One reason for increased seizure susceptibility of hippocampus compared with neocortex

In this study we compared the membrane resting potential and action potential (AP) activation thresholds of neocortical layer 2/3 and CA1 hippocampal pyramidal cells in brain slices from 6–8-day old mice. The activation threshold was −37 ± 2 mV in the neocortical pyramids (5 cells), and −50 ± 1 mV in the CA1 ones (5 cells). The observed difference in the AP activation thresholds may account for a higher excitability of hippocampus as compared to neocortex. The article is submitted by the author in English.     

Regulation of the phonotactic threshold of the female cricket, Acheta domesticus : juvenile hormone III, allatectomy, L1 auditory neuron thresholds and environmental factors

Juvenile hormone III (JHIII), when applied to the abdomen of 1-day-old female Acheta domesticus (in quantities that would create JHIII titers in the hemolymph that were within the range measured in females of this species) caused a significant decrease in phonotactic thresholds (Fig. 1). Removal of the corpora allata from 5-day-old females with low phonotactic thresholds caused significantly increased phonotactic thresholds 2–5 days later. After a temporary increase (24 h) of, on average, about 25 dB, the phonotactic thresholds drop to about 10 dB above preallatectomy levels (Fig. 2), but remain significantly higher than controls. Application of JHIII to allatectomized females, with a mean increase in thresholds of 20 dB, results in significantly decreased thresholds (mean of about 20 dB) over the next 6 h (Fig. 3). Exposure to males 1 week before the imaginal molt causes the phonotactic thresholds of postimaginal females to drop 1–2 days significantly earlier than controls (Fig. 4). One- and 3-day-old females, phonotactically tested only once, exhibit lower thresholds in the early morning than they do in the late afternoon (Fig. 5). Five-day-old females do not exhibit such a diurnal rhythm. Phonotactically testing females more than once a day significantly influences their phonotactic thresholds (Figs. 6, 7). In 1-day-old females, with high (above 70 dB) phonotactic thresholds, the threshold of their L1 auditory interneurons can be 30 dB or more below their phonotactic threshold (Fig. 8). In females with phonotactic thresholds of 70 dB or lower, the L1 threshold is within 10 dB of their phonotactic threshold. Both JHIII and allatectomy influence phonotactic and L1 thresholds in a similar manner. Accepted: 29 September 1997     

Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef

 Experimental studies of the upper thermal limits of corals from Orpheus Island, an inshore reef in the central Great Barrier Reef, show that Acropora formosa has a 5-day 50%-bleaching threshold of between 31 and 32 °C in summer, only 2 to 3 °C higher than local mean summer temperatures (29 °C). Summer bleaching thresholds for Pocillopora damicornis and A. elseyi were 1 °C higher (between 32 and 33 °C). The winter bleaching threshold of Pocillopora damicornis was 1 °C lower than its summer threshold, indicating that seasonal acclimatisation may take place. This seasonal difference raises the possibility that at least some corals may be capable of short-term thermal acclimatisation. Neither P. damicornis nor A. elseyi showed habitat-specific (reef flat versus reef slope) differences in bleaching thresholds. Further, colonies of P. damicornis collected from sites 3 km apart also showed no difference in bleaching threshold despite populations of this species responding differently at these two sites during a natural bleaching event. The bleaching thresholds determined in this study are best considered as the maximum tolerable temperatures for local populations of these species because they were determined in the absence of additional stressors (e.g. high light) which often co-occur during natural bleaching events. We consider the 5-day 50% bleaching thresholds determined in these experiments to be fair indicators of upper thermal limits, because >50% of a sample population died when allowed to recover in situ. We found a delay of up to a month in the bleaching response of corals following thermal stress, a result that has implications for identifying the timing of stressful conditions in natural bleaching events. Accepted: 26 May 1999     

