Localization and function of the electrical oscillation in electroreceptive ampullary epithelium from skates.

A steady, spontaneous current oscillation (1 nA p-p) occurs in voltage-clamped, isolated ampullary organs (canal, ampulla, and nerve) from skates (Raja). Spectral analysis showed that energy in the oscillation was confined to a narrow band of frequencies (3 Hz) about a fundamental frequency (32 Hz at 20 degrees C) and in harmonics. The frequency of the oscillation was temperature dependent (increasing from 21 to 33 Hz for increases in temperature from 13 degrees C to 21 degrees C). The addition of 0.5 microM tetrodotoxin to the basal side of the ampullary epithelium eliminated afferent nerve activity but had no effect on the epithelial oscillation, indicating that the oscillation is not generated or induced by afferent nerve activity. Nitrendipine (2 microM) added to the solution bathing the basal side of the ampullary epithelium abolished the oscillation rapidly (within minutes), but a steady-state negative conductance (i.e., real part of the complex admittance < 0) generated by the preparation remained for 36 min. Conversely, nitrendipine (50 microM) added to the perfusate (artificial sea water) of the apical side eliminated the negative conductance rapidly (18.5 min) but had no effect on the spontaneous oscillation for more than 1 h. The effect and the elapsed time for an effect of nitrendipine after separate applications to the basal and apical membrane surfaces of ampullary epithelium suggest that 1) the negative conductance and the oscillation are generated independently in apical and basal membranes, respectively, and 2) both processes involve L-type Ca channels. Furthermore, the addition of tetraethylammonium (2 mM) to the basal side eliminated both the oscillation and the postsynaptic response to voltage clamps (< or = 100 microV) of the ampullary epithelium in the operational voltage range of the afferent nerve. This result suggests that the basal membrane oscillation functions in neurotransmitter release from presynaptic (basal) membranes.     

Ion channels and transporters in the electroreceptive ampullary epithelium from skates.

Two ampullary epithelial properties necessary for electroreception were used to identify the types of ion channels and transporters found in apical and basal membranes of ampullary receptor cells of skates and to assess their individual role under voltage-clamp conditions. The two essential properties are (1) a steady-state negative conductance generated in apical membranes and (2) a small, spontaneous current oscillation originating in basal membranes (Lu and Fishman, 1995). The effects of pharmacological agents and ion substitutions on these properties were evaluated from transorgan or transepithelial complex admittance determinations in the frequency range 0.125 to 50 Hz measured in individual, isolated ampullary organs. In apical membranes, L-type Ca channels were found to be responsible for generation of the steady-state negative conductance. In basal membranes, K and Ca-dependent Cl (Cl(Ca)) channels were demonstrated to contribute to a net positive membrane conductance. L-type Ca channels were also evident in basal membranes and are thought to function in synaptic transmission from the electroreceptive epithelium to the primary afferent nerve. In addition to ion channels in basal membranes, two transporters (Na+/K+ pump and Na(+)-Ca+ exchanger) were apparent. Rapid (minutes) cessation of the current oscillation after blockage of any of the basal ion channels (Ca, Cl(Ca), K) suggests critical involvement of each of these channel types in the generation of the oscillation. Suppression of either Na+/K+ transport or Na(+)-Ca2+ exchange also eliminated the oscillation but at a slower rate, indicating an indirect effect.     

Fluctuation and linear analysis of Na-current kinetics in squid axon

The power spectrum of current fluctuations and the complex admittance of squid axon were determined in the frequency range 12.5 to 5,000 Hx during membrane voltage clamps to the same potentials in the same axon during internal perfusion with cesium. The complex admittance was determined rapidly and with high resolution by a fast Fourier transform computation of the current response, acquired after a steady state was attained, to a synthesized signal with predetermined spectral characteristics superposed as a continuous, repetitive, small perturbation on step voltage clamps. Linear conduction parameters were estimated directly from admittance data by fitting an admittance model, derived from the linearized Hodgkin-Huxley equations modified by replacing the membrane capacitance with a "constant-phase-angle" capacitance, to the data. The constant phase angle obtained was approximately 80 degrees. At depolarizations the phase of the admittance was 180 degrees, and the real part of the impedance locus was in the left-half complex plane for frequencies below 1 kHz, which indicates a steady-state negative Na conductance. The fits also yielded estimates of the natural frequencies of Na "activation" and "inactivation" processes. By fitting Na-current noise spectra with a double Lorentzian function, a lower and an upper corner frequency were obtained; these were compared with the two natural frequencies determined from admittance analysis at the corresponding potentials. The frequencies from fluctuation analyses ranged from 1.0 to 10.3 times higher than those from linear (admittance) analysis. This discrepancy is consistent with the concept that the fluctuations reflect a nonlinear rate process that cannot be fully characterized by linear perturbation analysis. Comparison of the real part of the admittance and the current noise spectrum shows that the Nyquist relation, which generally applies to equilibrium conductors, does not hold for the Na process in squid axon. The Na-channel conductance, gamma Na, was found to increase monotonically from 0.1 to 4.8 pS for depolarizations up to 50 mV from a holding potential of -60 mV, with no indication of a maximum value.     

