Duration Of Interspike Intervals Research Articles

Human motoneuron firing behavior and single motor unit F-wave

In motor control studies, the F-wave (a recurrent discharge evoked by an axonal antidromic volley) widely used for obtaining information on motoneuron pool behavior. However, such F-wave using is a matter of discussion and still has been not validated experimentally. The aim of the present study was investigation of F-wave properties of single firing motor units (MUs) in healthy humans, the properties, which could give evidence for F-wave origin in motoneuron soma and, therefore, could be used for estimation of a relation between MU firing and motoneuron firing behavior. In total, 91 MUs in five muscles of six healthy subjects, during gentle voluntary contractions, were studied. Peri-stimulus time histograms of single MUs were plotted. None of them revealed statistically significant increasing in MU firing probability at the F-wave latency. Analysis of relationships between characteristics of motoneuron firing behavior (mean firing frequency and target interspike interval duration) and properties of F-waves showed their independence. At the same time, it was found that F-waves were recorded in MUs, whose axons possessed the marked supernormal period in excitability recovery cycle after a discharge. Thus, the present results are in contrast to that which should be expected if the F-wave originated in the motoneuronal soma and could provide evidence for motoneuron firing behavior.

Effects of microgravity on characteristics of the accuracy control of movements

Effects of microgravity on characteristics of the accuracy control of movements

Relationship in Pacemaker Neurons Between the Long-Term Correlations of Membrane Voltage Fluctuations and the Corresponding Duration of the Inter-Spike Interval.

Several studies of the behavior in the voltage and frequency fluctuations of the neural electrical activity have been performed. Here, we explored the particular association between behavior of the voltage fluctuations in the inter-spike segment (VFIS) and the inter-spike intervals (ISI) of F1 pacemaker neurons from H. aspersa, by disturbing the intracellular calcium handling with cadmium and caffeine. The scaling exponent α of the VFIS, as provided by detrended fluctuations analysis, in conjunction with the corresponding duration of ISI to estimate the determination coefficient R 2 (48-50 intervals per neuron, N=5) were all evaluated. The time-varying scaling exponent α(t) of VFIS was also studied (20 segments per neuron, N=11). The R 2 obtained in control conditions was 0.683 ([0.647 0.776] lower and upper quartiles), 0.405 [0.381 0.495] by using cadmium, and 0.151 [0.118 0.222] with caffeine (P<0.05). A non-uniform scaling exponent α(t) showing a profile throughout the duration of the VFIS was further identified. A significant reduction of long-term correlations by cadmium was confirmed in the first part of this profile (P=0.0001), but no significant reductions were detected by using caffeine. Our findings endorse that the behavior of the VFIS appears associated to the activation of different populations of ionic channels, which establish the neural membrane potential and are mediated by the intracellular calcium handling. Thus, we provide evidence to consider that the behavior of the VFIS, as determined by the scaling exponent α, conveys insights into mechanisms regulating the excitability of pacemaker neurons.

Cav1.3 Channels as Key Regulators of Neuron-Like Firings and Catecholamine Release in Chromaffin Cells.

Neuronal and neuroendocrine L-type calcium channels (Cav1.2, Cav1.3) open readily at relatively low membrane potentials and allow Ca2+ to enter the cells near resting potentials. In this way, Cav1.2 and Cav1.3 shape the action potential waveform, contribute to gene expression, synaptic plasticity, neuronal differentiation, hormone secretion and pacemaker activity. In the chromaffin cells (CCs) of the adrenal medulla, Cav1.3 is highly expressed and is shown to support most of the pacemaking current that sustains action potential (AP) firings and part of the catecholamine secretion. Cav1.3 forms Ca2+-nanodomains with the fast inactivating BK channels and drives the resting SK currents. These latter set the inter-spike interval duration between consecutive spikes during spontaneous firing and the rate of spike adaptation during sustained depolarizations. Cav1.3 plays also a primary role in the switch from “tonic” to “burst” firing that occurs in mouse CCs when either the availability of voltage-gated Na channels (Nav) is reduced or the β2 subunit featuring the fast inactivating BK channels is deleted. Here, we discuss the functional role of these “neuron-like” firing modes in CCs and how Cav1.3 contributes to them. The open issue is to understand how these novel firing patterns are adapted to regulate the quantity of circulating catecholamines during resting condition or in response to acute and chronic stress.