Analysis of the jamming avoidance response in the electric fish Gymnotus carapo

The electric fish Gymnotus carapo emits brief (4 ms) electric pulses separated by much longer intervals of high regularity (coefficient of variation 0.01–0.02). Two main changes in the firing patterns of electric organ discharge were observed when two fish were placed together. (1) All fish pairs showed an increase in the frequency difference between the two fish, in comparison with the value observed in isolated fish, prior to the interaction. This change increased the number of beats per second between both discharge trains, i.e., the number of times per second that the higher rate discharge sweeps the lower rate one when displayed on an oscilloscope. (2) When the sweeping velocity fell below 2–3 sweeps/s, transient frequency increases were also observed in the electric organ discharge of the higher rate fish when they were about to discharge simultaneously. The contribution to jamming avoidance of these two changes was analyzed by comparing recordings of behavioral interactions with simulations produced by a computational model. The jamming effect of the firing of a conspecific located in the same tank was evaluated by counting the number of coincidences between both trains (occurrence of discharges of the two fish within 2 ms of one another). The number of coincidences was evaluated as a function of the sweeping velocity in both simulations (with and without transient frequency increases) and real fish. As the sweeping velocity increased, single coincidences increased slightly in simulations without transient frequency increases, whereas the successive coincidences (coincidences repeated in successive discharges) decreased abruptly. The simulation including transient frequency increases eliminated the successive coincidences and decreased the single ones. Only when the sweeping velocity was less than 2–3 sweeps/s, did transient frequency increases improve the coincidence-avoiding performance of the simulation. The number of coincidences observed in natural behavioral interactions for the different sweeping velocities coincided with the distributions obtained with the simulations. As successive coincidences are known to be more detrimental for electrolocation than single ones, the increase in the sweeping velocity may be considered a jamming avoidance strategy in Gymnotus carapo, in addition to the already described transient frequency increases. Received: 2 June 1998 / Accepted in revised form: 18 November 1998     

Analysis of the processes of summation of postsynaptic potentials on the membrane of motoneurons upon realization of the stretch reflex

Stretching of the m. triceps surae (TS) of decerebrated cats evokes reflex shifts of the membrane potential (MP) in spinal motoneurons resulting from summation of EPSPs. We carried out model analysis of summation of a great number of EPSPs and compared the respective results with changes in the MP observed in real experiments using intrasomatic microelectrode recording. Simulation was based on the supposition of the proximity of the time course of an excitatory postsynaptic current to the positive part of the EPSP derivative. Transformation of EPSPs was performed using low-frequency filtration with two values of the time constant, 7 and 20 msec, (models M1 and M2, respectively). The models obtained provided sufficiently adequate reflection of the ascending phase of the real EPSP but inadequately reflected the rate of its decline and slow components of the MP changes. The disagreement of simulations with the real MP shifts shows that, most probably, final postsynaptic effects are to a considerable extent provided by summation of a great number of EPSPs generated in distal dendrites, and EPSPs immediately recorded in trasomatically cannot provide one with adequate information on the entire pattern of natural synaptic activation of the neuron. In addition, simulation analysis demonstrated a high probability of the contribution of active inhibitory processes to the formation of resulting MP changes under conditions of the stretch reflex. Neirofiziologiya/Neurophysiology, Vol. 40, No. 3, pp. 260–263, May–June, 2008.     

In Planta Transformation of Tomato

In this study, in planta transformation of tomato (Solanum lycopersicum L.), using fruit injection and floral dip, is reported. Agrobacterium tumefaciens strain EHA 105 containing one of three constructs, i.e., pROKIIAP1GUSint (carrying the Apetala 1 [AP1] gene), pROKIILFYGUSint (carrying the LEAFY [LFY] gene), or p35SGUSint (carrying the β-glucuronidase [GUS] gene), was used for plant transformation. For fruit injection transformation, no significant effects (p > 0.05) of the construct used were observed. The highest frequency of transformation was obtained following 48-h incubation of tomato fruit with bacterial cells harboring either one of the three constructs; transformation frequencies of 17%, 19%, and 21% for AP1, LFY, and GUS gene constructs, respectively, were obtained. When fruit maturity was evaluated in fruit injection experiments, mature red fruit resulted in higher frequency of transformants than immature green fruit with 40%, 35%, and 42% for AP1, LFY, and GUS gene constructs, respectively. For floral dip transformation, a higher number of transformants was obtained when the GUS gene construct was used instead of either the AP1 or LFY gene construct, thus suggesting a possible inhibitory effect of the flowering genes used. When flowers were transformed prior to rather than following pollination, they yielded a higher transformation frequency, 12% for the LFY construct and 23% for the GUS construct (p < 0.05), although no transformant was obtained with the AP1 gene construct. All putative GUS-positive transformants were analyzed using polymerase chain reaction and confirmed for the presence of the transgene. Compared to control plants, transgenic plants carrying either the AP1 or LFY transgene flowered earlier and showed several different morphological characters.     

