Pattern of human motoneuron activity during voluntary muscular contraction

The pattern of motor unit activity of the rectus femoris muscle during isometric contraction was studied. Interspike interval sequences were analyzed. At frequencies below 8–10/sec the interval histograms were asymmetrical (long intervals predominated), the standard deviation was high and depended on the mean interval, and adjacent intervals were independent. At frequencies above 10–13/sec the interval distribution became normal in type, the standard deviation was low and independent of the mean interval duration, and negative correlation was found between adjacent intervals. During brief contraction specific for this particular (fast) muscle the activity of most neurons was of the second type, while during prolonged contraction it was of the first type. The change from activity of the first to the second type took place approximately when, with an increase in frequency, spikes began to appear against the background of after-hyperpolarization. A hypothesis is put forward to the effect that after-hyperpolarization creates the rhythm of activity (low variability of the intervals), and that the duration of after-hyperpolarization is correlated with the lower limit of working frequencies of the motoneurons. The motoneuron is regarded as performing not only a summating, but also a damping function, reducing the effect of random fluctuations of the synaptic inflow and coding information for transmission as a mean frequency.Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 609–619, November–December, 1971.     

Activity of single motor units in attention-demanding tasks: firing pattern in the human trapezius muscle

Activity of single motor units in relation to surface electromyography (EMG) was studied in 11 subjects in attention-demanding work tasks with minimal requirement of movement. In 53 verified firing periods, single motor units fired continuously from 30 s to 10 min (duration of the experiment work task) with a stable median firing rate in the range of 8–13 Hz. When the integrated surface EMG were stable, the motor units identified as a rule were continuously active with only small modulations of firing rate corresponding to low-amplitude fluctuations in surface EMG. Marked changes in the surface EMG, either sudden or gradual, were caused by recruitment or derecruitment of motor units, and not by modulations of the motor unit firing rate. Motor unit firing periods (duration 10 s-35 s) in low-level voluntary contractions (approximately 1%–5% EMG_max) performed by the same subjects showed median firing rates (7–12 Hz) similar to the observations in attention-related activation.     

Testing excitability in human firing motoneurons during the interspike interval

Monosynaptic testing of excitability in firing triceps surae muscle motoneurons activated during volitional contraction was performed using a technique for recording potentials from single motor units and by producing H-reflex. Motoneuronal excitability was assessed according to level of firing index. Motoneuronal firing index decreased during transition from a low background rhythmic firing rate of less than 6 spikes/sec to one of 6–8 spikes/sec. It hardly changed with a further rise in rate to 12 spikes/sec. The dependence between firing index and spike rate are put down to changes occurring in motoneuronal excitability during the interspike interval. Findings indicate that in the low frequency range of motoneuronal firing characteristic of natural muscle contraction, discharge rate may be considered one of the factors determining excitability in the motoneuron and hence its transmission qualities.Institute of Problems in Information Transmission, Academy of Sciences of the USSR. Translated from Neirofiziologiya, Vol. 19, No. 2, pp. 210–216, March–April, 1987.     

Dynamics of single unit responses of the rabbit visual cortex to repetitive photic stimulation

Single unit responses in the visual cortex of the waking rabbit to repetitive photic stimulation at a frequency of once every 2.5 sec were studied. Depending on the total number of spikes in the response, the dynamics of the responses could be divided into two types: "fast" and "slow." From 5 to 15 stimuli were required to establish a stable level of response with changes of the first type, but 50 to 100 stimuli were needed for the response with changes of the second type. About 50% of all neurons did not change the characteristics of response. In the group of neurons with changing responses, partial habituation was found in 55–59% of cells; there were 25% of neurons with sensitization of discharge and 17–20% with a humpbacked type of response dynamics. A "slow" dynamics of unit responses in most cases was accompanied by changes in the duration of inhibitory pauses in the response; negative correlation of these values was observed in 65% of neurons.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 10, No. 5, pp. 451–459, September–October, 1978.     

