Phase Plane Trajectories of the Muscle Spike Potential

To facilitate a study of the transmembrane action current, the striated muscle spike potential was recorded against its first time derivative. The specialized recording methods are described, as well as several mathematical transformations between a coordinate system in V, t, and the present coordinate system in V, dV/dt. The particular properties of the present recording method permitted an estimation for the “sodium” potential in muscle fibers at +39 mv. The maximum membrane conductance during spike production is in the order of 150 to 200 mmhos/cm². The changes in the shape of the recorded response indicate that the ionic currents and membrane conductances are reduced by Tris buffer or hypertonic Ringer's fluid. However, no marked changes in the properties of active membrane were observed when chloride ion was replaced by sulfate.     

Structure-Dependent Electrical and Concentration Processes in the Dendrites of Pyramidal Neurons of Superficial Neocortical Layers: Model Study

On mathematical models of pyramidal neurons localized in the neocortical layers 2/3, whose reconstructed dendritic arborization possessed passive linear or active nonlinear membrane properties, we studied the effect of morphology of the dendrites on their passive electrical transfer characteristics and also on the formation of patterns of spike discharges at the output of the cell under conditions of tonic activation via uniformly distributed excitatory synapses along the dendrites. For this purpose, we calculated morphometric characteristics of the size, complexity, metric asymmetry, and function of effectiveness of somatopetal transmission of the current (with estimation of the sensitivity of this efficacy to changes in the uniform membrane conductance) for the reconstructed dendritic arborization in general and also for its apical and basal subtrees. Spatial maps of the membrane potential and intracellular calcium concentration, which corresponded to certain temporal patterns of spike discharges generated by the neuron upon different intensities of synaptic activation, were superimposed on the 3D image and dendrograms of the neuron. These maps were considered “spatial autographs” of the above patterns. The main discharge pattern included periodic two-spike bursts (dublets) generated with relatively stable intraburst interspike intervals and interburst intervals decreasing with a rise in the intensity of activation. Under conditions of intense activation, the interburst intervals became close to the intraburst intervals, so the cell began to generate continuous trains of action potentials. Such a repertoire (consisting of two patterns of the activity, periodical dublets and continuous discharges) is considerably scantier than that described earlier in pyramidal neurons of the neocortical layer 5. Under analogous conditions of activation, we observed in the latter cells a variety of patterns of output discharges of different complexities, including stochastic ones. A relatively short length of the apical dendrite subtree of layer 2/3 neurons and, correspondingly, a smaller metric asymmetry (differences between the lengths of the apical and basal dendritic branches and paths), as compared with those in layer 5 pyramidal neurons, are morphological factors responsible for the predominance of periodic spike dublets. As a result, there were two combinations of different electrical states of the sites of dendritic arborization (“spatial autographs”). In the case of dublets, these were high depolarization of the apical dendrites vs. low depolarization of the basal dendrites and a reverse combination; only the latter (reverse) combination corresponded to the case of continuous discharges. The relative simplicity and uniformity of spike patterns in the cells, apparently, promotes the predominance of network interaction in the processes of formation of the activity of pyramidal neurons of layers 2/3 and, thereby, a higher efficiency of the processes of intracortical association.     

Statistical characteristics of unit activity in spinal locomotor centers

A statistical analysis of unit activity in spinal locomotor centers was undertaken on immobilized thalamic cats at rest and during generation of efferent discharges. Activation of the spinal locomotor generator was accompanied by shortening of interspike intervals in the spike sequences of neurons and a decrease in their fluctuations. Histograms of interspike intervals became more symmetrical under these circumstances and there was a considerable increase in the number of neurons whose activity showed regular fluctuations on autocorrelation histograms. Spike trains at rest were characterized by dependence of successive intervals, which increased during efferent discharge generation. The possible mechanisms of modification of the time structure of unit activity in spinal locomotor centers during their activation are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 192–198, March–April, 1980.     