Energy Efficient Neurons With Generalized Inverse Gaussian Conditional and Marginal Hitting Times

Neuronal information processing is energetically costly. Energy supply restrictions on information processing have resulted in the evolution of brains to compute and communicate information with remarkable energy efficiency. Indeed, energy minimization subject to functional constraints is a unifying principle. Toward better comprehension of neuronal information processing and communication from an information-energy standpoint, we consider a continuous time, continuous state-space neuron model with a generalized inverse Gaussian (GIG) conditional density. This GIG model arises from a Levy diffusion that contains both homogeneous Poisson processes and Wiener processes with drift as special cases. We show that, when the energy constraints consist of a tripartite family apropos of the GIG model, the distribution of input excitation, $\Lambda $ , that maximizes bits per Joule (bpJ) generates an output interspike interval duration $T$ that possesses a related GIG marginal distribution. Most importantly, we obtain the exact expression for the bpJ-maximizing distribution of $\Lambda $ in terms of Bessel functions that are readily evaluated numerically. This permits display of the strictly concave curves of bpJ-maximized mutual information $I(\Lambda ;T)$ . Because bpJ equals bits per second/watt, these curves serve as the Shannon-theory capacity v. power curves for GIG neuron models. We show that the noise in GIG channels is essentially multiplicative, so our results may have significance for broader classes of fading channels.

Predictions of energy efficient Berger-Levy model neurons with constraints

Information theory has been extensively applied to neuroscience problems. The mutual information between input and output has been postulated as an objective, which neuronal systems may optimize. However, only recently the energy efficiency has been addressed within an information-theoretic framework [1]. Here, the key idea is to consider capacity per unit cost (measured in bits per joule, bpj) as the objective. We are interested in how biologically plausible constraints affect predictions made by this new theory for bpj-maximizing model neurons. More specifically, in our contribution, in line with [1] and [2], a neuron is modeled as a memory-less constant communication channel with a Gamma conditional probability distribution function (PDF) [1]. In this setting, the channel input and output are the excitatory postsynaptic potential intensity, λ, and the inter spike interval (ISI), t, with PDFs f (λ) and fT (t), respectively. We then formulate two new constraints: First, we impose a lower bound tmin on the duration tof ISIs. The rational for this is to account for a maximal firing rate. Second, we consider a peak energy expenditure constraint per ISI as compared to only bounding the expected energy expenditure. This translates into an upper bound tmax on the ISI duration. We then derive the fT (t) (corresponding to valid f (λ)) of a bpj-maximizing neuron for the original unconstrained setting from [1] and in the presence of the above two constraints for different expected ISIs. (Details omitted here for brevity.) Figure 1 shows three fT (t)s obtained in the unconstrained (dashed curves) and constrained settings (solid curves) for tmin = 1and tmax = 5. While the constrained and unconstrained solutions have the same mean, the shape of their fT (t) differ. For comparison

Involvement of Na+/K+pump in fine modulation of bursting activity of the snail Br neuron by 10 mT static magnetic field

The spontaneously active Br neuron from the brain-subesophageal ganglion complex of the garden snail Helix pomatia rhythmically generates regular bursts of action potentials with quiescent intervals accompanied by slow oscillations of membrane potential. We examined the involvement of the Na(+)/K(+) pump in modulating its bursting activity by applying a static magnetic field. Whole snail brains and Br neuron were exposed to the 10-mT static magnetic field for 15min. Biochemical data showed that Na(+)/K(+)-ATPase activity increased almost twofold after exposure of snail brains to the static magnetic field. Similarly, (31)P NMR data revealed a trend of increasing ATP consumption and increase in intracellular pH mediated by the Na(+)/H(+) exchanger in snail brains exposed to the static magnetic field. Importantly, current clamp recordings from the Br neuron confirmed the increase in activity of the Na(+)/K(+) pump after exposure to the static magnetic field, as the magnitude of ouabain's effect measured on the membrane resting potential, action potential, and interspike interval duration was higher in neurons exposed to the magnetic field. Metabolic pathways through which the magnetic field influenced the Na(+)/K(+) pump could involve phosphorylation and dephosphorylation, as blocking these processes abolished the effect of the static magnetic field.

