Short Interpulse Intervals Research Articles

A Pilot Study on the Perceived Urgency of Multitone and Frequency-Modulated Warning Signals

In some environments, there is a serious mismatch between the perceived (psychoacoustic) urgency of a warning and its situational urgency. This pilot study investigated effect of pulse format, pulse duration, and time between pulses on the perceived urgency of warning signals. The intent was to determine the best combination of variables and levels of variables to use in a formal study on the perceived urgency of warning signals. The results indicated that only pulse format and time between pulses were significant. Subjects rated sequential pulses as being less urgent than any other format. Signals with shorter inter-pulse intervals were rated as significantly more urgent. Pulse format and time between pulses were determined to be variables which should be used in future research.

19F-NMR spin-spin relaxation (T2) method for characterizing volatile anesthetic binding to proteins. Analysis of isoflurane binding to serum albumin.

This paper characterizes the low-affinity ligand binding interactions of a fluorinated volatile anesthetic, isoflurane (CHF2OCHClCF3), with bovine serum albumin (BSA) using 19F-NMR transverse relaxation (T2). 19F-NMR spectra of isoflurane in aqueous BSA reveal a single isoflurane trifluoromethyl resonance, indicative of rapid exchange of isoflurane between protein-bound and aqueous (free) environments. The exchange is slow enough, however, that the chemical shift difference between bound and free isoflurane (delta omega = 0.545 ppm) contributes to the observed isoflurane T2. The contribution of delta omega to T2 can be minimized by shortening the interval between 180 degrees refocusing pulses in the Carr-Purcell-Meiboom-Gill pulse sequence used to monitor T2. Analysis of the dependence of T2 on interpulse interval additionally allows determination of the T2 (6.2 ms) and the average lifetime (tau b = 187 microseconds) of bound isoflurane molecules. By use of a short interpulse interval (less than 100 microseconds), T2 measurements can readily be used to analyze equilibrium binding of isoflurane to BSA. This analysis revealed a discrete saturable binding component with a KD = 1.4 mM that was eliminated either by coincubation with oleic acid (6 mol/mol of BSA) or by conversion of BSA to its "expanded" form by titration to pH 2.5. The binding was independently characterized using a gas chromatographic partition analysis (KD = 1.4 mM, Bmax = 3-4 sites). In summary, this paper describes a method whereby T2 measurements can be used to characterize equilibrium binding of low-affinity ligands to proteins without the confounding contributions of chemical shift.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the stimulation pattern on the fatigue of single motor units in adult cats.

1. The main purpose of this study was to examine the effects of two subtly different stimulus patterns on the force developed by fast-twitch, fatiguable motor units in a cat hindlimb muscle during control (pre-fatigue) and fatiguing contractions. 2. The peak force and the force-time integral responses of nineteen high fatigue (FF) and three intermediate fatigue (FI) motor units of the tibialis posterior muscle in five deeply anaesthetized adult cats were measured at selected times during the course of a 360-s fatigue test. 3. The fatigue test involved a pseudo-random alternation of two patterns of stimulation. One pattern (regular) was composed of a train of stimuli with constant interpulse intervals, set at 1.8 x the twitch contraction time of each unit (interval range, 27-51 ms), and delivered for 500 (or 400) ms. For the total (FF + FI) motor-unit sample, the mean (+/- S.D.) stimulation frequency was 26 +/- 4 Hz (range, 19-37 Hz). The other stimulus pattern (optimized) consisted of three initial stimuli with short (10 ms) interpulse intervals, followed by a constant interpulse-interval train that was adjusted (interval range, 29-62 ms; frequency, 23 +/- 5 Hz; frequency range, 16-36 Hz) such that the total train had the same number of pulses, and the same average frequency and duration as the regular train. 4. The stimulus trains were delivered at 1 s-1 for 360 s, using three-train sequences of each pattern, randomly alternating with one another. The response of the third train in each sequence was selected for the force measurements. The force profile obtained from the fatigue test was subsequently decomposed into two profiles: one attributable to regular and one to optimized stimulation. 5. During the initial responses to the fatigue test, the optimized stimulus pattern produced significantly more force than the regular stimulus pattern. For FF units, the mean increase in peak force (141%) was significantly greater than the increase in the force-time integral (59%). 6. All motor units exhibited an initial potentiation of peak force with the regular stimulation pattern, whereas peak force declined monotonically with the optimized pattern. In contrast, the force-time integral potentiated in the first 30 s for both regular and optimized stimulus patterns. 7. Each motor unit maintained an increased force response to optimized stimulation during the fatigue test, with the greatest relative increase occurring about 120 s into the test, well after the potentiation effect had subsided.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of Estradiol on the Secretion of Oxytocin and Prostaglandin F2α during Luteolysis in the Ewe1