Mechanisms for lateral turns in lamprey in response to descending unilateral commands: a modeling study

 Straight locomotion in the lamprey is, at the segmental level, characterized by alternating bursts of motor activity with equal duration and spike frequency on the left and the right sides of the body. Lateral turns are characterized by three main changes in this pattern: (1) in the turn cycle, the spike frequency, burst duration, and burst proportion (burst duration/cycle duration) increase on the turning side; (2) the cycle duration increases in both the turn cycle and the succeeding cycle; and (3) in the cycle succeeding the turn cycle, the burst duration increases on the non-turning side (rebound). We investigated mechanisms for the generation of turns in single-segment models of the lamprey locomotor spinal network. Activation of crossing inhibitory neurons proved a sufficient mechanism to explain all three changes in the locomotor rhythm during a fictive turn. Increased activation of these cells inhibits the activity of the opposite side during the prolonged burst of the turn cycle, and slows down the locomotor rhythm. Secondly, this activation of the crossing inhibitory neurons is accompanied by an increased calcium influx into the cells. This gives a suppressed activity on the turning side and a contralateral rebound after the turn, through activation of calcium-dependent potassium channels. Received: 28 June 2000 / Accepted for publication: 10 May 2001     

Absolute hearing thresholds and critical masking ratios in the European barn owl: a comparison with other owls

Absolute thresholds and critical masking ratios were determined behaviorally for the European barn owl (Tyto alba guttata). It shows an excellent sensitivity throughout its hearing range with a minimum threshold of −14.2 dB sound pressure level at 6.3 kHz, which is similar to the sensitivity found in the American barn owl (Tyto alba pratincola) and some other owls. Both the European and the American barn owl have a high upper-frequency limit of hearing exceeding that in other bird species. Critical masking ratios, that can provide an estimate for the frequency selectivity in the barn owl's hearing system, were determined with a noise of about 0 dB spectrum level. They increased from 19.1 dB at 2 kHz to 29.2 dB at 8 kHz at a rate of 5.1 dB per octave. The corresponding critical ratio bandwidths were 81, 218, 562 and 831 Hz for test-tone frequencies of 2, 4, 6.3 and 8 kHz, respectively. These values indicate, contrary to expectations based on the spatial representation of frequencies on the basilar papilla, increasing bandwidths of auditory filters in the region of the barn owl's auditory fovea. This increase, however, correlates with the increase in the bandwidths of tuning curves in the barn owl's auditory fovea. Accepted: 27 November 1997     

Membrane Potential Oscillations Produced by Excitatory Amino Acids in Isolated Spinal Neurons of the Lamprey Lampetra fluviatilis

Membrane potential (MP) oscillations produced by excitatory amino acids (EAA) have been studied in branching neurons isolated by an enzymatic-mechanical method from the lamprey spinal cord. It was shown that (1) all studied EAA (glutamate, kainate, NMDA, aspartate, and quisqualate) evoke an ion current and a short-term reversible depolarization in studied cells; (2) EAA added to perfusion solution may produce MP oscillations, with kinetic parameters and duration of the oscillation depending on the amino acid used (the most effective are kainate and NMDA, the least effective, quisqualate); (3) oscillations can be irregular (of the type of a synaptic noise or of a long-term plateau of depolarization with action potentials—AP) or regular, with frequency of 0.5–1.5 Hz. Amplitude of both oscillation types depends on MP level, frequency is more steady for each cell and less depends on MP. In 68 out of 128 studied cells, oscillations could be evoked, which indicates that a significant part of lamprey spinal neurons have intrinsic capability for MP oscillations and probably pacemaker properties. The functional role of oscillations can be different. They can take cells out from the profound inhibition state, synchronize activity of rhythm generation neurons and/or be the base for trigger signals (AP firing) sent by locomotor neuronal circuits to trunk muscles.     

