Temporally patterned pulse trains affect directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus

The directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, was studied under free field stimulation conditions with 3 temporally patterned trains of sound pulses which differed in pulse repetition rate and duration. The directional sensitivity curves of 92 neurons studied can be described as hemifield, directionally-selective, or non-directional according to the variation in the number of impulses with pulse train direction. When these neurons were stimulated with all 3 pulse trains, the directional sensitivity curves of 50 neurons was unchanged but that of the other 42 neurons changed from one type into another. When these pulse trains were delivered at high pulse repetition rate and short pulse duration, they significantly sharpened the directional sensitivity of two thirds of the neurons examined by reducing the angular range and increasing the slope of their impulse directional sensitivity curves. These pulse trains also sharpened the slope of the threshold directional sensitivity curves of 25 neurons studied. However, when directional sensitivity of collicular neurons was determined with pulse trains that differed only in pulse repetition rate or in pulse duration, significant sharpening of directional sensitivity was rarely observed in all experimental conditions tested. Possible mechanisms underlying these findings are discussed.     

Über die Impulsverarbeitung eines mathematischen Neuronenmodelles

Zusammenfassung Das Membranmodell von Hodgkin und Huxley wurde durch einfache lineare Modelle einer erregenden und einer hemmenden Synapse ergänzt. Die Impulsverarbeitung des so entstandenen mathematischen Neuronenmodells wurde mit Hilfe eines Digitalrechners untersucht. In erster Linie interessierten die Eingangs und Ausgangsbeziehungen, d. h. die Zusammenhänge zwischen den Daten der präsynaptischen Anregung und der postsynaptischen Antwort. Die Modellparameter und damit auch die postsynaptische Antwort hängen von der Intensität und vom Zeitverlauf der Anregung ab. Daher wurde das Modell mit Einzelimpulsen, Impulspaaren und Impulsfolgen sowie mit sprungartig und rampenförmig einsetzender Gleichspannung angeregt. Durch die Gleich- oder Daueranregung wird der Fall nachgebildet, daß viele Synapsen gleichzeitig von verschiedenen Impulsfolgen mit hinreichend hoher Folgefrequenz aktiviert werden.Die Gleichanregungskennlinien geben den Zusammenhang zwischen der präsynaptischen Gleichanregung und der stationären postsynaptischen De- oder Hyperpolarisation an (Abb. 5). Sie zeigen Sättigungscharakter; ihre Steilheit besitzt am Anfang den größten Wert und nimmt mit zunehmender De- und Hyperpolarisation ab. Im unterschwelligen Bereich bis zu Depolarisationen von etwa 4 mV ist das Modell stabil, es können nur lokale Antworten erzeugt werden. Im Zwischenbereich von 4–8,8 mV können je nach Anstiegsart der Dauererregung ein oder mehrere Aktionspotentiale hervorgerufen werden. Im instabilen Bereich von 8,8 mV bis etwa 18 mV ergeben sich in jedem Fall periodische Folgen von Aktionspotentialen, deren Folgefrequenz mit wachsender Depolarisation zunimmt.Die Impulshemmungskennlinien (Abb. 12) geben den Zusammenhang zwischen den Amplituden der Eingangsimpulse und der zugehörigen IPSP an. Sie haben ebenfalls Sättigungscharakter. Überlagert man IPSP und Dauerpolarisation, so erhält man mit zunehmender Depolarisation eine Verstärkung der IPSP.Durch erregende Eingangsimpulse können postsynaptische Antworten mit jeder beliebigen Amplitude zwischen 0 und 100 mV erzeugt werden. Die Impulserregungskennlinien (Abb. 7, 27) beginnen mit einer geringen Steigung, die in der Nähe des Schwellenwertes rasch zunimmt. In der Umgebung des Schwellenwertes, wo die postsynaptischen Potentiale in die Aktionspotentiale übergehen, verläuft sie sehr steil und nähert sich dann rasch einem Grenzwert. Überlagert man EPSP mit einer konstanten De- oder Hyperpolarisation, so erhält man in jedem Fall eine Abschwächung des EPSP.Bei der Überlagerung von postsynaptischen Potentialen können starke Impulsverformungen auftreten. Diese sind besonders ausgeprägt, wenn einem großen Hauptimpuls ein kleiner Testimpuls überlagert wird (Abb. 15–20).Aktionspotentiale werden gewöhnlich durch hinreichend starke erregende Eingangsimpulse ausgelöst. Sie können aber auch durch hemmende Eingangsimpulse hervorgerufen werden, wenn das erste positive Nachpotential des IPSP den Schwellenwert überschreitet (Abb. 30).Bei der Anregung des Modelles mit periodischen Impulsfolgen wurde insbesondere die Frequenz- bzw. Impulsratenteilung untersucht. Das Ergebnis ist in Form eines v-Diagrammes dargestellt (Abb. 32, 33).