Neural repetitive firing: modifications of the Hodgkin-Huxley axon suggested by experimental results from crustacean axons.

The Hodgkin-Huxley equations for space-clamped squid axon (18 degrees C) have been modified to approximate voltage clamp data from repetitive-firing crustacean walking leg axons and activity in response to constant current stimulation has been computed. The m infinity and h infinity parameters of the sodium conductance system were shifted along the voltage axis in opposite directions so that their relative overlap was increased approximately 7 mV. Time constants tau m and tau h, were moved in a similar manner. Voltage-dependent parameters of delayed potassium conductance, n infinity and tau n, were shifted 4.3 mV in the positive direction and tau n was uniformly increased by a factor of 2. Leakage conductance and capacitance were unchanged. Repetitive activity of this modified circuit was qualitatively similar to that of the standard model. A fifth branch was added to the circuit representing a transient potassium conductance system present in the repetitive walking leg axons and in other repetitive neurons. This model, with various parameter choices, fired repetitively down to approximately 2 spikes/s and up to 350/s. The frequency vs. stimulus current plot could be fit well by a straight line over a decade of the low frequency range and the general appearance of the spike trains was similar to that of other repetitive neurons. Stimulus intensities were of the same order as those which produce repetitive activity in the standard Hodgkin-Huxley axon. The repetitive firing rate and first spike latency (utilization time) were found to be most strongly influenced by the inactivation time constant of the transient potassium conductance (tau b), the delayed potassium conductance (tau n), and the value of leakage conductance (gL). The model presents a mechanism by which stable low frequency discharge can be generated by millisecond-order membrane conductance changes.     

Vasopressin induces depolarization and state-dependent firing patterns in rat thalamic paraventricular nucleus neurons in vitro

The thalamic midline paraventricular nucleus (PVT) is prominently innervated by vasopressin-immunoreactive neurons from the suprachiasmatic nucleus (SCN), site of the brain's biological clock. Using patch-clamp recordings in slice preparations taken from Wistar rats during the subjective day, we examined 90 PVT neurons for responses to bath-applied AVP (0.5-2 microM; 1-3 min). In current clamp at resting membrane potentials (-65 +/- 1 mV), PVT neurons displayed low-threshold spikes (LTSs) and burst firing patterns. In 50% of cells tested, AVP induced a slowly rising, prolonged membrane depolarization and tonic firing, returning to burst firing upon recovery. AVP modulated hyperpolarization-activated LTSs by decreasing the time to the initial sodium spike at the onset of LTS, also increasing the duration of the afterdepolarization. Responses were blockable with a V(1a) receptor antagonist (Manning compound). Under voltage clamp, AVP induced a TTX-resistant, slowly rising, and prolonged (approximately 15 min) inward current (<40 pA). Current-voltage relationship (I-V) analyses of the AVP responses revealed a decrease in membrane conductance to 73.1 +/- 6.2% of control, with net AVP current reversing at -106 +/- 4 mV, and decreased inward rectification at negative potentials. These observations are consistent with an AVP-induced closure of an inwardly rectifying potassium conductance. On the basis of these in vitro observations, we suggest that the SCN vasopressinergic innervation of PVT is excitatory in nature, possibly releasing AVP with circadian rhythmicity and contributing to state-dependent firing patterns in PVT neurons over the sleep-wake cycle.     