A model of the mechanisms of formation of unit activity

Dependence of the temporal structure of the spike discharge of a neuron in a weakly interacting network on the characteristics of excitatory and inhibitory input flows and on cell parameters was analyzed by a mathematical model. The intensity of communication between individual neurons corresponded to the intensity of synaptic communication between real spinal neurons. The temporal course of trace and accommodation processes in the model was similar to that of these processes in real spinal neurons. Connection of inhibitory inputs and an increase in the intensity of their influences were shown to be equivalent to a decrease in the intensity of excitatory input flows. Changes in cell parameters had a significant effect on the spike discharge only in the case of weak input influences (the ratio of the amplitude of the combined ESP evoked by the input spike train to the threshold value of membrane potential at rest was about 1.2:1.0 to 1.4:1.0). An increase in the input flow intensity led to considerable reorganization of the firing pattern: Mean values of interspike intervals and their fluctuations were reduced, histograms of interspike intervals became more symmetrical, and periodic waves appeared on the autocorrelation histograms. It is concluded on the basis of these results and of data in the literature that the main factor determining reorganization of the temporal structure of unit activity in a network of weakly interacting cells is the intensity of the input flow.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 199–207, March–April, 1980.     

Characterization of respiratory-related neurons in the isolated brainstem of the frog

Using intra- and extracellular recording techniques we examined the spontaneous discharge and membrane properties of respiratory-related neurons in isolated brainstem preparations of the frogs Rana catesbeiana and Rana pipiens that display spontaneous respiratory related activity in vitro. We observed neurons that depolarize during the fictive lung ventilation cycle as well as neurons that depolarize during the non-lung ventilation phase. Respiratory-related neurons demonstrated significant decreases in membrane input resistance during the fictive lung ventilation cycle but showed no evidence of voltage-dependent membrane conductances activated near resting membrane potential. Furthermore, respiratory neurons showed little spike frequency adaptation, their oscillatory activity was not dissociated from the global respiratory motor output following imposed changes in membrane potential, and spontaneous fluctuations in membrane potential were not observed following reversible interruption of respiratory burst activity by application of solutions low in calcium and high in magnesium. Taken together these results suggest that bulbar respiratory neurons in the isolated frog brainstem sampled in our study do not display endogenous bursting characteristics. Rather, they are strongly influenced by synaptic input. Accepted: 20 March 1997     

Changes in postsynaptic and spike potentials of cortical neurons during habituation

Changes in spike potentials and EPSPs and IPSPs of neurons in the general cortex of the turtle forebrain were investigated intracellularly during habituation to flashes. The amplitudes of all these potentials were reduced although the level of the membrane potential remained unchanged. Their dependence on membrane potential was disturbed. The lowering of amplitude of the short-latency spike in response to flashes was greater than that of the spontaneous spike or of the spike after an IPSP. Considering that with extracellular recording only a selective lowering of the short-latency spike is observed, it can be concluded that depression of the spontaneous spike and of the post-IPSP spike reflects a nonspecific decrease in neuron excitability on account of prolonged intracellular recording, whereas the lowering of the short-latency spike reflects habituation at the neuronal level. Disinhibition of the amplitude of spikes and postsynaptic potentials was observed. The hypothesis that a population of synapses activated by a particular stimulus when applied repeatedly induces a short-term change in the electrogenic prperties of the nonreceptor neuron membrane, which determines the depression of the electrical responses, is put forward and discussed.M. V. Lomonosov State University, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 22–29, January–February, 1976.     

Electrophysiological properties of neurons and neuronal organization of the crayfish somatogastric ganglion

Experiments with intracellular recording from neurons of the isolated crayfish somatogastric ganglion established that the membrane potential of the neurons is 53±3 mV. Single stimulation of the central branches of the ganglion evoked EPSP and a spike in the neurons. The spike amplitude was 7.5±0.6 mV. The small amplitude of the spike is explained by the fact that it arises at some distance from the body of the neuron and propagates electrotonically in it. Summation of several EPSP is necessary in most cases for initiation of the spike. When the orthodromic stimulus was strong enough, and IPSP occurred in some cells in addition to the EPSP and spike. Stimulation of the peripheral nerves of the ganglion induced in most neurons antidromic excitation and in some neurons orthodromic excitation. Some neurons spontaneously discharged rhythmically with an unstable frequency (11–27 impulses/sec). An investigation of the effect on neurons of chemical agents [acetylcholine, adrenalin, noradrenalin, gamma-aminobutyric acid (GABA), glutamic acid, and dopamine] showed that acetylcholine has the strongest and most stable depolarizing action and apparently is a synaptic transmitter in the ganglion. The other agents excited some neurons — depolarized them and evoked rhythmic discharges — and, coversely, hyperpolarized and suppressed the rhythmic activity of other neurons. A scheme of neuronal organization of the somatogastric ganglion of the crayfish is proposed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 307–313, May–June, 1970.     