Twenty ewes of mixed breeds were randomly assigned in equal numbers to one of four groups in a 2 x 2 factorial design. The factors were x-irradiation to destroy ovarian follicles or sham irradiation and the administration of estradiol-containing or empty (placebo) implants. Surgery for irradiation was performed on Day 8 of the cycle. Blood samples were withdrawn from jugular catheters at 1.5-h intervals from Day 10 to Day 17. Luteolysis was not observed by Day 17 in 4 of 5 placebo-treated ewes after destruction of ovarian follicles. Luteolysis was observed in 4 of 5 ewes of the sham-irradiated, placebo-treated group and in all ewes that received estradiol whether or not ovarian follicles had been destroyed. The longest (p less than 0.07) interval between peaks of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) was observed in the x-irradiated, placebo-treated group, whereas the administration of estradiol reduced (p less than 0.01) the interval between PGFM peaks. These findings indicate that a short interpulse interval in the secretion of prostaglandin F2 alpha (PGF2 alpha) is associated with luteolysis. It is possible that the reduced interpulse interval was either an effect of estradiol that caused luteolysis or a secondary event resulting from luteolysis. The administration of estradiol decreased (p less than 0.05) the number of episodes of oxytocin secretion during luteolysis and increased (p less than 0.01) the interval between episodes.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of repetitive depolarization on sodium current in chick myocytes.

Acute effects of repetitive depolarization on the inward Na+ current (INa) of cultured embryonic chick atrial cells were studied using the whole cell patch-clamp technique. Stimulation rates of 1 Hz or greater produced a progressive decrement of peak INa. With depolarizations to 0 mV of 150-ms duration, applied at 2 Hz from a holding potential of -100 mV, the steady-state decrement was approximately 20%. The magnitude of this effect increased with stimulation frequency and with test potential depolarization and decreased with membrane hyperpolarization. Analysis of INa kinetics revealed that reactivation was sufficiently slow to preclude complete recovery from inactivation with interpulse intervals less than 1,000 ms. Moreover, reactivation accelerated markedly with membrane hyperpolarization, in parallel with the response to repetitive stimulation. The multiexponential time course of recovery of peak INa from repetitive depolarization was similar to that observed after single stimuli; however, there was a shift toward a greater proportion of current recovering with the slower of two time constants. It is concluded that incomplete recovery from inactivation is responsible for the decrement in INa observed with short interpulse intervals.

Enhancement of excitability and inhibitory processes in hippocampal dentate gyrus by noradrenaline: A pharmacological study in awake, freely moving rats

Idazoxan (IDA), an α 2 receptor antagonist which increases firing rate of noradrenergic neurons in the locus coeruleus (LC) and release of noradrenaline (NA) in target structures, was used to study the neuromodulatory effects of NA in the hippocampus in awake rats. After IDA the population spike in the dentate gyrus (DG), evoked by a single pulse to the perforant path, was greatly enhanced with no effect of the drug on excitatory postsynaptic potentials (EPSPs). Paired pulses with short interpulse intervals (25–30 ms) produced inhibition of the response to the second pulse which was increased by IDA. This drug effect was independent of its effect on the amplitude of the first spike, since the increase in inhibition was seen at stimulation intensities which did not increase the response amplitude to the initial pulse. Thus both excitability and inhibitory processes can be enhanced in the same population of neurons by an α 2 adrenoceptor antagonist.