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Summary A simple neuron model has been developed by adding simple linear excitatory and inhibitory synapse models to Hodgkin and Huxley's model of the giant axon. Some cases of pulse processing of this model have been investigated by an electronic digital computer. The postsynaptic response of the model depends on the shape and on the intensity of the presynaptic impulses. Therefore, it has been activated by single and double presynaptic impulses, by pulse trains and by DC excitation (inhibition). DC excitation (inhibition) simulates the case that there are very many excitatory (inhibitory) synapses activated alternately by pulse trains of high repetition rate.The characteristic curves of the model for DC excitation (inhibition) represent the relationship between the presynaptic DC excitation (inhibition) and the depolarisation (hyperpolarisation) of the postsynaptic membrane potential (Fig. 5). These curves show saturation. At the beginning they are rather steep. With increasing depolarisation (hyperpolarization) the steepness decreases.With DC excitation four states of different stability can be distinguished. Up to 4 mV depolarisation of the postsynaptic membrane the model is absolutely stable; no actionpotentials can be evoked. A quasi stable state exists for depolarisation between 4 mV and 8.8 mV. In this range single actionpotentials or trains of actionpotentials may be evoked. For depolarisations between 8.8 and 18 mV the model is instable; each displacement of the membrane potential in this range generates a periodic tram of impulses. Another quasi stable state exists for depolarisations above 18 mV. In this range single actionpotentials may be evoked which are followed by a damped oscillation (Figs. 21, 23).The characteristic curve for inhibition by single presynaptic impulses represents the relationship between the amplitude of the IPSP and the intensity of inhibition (in the neuron: number of synchronously activated synapses; in the model: amplitude of presynaptic impulse). Inhibitory impulses have been superimposed on DC excitation or DC inhibition. The amplitude of the IPSP increases with increasing depolarisation and decreases with increasing hyperpolarisation (Figs. 12, 14).The characteristic curve for excitation by single presynaptic impulses gives the relationship between the amplitudes of the postsynaptic response and of the presynaptic impulse (Figs. 7, 27). This curve raises slowly in the subthreshold range, very fast close to the threshold and remains almost constant above threshold (Fig. 27). If subthreshold presynaptic impulses are superimposed on DC excitation or inhibition, the amplitude of the postsynaptic response (EPSP) decreases in both cases (Figs. 9, 10).Because of the nonlinearity of the model there are many eases where postsynaptic responses of single presynaptic impulses cannot been added linearly (Figs. 15–20).In general actionpotentials are evoked by sufficiently high excitatory input impulses. They also may be evoked by sufficiently high inhibitory input impulses. In this case the first positive afterpotential may exceed the threshold and evoke a spike (Fig. 30).The model has been activated by periodic pulse trams. Depending on the input intensities and on the repetition rates the model shows three modes of operation: 1. full (one to one) transmission, 2. partial transmission (division of repetition rate), 3. no transmission at all (subthreshold activation) (Figs. 32, 33).