The effect of severe eccentric exercise-induced muscle damage on plasma elastase, glutamine and zinc concentrations

The aim of this study was to determine if severe exercise-induced muscle damage alters the plasma concentrations of glutamine and zinc. Changes in plasma concentrations of glutamine, zinc and polymorphonuclear elastase (an index of phagocytic cell activation) were examined for up to 10 days following eccentric exercise of the knee extensors of one leg in eight untrained subjects. The exercise bout consisted of 20 repetitions of electrically stimulated eccentric muscle actions on an isokinetic dynamometer. Subjects experienced severe muscle soreness and large increases in plasma creatine kinase activity indicative of muscle fibre damage. Peak soreness occurred at 2 days post-exercise and peak creatine kinase activity [21714 (6416) U · l⁻¹, mean (SEM)] occurred at 3 days post-exercise (P < 0.01 compared with pre-exercise). Plasma elastase concentration was increased at 3 days post-exercise compared with pre-exercise (P < 0.05), and is presumably indicative of ongoing phagocytic leucocyte infiltration and activation in the damaged muscles. There were no significant changes in plasma zinc and glutamine concentrations in the days following eccentric exercise. We conclude that exercise-induced muscle damage does not produce changes in plasma glutamine or zinc concentrations despite evidence of phagocytic neutrophil activation. Accepted: 3 November 1997     

Biomechanical properties and a kinetic simulation model of the smooth muscle I2 in the buccal mass of Aplysia

The muscle I2 is a smooth muscle from the buccal mass of the marine mollusc Aplysia californica whose neural control, in vivo kinematics, and behavioral role have been extensively analyzed. In this study, we measured the activation and contractile dynamics of the muscle in order to construct a Hill-type kinetic model of the muscle. This is the first study to our knowledge, of Aplysia muscle contractile dynamics. The isometric force-frequency relationship of I2 had a frequency threshold of about 6–8 Hz, and its force output saturated at 20–25 Hz, properties that match the high frequency (20 Hz) bursts generated by the B31/B32 neurons that innervate it. Peak isometric force was generated at about 118% of the in situ relaxed length. These results and I2's estimated in vivo kinematics suggest that it generates maximum force at the onset of protraction. The muscle tension during iso-velocity lengthening and shortening was an asymmetric function of velocity. Short range stiffness and yielding responses were observed in lengthening, whereas muscle tension decreased smoothly in shortening. These visco-elastic properties suggest that the I2 muscle can serve to brake forceful retraction movements. A Hill-type model, parameterized from the measurements, captured many of the mechanical properties of I2. Our results provide a quantitative understanding of the biomechanical significance of the muscle's neural control and provide a basis for simulation studies of the control of feeding behavior. Received: 5 February 1999 / Accepted in revised form: 18 May 1999     

Intense cold and mortality in Castile-La Mancha (Spain): study of mortality trigger thresholds from 1975 to 2003

Studies on temperature–mortality time trends especially address heat, so that any contribution on the subject of cold is necessarily of interest. This study describes the modification of the lagged effects of cold on mortality in Castile-La Mancha from 1975 to 2003, with the novelty of also approaching this aspect in terms of mortality trigger thresholds. Cross-correlation functions (CCFs) were thus established with 15 lags, after application of ARIMA models to the mortality data and minimum daily temperatures (from November to March), and the results for the periods 1975–1984, 1985–1994 and 1995–2003 were then compared. In addition, daily mortality residuals for the periods 1975–1989 and 1990–2003 were related to minimum temperatures grouped in 2°C intervals, with a cold threshold temperature being obtained in cases where such residuals increased significantly (p < 0.05) with respect to the mean for the study period. A cold-related mortality trigger threshold of −3°C was obtained for Ciudad Real for the period 1990–2003. The significant number of lags (p < 0.05) in the CCFs declined every 10 years in Toledo (5–2–0), Cuenca (4–2–0), Albacete (4–3–0) and Ciudad Real (3–2–1). This meant that, while the trend in cold-related mortality trigger thresholds in the region could not be ascertained, it was possible to establish a reduction in the lagged effects of cold on mortality, attributable to the improvement in socio-economic conditions over the study period. Evidence was shown of the effects of cold on mortality, a finding that renders the adoption of preventive measures advisable in any case where intense cold is forecast.     