ON-OFF units in the mustached bat inferior colliculus are selective for transients resembling ?acoustic glint? from fluttering insect targets

Of 311 single units studied in the central nucleus of the inferior colliculus (ICC) in 18 mustached bats (Pteronotus parnelli), a small but significant population (13%) of cells with on-off discharge patterns to tone bursts at best frequency (BF) was found in the dorsoposterior division. In contrast to units with the same BF's but other discharge patterns, the majority of ON-OFF units were unresponsive to sinusoidally amplitude-modulated tone bursts (SAM). To define the contribution of linear and nonlinear components to the responses of ICC neurons to amplitude modulation, we tested some of these neurons with a long, seamlessly repeating pseudorandom sequence of ternary amplitude-modulated tones at BF. Wiener-like kernels were subsequently derived from cross-correlation of spikes with acoustic events in the sequence. These kernels provided estimates of neural impulse responses that proved unusual in SAM-unresponsive ON-OFF units. First, their estimated impulse response had no linear component. Second, the predicted second-order impulse responses to both increments and decrements in stimulus intensity were long (about 20 ms) and nearly identical in shape: triphasic, with the positive phase bounded by leading and trailing negative periods. The similar shape of responses to increments and decrements in these neurons suggests a full-wave rectifier. The triphasic, initially negative second-order prediction of the impulse response accounted for an unusual result in experiments measuring the recovery cycle of ON-OFF units using a pair of identical stimulus pulses separated by various time delays. This recovery cycle can be related to their response to amplitude modulation. As the delay between two brief, near-threshold BF tone bursts decreased, the response to the first tone diminished, rather than to the second. The second-order prediction of this experiment derived from impulse responses obtained with pseudorandom noise suggests that, at short interpulse intervals, the initial negative phase of the response to the later stimulus cancels the positive phase of the response to the first. Such cancellation at short interpulse intervals may help explain why the majority of ON-OFF units are unresponsive to SAM. The unusual properties of these ON-OFF units make them ideally suited to respond selectively to infrequent acoustic transients superimposed on an ongoing background of modulation. Such patterns are commonly encountered by mustached bats foraging in cluttered habitats for small, fluttering insects, which generate "acoustic glints" upon a background of modulated echoes from the surroundings (Schnitzler et al. 1983; Henson et al. 1987).

Differential block of sodium and calcium channels by chlorpromazine in mouse neuroblastoma cells.

1. The effects of chlorpromazine on the voltage-activated sodium and type I (transient) calcium channels were studied in cultured mouse neuroblastoma cells (N1E-115) using the whole-cell patch-clamp technique. 2. Chlorpromazine (2-10 x 10(-6) M) blocked both the sodium channel current and the calcium channel current as carried by Ba2+ in a reversible and dose-dependent manner. 3. The block was not associated with any change in the time course of the activation and inactivation of the sodium and calcium channel currents. 4. The dose-response relationships for the block of these channels measured with a holding potential of -120 mV indicated a 1:1 binding stoichiometry with apparent dissociation constants of 2.5 +/- 10(-6) M and 1.5 +/- 10(-5) M for the sodium and calcium channels, respectively. 5. The block was dependent on the holding potential for both channel currents. The apparent dissociation constant for the sodium channel was decreased to 0.65 +/- 10(-6) M when the membrane was held at -80 mV. The apparent dissociation constant for the calcium channel was decreased to 3.2 +/- 10(-6) M when the membrane was held at -60 mV. 6. The steady-state inactivation curve for the sodium channel was shifted by 12.4 +/- 1.8 mV to more negative potentials by exposure to 1 x 10(-6) M-chlorpromazine. The inactivation curve for the calcium channel was also shifted by 15.4 +/- 3.2 mV to more negative potentials by exposure to 1 x 10(-5) M-chlorpromazine. These results indicate a greater affinity of chlorpromazine for the inactivated state of the channels than for the resting state. 7. Chlorpromazine caused a marked use-dependent block of the sodium channel current, as demonstrated by a cumulative increase of the block during a train of depolarizing pulses. The use-dependent block was observed even with an interpulse interval as long as 2 s. 8. On the other hand, the block of the calcium channel current did not notably accumulate during a train of depolarizing pulses even when extremely prolonged (1 s) pulses were applied at a very short interpulse interval (200 ms). 9. The marked use dependence of the sodium channel block was due to a very slow repriming of the drug-bound sodium channels from inactivation, whereas the lack of use dependence of the calcium channel block was due to a rapid repriming of the drug-bound calcium channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Conduction along myelinated and demyelinated nerve fibres during the recovery cycle: Model investigations

