferents were not directionally sensitive.Central (midbrain) lateral line units of weakly electric fish (Apteronotus) showed a jittery response if an object moved by. In midbrain mechanosensory lateral line, ampullary, and tuberous units the response to a rostral-tocaudal object movement may be different from that elicited by a caudal-to-rostral object motion. Central units of Apteronotus may receive input from two or more sensory modalities. Units may be lateral line-tuberous or lateral line-ampullary. Multimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. The receptive fields of central units demonstrate a weak somatotopic organization of lateral line input: anterior body areas project to rostral midbrain, posterior body areas project to caudal midbrain.Abbreviation EOD electric organ discharge     

神经元放电阈值的可变性及其意义

神经元能够将不同时空模式的突触输入转化为时序精确的动作电位输出，这种灵活、可靠的信息编码方式是神经集群在动态环境或特定任务下产生所需活动模式的重要基础。动作电位的产生遵循全或无规律，只有当细胞膜电压达到放电阈值时，神经元才产生动作电位。放电阈值在细胞内和细胞间具有高度可变性，具体动态依赖于刺激输入和放电历史。特别是，放电阈值对动作电位起始前的膜电压变化十分敏感，这种状态依赖性产生的生物物理根源包括Na⁺失活和K⁺激活。在绝大多数神经元中，动作电位的触发位置是轴突起始端，这个位置处的阈值可变性是决定神经元对时空输入转化规律的关键因素。但是，电生理实验中动作电位的记录位置却通常是胞体或近端树突，此处的阈值可变性高于轴突起始端，而其产生的重要根源是轴突动作电位的反向传播。基于胞体测量的相关研究显示，放电阈值动态能够增强神经元的时间编码、特征选择、增益调控和同时侦测能力。本文首先介绍放电阈值的概念及量化方法，然后详细梳理近年来国内外关于放电阈值可变性及产生根源的研究进展，在此基础上归纳总结放电阈值可变性对神经元编码的重要性，最后对未来放电阈值的研究方向进行展望。     

Parallel processing by a homogeneous group of coupled model neurons can enhance, reduce and generate signal correlations

 Correlated activities have been proposed as correlates of flexible association and assembly coding. We addressed the basic question of how signal correlations on parallel pathways are enhanced, reduced and generated by homogeneous groups of coupled neurons, and how this depends on the input activities and their interactions with internal coupling processes. For this we simulated a fully connected group of identical impulse-coded neurons with dynamic input and threshold processes and additive or multiplicative lateral coupling. Input signals were Gaussian white noise (GWN), completely independent or partially correlated on a subgroup of the parallel inputs. We show that in states of high average spike rates input-output correlations were weak while the network could generate correlated activities of stochastic, oscillatory and rhythmic bursting types depending exclusively on lateral coupling strength. In states of low average spike rates input-output correlations were high and the network could effectively enhance or reduce differences in spatial correlation applied to its parallel inputs. The correlation differences were more pronounced with multiplicative lateral coupling than with the additive interactions commonly used. As the different modes of correlation processing emerged already by global changes in the average spike rate and lateral coupling strength, we assume that in real cortical circuits changes in correlational processing may also be induced by unspecific modulations of activation and lateral coupling. Received: 11 December 1995 / Accepted in revised form: 29 November 1996     

Tentacle regulating potentials inNoctiluca miliaris: their generation sites and ionic mechanisms

Summary Membrane potential responses that regulate movement of the food-gathering tentacle ofNoctiluca miliaris (tentacle regulating potentials, TRPs) were examined electrophysiologically under various ionic conditions. These spontaneous TRPs were modified by changing the external ionic conditions. Positive spike appeared as external Ca²⁺ concentration was lowered. The peak of the spike became more positive with increasing external Na⁺ concentration. The spike could be evoked by injecting a depolarizing current when the membrane was hyperpolarized. The positive spike is assumed to be caused by regenerative activation of depolarization-sensitive Na channels. The peak of the negative spike, reported by previous workers, became more negative with increasing external Cl^– concentration. The spike was evoked by injecting a hyperpolarizing current when the membrane was depolarized. The negative spike is assumed to be caused by regenerative activation of hyperpolarization-sensitive Cl^– channels. The waveforms and amplitudes of the TRPs recorded from the nucleus were identical to those recorded from the flotation vacuole. This suggests that the TRPs are generated on the membrane facing the external solution. Possible roles of the TRPs in the control of tentacle movement are discussed.Abbreviations ASW artificial sea water - FTP flash-triggering potential - TRP tentacle regulating potential     