Extra spike formation in sensory neurons and the disruption of afferent spike patterning

The peculiar pseudounipolar geometry of primary sensory neurons can lead to ectopic generation of "extra spikes" in the region of the dorsal root ganglion potentially disrupting the fidelity of afferent signaling. We have used an explicit model of myelinated vertebrate sensory neurons to investigate the location and mechanism of extra spike formation, and its consequences for distortion of afferent impulse patterning. Extra spikes originate in the initial segment axon under conditions in which the soma spike becomes delayed and broadened. The broadened soma spike then re-excites membrane it has just passed over, initiating an extra spike which propagates outwards into the main conducting axon. Extra spike formation depends on cell geometry, electrical excitability, and the recent history of impulse activity. Extra spikes add to the impulse barrage traveling toward the spinal cord, but they also travel antidromically in the peripheral nerve colliding with and occluding normal orthodromic spikes. As a result there is no net increase in afferent spike number. However, extra spikes render firing more staccato by increasing the number of short and long interspike intervals in the train at the expense of intermediate intervals. There may also be more complex changes in the pattern of afferent spike trains, and hence in afferent signaling.     

Ion conductances of the surface and transverse tubular membranes of skeletal muscle

Summary A combination voltage clamp and admittance analysis of single skeletal muscle fibers showed that moderate depolarizations activated a steady-state negative sodium conductance in both the surface and transverse tubular membranes. The density of the voltage-dependent channels was similar for the surface and tubular conductances. The relaxation times associated with the negative conductance were in the millisecond range and markedly potential dependent. The negative tubular conductance has the consequence of increasing the apparent steady-state radial space constant to large values. This occurs because the positiv conductance is counterbalanced by the maintained inward-going sodium current. The enhancement of the space constant by a negative conductance provides a means for the nearly simultaneous activation of excitation-contraction coupling.     

Electroreceptor mechanisms of the ampullae of lorenzini in skates

Responses of the receptor epithelium of single electrically isolated ampullae of Lorenzini and spike responses of nerve fibers connected to them to electrical stimulation under voltage clamping conditions were studied in experiments on the Black Sea skateRaja clavata. The preparations had an input resistance of 200–800 KΩ, a transepithelial resting potential of between 0 and ?2 mV, and the usual spontaneous spike activity in their afferent fibers. Thresholds of the electroreceptors were 2–10 µV (current about 10^?11 A). Within the working range of the electroreceptors (up to ±500 µV, current up to 1 nA) the current-voltage characteristic curve of the epithelium was linear without any evidence of spike or wave activity of the receptor cells. With negative currents of over 1–10 nA a regenerative spike appeared on the epithelium and was accompanied by an uncharacteristic pattern of spike discharge of the nerve fiber. It is concluded that, contrary to Bennett's hypothesis, spike or oscillatory activity bears no relationship to normal working of electroreceptors. It is postulated that "secondarily sensitive" receptor cells share a common functional organization, which is based on a chemical synapse with high electrical sensitivity.     

Spiking properties of olfactory receptor cells in the slice preparation

The whole-cell, patch clamp [corrected] method was applied to olfactory receptor cells in slice preparations made from bullfrog olfactory epithelium. Under voltage-clamp conditions, olfactory receptor cells showed a transient inward current followed by a steady outward current in response to depolarizing voltage steps, as has been shown in the isolated preparation. The input resistance was 5.4 +/- 3.9 GOmega and capacitance 21.9 +/- 9.7 pF. Under current-clamp conditions, depolarization of cells by current injection induced action potentials. In 13 out of 20, spike generation was repetitive with a maximum frequency of 24 Hz. The frequency of the repetitive discharges increased as the injected current was increased. The relationship between the size of the injected current and firing frequency could be well fitted by the Michaelis-Menten equation, indicating that the spike generation site lacks the non-linear boosting system. The slice preparation developed here would provide a powerful tool to study the spike encoding system of the olfactory receptor cells.     

Analysis of the electrical responses of antennal thermo- and hygroreceptors of Antheraea (Saturniidae,Lepidoptera) to thermal,mechanical, and electrical stimuli