Motoneuron afterhyperpolarisation duration in amyotrophic lateral sclerosis

Motor unit (MU) potentials were registered from 20 ALS patients and 13 age-matched control individuals during isometric constant force contractions of brachial biceps (BB). The registered signals were decomposed into single MU potential trains. The estimates of duration of the afterhyperpolarisation (AHP) in MNs, derived from the interspike interval variability, was compared between ALS patients (124 MNs) and control subjects (111 MNs) and no significant differences were encountered. However, the relationship between TI and age for patients appeared to be qualitatively different from that of the control group. The dependence of patients' AHPs on relative force deficit (RFD), which quantified muscle involvement, was more specific. For RFDs below 30%, the AHP estimate was significantly lower than control values and then increased thereafter with increasing RFDs. Moreover, firing rates of patients with the smallest RFDs were significantly higher while firing rates of patients with the greatest RFDs were significantly lower than control values. The AHP shortening in the early stages of muscle impairment is consistent with the decrease in firing threshold of ‘fast' MNs found in spinal cord slices from neonatal SOD1 mice. The later elongation of the AHP may be caused by the higher vulnerability of ‘fast' MNs to degeneration and by the influence of reinnervation. Our results are comparable to what has been observed in acute experiments in animal models, providing a bridge between animal and clinical research that may be relevant for identification of mechanism(s) underlying neurodegeneration in ALS.

Influence of contraction strength on single motor unit synchronous activity

The influence of contraction strength on motoneurone (MN) synchrony is poorly documented. With stronger contraction, more common and/or synchronized inputs might contribute to greater MN drive and generate more synchronous firings. This effect might be counterbalanced, however, by a negative impact of MN faster firing rates on synaptic effectiveness. Pairs of motor units (MUs) were tested at various force levels, in 2-s sequences. MN synchrony was assessed using the index k', the synchronous impulse probability (SIP), and the synchronous impulse frequency (SIF) in cross-correlograms. MU inter-spike interval duration and variability, surface EMG activity and force output were evaluated concurrently. Both SIP and SIF increased with contraction strength, whereas k' remained unaffected. Faster firing rates and stronger contraction had the greatest effects on SIF. By testing the same MUs at different force levels, we showed that contraction strength does influence MN synchrony. The enhancement of MU synchrony with stronger contraction suggests an efficient contribution of more common and/or synchronized inputs. Force output must be controlled when assessing MN synchrony. Normalizing MU synchronous activity per reference spike is preferable to minimize the influence of firing rate. This is particularly relevant for clinical research, in conditions of poorer neuromuscular control.

Enhancement of single motor unit inhibitory responses to transcranial magnetic stimulation in amyotrophic lateral sclerosis

In healthy human subjects, transcranial magnetic stimulation (TMS) applied to the motor cortex induces concurrent inhibitory and excitatory effects on motoneurone activity. In amyotrophic lateral sclerosis (ALS), a neurodegenerative disease affecting both cortical and spinal motor neurons, paired-pulse studies based on electromyographic (EMG) recording have revealed a decrease in TMS-induced inhibition. This suggested that inhibition loss may promote excito-toxicity in this disease. Against this hypothesis, an abnormally high incidence of inhibitory responses to TMS has been observed in the peristimulus time histograms (PSTHs) in ALS single motor unit studies. The disappearance of cortico-motoneuronal excitatory inputs might, however, have facilitated the detection of single motor unit inhibitory responses in the PSTHs. This question was addressed here using a new approach, where the strength of the excitatory and inhibitory effects of TMS on motoneurone activity was assessed from the duration of inter-spike intervals (ISIs). This analysis was conducted on single motor unit (MU), tested on healthy subjects and patients with ALS or Kennedy's disease (KD), a motor neuron disease which unlike ALS, spares the cortico-spinal pathway. MUs tested on KD patients behaved like those of healthy subjects unlike those tested on ALS patients. The present data reveal that in ALS, the TMS-induced inhibitory effects are truly enhanced during voluntary contractions and not reduced, as observed in paired-pulse TMS studies under resting conditions. The possible contribution of inhibitory loss to the physiopathology of ALS therefore needs to be reconsidered. The present data do not support the idea that inhibition loss may underlie excito-toxicity in ALS.