Intensity Characteristics of the Noctuid Acoustic Receptor

Spiking activity of the more sensitive acoustic receptor is described as a function of stimulus intensity. The form of the intensity characteristic depends strongly on stimulus duration. For very brief stimuli, the integral of stimulus power over stimulus duration determines the effectiveness. No response saturation is observed. With longer stimuli (50 msec), a steady firing rate is elicited. The response extends from the spontaneous rate of 20–40 spikes/sec to a saturated firing rate of nearly 700 spikes/sec. The characteristic is monotonic over more than 50 db in stimulus intensity. With very long stimuli (10 sec), the characteristics are nonmonotonic. Firing rates late in the stimulus decrease in response to an increase in stimulus intensity. The non-monotonic characteristics are attributed to intensity-related changes in response adaptation.     

Auditory units in the medulla of the marsh frog with unusual patterns of spontaneous activity

Statistical properties of spontaneous firing were studied in 79 single auditory units located in the dorsal medullar (cochlear) nucleus of unanaesthetized curarized marsh frogs (Rana ridibunda). The great majority of these units showed irregular spontaneous activity with mean rates in the range 1–30 spikes · s^–1. In 53% of the cells the auto-renewal functions of the spontaneous activity monotonically rose to an asymptotic value, but 41% of the cells produced auto-renewal functions which showed a pronounced peak after a dead-time period. Five low-frequency auditory neurons revealed periodic firing in the absence of controlled stimuli. The preferred period did not correspond to the unit's best frequency but demonstrated a modest correlation with the best modulation frequency of the unit's response to amplitude-modulated tones and with the duration of the after-onset dip in peri-stimulus time histograms.Abbreviations AM amplitude modulation - ARF auto-renewal function - DMN dorsal medullar nucleus - PST peristimulus time - SA spontaneous activity - TID time interval distribution - RMG response modulation gain     

A Simulation Study to Examine the Effect of Common Motoneuron Inputs on Correlated Patterns of Motor Unit Discharge

The influence of common oscillatory inputs to the motoneuron pool on correlated patterns of motor unit discharge was examined using model simulations. Motor unit synchronization, in-phase fluctuations in mean firing rates known as ‘common drive’, and the coefficient of variation of the muscle force were examined as the frequency and amplitude of common oscillatory inputs to the motoneuron pool were varied. The amount of synchronization, the peak correlation between mean firing rates and the coefficient of variation of the force varied with both the frequency and amplitude of the common input signal. Values for ‘common drive’ and the force coefficient of variation were highest for oscillatory inputs at frequencies less than 5 Hz, while synchronization reached a maximum when the frequency of the common input was close to the average motor unit firing rate. The frequency of the common input signal for which the highest levels of synchronization were observed increased as motoneuron firing rates increased in response to higher target force levels. The simulation results suggest that common low-frequency oscillations in motor unit firing rates and short-term synchronization result from oscillatory activity in different bands of the frequency spectrum of shared motoneuron inputs. The results also indicate that the amount of synchronization between motor unit discharges depends not only on the amplitude of the shared input signal, but also on its frequency in relation to the present firing rates of the individual motor units.     

Depression of evoked potentials in rat thalamic ventro-basal complex and somatosensory cortex after reticular stimulation

Experiments on unanesthetized rats immobilized with D-tubocurarine showed that electrical stimulation (100/sec) of the central gray matter and the mesencephalic and medullary reticular formation considerably depressed potentials in the somatic thalamic relay nucleus and somatosensory cortex evoked by stimulation of the forelimb or medial lemniscus. The mean threshold values of the current used for electrical stimulation of these structures did not differ significantly and were 70 (20–100), 100 (20–120), and 120 (50–200) µA, respectively. On comparison of the amplitude-temporal characteristics of inhibition of evoked potentials during electrical stimulation of the above-mentioned structures by a current of twice the threshold strength, no significant differences were found. Immediately after the end of electrical stimulation the amplitude of the cortical evolved potential and the post-synaptic components of the thalamic evoked potential was 50–60% (P<0.01) below the control values. The duration of this depression varied from 0.5 to 1 sec. An increase in the intensity of electrical stimulation of brain-stem structures to between three and five times the threshold led to depression of the presynaptic component of the thalamic evoked potential also. Depression of the evoked potential as described above was found with various ratios between the intensities of conditioning and testing stimuli.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 467–475, September–October, 1976.     

