Single Spike Trains Research Articles

Coding of time-varying signals in spike trains of linear and half-wave rectifying neurons

The encoding of time-varying stimuli in linear and half-wave rectifying neurons is studied. The information carried in single spike trains is assessed by reconstructing part of the stimulus using mean square estimation methods. For the class of models considered here, the mean square error in the reconstructions and estimates of the rate of information transmission are computed analytically. The optimal encoding of stimuli having statistical properties of natural images predicts a change in the temporal filtering characteristics with mean firing rate. This change relates to those observed experimentally at the early stages of visual processing. The transmission of information by model neurons is shown to be fundamentally limited to a maximum of 1.13 bit/spike and it is conjectured that nonlinear processing is necessary to explain higher rates which have been observed experimentally in certain preparations. In spite of the fact that single neurons might not transmit information efficiently, a substantial part of a time-varying stimulus can be recovered from single spike trains. In particular, our results demonstrate that a small number of 'noisy' neurons can carry precise temporal information in their spike trains.

Bursting and oscillating neurons of the cat basolateral amygdaloid complex in vivo: electrophysiological properties and morphological features.

1. To characterize the physiological properties of lateral and basolateral (BL) amygdaloid neurons, intracellular recordings were performed in barbiturate-anesthetized cats. Morphological identification of recorded cells was achieved by intracellular injection of neurobiotin. Two types of physiologically identified projection neurons were distinguished in the BL and lateral nuclei. 2. The first type of neurons prevailed in the BL nucleus (80% of BL cells). Their resting membrane potential (Vm) averaged -66 +/- 4.9 (SE) mV. They generated stereotyped spike doublets or bursts in response to threshold depolarizing pulses. In most cells, depolarizing pulses of higher amplitude elicited spike bursts or doublets at a shorter latency followed by a nonadapting train of single spikes whose frequency rose with the amplitude of the current pulses. However, 15% of BL bursting neurons generated repetitive spike bursts or doublets in response to prolonged depolarizing current pulses. The response of BL bursting neurons to hyperpolarizing current pulses revealed the presence of slow inward rectification in the form of a depolarizing sag, thus suggesting the presence of a hyperpolarization-activated current. 3. The second type of neurons prevailed in the lateral nucleus. Their resting Vm was quite polarized (-74 +/- 2.85 mV) and they generated slow Vm oscillations (2-10 Hz) upon steady depolarization beyond congruent to -62 mV. The frequency of the oscillation increased with the amount of depolarizing current. In the majority of cells, analysis of voltage responses to subthreshold current pulses revealed the presence of voltage- and time-dependent rectification in the depolarizing direction. Current pulses that brought the Vm to -65 mV and beyond elicited a voltage response that reached an early peak and then decayed. Increasing the amplitude of the pulse decreased the latency of the early peak until it triggered an action potential. Current-voltage plots demonstrated inward rectification in the depolarizing direction. At the break of hyperpolarizing current pulses applied at depolarized levels, the Vm overshot prepulse values and generated one or more oscillatory cycles. 4. An important proportion of bursting and oscillating neurons (45.8% and 29%, respectively) were physiologically identified as projection neurons by antidromic invasion from the basal forebrain, entorhinal cortex, or perirhinal cortex. The conduction velocity of bursting and oscillating neurons estimated from the latency of antidromic spikes was low (< or = 2.5 m/s). 5. Most bursting and oscillating neurons of the BL nucleus were spiny cells with a pyramidal morphology. Four to eight dendritic trunks emerged from the apex, base, and sides of their triangular soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Phase locking of neuronal responses to the vertical refresh of computer display monitors in cat lateral geniculate nucleus and striate cortex

The proliferation of low-cost microcomputer systems has led to the use of these systems as alternatives to expensive display devices for visual physiology and psychophysics experiments. The video displays of these systems often lack the flexibility of achieving wide linear luminance ranges and high vertical refresh rates — two parameters which may influence data acquisition. We have examined the responses of neurons and pairs of neurons in cat LGN and striate cortex to bar and sinusoidal grating stimuli generated by a conventional PC-based VGA graphics card and displayed on a NEC Multisync + color monitor with a 60 Hz vertical (display) refresh rate. Responses to these stimuli were autocorrelated and power spectral densities (PSD) were calculated, revealing that the majority of simple and complex cortical cells and nearly all LGN cells exhibited significant peaks in their autocorrelations at 16.7 ms and in the PSD at 60 Hz. Responses to identical stimuli generated with an optical bench using an incandescent light source contained no power at 60 Hz. Furthermore, cross-correlations between the spike trains of neuron-pairs were severely contaminated by peaks directly attributable to the entrainment of the two elements of the pair to the vertical refresh signal. Thus, we suggest that the use of conventional computer displays introduces a temporal artifact into neuronal spike trains in both single and multiple spike train analysis.