Evidence for different nonlinear summation schemes for lines and gratings at threshold

Within a wide class of multichannel models of the visual system it is suggested that spatial distributions of luminance are processed by the independent activation of grating detectors, or spatial frequency channels. Probability summation is often described in terms of Quick's nonlinear pooling model [Quick RF (1974) Kybernetik 16:65–67]. Using this model, we find evidence for the existence of different kinds of nonlinear summation at threshold; for compound gratings with well-separated spatial frequency components, the threshold functions indicate nonlinear summation which is not compatible with probability summation, while for line patterns well separated in the spatial domain the probability summation rule proves compatible with the data. Received: 24 June 1998 / Accepted in revised form: 16 March 1999     

A model for steady isometric muscle activation

The present model of the motoneuronal (MN) pool – muscle complex (MNPMC) is deterministic and designed for steady isometric muscle activation. Time-dependent quantities are treated as time-averages. The character of the model is continuous in the sense that the motor unit (MU) population is described by a continuous density function. In contrast to most already published models, the wiring (synaptic weight) between the input fibers to the MNPMC and the MNs (about which no detailed data are known) is deduced, whereas the input–force relation is given. As suggested by experimental data, this relation is assumed to be linear during MU recruitment, but the model allows other, nonlinear relations. The input to the MN pool is defined as the number of action potentials per second in all input fibers, and the excitatory postsynaptic potential (EPSP) conductance in MNs evoked by the input is assumed to be proportional to the input. A single compartment model with a homogeneous membrane is used for a MN. The MNs start firing after passing a constant voltage threshold. The synaptic current–frequency relation is described by a linear function and the frequency–force transformation of a MU by an exponential function. The sum of the MU contraction forces is the muscle force, and the activation of the MUs obeys the size principle. The model parameters were determined a priori, i.e., the model was not used for their estimation. The analysis of the model reveals special features of the activation curve which we define as the relation between the input normalized by the threshold input of the MN pool and the force normalized by the maximal muscle force. This curve for any muscle turned out to be completely determined by the activation factor, the slope of the linear part of the activation curve (during MU recruitment). This factor determines quantitatively the relation between MU recruitment and rate modulation. This property of the model (the only known model with this property) allows a quantification of the recruitment gain (Kernell and Hultborn 1990). The interest of the activation factor is illustrated using two human muscles, namely the first dorsal interosseus muscle, a small muscle with a relatively small force at the end of recruitment, and the medial gastrocnemius muscle, a strong muscle with a relatively large force at the end of recruitment. It is concluded that the present model allows us to reproduce the main features of muscle activation in the steady state. Its analytical character facilitates a deeper understanding of these features. Received: 24 November 1997 / Accepted in revised form: 30 November 1998     