Neuronal electrical activity in the epileptic focus produced by electrical stimulation in an isolated cortical slab

Characteristics of neuronal activity in an isolated cortical slab were investigated during the onset of seizure spikes induced by frequent and powerful stimulation of the slab during experiments on unanesthetized immobilized cats. A high degree of coordination between the activity of cellular elements was found in the focus of epileptiform activity studied: convulsive shifts in membrane potential exactly corresponding to electrocorticograms of convulsive activity waves were observed in all neurons studied using intracellular techniques. No action potentials occurred in the soma of any of these neurons, moreover. Bursting spike discharges were recorded from neurons of the isolated slab at the same time. Findings from extra- and intracellular recordings of activity in the same neurons showed that action potentials are generated during convulsive activity at certain trigger zones remote from the cell in question without involving the soma, from which convulsive shifts in membrane potentials were recorded simultaneously. Mechanisms possibly underlying the generation of spike activity in neurons of the isolated slab undergoing development of generalized convulsive state are discussed.I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 357–365, May–June, 1988.     

Spontaneous unit activity in anterior (limbic) nuclei of rabbit thalamus

Spontaneous spike activity in three anterior (limbic) neurons of the thalamic nucleus was studied by means of extracellular recording during chronic experiments on anesthetized rabbits. Neurons of the anteroventral nucleus showed high mean rate (24.8±5.8 spikes/sec) and varying structure of spike discharges ("inactivating" bursts of discharges, modulations in delta- and theta-rhythms, and bursts of discharges with a spindle rhythm of 12–14 Hz). "Inactivating" bursts of discharges alternating with single discharges predominated in the activity of neurons of the anteromedial nucleus (mean rate 10.0±1.4 spikes/sec). Activity of the anterodorsal nucleus could be clearly distinguished by the predominance of high-frequency groups of spikes (mean group frequency 67±5 spikes/sec) with prolonged intervals between groups.Institute of Biological Physics, Academy of Sciences of the USSR, Puschino, Moscow Province. Translated from Neirofiziologiya, Vol. 17, No. 5, pp. 579–586, September–October, 1985.     

The stochastic properties of input spike trains control neuronal arithmetic

In the nervous system, the representation of signals is based predominantly on the rate and timing of neuronal discharges. In most everyday tasks, the brain has to carry out a variety of mathematical operations on the discharge patterns. Recent findings show that even single neurons are capable of performing basic arithmetic on the sequences of spikes. However, the interaction of the two spike trains, and thus the resulting arithmetic operation may be influenced by the stochastic properties of the interacting spike trains. If we represent the individual discharges as events of a random point process, then an arithmetical operation is given by the interaction of two point processes. Employing a probabilistic model based on detection of coincidence of random events and complementary computer simulations, we show that the point process statistics control the arithmetical operation being performed and, particularly, that it is possible to switch from subtraction to division solely by changing the distribution of the inter-event intervals of the processes. Consequences of the model for evaluation of binaural information in the auditory brainstem are demonstrated. The results accentuate the importance of the stochastic properties of neuronal discharge patterns for information processing in the brain; further studies related to neuronal arithmetic should therefore consider the statistics of the interacting spike trains.     

Mechanism of inhibition following reflex discharge in spinal cord interneurons

Using the method of microelectrode (intracellular and extracellular) recording, the mechanism of inhibition following reflex discharge in interneurons of the lumbosacral section of the spinal cord of cats on activation of cutaneous and high-threshold muscle afferents was studied. It was shown that the postdischarge depression of the reflex responses 10–20 msec after the moment of activation of the neuron is due to afterprocesses in the same neuron and presynaptic pathways. The depression of spike potentials from the 20th to the 100th msec is produced by inhibitory postsynaptic potentials (IPSP). During the development of IPSP the inhibition of spike potentials can be due to both a decrease of the depolarization of the postsynaptic membrane below the critical threshold and a decrease of sensitivity of the cell membrane to the depolarizing action of the excitatory postsynaptic potential (EPSP). At intervals between the stimuli of 30–100 msec the duration of EPSP after the first stimulus does not differ from that after the second stimulus. Hence, it is suggested that the presynaptic mechanisms do not play an essential part in this type of inhibition of interneurons. The inhibition following the excitation favors the formation of a discrete message to the neurons of higher orders.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 3–9, January–February, 1970.     