Interaction between central and peripheral temperature signals on temperature-sensitive hypothalamic neurons

Changes in the mean firing rate of posterior hypothalamic neurons were studied in experiments on unanesthetized cats in response to elevation of the brain temperature by 0.7–1.5°C and the skin temperature by 3–5°C separately or simultaneously. Altogether 85 neurons were studied in 14 animals: 11 responded to only one form of temperature stimulation, whereas in 16 neurons changes in the firing pattern (in most cases in the same direction) were observed in response to both forms of temperature stimulation. Different types of responses of these neurons were established. Sensitivity to the central temperature stimulus was increased in some neurons of this group when skin temperature stimulation was intensified.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 8, No. 6, pp. 613–619, November–December, 1976.     

Effects of microwave irradiation and temperature on spontaneous ventral root discharges in prog spinal cord

Aperiodic firing independent of extraneous stimuli at rates varying between 3–8 and 65–100 Hz (spontaneous firing activity, or SFA) was recorded at ventral root filaments of isolated, sagitally hemisected frog spinal cord. Lowest activity level was observed at temperatures of 7–11°C and an increased rate at either higher or lower temperatures. Some consistent short-lasting changes in SFA were noted straight away during the course of thermal changes: heating and cooling the preparation increased and reduced discharge rate, respectively. Characteristic activity rate for a given temperature level would set in 1–3 min after this level had stabilized. Microwave radiation of the spinal cord (6.45 GHz; specific absorption rates: 0.1, 0.4, and 2.0 W/g; duration 5 min) brought about no significant alteration in SFA at a steady temperature level. Microwave heating of the preparation and raised temperature both produced the same effects in all trials. Results would indicate that a thermal mechanism underlies the microwave effects on SFA at the ventral roots of frog spinal cord.Institute of Medical Radiology, Academy of Medical Sciences of the USSR, Obninsk. Translated from Neirofiziologiya, Vol. 20, No. 6, pp. 723–728, November-December, 1988.     

Conduction velocity and excitability of A and C fibers of cat mesenteric nerves

The conduction velocity and excitability of fibers running from the mesenteric into the splanchnic nerves were studied in experiments on cats. Among the A fibers of these nerves there were shown to be: 1) fibers with an excitation threshold of 0.06–0.10 V (stimulus duration 0.1 msec) and a maximal conduction velocity of 48–85 m/sec; 2) fibers with an excitation threshold of 0.3–0.7 V, impulses of which form up to five waves in the composition of the action potential, with maximal conduction velocities of between 8–10 and 33–39 m/sec; 3) fibers with an excitation threshold of over 1 V and a conduction velocity of between 1.8 and 7 m/sec. The excitation threshold of the group C fibers was 6–8 V. Impulses of these fibers form a low-amplitude wave in the composition of the action potential of the mesenteric and splanchnic nerves with a conduction velocity of 1.0–1.8 m/sec, several waves of higher amplitude with a conduction velocity of 0.5–1.2 m/sec, and several low-amplitude waves with a conduction velocity of 0.35–0.55 m/sec. The results of experiments with different combinations of arrangement of the stimulating and recording electrodes on the mesenteric and splanchnic nerves indicate that sympathetic postganglionic C fibers of the mesenteric nerves occur only in the second group, whereas afferent C fibers occur in all three of the groups distinguished.Institute of Normal and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 7, No. 3, pp. 272–278, May–June, 1975.     

Characteristics of Gamma-Oscillations in Responses of Neurons of the Cat Lateral Geniculate Body