The membrane excitability changes as well as the underlying mechanisms of these changes in a normal and in a systematically paranodally demyelinated nerve fibre have been investigated by paired stimulation during the first 30 ms of the recovery cycle. The ionic current kinetics determining the observed changes in the action potential parameters are presented also. The simulation of the conduction in the normal fibre is based on the Frankenhaeuser and Huxley (1964) and Goldman and Albus (1968) equations, while in the case of a demyelinated fibre according to the same equations modified by Stephanova (1988a). It has been shown for the demyelinated membrane that increased demyelination increases both the threshold current for the second potential as well as the absolute refractory period. With increasing interpulse interval, the subnormality of the membrane excitability is followed by supernormality in the case of the demyelinated membrane. For the recovery cycle of 30 ms under consideration no supernormality of the normal membrane excitability is obtained. With interpulse interval from 8.8 to 10.9 ms, the highest degree of demyelination (l = 30 microns) is accompanied by a refractory period of transmission. The membrane properties of the normal and demyelinated fibres recover 20 ms after the first pulse. For short interpulse intervals, the amplitude of the second action potential is decreased, and a slower propagation velocity is obtained. The most sensitive phenomenon is the excitability of the demyelinated membrane, which remains unrecovered 30 ms after the first pulses has been applied.

Paired-pulse facilitation and inhibition in the dentate gyrus is dependent on behavioral state

It is well established that neuronal transmission from the entorhinal cortex through the dentate gyrus via the perforant path is dependent on behavioral state. To further study the modulation of neuronal transmission by behavioral state we employed the paired-pulse technique to study interneuronally-mediated inhibition and short-term facilitation in the dentate gyrus of freely-moving rat preparations. Precisely timed double pulses of electrical stimulation were delivered to the perforant path in the chronically implanted rat preparation during each of four well-defined behavioral states: slow-wave sleep (SWS), REM sleep (REM), immobile waking (IW) or active waking with voluntary movements (AW). Evoked field potentials were recorded in the dentate gyrus and analyzed to measure the population spike amplitude which represents the total number of dentate granule cells firing in synchronous response to perforant path stimulation. The paired-pulse index (PPI) was used as a measure of the net short-term facilitation or interneuronally-mediated inhibition effective at the time of the paired-pulse test and is computed by dividing the amplitude of the second population spike (p2) by the amplitude of the first population spike (p1). During the course of this study 3754 paired-pulse tests were performed in 9 rat preparations. The three interpulse interval (IPI) values used in these studies were 25, 30 and 35 ms. The results showed that the PPI was greater during AW and REM as compared to SWS and IW. The PPI was significantly greater during AW than during SWS and IW regardless of p1 amplitude or IPI value. The PPI was significantly greater during AW than during REM under most conditions except those corresponding to low p1 amplitude and long IPI. The PPIs measured during REM were significantly greater than those measured during SWS and IW at short IPIs (25 and 30 ms) but not at an IPI of 35 ms. These results indicate that short-term facilitation is the dominant response during AW especially when observed using an IPI of 35 ms. In contrast, interneuronally-mediated inhibition was observed to be dominant during SWS and IW. The net effect during REM was observed to lie between these two extremes using an IPI of 25 ms and tended toward short-term facilitation at longer IPIs of 30 and 35 ms. Septal disinhibition of dentate granule cells is proposed as the mechanism for this effect. The behavioral state modulation of neuronal transmission through the dentate gyrus is discussed in terms of this hypothesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular neurophysiological analysis reveals alterations in excitation in striatal neurons in aged rats

Intracellular recordings were used to characterize the physiological changes underlying decreases in excitation observed in striatal neurons during the aging process. Rats were divided into 3 age groups: young (3–5 months), middle-aged (10–12 months) and aged (>24 months). All experiments were performed in urethane-anesthetized rats. Recordings were obtained from 33 neurons in young, 17 in middle-aged and 20 in aged rats. When identified by intracellular injections of Lucifer yellow all recorded neurons were medium-sized spiny cells. Resting membrane potentials were at least −40 mV and action potentials greater than 35 mV. Postsynaptic responses were evoked by stimulation of frontal cortex. In all recorded neurons, regardless of age, excitatory postsynaptic potentials (EPSPs) could be evoked. However, the threshold currents for eliciting both EPSPs and synaptically driven action potentials were significantly higher in neurons obtained from aged rats than those recorded in the other two groups. Other changes in excitation in aged striatal neurons consisted of absence of spontaneously occurring EPSPs, higher current to induce firing by intracellular injections of depolarizing current and an inability of orthodromically induced action potentials to follow paired stimulation pulses to the cortex at short interpulse intervals. These data were interpreted to indicate that a combination of changes in synaptic connectivity and in membrane properties underlie the decreases in excitation. Together with our previous findings obtained from aged cats these results indicate that decreased neuronal excitability is a major effect of aging in the striatum.