Unit activity in an epileptic focus formed in the cat motor cortex with tetanus toxin

Single unit activity was recorded intracellularly in the zone of an epileptic focus produced by injection of tetanus toxin into the cerebral cortex of cats. Epileptic activity of all neurons tested correlated with cortical discharges between fits. A group of neurons with continuous spontaneous activity, in which a steady fall of membrane potential and cyclic changes in the frequency of the spike discharges were observed was distinguished. In these neurons paroxysmal depolarization changes of membrane potential were found in the discharges between fits, without subsequent hyperpolarization of the membrane. Hyperpolarization potentials after paroxysmal depolarization shifts could be observed in neurons of other groups. The role of neurons of the different groups in the formation of an "epileptic aggregate," the main generator of pathologically enhanced excitation, is discussed.Institute of Normal and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Institute of Clinical and Experimental Neurology, Ministry of Health of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 582–589, November–December, 1978.     

Fluctuations in nuclear envelope’s potential mediate synchronization of early neural activity

Neural progenitor cells and developing neurons show periodic, synchronous Ca²⁺ rises even before synapse formation, and the origin of the synchronous activity remains unknown. Here, fluorescence measurement revealed that the membrane potential of the nuclear envelope, which forms an intracellular Ca²⁺ store, changed with a release of Ca²⁺ and generated spontaneous, periodic bursts of fluctuations in potential. Furthermore, changes in the nuclear envelope’s potential underlay spike burst generations. These results support the model that voltage fluctuations of the nuclear envelope synchronize Ca²⁺ release between cells and also function as a current noise generator to cause synchronous burst discharges.     

Postsynaptic neuronal response of a cat sensorimotor cortex during evoked and self-sustained rhythmic "spike and wave" activity

Intracellular correlates of complex sets of rhythmic cortical "spike and wave" potentials evoked in sensorimotor cortex and of self-sustained rhythmic "spike and wave" activity were examined during acute experiments on cats immobilized by myorelaxants. Rhythmic spike-wave activity was produced by stimulating the thalamic relay (ventroposterolateral) nucleus (VPLN) at the rate of 3 Hz; self-sustained afterdischarges were recorded following 8–14 Hz stimulation of the same nucleus. Components of the spike and wave afterdischarge mainly correspond to the paroxysmal depolarizing shifts of the membrane potential of cortical neurons in length. After cessation of self-sustained spike and wave activity, prolonged hyperpolarization accompanied by inhibition of spike discharges and subsequent reinstatement of background activity was observed in cortical neurons. It is postulated that the negative slow wave of induced spike and wave activity as well as slow negative potentials of direct cortical and primary response reflect IPSP in more deep-lying areas of the cell bodies, while the wave of self-sustained rhythmic activity is due to paroxysmal depolarizing shifts in the membrane potential of cortical neurons.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 298–306, May–June, 1986.     

Effect of cortisol microiontophoresis on discharge time structure of dorsal hippocampal neurons in rabbits

The effect of microiontophoretic application of cortisol to single neurons of the dorsal hippocampus on the character of distribution of interspike intervals in their discharges was studied in chronic experiments on rabbits. Cortisol modified the time structure of regular and rhythmic discharges of hippocampal neurons. Regularization of discharges in the form of bursting activity appeared as the result of cortisol in cells with irregular spontaneous activity. Activity of more than half of the neurons, in which bursting discharges corresponded in frequency to the theta-rhythm, was intensified as a result of microapplication of cortisol. In neurons discharging complex spikes, in which under normal conditions a phenomenon of reduction of spike amplitude was observed within each burst, no definite rule as regards changes in the time structure of the discharges could be observed after administration of the hormone. It is suggested that cortisol plays a modulating role in mechanisms of generation of spike activity by hippocampal neurons.P. K. Anokhin Research Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 628–635, November–December, 1981.     