There were studied characteristics of gamma-oscillations in responses of neurons of the lateral geniculate body (LGB) in cat to exposure in their receptive fields (RF) of half-tone and binary test images. The gamma-oscillations were observed in 38.8% of cases (69 cells). The spectral characteristics (SC) (the band 20–100 Hz) of the neuronal responses to adequate stimuli (on- and off-responses correspondingly of on- and off-neurons) were analyzed. The total of 5930 poststimulus histograms (PSTH) of responses constructed from 177 900 neuronal impulse responses were considered. The mean value of the SC dominant frequencies of the whole sample of the neuronal responses amounted to 44.74 ± 21.46 Hz. In this cell sample, the neurons were revealed, which generated oscillations with markedly different frequencies in response to the same stimuli. Based on this property, three types of neurons were determined, with the mean oscillation frequencies of 26.95 ± 4.35, 52.02 ± 9.05, and 85.79 ± 7.19 Hz. The histograms of distribution of peak frequency values in SC of the neuronal responses and of index values of these oscillation peaks also revealed three maxima that corresponded to the frequencies of the three described types of neurons. The mean values of dominant frequencies of gamma-oscillations in responses of all three types of neurons remained constant (within the limits of dispersion) at changes of spatial-brightness parameters of test stimuli as well as at changes of the neuronal excitation level (the number of impulses in responses). The oscillation index values of dominant frequencies depended on parameters of the test images and correlated with the neuronal excitation level (the coefficient of correlation was 0.78 from data of 5930 CX). The suggestion is made about the existence in the neuronal network of the synchronization mechanisms functioning on the principle of multiple synchronization.      

Detection and analysis of recurrent inhibition of firing motoneurons in man

During regular firing of "small" motor units, activated during weak voluntary contraction of the human soleus muscle, thick efferent fibers of n. tibialis were stimulated (a small M response was evoked, in which the small units did not participate). Peristimulus histograms of potentials of single motor units were constructed and the effect of stimulation on interspike interval duration was analyzed. The firing rate of the motor units was 4.5–7.6 spikes/sec. Stimulation of the nerve led to a sharp decrease in probability of their discharge or even complete temporary cessation of firing, i.e., it had a well marked inhibitory effect (lasting 10–20 msec). The latent period of inhibition (35–40 msec) was only a little longer than the latent period of the monosynaptic reflex of the soleus muscle. The effect of an inhibitory volley on duration of the interspike interval of the motor units depended on the time when the volley arrived during the interval. Lengthening of the interval was observed only if the inhibitory volley arrived in the second half or at the end of the interval. It is concluded that inhibition of firing of small motor units is due to Renshaw cells, activated on stimulation of axons of large motoneurons. The efficiency of a short (compared with the duration of the interspike interval) inhibitory volley reaching a motoneuron firing at low frequency characteristic of its adequate activation, is discussed.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 88–96, January–February, 1984.     

Role of the midbrain periaqueductal gray matter in conditioned reflexes

Unit activity in the midbrain periaqueductal gray matter (PGM) during an instrumental placing reflex, its extinction, differentiation, and conditioned inhibition, was studied in chronic experiments on cats. Spike responses 1–2 sec in duration in 69 (36.7%) of 182 neurons preceded by 400–800 msec the beginning of conditioned-reflex and voluntary intertrial movements. These advanced responses appeared 200 msec before the corresponding advance responses of motor cortical neurons. Fifty-eight neurons (30.9%) responded directly to acoustic stimulation with a latent period of 10–50 msec for 2–6 sec, 19 neurons (10.1%) generated double responses, linked with both the acoustic stimulus and subsequent conditioned-reflex movement, and 42 neurons (22.3%) did not respond to acoustic stimulation, although individual neurons of this group changed the level of their spontaneous activity in response to repeated conditioned stimulation, and this change was maintained for some tens of minutes. Extinction, differentiation, and conditioned inhibition all abolished conditioned-reflex movements, but each type of internal inhibition was accompanied by its own characteristic changes in the firing pattern of PGM neurons. Functional independence of neurons of the first and second groups was demonstrated during extinction and recovery of the conditioned-reflex. The results indicate the important role of PGM not only in the mechanism of the conditioned reflex, but also in the development of its internal inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 403–419, May–June, 1984.     