Effects of Some Organic Cations on Generator Potential of Crayfish Stretch Receptor

The generator potential of both slowly and rapidly adapting crayfish stretch receptor cells can still be elicited by mechanical stimuli when all the Na of the bathing medium is replaced by various organic cations. In the presence of tris(hydroxymethyl)aminomethane (Tris), the generator potential is particularly large, about 30–50 % of that in the control saline, while spike electrogenesis of the cell is abolished. Persistence of the generator response is not due to retention of Na by a diffusion barrier, and ionic contributions to the electrogenesis by Ca and Cl can also be excluded. Thus, whereas the electrogenesis of the generator membrane must be due to an increased permeability to monovalent cations, the active receptor membrane appears to be less selective for different monovalent cations than is the receptor component of some other cells, or the conductile component of the stretch receptor neuron.     

Physiological characteristics and reflex activation of DUM (octopaminergic) neurons of locust metathoracic ganglion

Intracellular recordings were made from single or pairs of somata of the dorsal unpaired median (DUM) neurons of the metathoracic ganglion of the locust Schistocerca gregaria and the grasshopper Romalea microptera, during reflex actions, direct electric excitation and orthodromic and antidromic neural stimulation. Some, possibly all, of these neurons are unique, identifiable individuals in regard to their targets, which are specific peripheral muscles. Their physiological properties and the ways they are activated synaptically are, however, similar. Large, overshooting action potentials, comprising three components, occur. The first component in time is small and represents an excitatory synaptic potential for orthodromic stimulation or an axon spike (AS) for antidromic stimulation, electrotonically conducted into the soma. The second component is larger, being an electrotonically conducted integrating segment spike (ISS). The final component is the soma spike (SS). Neither AS nor ISS have a late positive phase, but there is a large, prolonged one for SS. The latter, combined with rapid accommodation, determine a low maximum firing rate for the neurons. Most nerves entering the ganglion make excitatory inputs onto each DUM neuron, which is readily driven to spike by electric excitation of either connective. There is a great deal of spontaneous excitatory synaptic input to each DUM neuron and a high proportion of it is common. Although there is no detectable electrical coupling between the cells, there is about 30% synchronous firing, apparently due to the common inputs; independent excitation and inhibition also occur. All sensory modalities tested have inputs to the neurons, which tend to fire constantly at a low rate (1 per 3–4 sec). In reflex actions, DUM neurons tend to fire before motor output occurs. It is suggested that the cells will be found to have many functions serving a general role comparable to that achieved by the release of adrenaline in vertebrates.     

EGTA induces prolonged summed depolarizations in Mytilus gill coupled ciliated epithelial cells: implications for the control of ciliary motility

Abfrontal ciliated cells of Mytilus edulis gill beat when mechanically stimulated, a consequence of a Ca++-based generator potential and regenerative response. In contrast, the lateral ciliated epithelial cells arrest when stimulated, a consequence of a Ca++-based generator potential and a Na+/Ca++-based regenerative response. Iontophoretic injection of EGTA in abfrontal cells, followed by mechanical stimulation, results in a large, prolonged depolarization that returns to the resting level stepwise. It has been hypothesized that this phenomenon is caused by successive Ca++-dependent repolarizations in coupled cells, first in adjacent cells and then in the injected cell, in accord with relative EGTA loading. We have now demonstrated this same stepwise repolarization phenomenon in the Na+/Ca++-dependent lateral ciliated cells. In this case, each repolarization step is often preceded by a small spike. With either cell type, using two-electrode recording techniques, we can detect the stepwise repolarization in distant cells, proportionately decremented when the second (KCl) electrode is some distance from the injection (EGTA) electrode and stimulus. When force is applied between the electrodes and nearest the KCl electrode, a greater initial response is recorded from this electrode, presumably resulting from depolarization of its impaled cell, prolonged by EGTA diffusion through the intervening cell junctions. The subsequent repolarization steps are of approximately the same size, suggesting repolarization of cells between the two electrodes. These observations are consistent with the cell coupling/EGTA loading hypothesis and indicate that both cell types mediate repolarization through Ca++ and propagate ciliary beat or arrest through intracellular coupling.