Changes of single unit activity in the cat's auditory thalamus and cortex associated to different anesthetic conditions

Single unit spike trains were recorded in the auditory cortex ( n=78) and in the auditory thalamus ( n=55) of nitrous oxide anesthetized cats. The electrophysiological activity was studied before and during the application of pentobarbital (P, 7 mg/kg), ketamine (K, 12 mg/kg) and a mixture of these anesthetics (KP). The units were characterized during the spontaneous and acoustically driven activity (‘white’ noise and pure tone bursts). For the majority of cortical (61%) and thalamic (83%) units both drugs tended to decrease the spontaneous firing rate, but affected differently its time structure: P tended to increase the average size of burst discharges, whereas K and KP tended to decrease it. In the cortex the peak firing rate evoked by ‘white’ noise tended to be decreased, whereas stronger excitatory responses were observed in the thalamus after injection of K or KP. The overall effect of the anesthetics during stimulation by pure tones was an increase in tonal selectivity due to a decrease in the response bandwidth. The response pattern to tones was also sometimes affected by the drugs. The direct evidence reported here for significant alterations of the discharge properties of auditory neurons in the thalamus and cortex resulting from low dose administration of K and/or P emphasizes difficulties in comparing data derived from experiments conducted in various conditions of anesthesia or in the awake state.

Oscillation and noise determine signal transduction in shark multimodal sensory cells.

Oscillating membrane potentials that generate rhythmic impulse patterns are considered to be of particular significance for neuronal information processing. In contrast, noise is usually seen as a disturbance which limits the accuracy of information transfer. We show here, however, that noise in combination with intrinsic oscillations can provide neurons with particular encoding properties, a discovery we made when recording from single electro-sensory afferents of a fish. The temporal sequence of the impulse trains indicates oscillations that operate near the spike-triggering threshold. The oscillation frequency determines the basic rhythm of impulse generation, but whether or not an impulse is actually triggered essentially depends on superimposed noise. The probability of impulse generation can be altered considerably by minor modifications of oscillation baseline and amplitude, which may underlie the exquisite sensitivity of these receptors to thermal and electrical stimuli. Additionally, thermal, but not electrical, stimuli alter the oscillation frequency, allowing dual sensory messages to be conveyed in a single spike train. These findings demonstrate novel properties of sensory transduction which may be relevant for neuronal signalling in general.

Single neurons are differently involved in stimulus-specific oscillations in cat visual cortex.

Synchronised oscillatory population events (35-80 Hz; 60-300 ms) can be induced in the visual cortex of cats by specific visual stimulation. The oscillatory events are most prominent in local slow wave field potentials (LFP) and multiple unit spikes (MUA). We investigated how and when single cortical neurons are involved in such oscillatory population events. Simultaneous recordings of single cell spikes, LFP and MUA were made with up to seven microelectrodes. Three states of single cell participation in oscillations were distinguished in spike triggered averages of LFP or MUA from the same electrode: (1) Rhythmic states were characterised by the presence of rhythmicity in single cell spike patterns (35-80 Hz). These rhythms were correlated with LFP and MUA oscillations. (2) Lock-in states lacked rhythmic components in single cell spike patterns, while spikes were phase-coupled with LFP or MUA oscillations. (3) During non-participation states LFP or MUA oscillations were present, but single cell spike trains were neither rhythmic nor phase coupled to these oscillations. Stimulus manipulations (from "optimal" to "suboptimal" for the generation of oscillations) often led to systematic transitions between these states (from rhythmic to lock-in to non-participation). Single cell spike coupling was generally associated with negative peaks in LFP oscillations, irrespective of the cortical separation of single cell and population signals (0-6 mm). Our results suggest that oscillatory cortical population activities are not only supported by local and distant neurons with rhythmic spike patterns, but also by those with irregular patterns in which some spikes occur phase-locked to oscillatory events.

Changes in motor unit synchronization following central nervous lesions in man.