Characteristics of background unit activity in the nucleus reticularis of the human thalamus

Background firing activity was examined in 240 neurons belonging to the thalamic nucleus reticularis (Rt) in the unanesthetized human brain by extracellular microelectrode recording techniques during stereotaxic surgery for dyskinesia. The cellular organization of Rt was shown to be nonuniform, and distinguished by the presence of three types of neuron: one with arrhythmic single discharge (A-type, 40%), another with rhythmic (2–5 Hz) generation of short high-frequency (of up to 500/sec) burster discharges (B-type, 49%) and a third with aperiodic protracted high-frequency (of up to 500/sec) bursting discharges separated by "silent" intervals of a constant duration of 80–150 msec (i.e., C-type, 11%). Differences between the background activity pattern of these cell types during loss of consciousness under anesthesia are described. Tonic regulation of neuronal type was not pronounced but a tendency was noticed in the cells towards a consistent rise in firing rate, rhythmic frequency and variability, etc. in both A and B units, especially in the latter. Findings pointing to the absence of a direct relationship between rhythmic activity in the Rt and parkinsonian tremor were confirmed. Background activity in B-type cells was found to increase and then stabilize with a rise in the degree of tremor. The nature of regular bursting activity patterns in B and C neurons is discussed in the light of our findings.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Institute of Neurosurgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 19, No. 4, pp. 456–466, July–August, 1987.     

Analysis of firing behaviour of human motoneurones within 'subprimary range'.

Firing behaviour of human motoneurones within a low range of frequencies was studied during voluntary muscle contraction. It was found that, in contrast to the higher 'primary range', both excitability and inhibitibility of these motoneurones were significantly higher. As to their minimal firing rates, no correlation between them and the reciprocal values of afterhyperpolarization (AHP) duration was found. This suggests that AHP can hardly be regarded as the main factor controlling the behaviour of human motoneurones within the low-frequency range of firing and that this range (termed here 'subprimary range') should be kept apart from the 'primary range'.     

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.     

Effects of experimentally induced deficiency of catecholaminergic transmission on neuronal activity in the ventrolateral thalamic nucleus

Background activity was investigated in 272 neurons of the ventrolateral thalamic nucleus (VLTN) before and after systemic administration of neuroleptics (haloperidol and droperidol) at cataleptic doses by means of extracellular techniques during chronic experiments on cats. Autocorrelation and spectral analysis revealed regularly-occurring changes in the background activity rate of VLTN neurons, the periodicity of which changed by fractions of seconds (0.2–0.8 sec), seconds (1.5–10 sec), or tens of seconds (12–30 sec). While numbers of neurons with individual types of periodic activity did not exceed 6–8% in intact animals, it did increase to 18–30% after administering neuroleptics. Raised numbers of neurons with two types of regularly occurring processes within a single spike train were also noted. Experimentally-produced data were compared with findings from clinical observations. Quantities of neurons with different variations in the periodicity of their firing activity reached 19–46% in patients with parkinsonism but did not exceed 4–8% in those with torsion dystonia. The genesis of raised rhythmic firing in thalamic neurons occurring with parkinsonism is thought to be associated with impaired catecholaminergic (both dopaminergic and -adrenergic) transmission.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 3, pp. 359–368, May–June, 1990.     

Protective Role of the Polysaccharide-containing Capsular Components of Azospirillum brasilense

The involvement of the carbohydrate components of the Azospirillum brasilenseSp245 capsules in bacterial protection from the action of extreme factors was investigated. The survival of encapsulated and non-encapsulated azospirilla exposed to elevated (46–48°C) and below-freezing (–20 and –70°C) temperatures, extreme pH values (2 and 10), and to drying was studied. High-molecular-weight carbohydrate-containing complexes (lipopolysaccharide–protein complex and polysaccharide–lipid complex) were isolated from the capsular material of azospirilla. It was shown that the addition of these complexes to the suspension of decapsulated cells before exposing them to extreme factors enhanced their survival rates by 15 to 51%.     

Inhibition in the fish olfactory bulb

Inhibition in the olfactory bulb of the carp was studied by recording potentials from secondary neurons intracellularly. Three types of inhibition — trace, early, and late — can arise in neurons of the olfactory bulb. Trace inhibition corresponds to hyperpolarization about 20 msec in duration, which is closely connected with the spike, but it is not after-hyperpolarization but an IPSP. Early and late inhibition correspond to IPSPs of different parameters. The first has a latency of 0–50 msec (relative to the spike) and a duration of 60–400 msec; the corresponding values for the second are 100–400 msec and 0.5–3 sec. The possible mechanisms of these types of inhibition are discussed.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 650–656, November–December, 1971.