     

The potential coding utility of intercell cross-correlations in the retina

The action potentials (impulses) produced by pairs of neighboring retinal ganglion cells often show a tendency either to fire in close temporal synchrony or to avoid temporal synchrony. This cross-correlation (a rate of coincidences that differs from that expected by chance) has been exploited as a window into retinal processing, but its possible functional significance has proven elusive. Previous work has failed to show that the coincidences serve as a direct code for visual stimuli. In this analysis it is shown that the coincidences serve neither as a key for reducing variability nor as a key for improving the coding by the individual cells. The residual impulse trains (trains with coincidences deleted) are more variable than the raw impulse trains and provide an inferior coding to that of the raw impulse trains. There is negative correlation between the firing rate of the residual impulse trains and that of the coincidence impulse trains, which is consistent with the lower variance of the raw impulse trains. There is no consistent cross-correlation between the rates of residual impulse trains of cells in pairs showing cross-correlation; however, it is found that this observation does not discriminate among models for generating coincidences.     

The facilitated probability of quantal secretion within an array of calcium channels of an active zone at the amphibian neuromuscular junction

A Monte Carlo analysis has been made of the phenomenon of facilitation, whereby a conditioning impulse leaves nerve terminals in a state of heightened release of quanta by a subsequent test impulse, this state persisting for periods of hundreds of milliseconds. It is shown that a quantitative account of facilitation at the amphibian neuromuscular junction can be given if the exocytosis is triggered by the combined action of a low-affinity calcium-binding molecule at the site of exocytosis and a high-affinity calcium-binding molecule some distance away. The kinetic properties and spatial distribution of these molecules at the amphibian neuromuscular junction are arrived at by considering the appropriate values that the relevant parameters must take to successfully account for the experimentally observed amplitude and time course of decline of F1 and F2 facilitation after a conditioning impulse, as well as the growth of facilitation during short trains of impulses. This model of facilitation correctly predicts the effects on facilitation of exogenous buffers such as BAPTA during short trains of impulses. In addition, it accounts for the relative invariance of the kinetics of quantal release due to test-conditioning sequences of impulses as well as due to change in the extent of calcium influx during an impulse.     

Lineare Systeme mit gesteuerten statistischen Impulsquellen

Summary From the communication engineers point of view the paper deals with networks consisting of linear filters and a special sort of controlled statistical pulse generators (SIG). The SIG responds to an analog signal with a sequence of Dirac impulses, where the probability for an output impulse at a given time depends on the instantaneous value of the input signal only. In connection with linear filters, however, systems can be realized in which the whole past of the input determines the statistical structure of the output. Therefore systems consisting of linear filters and SIGs (SIG networks) become interesting as models of biological systems, because in biological information processing transformations from analog signals into pulse trains occur very often. An example concerning the application of SIG systems in behavioural sciences will be discussed in a subsequent paper. In the present paper a theoretical analysis of the SIG and SIG networks is carried out by means of Statistical Communication Theory and Theory of Stochastic Processes. It is shown that under certain conditions very complex SIG networks can be treated with Correlation Theory. For one case not satisfying these conditions a solution on the base of Markoff processes is given.

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Auszug aus einer von der Fakultät für Maschinenwesen und Elektrotechnik der Technischen Hochschule München genehmigten Dissertation.Herrn Prof. Dr.-Ing. H. Marko danke ich für das Interesse und die fördernde Kritik während des Entstehens der dieser Arbeit zugrundeliegenden Dissertation. Der Deutschen Forschungsgemeinschaft ist für die finanzielle Förderung der Untersuchungen zu danken. Die numerischen Berechnungen wurden auf der Rechenanlage TR4 des Leibniz-Rechenzentrums der Bayerischen Akademie der Wissenschaften durchgeführt.     