Tetanic responses of electrically stimulated paralyzed muscle at varying interpulse intervals.

The influence of stimulus interpulse interval (IPI) on torque output during electrically-evoked contractions was investigated for the knee extensor muscles of paralyzed subjects. The parameters measured were the rise time, magnitude, and relaxation time of the contraction at stimulus IPI's ranging from 62 to 7 ms. Torque output increased as IPI's were decreased from 62 to 15 ms. Peak torques were recorded at IPI's of 12-15 ms; IPI's less than these resulted in an insignificant loss of torque. Rise times decreased as IPI's were decreased. Relaxation time generally increased as IPI's were decreased with the longest relaxation times occurring with stimulation at an IPI of 12 ms. To demonstrate the influence of IPI on muscle fatigue, the effect of prolonged stimulation at short (12 ms) and long (50 ms) IPI's was also compared. After 30 s of stimulation with an IPI of 12 ms, mean torque had declined to 5 +/- 3 percent and after 30 s of stimulation with an IPI of 50 ms, mean torque had declined to 82 +/- 4 percent of the initial value. Knowledge of how stimulus IPI influences the response of paralyzed muscle to electrical stimulation may assist in the development of rehabilitation devices which utilize these technologies.

Inhibitory processes in normal and epileptic-like rat hippocampal slices: the role of GABA B receptors

The purpose of this study was to obtain more insight into the role of GABA B receptors in the rat hippocampus using the selective GABA B blocker phaclofen. In the CA1 region in normal slices, phaclofen (1 mM) had no effect on population spike amplitude. Moreover, phaclofen did not affect paired-pulse inhibition when the paired-pulses were delivered at short interpulse intervals (20–30 ms). At longer interpulse intervals (50–150 ms) phaclofen moderately reduced paired-pulse inhibition. In slices treated with submaximal concentrations of bicuculline methiodide or penicillin, the frequency and amplitude of electrically induced multiple population spikes were not affected by phaclofen. The GABA B agonist baclofen, however, attenuated paired-pulse inhibition and this effect was antagonized by phaclofen. In the CA3 region, phaclofen reduced the inhibitory effect of baclofen on penicillin-induced burst discharges. In conclusion, although phaclofen antagonized the effects of baclofen it did not show proconvulsant activity. Furthermore, phaclofen did not induce any pronounced signs of disinhibition of hippocampal pyramidal cells as is usually seen with the GABA A receptor-blocker bicuculline methiodide.

The echolocation and hunting behavior of Daubenton's bat, Myotis daubentoni

The echolocation and hunting behavior of Daubenton's bat (Myotis daubentoni) were studied in the field under completely natural conditions using a multiflash photographic system synchronized with high-speed tape recordings. The hunting behavior of M. daubentoni is separated into four stages. In the search flight stage Daubenton's bat flies with an average speed of 3.4±0.6 m/s SD usually within 30 cm over water surfaces searching for insects. After the detection of potential prey, the approach flight stage occurs, during which the bat approaches the target in a goal-directed flight. The stage tail down indicates that M. daubentoni is close to the potential prey (approximately 10–22 cm) and is preparing for the catch. The insects are caught with the interfemoral membrane, the feet, and sometimes with the additional aid of a wing. In the stage head down, the bat seizes the prey during flight. Immediately afterwards, Daubenton's bat returns to search flight. M. daubentoni shows the typical echolocation behavior of a vespertilionid bat, emitting frequency-modulated (FM) echolocation signals. The three behavioral stages search, approach, and terminal phase (Griffin et al. 1960) are used to describe the pulse pattern of foraging M. daubentoni in the field. The terminal phase (or buzz) of Daubenton's bat is separated into two parts: buzz I and buzz II. Buzz II is distinguished from buzz I by the following characteristics: a sharp drop in terminal frequency, a distinct reduction in the bandwidth of the first harmonic, a continuous high repetition rate throughout the phase in the range 155–210 Hz, very short pulses (0,25–0.3 ms) and interpulse intervals (4.5–5.0 ms) at the end of the phase, and a distinct decrease in duty cycle. A pause in echolocation separates the end of the terminal phase from the ongoing search phase. The reduction in sound duration after the detection of a target and during pursuits with successfull or attempted catches is discussed in relation to the actual distance of the bat to the target at each stage. It is likely that Daubenton's bat reduces sound duration during approach and terminal phase in order to prevent an overlap of an outgoing pulse with the returning echo from the target. It is argued that the minimum detection distance can be estimated from the sound duration during search flight. Estimates of detection and reaction distances of M. daubentoni based upon synchronized photos and echolocation sequences are given to corroborate this hypothesis. An average detection distance of 128 cm and an average reaction distance of 112 cm were determined. Each behavioral stage of foraging M. daubentoni is characterized by a distinct pattern of echolocation signals and a distinct stage in hunting behavior. The approach flight in hunting behavior coincides with the approach phase and with buzz I in echolocation behavior. The stage tail down corresponds to buzz II. The stage head down is correlated with a pause in echolocation. Immediately afterwards, the bat returns into search flight and into the search phase, emitting search signals.