Firing pattern of fasciculations in ALS

In amyotrophic lateral sclerosis (ALS), the origin of fasciculations is disputed. We hypothesized that the discharge pattern of fasciculation potentials (FPs) would be different for FPs arising in the motor axon or in the spinal motor neuron. FPs were recorded by high-density surface EMG of the biceps brachii or vastus lateralis muscle for 15 minutes in 10 patients with ALS. Records were decomposed into different FP waveforms and their firing moments. Interspike interval (ISI) histograms were constructed for FPs that fired more than 100 times. Two types of ISI histograms were found. 1) In 23 of 30 different FPs with a total of 8,597 ISIs, the refractory period was 3 to 4 msec. ISIs longer than 15 msec had a Poisson distribution. Five of these 23 FPs discharged doublets with an ISI of approximately 5 msec, indicative of supernormality. This is consistent with the FPs arising in motor axons. 2) In the other 7 FPs, accounting for 11,266 ISIs, the refractory period was 17 to 46 msec. The preferred ISI duration was around 80 msec. Both timing factors are consistent with origin in the spinal motor neuron. Firing pattern analysis, based on high-density surface EMG, can detect fasciculation potentials (FPs) of axonal and neuronal origin in amyotrophic lateral sclerosis. The two FP types coexist within the same muscle. The recognition that clinically identical fasciculations conceal the existence of two types of FP that can be studied in a noninvasive manner will introduce a new aspect in the research of motor neuron disease.

Effects of the action of microwave-frequency electromagnetic radiation on the spike activity of neurons in the supraoptic nucleus of the hypothalamus in rats

Acute experiments on white rats anesthetized with Nembutal (40 mg/kg, i.p.) were performed with extracellular recording and analysis of background spike activity from neurons in the supraoptic nucleus of the hypothalamus after exposure to electromagnetic radiation in the millimeter range. The distribution of neurons was determined in terms of the degree of regularity, the nature of the dynamics of neural streams, and the modalities of histograms of interspike intervals; the mean neuron spike frequency was calculated, along with the coefficient of variation of interspike intervals. These studies demonstrated changes in the background spike activity, predominantly affecting the internal structure of the spike streams recorded. The major changes were in the duration of interspike intervals and the degree of regularity of spike activity. Statistically significant changes in the mean spike frequencies of neuron populations in individual frequency ranges were also seen.

Mechanisms of Disorders of the Characteristics of Fine Movements in Long-Term Hypokinesia

The precision and electromyographic characteristics of single-joint voluntary movements of the human foot, of the programmed and tracking types, were studied, along with the characteristics of rhythmic activity of motor units before and during 120-day antiorthostatic hypokinesia. This latter was accompanied by significant decreases in the precision of the control system, evident as decreases in the number of discriminable force gradations, increases in the absolute and differential thresholds for movements of the programmed type, and sharp increases in the variability of integrated EMG traces from the working muscles during tracking movements. The direction and dynamics of changes in the activity of motor units at different stages of antiorthostatic hypokinesia were different: during the first 14-30 days (stage I), there was a sharp increase in the variability of interspike intervals and an increase in the extent of synchronization of motor unit activity; from day 30 onwards (stage II), there was a reproducible decrease in the duration of interspike intervals, along with disappearance of the synchronization of motor units, while the high level of variability of spike activity persisted. The results obtained here suggest that impairments of precision during stages I and II of antiorthostatic hypokinesia are different in nature and are associated with reflex responses to support unloading at stage I and with atrophic processes in muscles in stage II.

Rhythmic properties of neurons in the rostral ventrolateral medulla of the rat in vitro: effects of clonidine.

The rostral ventrolateral medulla (RVLM) is thought to be the main central site for generation of tonic sympathetic activity. In the rat in vitro slice preparation, we used intracellular recordings to identify different populations of neurons in the RVLM: 43 spontaneously active neurons with regular (R) or irregular (I) patterns of spike firing and 10 silent neurons. The degree of regularity was quantified by the coefficient of variation (CV = SD/mean) of interspike interval durations, as well as by the rhythmic properties of the spike autospectrum and autocorrelation. The distribution of CVs was clustered: R and I neurons were defined as those with CVs </=12% (n = 21) or >12% (n = 22), respectively. The R-type and I-type neurons resemble the type II and type I neurons, respectively, which were previously characterized in the RVLM in vivo as barosensitive and bulbospinal. Both types may be important in generation of sympathetic tone. Clonidine (1-100 microM) was applied to 10 R-type neurons and 16 I-type neurons. The firing of 21/26 was depressed to the point of silence. However, 18/26 neurons were excited earlier in the perfusion. The later depression of firing occurred in both I and R neurons and in different cases was associated with either hyperpolarization or depolarization.