Changes in interspike interval during propagation: Quantitative description

The intervals between nerve impulses can change substantially during propagation because of conduction velocity aftereffects of previous impulse activity. Effects of such changes on interval histograms and on statistical parameters of spike trains were evaluated for Poisson spike trains and for trains generated by a clock with added random delays. The distribution of short intervals was significantly changed during propagation for these spike trains. Substantial changes in serial correlation coefficients were found in trains with certain initial interval distributions. The relevance of these effects to neural coding is discussed.     

Zur Berechnung statistisch schwankender Impulsfolgen und ihrer Überlagerung

Summary In the central nervous system information transmission and processing are accomplished by pulses and pulse trains. The superposition of pulse trains is essential for information processing as it allows the execution of several logical operations, e.g. the multiplication of afferent signals. Jenik (1961) has pointed out that for the superposition of periodic pulse trains the rate of coincidence is proportional to the product of the pulse repetition frequencies (multiplication law). Furtheron he has shown that this simple principle is not always applicable. Errors may occur for certain repetition frequencies of the pulse trains. If the product of signals is accomplished by superposition, mechanisms must exist reducing these errors. Since pulse trains in the nervous system always vary stochastically the following paper is concerned with the effect of random pulse trains in superposition. Two different types of random pulse trains are investigated, trains with random phase and trains with random interval between the pulses. For these two cases calculation methods are given. It is also shown that the deviations from the multiplication law may disappear when superimposing random pulse trains.

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Auszug aus einer von der Fakultät für Elektrotechnik der Technischen Hochschule Darmstadt genehmigten Dissertation.

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Meinem verehrten Lehrer, Herrn Professor Dr.-Ing. E.h. K. Küpfmuller, danke ich herzlich für sein ständiges Interesse an meiner Arbeit und für viele anregende Diskussionen.     

Effect of single impulses on cochlear microphone potentials of the guinea-pig cochlea]

The influence of highly intensive single impulses on the cochlea of guinea pig was studied in an acute experiment. Very short impulses of less than or equal to 0.1 ms duration were produced by a sparknoise generator. The cochlear microphonics (CM) to a test stimulus (sinus tone, 3150 Hz) were recorded from the round window and measured prior to, during, and following impulse treatment. During the impulse treatment, the greatest amplitude reduction of CM occurred after the first impulse, while the further impulses caused a decreasing reduction. At first the number of impulses was varied: 1, 3, and 5 impulses were applied at intervals of 15 s each, at an impulse sound level of 164 dB sound pressure level re. 0.002 mubar (SPL). After these impulse treatments, in all cases a continual decrease of CM amplitudes up to a constant end value without recovery was found within a 2-hrs period of observation. The height of the end value depends on the number of impulses applied. Subsequently, at an exposure to 5 impluses the impulse sound level was stepwise reduced (164, 153, 144, 139 and 133 dB SPL). Again, a characteristic decrease of CM amplitudes was observed during the 2-hrs period of observation. The height of the end value is now dependent on the impluse sound level. Impulses of 164, 153 and 144 dB SPL cause a strong decrease of CM while the effect of impulses of 139 and 133 dB SPL is distinctly lower.     

The topography and dynamics of cholinergic transmission in the myenteric plexus-smooth muscle preparation of the guinea-pig ileum; the effect of ketocyclazocine, a kappa opiate ligand