Correlation between auditory sensitivity and vocalization in anabantoid fishes

Several anabantoid species produce broad-band sounds with high-pitched dominant frequencies (0.8–2.5 kHz), which contrast with generally low-frequency hearing abilities in (perciform) fishes. Utilizing a recently developed auditory brainstem response recording-technique, auditory sensitivities of the gouramis Trichopsis vittata, T. pumila, Colisa lalia, Macropodus opercularis and Trichogaster trichopterus were investigated and compared with the sound characteristics of the respective species. All five species exhibited enhanced sound-detecting abilities and perceived tone bursts up to 5 kHz, which qualifies this group as hearing specialists. All fishes possessed a high-frequency sensitivity maximum between 800 Hz and 1500 Hz. Lowest hearing thresholds were found in T. trichopterus (76 dB re 1 μPa at 800 Hz). Dominant frequencies of sounds correspond with the best hearing bandwidth in T. vittata (1–2 kHz) and C. lalia (0.8–1 kHz). In the smallest species, T. pumila, dominant frequencies of acoustic signals (1.5–2.5 kHz) do not match lowest thresholds, which were below 1.5 kHz. However, of all species studied, T. pumila had best hearing sensitivity at frequencies above 2 kHz. The association between high-pitched sounds and hearing may be caused by the suprabranchial air-breathing chamber, which, lying close to the hearing and sonic organs, enhances both sound perception and emission at its resonant frequency. Accepted: 26 November 1997     

Computer simulation of the motoneuron pool–muscle complex. I. Input system and motoneuron pool

 The aim of the present study was to simulate the input system and the motoneuron (MN) pool of the MN pool–muscle complex (MNPMC). Input fibers, which can originate from command centers in the central nervous system or from sensory organs, activate the MN pool. They generate sequences of action potentials, the frequency of which is proportional to a time-dependent activation factor (which is an input to the model). Different connection patterns between the input fibers and motor units (MUs) are allowed. For simplicity and since no precise experimental data are available, 70 input fibers and 4 boutons per fiber and MN are simulated (this corresponds approximately to the monosynaptic group-Ia input of the cat medial gastrocnemius muscle). Each bouton generates the same conductance change in the postsynaptic membrane. The MNs are modeled with a single compartment and a homogenous membrane. According to experimental data, the membrane leakage conductance and capacitance are MU dependent. Since the precise relation is unknown: (a) the computed relation between MU contraction force and the MN leakage conductance was taken from a steady-state MNPMC model, and (b) the capacitance was assumed to be proportional to the leakage conductance. The MN membrane includes time- and voltage-dependent ionic channels (fast and slow K⁺ and low- and high-threshold Ca²⁺ channels). The density and time constant of the slow K⁺ channels and the density of the Ca²⁺ channels were fitted to approximate afterhyperpolarization characteristics and frequency-injected current relations of type-identified cat MNs. If the membrane reaches a voltage threshold the MNs generate action potentials, which were simulated by voltage pulses. The activation of the MN pool of the human first dorsal interosseus muscle was simulated with injected and synaptic currents in order to illustrate the size principle, synaptic noise, and other features of muscle activation. It is concluded that the present model reproduces the main properties of the input–output relations of different MN types within a muscle. Together with the simulation of the muscle force and the surface EMG, which will be published in subsequent papers, it will be a powerful tool for reproducing experiments on the motor system and investigating functional mechanisms of motor control. Received: 17 April 2001 / Accepted in revised form: 6 November 2001     

Alteration of Cpn60 expression in pancreatic tissue of rats with acute pancreatitis

The expression of heat-shock protein 60 (also known as chaperonin 60, Cpn60) in experimental acute pancreatitis (AP) is considered to play an active role in the prevention of abnormal enzyme accumulation and activation in pancreatic acinar cells. However, there are controversial results in the literature regarding the relationship between the abnormality of Cpn60 expression and AP onset and development. The purpose of this study was to investigate the alternations of Cpn60 expression and the relationship between the abnormal expression of Cpn60 and AP progression in rat severe acute pancreatitis (SAP) models. In this report, we induced SAP in Sprague–Dawley (SD) rats by reverse injection of sodium deoxycholate into the pancreatic duct, and examined the dynamic changes of Cpn60 expression in pancreatic tissues from different time points and at different levels with techniques of real-time PCR, western blotting, and immunohistochemistry. At 1 h after SAP induction, the expression of Cpn60 mRNA in the AP pancreatic tissues was higher than those in the sham-operation group and normal control group, but decreased sharply as the time period was extended, and there was a significant difference between 1 h and 10 h after SAP induction (p < 0.05). In the AP process, Cpn60 protein expression showed transient elevation as well, and the increased protein expression occurred predominantly in affected, but not totally destroyed, pancreatic acinar cells. As AP progressed, the pancreatic tissues were seriously damaged, leading to a decreased overall Cpn60 protein expression. Our results show a complex pattern of Cpn60 expression in pancreatic tissues of SAP rats, and the causality between the damage of pancreatic tissues and the decrease of Cpn60 level needs to be investigated further. Xue-Li Li and Kun Li contributed equally to this work.     