Summary Antennal styloconic thermo-hygro sensilla of Antheraea were studied with DC-coupled transepithelial recordings. — The transepithelial voltage changed by about 2 mV · °C^–1. The spike frequency of the cold cell reached 300 Hz at the onset of negative temperature steps, but only 30 Hz at static temperatures (as with metal electrodes). The cold cell spikes showed a brief afterhyperpolarization that increased with temperature. The spikes of the cold- and warm-stimulated cells facilitated each other at low temperature. Mechanical stimuli (push against the sensillum, hydrostatic pressure of < ± 50 kPa, ultrasonic vibrations 120 kHz) modified the responses of the cold- and the warm-stimulated cells. Latency of cold cell responses to ultrasonic stimulation was occasionally less than 3 ms. — The impulse frequencies of the warm and the cold cells depend on the temperature and the magnitude of temperature change. When the firing rate is high enough by either or both of these parameters, it can be forced still higher by application of clamp current (outside positive). The higher the firing rate prior to clamping, the greater the effect of the current. — By analogy with sensilla for other modalities, this relationship between frequency and clamp current strongly suggests that stimulus-dependent changes in the conductance of dendritic membranes control the excitation of the warm and cold cells.Abbreviations DC direct voltage - TER transepithelial slope resistance between recording electrode and reference electrode in the hemolymph - NTC thermistor with negative temperature coefficient - TEV transepithelial voltage between electrodes - THS thermo-hygro sensillum     

Computed Membrane Currents in Cardiac Purkinje Fibers during Voltage Clamps

Recent measurements have indicated that some of the cardiac cell electrical capacitance is in series with a resistance. The computations of currents in a voltage clamp presented below show that, in this case, there is a danger that capacitive transient currents recorded during voltage clamp experiments may be confused with currents arising through rapid active membrane conductance changes. Secondly, a voltage clamp technique aimed at avoiding capacitive transients, namely the linear or ramp clamp, has recently been introduced. An attempt has been made here to evaluate the usefulness of ramp clamps in studying membrane electrical properties, by computing ramp clamp results and considering the difficulties in reconstructing the original model from these results. It is concluded that such a reconstruction is not feasible.     

Complex admittance of Na+ conduction in squid axon

Summary The complex admittance,Y(p), of squid axon was measured (4-1000 Hz) during step voltage clamp to obtain linear data on Na⁺ conduction.Y(p) is used as a spectroscopic tool to identify Na⁺ and K⁺ conduction, which dominateY(p) at low frequencies and can be separated from each other and from the static capacitance. Na⁺ conduction is readily distinguishable from K⁺ conduction in that it produces a steady-state negative conductance. The admittance of the Na⁺ system can show an anomalous resonance or an antiresonance depending on whether the net shunt conductance is negative or positive. Use of the Na⁺ negative conductance to neutralize leakage yields a measurement of dielectric capacitance at low frequency. A 90^o phase angle suggests that the capacitance is ideal.     

Electrophysiological and biochemical responses of mouse vomeronasal receptor cells to urine-derived compounds: possible mechanism of action

Receptor cells of the vomeronasal organ (VNO) are thought to detect pheromone-like molecules important for reproductive physiology. Several compounds derived from male mouse urine have been demonstrated to affect endocrine events in female mice. In the present study, the ability of these compounds to affect VNO activity was tested. In dissociated VNO cells held under voltage clamp conditions, application of dehydro-exo-brevicomin (DHB) evoked an outward current at negative holding potentials and an inward current at positive holding potentials. Under current clamp, DHB reduced action potential firing. Since DHB application caused a decrease in membrane conductance, this compound appeared to act by reducing inward current through closing an ion channel. Biochemical experiments tested the effects of DHB and 2- (sec-butyl)-4,5-dihydrothiazole (SBT) on cAMP levels in the VNO. A mixture of DHB and SBT decreased cAMP levels in VNO sensory tissue and had no effect on VNO non-sensory tissue. The results suggest that pheromones have an inhibitory influence on action potential generation and on cAMP levels in receptor cells of the VNO.      

Analysis of bursting in a thalamic neuron model

We extend a quantitative model for low-voltage, slow-wave excitability based on the T-type calcium current (Wang et al. 1991) by juxtaposing it with a Hodgkin-Huxley-like model for fast sodium spiking in the high voltage regime to account for the distinct firing modes of thalamic neurons. We employ bifurcation analysis to illustrate the stimulus-response behavior of the full model under both voltage regimes. The model neuron shows continuous sodium spiking when depolarized sufficiently from rest. Depending on the parameters of calcium current inactivation, there are two types of low-voltage responses to a hyperpolarizing current step: a single rebound low threshold spike (LTS) upon release of the step and periodic LTSs. Bursting is seen as sodium spikes ride the LTS crest. In both cases, we analyze the LTS burst response by projecting its trajectory into a fast/slow phase plane. We also use phase plane methods to show that a potassium A-current shifts the threshold for sodium spikes, reducing the number of fast sodium spikes in an LTS burst. It can also annihilate periodic bursting. We extend the previous work of Rose and Hindmarsh (1989a–c) for a thalamic neuron and propose a simpler model for thalamic activity. We consider burst modulation by using a neuromodulator-dependent potassium leakage conductance as a control parameter. These results correspond with experiments showing that the application of certain neurotransmitters can switch firing modes. Received: 18 July 1993/Accepted in revised form: 22 January 1994     