Output of acetylcholine (ACh), neurogenic electromyogram (NEMG) and contractions of guinea-pig ileum preparations were studied during stimulation by high-frequency trains of impulses. Under control conditions the output of ACh per impulse after 2nd to 4th impulses during train stimulation (30 Hz) was higher by 20-40% than the level of ACh output during the first impulse. In the presence of ketocyclazocine (KTZ, 80 nmol x l-1) the output of ACh evoked by the first impulse was more effectively inhibited than that after impulses 2 to 4 so that the increase was higher (80-170%). NEMG, a direct consequence of the localized action of released transmitter (ACh), was recorded in the longitudinal muscle 4 and 10 mm aborally from the focal stimulation site. The incidence of NEMG responses was higher at the proximal than at the distal site and was proportional to the number of impulses in a train (100 Hz). At the distal site KTZ suppressed the appearance of NEMG responses to single impulses whereas at the proximal site its effect was much less; and so was its effect at either site during train stimulation. It is concluded that in the course of train stimulation, sites of transmission more distant from the stimulation focus were recruited, and consequently the secretion of ACh in succeeding impulses was enhanced. KTZ might preferentially inhibit the propagation of excitation by the very first impulse.     

In Vivo Studies on Fast and Slow Muscle Fibers in Cat Extraocular Muscles

In anesthetized in vivo preparations, responses of two types of extraocular muscle fibers have been studied. The small, multiply innervated slow fibers have been shown to be capable of producing propagated impulses, and thus have been labeled slow multi-innervated twitch fibers. Fast and slow multi-innervated twitch fibers are distinguished by impulse conduction velocities, by ranges of membrane potentials, by amplitudes and frequencies of the miniature end plate potentials, by responses to the intravenous administration of succinylcholine, by the frequency of stimulation required for fused tetanus, and by the velocities of conduction of the nerve fibers innervating each of the muscle fiber types.     

The distribution of the intervals between neural impulses in the maintained discharges of retinal ganglion cells

Simulated neural impulse trains were generated by a digital realization of the integrate-and-fire model. The variability in these impulse trains had as its origin a random noise of specified distribution. Three different distributions were used: the normal (Gaussian) distribution (no skew, normokurtic), a first-order gamma distribution (positive skew, leptokurtic), and a uniform distribution (no skew, platykurtic). Despite these differences in the distribution of the variability, the distributions of the intervals between impulses were nearly indistinguishable. These inter-impulse distributions were better fit with a hyperbolic gamma distribution than a hyperbolic normal distribution, although one might expect a better approximation for normally distributed inverse intervals. Consideration of why the inter-impulse distribution is independent of the distribution of the causative noise suggests two putative interval distributions that do not depend on the assumed noise distribution: the log normal distribution, which is predicated on the assumption that long intervals occur with the joint probability of small input values, and the random walk equation, which is the diffusion equation applied to a random walk model of the impulse generating process. Either of these equations provides a more satisfactory fit to the simulated impulse trains than the hyperbolic normal or hyperbolic gamma distributions. These equations also provide better fits to impulse trains derived from the maintained discharges of ganglion cells in the retinae of cats or goldfish. It is noted that both equations are free from the constraint that the coefficient of variation (CV) have a maximum of unity. The concluding discussion argues against the random walk equation because it embodies a constraint that is not valid, and because it implies specific parameters that may be spurious.     

Effects of varying impulse number on cotransmitter contributions to sympathetic vasoconstriction in rat tail artery

We examined the contributions of the cotransmitters norepinephrine (NE), ATP, and neuropeptide Y (NPY) to sympathetically evoked vasoconstriction in the rat tail artery in isolated vascular rings by using 1-100 stimulation impulses at 20 Hz. Phentolamine (2 microM), the alpha-adrenoceptor antagonist, markedly reduced responses to all stimuli, although responses to lower impulse numbers were reduced less than responses to longer trains. The purinergic receptor antagonist suramin (100 microM) reduced all responses, but to a much greater extent with few impulse trains. Responses were further reduced or abolished by addition of the second antagonist. Any remaining responses were abolished by the NPY-Y(1) receptor antagonist BIBP-3226 (75 nM). NPY had a direct agonist action and potentiated sympathetically mediated responses. NPY (75 nM) potentiated responses and BIBP-3226 decreased responses to 2- and 20-impulse trains. Both affected responses from 2 impulses to >20 impulses, but there was no preferential effect on purinergic contributions to responses because neurally released NPY potentiated both "pure" NE and ATP responses equally. We conclude that all three cotransmitters contribute significantly to vascular responses and their contribution varies markedly with impulse numbers. There is considerable synergy between cotransmitters, especially with lower impulse numbers where NPY contributions are greater than expected.     