THE GENETIC BASIS OF EVOLUTION OF THE MALE COURTSHIP SOUNDS IN THE DROSOPHILA VIRILIS GROUP.

When courting, males of the Drosophila virilis group vibrate their wings and emit species-specific courtship sounds consisting of trains of polycyclic sound pulses. To analyze the genetic basis of evolutionary changes in the sounds we made an F1 diallel set of reciprocal crosses between the members of the virilis phylad of the group (two stocks of D. virilis and one of D. americana americana, D. a. texana, D. novamexicana, and D. lummei). We also crossed the D. virilis stocks with the members of the montana phylad of the same group (D. kanekoi, D. littoralis, D. borealis, D. flavomontana, D. lacicola, and D. montana) and made a backcross (D. virilis x D. littoralis) x D. virilis using a D. virilis marker stock (b; sv t tb gp; cd; pe). The sounds of the hybrids were analyzed using the following parameters: the length of a pulse train (PTL), the number of pulses in a train (PN), the interpulse interval (IPI), the length of a pulse (PL), the number of cycles in a pulse (CN), and the length of a cycle (CL). In the virilis phylad, the differences between species appeared to be determined mainly by autosomal genes in each sound trait. The heritabilities (narrow-/broad-sense) obtained from the diallel tables were the following: PTL 0.662/0.817, PN 0.651/0.841, IPI 0.193/0.546, PL 0.408/0.552, CN 0.425/0.719, and CL 0.361/0.764. The direction of dominance is for longer PTL, higher PN and CN, and shorter IPI and CL. PL shows ambidirectional dominance. In the sounds of the virilis phylad species, PTL and PL seem to be phenotypically the most important parameters, since their components (PN and IPI for PTL, CN and CL for PL) are negatively correlated. In crosses between D. virilis and D. littoralis or D. flavomontana reciprocal hybrids differed from each other in PTL, IPI, PL, and CN indicating X-chromosomal or cytoplasmic inheritance. In the backcrosses between D. virilis and D. littoralis the role of the X chromosome was ascertained to be decisive. We conclude that an X-chromosomal major change allowing variation in IPI has occurred during the separation of the two D. virilis group phylads, the long IPI allowing variation also in PL (and CN). The evolution of the sounds in the virilis phylad has probably gone towards longer and denser pulse trains, while in the montana phylad the sounds have evolved in different directions.

Fluttering target detection in Hipposiderid bats

Two species of Hipposiderid bats,Hiposideros speoris andH. lankadiva, which both emit short CF-FM echolocation calls, were trained in a two-alternative forced-choice procedure to discriminate between an oscillating target and a motionless one. Two different targets were used: (1) the membrane of a low-frequency loudspeaker, producing sinusoidal frequency- and amplitude modulations and (2) a small rotating propeller, which produced short acoustical ‘glints’. In both casesH. lankadiva learned to discriminate between the oscillating and the motionless target. When the loudspeaker was used, thresholds for minimal modulation depths at different oscillation frequencies were determined. At loudspeaker membrane oscillation frequencies of 10 to 100 Hz the 75% correct thresholds lay between 90 and 300 Hz (Fig. 3).H. speoris could not be trained to react to the moving membrane, even at very high oscillation amplitudes. When the rotating propeller was the positive target, however,H. speoris learned very quickly to discriminate it from a motionless one. By decreasing the rotation speed it was possible to measure the minimal detectable glint-frequency for each bat. It lay at 67, 44, and just under 15 glints/s for the three specimens (Fig. 4). During the discrimination task both bat species increased their duty-cycle just prior to a decision by emitting long sequences of echolocation calls with short inter-pulse intervals. The duration of individual pulses remained relatively constant (Figs. 6 and 7). Possible mechanisms for discrimination of the oscillating targets are discussed and the importance of ‘glints’ in the echoes for fluttering target detection is emphasized.