Time Structure of Impulsation of Spinal Motoneurons of Cats under Conditions of Realization of the Stretch Reflex

We analyzed the characteristics of impulse activity recorded intracellularly from motoneurons of the mm. gastrocnemius + soleus in decerebrated cats; this activity was induced by controlled stretching of the above muscle group. Long-lasting (30 min long or more) sufficiently qualitative records (membrane potential, MP, greater than −50 mA throughout the entire recording period) were obtained from 17 motoneurons. Muscle stretching (sinusoidal half-waves of different amplitudes and combinations of such waves) allowed us to obtain well-reproducible successions of impulses with no significant changes in the resting MP of the studied motoneurons. In 14 of such cells, we found significant correlation between normalized variations of the instantaneous frequency (in a sequence of preceding and subsequent action potentials, APs), indicative of a considerable probability of alternation of high and low values of this parameter. The same trend (toward alternation of high and low values) was observed in successive magnitudes of afterhyperpolarization (AH) that followed APs. Between the values of the extrema of AH trajectories and values of the instantaneous frequency of impulsation, we observed significant negative correlation. The mechanisms underlying the formation of AH in spinal motoneurons and also the effects of this hyperpolarization on the characteristics of impulse activity generated by these cells are discussed.     

Changes in the levels of activity of spinal neurons after long-lasting vibrational stimulation of the shin muscles in rats

Vibrational stimulation of the tendon of the mm.gastrocnemius+soleus (100 sec^–1) in rats anesthetized with chloral hydrate (400 mg/kg) resulted in the appearance of considerable Fos immunoreactivity in the lumbar spinal cord (L1-L6), as compared with that in intact animals. Total densities of Fos-immunopositive (Fos-ip) neurons in each of the examined segments were higher than 40 units per 40-μm-thick slice; the respective index reached the maximum at the L4 level (78.9 ± 2.3 cells). Most Fos-ip neurons were localized in laminae 4 to 7 of the gray matter, both ipsi- and contralaterally with respect to the side of stimulation (28.5 ± 0.6 and 28.4 ± 0.6, respectively). Single Fos-ip motoneurons were found bilaterally in the ventral horn motor nuclei. Thus, activation of muscle spindle receptors induced by vibrational stimulation applied to the Achilles tendon induces noticeable bilateral c-fos expression in spinal neuronal networks related to transmission of proprioceptive muscle-born impulsation.     

Mauthner cell-evoked synaptic actions on pacemaker medullary neurons of a weakly electric fish

The present study was designed to examine the synaptic events in neurons of the pacemaker nucleus of Gymnotus carapo during the increase in rate of the electric organ discharge following activation of Mauthner cells. Pacemaker and relay cells were investigated using intracellular recordings which were performed under two different conditions: (1) with the pacemaker nucleus spontaneously discharging and (2) after its activity was abolished by anesthesia. Mauthner axon activation induced an increase in the rate of pacemaker cell discharges. This response was accompanied by an increase in the slope of the pacemaker potential (up to 110%) and a depolarization of these cells. The discharges of relay cells followed one to one those of pacemaker cells. In contrast to that observed in pacemaker cells, only brief depolarizing antidromic effects could be evoked in relay cells after Mauthner axon activation. In quiescent pacemaker cells, Mauthner cell activation induced a prolonged (up to 500 ms) depolarizing potential with an average amplitude of 1.92 ± 0.82 mV; its latency was 4.43 ± 1.14 ms. Our data indicate that, within the pacemaker nucleus, the population of pacemaker cells is the only target for Mauthner cell-evoked, short-latency excitatory synaptic actions. Accepted: 1 March 1997