The amplitude principle of the structural-functional classification of cortical neurons

Some literature and author's experimental data are presented on functional and morphological characteristics of nerve cells by their electrophysiological parameters. The proposition is substantiated that in multineuron discharges reflecting the activity of microgroups of neighbouring cells, the absolute size of impulses indicates the distance of cortical neurones from the tip of the electrode, while amplitudes correlation may characterize the size of the cell and some of its functional properties. Arguments for this proposition are: latencies of impulse responses to afferent and antidromic stimulations and amplitude changes of multineuronal activity during movement of the electrode through the cortex thickness in acute experiments. As the electrode (with the tip diameter of 50 mcm) approaches a microgroup of neurons, the amplitude of the recorded impulse discharges increases, while the relation of big spikes to small ones remains almost the same.     

Cooperative phenomena in the activity of single ion channels

Using the patch-voltage-clamp method kinetics of the fast potential-dependent K+-channels in molluscan neurones was investigated. It was found that under given experimental conditions the amplitudes of single current impulses have a wide spectrum. The amplitudes are proportional to a number of the current substates involved. Averaged fronts of the current impulses are S-shaped, and have duration greater than 1 ms. Averaged duration of the current impulses increases (from 0.25 to 30-40 ms) with the impulse amplitude (or with the number of the substates involved). There is a sharp bend of the dependence at the impulse amplitude 0.6-0.7 of maximal value. The phenomena investigated reflect, probably, cooperativity of the channel transitions between the substates. The degree of the cooperativity depends on the membrane potential value.     

Superposition of impulse activity in a rapidly-adapting afferent unit model

Mechanosensory afferent units consist of a parent axon, the peripheral axonal arborization, and the branch terminal mechanoreceptors. The present work uses a mathematical model to describe the contribution of a given number of rapidly-adapting mechanoreceptors to the impulse pattern of their parent axon. In the model impulses initiated by any driven mechanoreceptor instantaneously propagate orthodromically and antidromically. The model also incorporates the axonal absolute refractory period as well as ortho-and antidromically elicited recovery cycles. In separate computations, periodic or random (Poisson process) trains of short-duration stimuli at constant amplitude are delivered to a given number (N=2–30) of co-innervated mechanoreceptors. The superposition of component impulse trains always departs from the theoretical ideal (Poisson process). Such departures are attributable to: (i) the number of driven mechanoreceptors, when N is small, (ii) axonal absolute refractory period, during maximal amplitude stimulation, and (iii) antidromic recovery cycles as well as absolute refractoriness, during submaximal-amplitude stimulation. Computations reveal that this instantaneous reset model results in the elimination of information extracted by driven mechanoreceptors. Model predictions with Poisson stimulation at varied amplitudes are compared to G-hair afferent unit responses to analogous stimulation. Qualitatively opposite results with respect to parent axonal impulse patterns imply that the axonal arborization is not simply a substrate for impulse propagation from branch terminals to parent axon.     

耳蜗电位的持续反应与其中单个脉冲的反应

研究了豚鼠耳蜗电位中持续反应与其中的单个脉冲反应的关系。由于听学系统存在着非线性,因此仅仅知道由单个短声诱发的耳蜗电位脉冲反应还无法预测一串等间隔重复短声诱发的持续反应。然而,等间隔重复短声串中第5个以后的每个短声受前面所有短声的掩蔽都相同,诱发的反应都相同,因此持续反应的稳态部分可以由掩蔽作用达到饱和时单个短声的反应通过延时相加得到。本文在时间域和频率域上定量地证明了这点。     

Energy filters,motion uncertainty,and motion sensitive cells in the visual cortex: a mathematical analysis