Human motor unit recruitment and derecruitment during long lasting intermittent contractions

Seven healthy subjects were investigated in cyclic ramp-and-hold long lasting isometric contractions. Wire branched electrodes were used for selective recording of single motor unit (MU) potentials from m. biceps brachii. MU behaviour was defined in terms of recruitment/derecruitment thresholds (RT and DT) and the duration of interspike intervals (ISI). A total of 63 MUs was investigated: 40 units were active from the beginning of the task performance and another 23 were recruited later. There were no changes in the recruitment pattern of MUs with fatigue development — a short first ISI followed by a very long second one and an almost constant firing rate after this transient phase. The tendency of RT to gradually decrease dominates the results. Thus, the required constant rate of force increase with fatigue development was maintained mostly by the mechanisms of space coding (i.e., decrease of RT and recruitment of additional MUs). Oppositely, the time behaviour of the DT changes was not uniform and rate coding was an essential mechanism in the adaptation of MU activity to muscle fatigue during relaxation phases. The recruitment pattern and fatigue related behaviour of the additionally recruited MUs were similar to those of MUs active from the first cycle of the motor task performance.

Interspike Intervals, Receptive Fields, and Information Encoding in Primary Visual Cortex

In the primate primary visual cortex (V1), the significance of individual action potentials has been difficult to determine, particularly in light of the considerable trial-to-trial variability of responses to visual stimuli. We show here that the information conveyed by an action potential depends on the duration of the immediately preceding interspike interval (ISI). The interspike intervals can be grouped into several different classes on the basis of reproducible features in the interspike interval histograms. Spikes in different classes bear different relationships to the visual stimulus, both qualitatively (in terms of the average stimulus preceding each spike) and quantitatively (in terms of the amount of information encoded per spike and per second). Spikes preceded by very short intervals (3 msec or less) convey information most efficiently and contribute disproportionately to the overall receptive-field properties of the neuron. Overall, V1 neurons can transmit between 5 and 30 bits of information per second in response to rapidly varying, pseudorandom stimuli, with an efficiency of approximately 25%. Although some (but not all) of our results would be expected from neurons that use a firing-rate code to transmit information, the evidence suggests that visual neurons are well equipped to decode stimulus-related information on the basis of relative spike timing and ISI duration.

Temporally structured impulse activity in spontaneously discharging somatosensory cortical neurons in the awake cat:: recognition and quantitative description of four different patterns of bursts, post-recording GFAP immunohistology and computer reconstruction of the studied cortical surface

We elaborated two methods used in two previous publications [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129–140 [16]; H.H. Webster, I. Salimi, A.A. Myasnikov, R.W. Dykes, The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats, Brain Res. 750 (1997) 109–121 [21]]: (A) a procedure for detecting and classifying brief epochs of high-frequency extracellular impulse activity (bursts) recorded chronically in the somatosensory cortex of the awake cat, and (B) a modification of an immunohistochemical technique [L.A. Bevento, L.B. McCleary, An immunochemical method for marking microelectrode tracks following single-unit recordings in long surviving, awake monkeys, J. Neurosci. Meth. 41 (1992) 199–204 [5]] for visualization of electrode tracks and electrolytic lesions around the tip of tungsten-in-glass microelectrodes [D.M.D. Landis, The early reactions of non-neuronal cells to brain injury, Annu. Rev. Neurosci. 17 (1994) 133–151 [15]] weeks after lesions were made in cortex. The burst recognition and classification method uses an interval threshold to determine the beginning and end of one epoch [M. Armstrong-James, K. Fox, Effects of ionophoresed noradrenaline on spontaneous activity of neurons in rat primary somatosensory cortex, J. Physiol. (London), 335 (1983) 427–447 [3]] in the original sequence of interspike intervals (ISIs) to segregate and analyze separately a burst. The threshold is based on the duration of the shortest modal ISI found in the autocorrelogram [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129–140 [16]]. The technique allowed recognition of bursts with several distinctive patterns: (i) an initial, longer ISI followed by progressively shorter ones; (ii) an initially shorter ISI followed by progressively longer ones; (iii) patterns where the intermediate ISI could be either longer or shorter than surrounding ones; and (iv) consecutive ISIs of relatively equal duration. Among the cells discharging in bursts with equal ISIs, the technique allows recognition of cells generating only short (up to three to five intervals) bursts, and others generating mixtures of a short and long (up to six or more intervals) bursts. Finally, frequency distributions of the probability of encountering bursts having intervals of a stated length is described. The visualization of tracks from chronic recording experiments is important for relating neuronal function to a specific cytoarchitectural region and a specific cortical layer. Several modifications of the procedure of immunostaining for GFAP allows identification of recording sites in clearer relationship to the cytoarchitectonic details of cat somatosensory cortex. Themes: Sensory systems Topics: Somatosensory cortex and thalamocortical relationships