Prenatal exposure to alcohol alters short-term plasticity in hippocampus

Paired-pulse potentiation was studied in the CA1 region of rats exposed prenatally to alcohol. Pregnant rats were fed a liquid diet containing 35% ethanol-derived calories on days 3 to 21 of gestation. Control animals were fed liquid diets without ethanol. When the pups of both groups were 40 to 60 days old, hippocampal slices were prepared and maintained in vitro. Paired-pulse potentiation was examined at intervals from 5 to 400 ms. Recordings made from these slices showed that the response inhibition typically seen at short interpulse intervals was minimal or absent in the animals with prenatal exposure to alcohol. Potentiation of population spike amplitude was enhanced at intervals to 100 ms.

Nonlinear summation of contractions in striated muscle. II. Potentiation of intracellular Ca2+ movements in single barnacle muscle fibres

Nonlinear summation of contractions is studied in single barnacle (Balanus nubilus) muscle fibres, loaded with the photoprotein aequorin. The results indicate that nonlinear summation of aequorin transients is indeed present and for short interpulse intervals (25-250 ms), a more-than-linear summation of transients, which suggest an increase of the cytosolic Ca2+ concentration in the second response, is observed. This augmented Ca2+ concentration is not merely due to summation with the preceding conditioning transient, but to an enlargement of the second transient in its own right. Furthermore, the enlargement of the second Ca2+ response is not the result of prolonged release, or slowing of re-uptake by intracellular organelles. On the contrary, Ca2+ release is found to be enhanced and for short depolarizations (20 ms), its time to half re-uptake is reduced. The intensified Ca2+ release, triggered by the second standard depolarization, is related to the level of cytosolic Ca2+ concentration reached in the conditioning response and, for example, appears to be larger in the presence of Dantrolene-sodium, which is known to reduce Ca2+ movements in a single twitch. It is concluded that contractile potentiation observed during nonlinear summation of contractions, is associated with a potentiation of intracellular Ca2+ movements, which interact to regulate the cytosolic Ca2+ concentration during contraction.

Paired impulses in lobster claw motoneurons: in vitro and in vivo production

The somata of the claw opener excitor (OE), the slow closer excitor (SCE), and the fast closer excitor (FCE) motoneurons were physiologically identified in the lobster, Homarus americanus, using an isolated claw ganglion preparation. Depolarization of the soma produced paired impulses with an interpulse interval of 10–12 ms, which demonstrates that doublet production is an intrinsic motoneuron property. Similar paired impulses were found for the FCE axon in intact lobsters where they occurred with a shorter interpulse interval for the cutter than for the crusher claw.

Inactivation of voltage-gated delayed potassium current in molluscan neurons. A kinetic model

Voltage-gated delayed potassium current in molluscan neurons is characterized by a marked inactivation. Inactivation can accumulate between repetitive pulses, giving rise to current patterns in which the maximum current during a second voltage pulse is less than the current at the end of the preceding pulse (cumulative inactivation). Other features of inactivation of this current include an onset time-course that can be characterized by the sum of two exponential processes and an early minimum in the recovery-vs.-time curve. A simple four-state model is developed that can, when supplied with rate constants derived from voltage-clamp experiments, reproduce these features of inactivation. The model incorporates state-dependent inactivation rates. Upon depolarization, both open and closed channels can be inactivated, although inactivation of closed channels is much faster. Upon repolarization, recovery from inactivated states is sufficiently slow that little recovery occurs during a short interpulse interval. Cumulative inactivation comes about as a result of fast inactivation during the second pulse, further limiting the peak current from the level at the end of the previous pulse.