Energy filters are tuned to space-time frequency orientations. In order to compute velocity it is necessary to use a collection of filters, each tuned to a different space-time frequency. Here we analyze, in a probabilistic framework, the properties of the motion uncertainty. Its lower bound, which can be explicitly computed through the Cramér-Rao inequality, will have different values depending on the filter parameters. We show for the Gabor filter that, in order to minimize the motion uncertainty, the spatial and temporal filter sizes cannot be arbitrarily chosen; they are only allowed to vary over a limited range of values such that the temporal filter bandwidth is larger than the spatial bandwidth. This property is shared by motion sensitive cells in the primary visual cortex of the cat, which are known to be direction selective and are tuned to spacetime frequency orientations. We conjecture that these cells have larger temporal bandwidth relative to their spatial bandwidth because they compute velocity with maximum efficiency, that is, with a minimum motion uncertainty.     

Localization of a sum of acoustic signals in air by the northern fur seal]

The localization of a sum of acoustic signals by two northern fur seals in air depending on sound parameters was investigated using the method of instrumental conditioned reflexes with food reinforcement. It was found that sound perception of northern fur seal proceeds by the binaural mechanism. The time/intensity interchange coefficient was 570 microseconds/dB for series of clicks (with amplitude maximum at 1 kHz) and 250 microseconds/dB for tonal impulses with a frequency of 1 kHz. With click amplitudes being equal, the number of approaches of the animal to the source of the first signal reached a 75% level at a delay of the second signal 0.07 ms (the minimum delay); with a delay of 6 ms (the maximum delay) and more, the fur seal, probably hears two separate signals. The minimum delay depended little on the duration of tonal impulses (with a frequency of 1 kHz) and was 0.3-0.7 ms; the maximum delay was 9-11 ms for tonal impulses with a duration of 3 ms and 37-40 ms with impulse duration 20 ms. The precedence effect became apparent at a greater delay for smooth fronts of impulses than for rectangular fronts.     

Multiple spike initiation zones in a neuron implicated in learning in the leech: a computational model

Sensitization of the defensive shortening reflex in the leech has been linked to a segmentally repeated tri-synaptic positive feedback loop. Serotonin from the R-cell enhances S-cell excitability, S-cell impulses cross an electrical synapse into the C-interneuron, and the C-interneuron excites the R-cell via a glutamatergic synapse. The C-interneuron has two unusual characteristics. First, impulses take longer to propagate from the S soma to the C soma than in the reverse direction. Second, impulses recorded from the electrically unexcitable C soma vary in amplitude when extracellular divalent cation concentrations are elevated, with smaller impulses failing to induce synaptic potentials in the R-cell. A compartmental, computational model was developed to test the sufficiency of multiple, independent spike initiation zones in the C-interneuron to explain these observations. The model displays asymmetric delays in impulse propagation across the S–C electrical synapse and graded impulse amplitudes in the C-interneuron in simulated high divalent cation concentrations.     

Temporally patterned pulse trains affect duration tuning characteristics of bat inferior collicular neurons

 This study examines the effect of temporally patterned pulse trains on duration tuning characteristics of inferior collicular neurons of the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. Using a 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of inferior collicular neurons determined with pulse trains of different pulse durations are described as band-pass, long-pass, short-pass, and all-pass. Each band-pass neuron discharged maximally to a specific pulse duration that was at least 50% larger than the neuron's responses to a long- and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long- or short-duration pulses that were at least 50% larger than the minimal responses. The number of impulses of an all-pass neuron never differed by more than 50%. When pulse trains were delivered at different pulse repetition rates, the number of short-pass and band-pass neurons progressively increased with increasing pulse repetition rates. The slope of the duration tuning curves also became sharper when determined with pulse trains at high pulse repetition rates. Possible mechanisms underlying these findings are discussed. Accepted: 25 August 1999