Relationships between voltage and tension in sheep cardiac Purkinje fibers

The two-microelectrode technique of voltage clamping sheep cardiac Purkinje fibers was used to examine the changes in contraction which occur during trains of voltage clamps. (A "train" is defined as a series of voltage clamps delivered at a particular rate, beginning after a rest long enough that the effects of previous stimulation have died away.) Contractions showed striking staircases, or progressive changes in peak isometric tension, during trains. Short clamps, clamps to voltages more negative than --20 or --30 mV, or holding potentials less negative than the resting potential favored negative staircases, while long clamps, clamps to positive voltages, and holding potentials near the resting potential each favored positive staircases. The staircase behavior appeared to be due to changes in the initial rate of recovery of the ability to contract. The changes in staircase behavior as a function of clamp voltage suggested that the relationship between peak tension and clamp voltage should depend on the experimental design. When the steady-state contraction was plotted as a function of clamp voltage, voltage-tension relations like those recently reported for working ventricle were obtained, with a threshold between --30 and - -40 mV and a steep relation between tension and voltage. When the first contraction after a rest was plotted, the threshold voltage was more negative, the curve was flatter, and the peak tensions at inside positive voltages were reduced.     

Reliability of spike and burst firing in thalamocortical relay cells

The reliability and precision of the timing of spikes in a spike train is an important aspect of neuronal coding. We investigated reliability in thalamocortical relay (TCR) cells in the acute slice and also in a Morris-Lecar model with several extensions. A frozen Gaussian noise current, superimposed on a DC current, was injected into the TCR cell soma. The neuron responded with spike trains that showed trial-to-trial variability, due to amongst others slow changes in its internal state and the experimental setup. The DC current allowed to bring the neuron in different states, characterized by a well defined membrane voltage (between ?80 and ?50 mV) and by a specific firing regime that on depolarization gradually shifted from a predominantly bursting regime to a tonic spiking regime. The filtered frozen white noise generated a spike pattern output with a broad spike interval distribution. The coincidence factor and the Hunter and Milton measure were used as reliability measures of the output spike train. In the experimental TCR cell as well as the Morris-Lecar model cell the reliability depends on the shape (steepness) of the current input versus spike frequency output curve. The model also allowed to study the contribution of three relevant ionic membrane currents to reliability: a T-type calcium current, a cation selective h-current and a calcium dependent potassium current in order to allow bursting, investigate the consequences of a more complex current-frequency relation and produce realistic firing rates. The reliability of the output of the TCR cell increases with depolarization. In hyperpolarized states bursts are more reliable than single spikes. The analytically derived relations were capable to predict several of the experimentally recorded spike features.     

Inward rectifier K+-channel kinetics from analysis of the complex conductance of Aplysia neuronal membrane.

Conduction in inward rectifier, K+-channels in Aplysia neuron and Ba++ blockade of these channels were studied by rapid measurement of the membrane complex admittance in the frequency range 0.05 to 200 Hz during voltage clamps to membrane potentials in the range -90 to -40 mV. Complex ionic conductances of K+ and Cl- rectifiers were extracted from complex admittances of other membrane conduction processes and capacitance by vector subtraction of the membrane complex admittance during suppressed inward K+ current (near zero-mean current and in zero [K+]0) from complex admittances determined at other [K+]0 and membrane potentials. The contribution of the K+ rectifier to the admittance is distinguishable in the frequency domain above 1 Hz from the contribution of the Cl- rectifier, which is only apparent at frequencies less than 0.1 Hz. The voltage dependence (-90 to -40 mV) of the chord conductance (0.2 to 0.05 microS) and the relaxation time (4-8 ms) of K+ rectifier channels at [K+]0 = 40 mM were determined by curve fits of admittance data by a membrane admittance model based on the linearized Hodgkin-Huxley equations. The conductance of inward rectifier, K+ channels at a membrane potential of -80 mV had a square-root dependence on external K+ concentration, and the relaxation time increased from 2 to 7.5 ms for [K+]0 = 20 and 100 mM, respectively. The complex conductance of the inward K+ rectifier, affected by Ba++, was obtained by complex vector subtraction of the membrane admittance during blockage of inward rectifier, K+ channels (at -35 mV and [Ba++]0 = 5 mM) from admittances determined at -80 mV and at other Ba++ concentrations. The relaxation time of the blockade process decreased with increases in Ba++ concentration. An open-closed channel state model produces the inductive-like kinetic behavior in the complex conductance of inward rectifier, K+ channels and the addition of a blocked channel state accounts for the capacitive-like kinetic behavior of the Ba++ blockade process.     