The Mechanism of Discharge Pattern Formation in Crayfish Interneurons

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.     

Spike generation by spinal neurons evoked by sinusoidal depolarization in cats

Dynamic characteristics of transformation of cell membrane depolarization by spinal neurons into spike discharge frequency were investigated in anesthetized cats. Neurons were activated by sinusoidally modulated currents passed through an intracellular microelectrode. Frequency analysis of this transformation for motoneurons was carried out within a modulation frequency range of 0.2–10 Hz. Frequency characteristics were determined with respect to parameters of the first harmonic of the evoked firing rate; the region of values of current fluctuations was chosen on the linear part of the current intensity versus firing rate characteristic curve. Changes in amplitude characteristics did not exceed 5 dB in absolute terms; at the same time the phase lead of the output signal increased with a rise of frequency. At a frequency of 0.2 Hz phase shifts were virtually absent, but at frequencies of 1, 5, and 10 Hz they amounted to 32, 50, and 83° on average respectively. Transformation of membrane depolarization by neurons into spike discharge frequency is characterized by essentially nonlinear properties, due in particular to the absence of a dynamic component of the response to a negative rate of change of depolarizing current. The frequency characteristics of spike activity of neurons of the motor system are discussed from the standpoint of possible correction of dynamic properties of the whole system at the single unit level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 35–42, January–February, 1982.     

Membrane Potential Fluctuations Determine the Precision of Spike Timing and Synchronous Activity: A Model Study

It is much debated on what time scale information is encoded by neuronal spike activity. With a phenomenological model that transforms time-dependent membrane potential fluctuations into spike trains, we investigate constraints for the timing of spikes and for synchronous activity of neurons with common input. The model of spike generation has a variable threshold that depends on the time elapsed since the previous action potential and on the preceding membrane potential changes. To ensure that the model operates in a biologically meaningful range, the model was adjusted to fit the responses of a fly visual interneuron to motion stimuli. The dependence of spike timing on the membrane potential dynamics was analyzed. Fast membrane potential fluctuations are needed to trigger spikes with a high temporal precision. Slow fluctuations lead to spike activity with a rate about proportional to the membrane potential. Thus, for a given level of stochastic input, the frequency range of membrane potential fluctuations induced by a stimulus determines whether a neuron can use a rate code or a temporal code. The relationship between the steepness of membrane potential fluctuations and the timing of spikes has also implications for synchronous activity in neurons with common input. Fast membrane potential changes must be shared by the neurons to produce synchronous activity.     

Action potential generation by turtle cortical neurons. Dendritic and somatic spikes

Action potentials of neurons of the turtle general cortex and the pattern of their generation were studied by an intracellular recording method. Besides the complete action potential, the cells also generate partial spikes of varied amplitude which compose the complete action potential. The threshold of generation and the discrete amplitude of each partial spike are not strictly constant but they fluctuate gradually and spontaneously within certain limits without any change in membrane potential of the cell. Somatic and dendritic spikes are distinguished. The trigger zones of the latter are located at various distances from the soma. During orthodromic activation of cortical neurons dendritic spikes are generated consecutively and spread to the some electrotonically with a decrement. They are the immediate cause of generation of the somatic spike.M. V. Lomonovsov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 237–242, May–June, 1976.     

Integrative Synaptic Mechanisms in the Caudal Ganglion of the Crayfish

A study of activity recorded with intracellular micropipettes was undertaken in the caudal abdominal ganglion of the crayfish in order to gain information about central fiber to fiber synaptic mechanisms. This synaptic system has well developed integrative properties. Excitatory post-synaptic potentials can be graded, and synaptic potentials from different inputs can sum to initiate spike discharge. In most impaled units, the spike discharge fails to destroy the synaptic potential, thereby allowing sustained depolarization and multiple spike discharge following single pulse stimulation to an afferent input. Some units had characteristics which suggest a graded threshold for spike generation along the post-synaptic fiber membrane. Other impaled units responded to afferent stimulation with spike discharges of two distinct amplitudes. The smaller or "abortive" spikes in such units may represent non-invading activity in branches of the post-synaptic axon. On a few occasions one afferent input was shown to inhibit the spike discharge initiated by another presynaptic input.