The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats

Single neurons ( n=356) were studied in the forelimb representation of awake, quietly resting cats. Thirty-five spontaneously bursting neurons in a sample of 206 cells recorded before forelimb deafferentation were compared to 39 spontaneously bursting neurons in a sample of 127 neurons studied 1–3 weeks after deafferentation. The probability of encountering bursting neurons increased significantly following deafferentation from 17% to 31% of the sample ( P<0.005). The same 5 classes of bursting cells were observed after deafferentation but there were significant changes in the duration of interspike intervals in some classes, in the probability of observing certain classes, and in the proportion of spikes found in bursts. The probability of encountering class III cells, a class thought to consist primarily of non-inactivating pyramidal burst neurons, nearly doubled and the average interspike interval length within the burst increased from 1.9 to 3.0 ms. The burst structure in the other classes did not change but they were found less frequently. These other classes may include inhibitory interneurons which receive less excitatory drive after deafferentation and therefore provide less inhibition to class III cells. The differential behavior of the different classes of bursting cells may be one reason why the overall level of spontaneous activity does not change after deafferentation and it suggests that there are homeostatic mechanisms in primary somatosensory cortex that maintain a certain level of neural activity.

Computation of long-distance propagation of impulses elicited by Poisson-process stimulation

1. The purpose of this work was to determine whether computed temporally coded axonal information generated by Poisson process stimulation were modified during long-distance propagation, as originally suggested by S. A. George. Propagated impulses were computed with the use of the Hodgkin-Huxley equations and cable theory to simulate excitation and current spread in 100-microns-diam unmyelinated axons, whose total length was 8.1 cm (25 lambda) or 101.4 cm (312.5 lambda). Differential equations were solved numerically, with the use of trapezoidal integration over small, constant electrotonic and temporal steps (0.125 lambda and 1.0 microsecond, respectively). 2. Using dual-pulse stimulation, we confirmed that for interstimulus intervals between 5 and 11 ms, the conduction velocity of the second of a short-interval pair of impulses was slower than that of the first impulse. Further, with sufficiently long propagation distance, the second impulse's conduction velocity increased steadily and eventually approached that of the first impulse. This effect caused a spatially varying interspike interval: as propagation proceeded, the interspike interval increased and eventually approached stabilization. 3. With Poisson stimulation, the peak amplitude of propagating action potentials varied with interspike interval durations between 5 and 11 ms. Such amplitude attenuation was caused by the incomplete relaxation of parameters n (macroscopic K-conductance activation) and h (macroscopic Na-conductance inactivation) during the interspike period. 4. The stochastic properties of the impulse train became less Poisson-like with propagation distance. In cases of propagation over 99.4 cm, the impulse trains developed marked periodicities in Interevent Interval Distribution and Expectation Density function because of the axially modulated transformation of interspike intervals. 5. Despite these changes in impulse train parameters, the arithmetic value of the mean interspike interval did not change as a function of propagation distance. This work showed that in theory, whereas the pattern of Poisson-like impulse codes was modified during long-distance propagation, their mean rate was conserved.

Reciprocal inhibition of human motoneurons with different firing frequencies

A study was made of the effect of reciprocal inhibition on individual firing motoneurons in the extensor carpi ulnaris and soleus muscle in human subjects. Peristimulus histograms (PSH) were plotted at different average frequency of motoneuron firing (\(\bar f\)) and the change in duration of interspike intervals (ISI) was analyzed. For reciprocal inhibition, as for other types of inhibition,\(\bar f\) is a factor in the effectiveness of motoneuron inhibition. The duration of inhibition apparent in the PSH, the sizes of zones of inhibition effectiveness in the ISIs and lengthening of the intervals are dependent on\(\bar f\). For all motoneurons, the low\(\bar f\) range is most favorable for effective inhibition. The dependence of effectiveness of a volley on the time of its arrival within the ISI is also analyzed.