Mechanisms underlying burst generation of the pyloric muscle in the mantis,shrimp, Squilla oratoria

The pyloric constrictor muscles of the stomach in Squilla can generate spikes by synaptic activation via the motor nerve from the stomatogastric ganglion. Spikes are followed by slow depolarizing afterpotentials (DAPs) which lead to sustained depolarization during a burst of spikes. 1. The frequency of rhythmic bursts induced by continuous depolarization is membrane voltage-dependent. A brief depolarizing or hyperpolarizing pulse can trigger or terminate bursts, respectively, in a threshold-dependent manner. 2. The conductance increases during the DAP response. The amplitude of DAP decreases by imposed depolarization, whereas it increases by hyperpolarization. DAPs from successive spikes sum to produce a sustained depolarizing potential capable of firing a burst. 3. The spike and DAP are reduced in amplitude by decreasing [Ca]o, enhanced by Sr2+ or Ba2+ substituted for Ca2+, and blocked by Co2+ or Mn2+. DAPs are selectively blocked by Ni2+, and the spike is followed by a hyperpolarizing afterpotential. 4. The spike and DAP are prolonged by intracellular injection of the Ca2+ chelator EGTA. A hyperpolarizing afterpotential is abolished by EGTA and enhanced by increasing [Ca]o. The DAP is diminished in Na(+)-free saline and reduced by tetrodotoxin. 5. It is concluded that the muscle fiber is endowed with endogenous oscillatory properties and that the oscillatory membrane events result from changes of a voltage- and time-dependent conductance to Ca2+ and Na+ and a Ca2+ activated conductance to K+.     

A voltage clamp study of the sodium, calcium and chloride spikes of chick skeletal muscle cells grown in tissue culture.

Conventional voltage clamp techniques with microelectrodes were applied to chick muscle cells grown in tissue culture. The similarities and differences in electrophysiological data obtained from normal myotubular muscle fibers and from rounded myosacs, produced by incubation with colchicine, were examined. Under voltage clamp both cellular types generated three distinguishable voltage and time-dependent currents which corresponded, respectively, to the Na⁺, Ca²⁺, and Cl^? spikes evoked under constant current conditions. The presumed Ca²⁺ currents were too small to allow quantitative comparisons. In myosacs, but not in myotubes, there was good correspondence, for both the Na⁺ and Cl^? systems, between their spike thresholds and peak membrane potentials, measured under constant current conditions, and their current thresholds and reversal potentials, measured under voltage clamp conditions. This correspondence is attributed to the isopotentiality of the myosac intracellular space and suggests that myosacs provide more accurate quantitative data in voltage clamp studies than myotubes.     

A cross-interval spike train analysis: the correlation between spike generation and temporal integration of doublets

A stochastic spike train analysis technique is introduced to reveal the correlation between the firing of the next spike and the temporal integration period of two consecutive spikes (i.e., a doublet). Statistics of spike firing times between neurons are established to obtain the conditional probability of spike firing in relation to the integration period. The existence of a temporal integration period is deduced from the time interval between two consecutive spikes fired in a reference neuron as a precondition to the generation of the next spike in a compared neuron. This analysis can show whether the coupled spike firing in the compared neuron is correlated with the last or the second-to-last spike in the reference neuron. Analysis of simulated and experimentally recorded biological spike trains shows that the effects of excitatory and inhibitory temporal integration are extracted by this method without relying on any subthreshold potential recordings. The analysis also shows that, with temporal integration, a neuron driven by random firing patterns can produce fairly regular firing patterns under appropriate conditions. This regularity in firing can be enhanced by temporal integration of spikes in a chain of polysynaptically connected neurons. The bandpass filtering of spike firings by temporal integration is discussed. The results also reveal that signal transmission delays may be attributed not just to conduction and synaptic delays, but also to the delay time needed for temporal integration. Received: 3 March 1997 / Accepted in revised form: 6 November 1997