1. Single motor unit spike trains have been recorded during voluntary isometric contraction of the affected intrinsic hand muscles of patients with unilateral central nervous lesions. These have been compared with similar recordings made from the patients' unaffected hand muscles and with recordings made from the hand muscles of healthy subjects. 2. Cross-correlation analysis was performed between the times of occurrence of the motor unit spike trains. The time course of central cross-correlogram peaks constructed for normal subjects and stroke patients was used to infer properties of the underlying common EPSPs and the impulse-generating properties of the motoneurones. The results of this analysis were compared between the two groups. In addition, the size and time course of cross-correlogram peaks obtained from the patients were related both to the patients' clinical state and to their hand and fine finger function. 3. Central nervous lesions were found to result in either a narrowing or broadening of the time course of motor unit synchronization. These changes were attributed either to an increase in the size of common EPSPs with respect to synaptic noise, or to the effects of presynaptic synchronization of motoneurone inputs. 4. Longitudinal studies of motor unit discharges in the year following the stroke demonstrated, in some patients, differences in the level of motor unit synchronization. These paralleled improvements in the patients' fine motor control. Pooled data from patients with varying deficits of fine motor control confirmed that loss or reduction of motor unit synchronization was associated with a corresponding slowing in the performance of rapidly alternating finger movements. 5. The results of the present study suggest that the branched common presynaptic inputs that generate motor unit synchronization are either of corticospinal tract origin or are intimately dependent on its function. Differences in the strength and time course of motor unit synchronization are demonstrated that may reflect the altered behaviour of presynaptic inputs to motoneurones following central nervous damage in man.

Ultrastructure of the extraocular photoreceptor in the genitalia of a butterfly, Papilio xuthus

This paper describes the ultrastructure of a sensory neuron found in the extraocular photoreceptive site on the butterfly genitalia. Our previous studies have shown that there are two pairs of the photoreceptive sites in a butterfly (four per individual). Each photoreceptive site is recognizable by a transparent area in the pigmented cuticle of the genitalia. From the nerve that extends from the transparent cuticle to the last abdominal ganglion, a sustained train of single unit spikes can be recorded in response to a light flash. The single unit spikes disappear when the transparent cuticle is covered, thus indicating that a single photoreceptor exists close to it. Here, we examined complete serial semithin sections of plastic-embedded photoreceptive sites of both male and female, and observed an ovoid structure close to the transparent cuticle. The structure contained the cell body of a sensory neuron whose axon was in the nerve branch from which the photoresponse had been recorded. Further electron microscopy revealed that the cell body extended a few distal processes that protrude tubular membrane from the tip, forming a structure resembling the phaosome (from Greek; phaos = light, some = body) which was first described in the earthworm dermal photoreceptors. The sensory neuron was also found in a surgically isolated nerve-photoreceptor preparation that responded to the light. We therefore propose that the phaosome-containing sensory neuron is the butterfly genital photoreceptor.

Delta frequency (1–4 Hz) oscillations of perigeniculate thalamic neurons and their modulation by light

Neurons in the perigeniculate sector of the reticular thalamic nuclear complex were recorded extra- and intracellularly under deep urethane anesthesia. They were identified by burst responses to optic chiasm stimulation and depolarizing spindle oscillations in response to internal capsule stimulation. Perigeniculate neurons displayed oscillations within the frequency range of electroencephalogram delta waves (1–4 Hz). One-third of extracellularly recorded neurons discharged rhythmic (2.5–4 Hz), highfrequency (150–200 Hz) spike bursts. This was similar to an intrinsic oscillation that was recently observed in dorsal lateral geniculate cells studied in vitro and in vivo. Other oscillating neurons displayed trains of single spikes (20–50 Hz) crowning rhythmic (2.5–4 Hz) depolarizing envelopes that were best expressed at the “resting” membrane potential (−60 to −65 mV). It is suggested that this oscillation reflects synaptic drives from dorsal lateral geniculate neurons. Changes in ambient room luminosity disrupted both types of delta rhythms. These data demonstrate for the first time that delta oscillations are present in the visual sector of the reticular thalamic nucleus. The results suggest that the two types of delta rhythmicity result from intrinsic and network properties of visual thalamic neurons and that perigeniculate cells may synchronize, through backward connections, the activity of dorsal lateral geniculate cells during deep stages of resting sleep.

A comparison of the electrophysiological properties of morphologically identified cells in layers 5B and 6 of the rat neocortex

In vitro studies performed in mammalian brain slices have shown that cortical neurons differ in their intrinsic membrane properties. In the rodent cortex these properties are related to a specific cell morphology and synaptic connectivity in some cells but not in others. Due to their small size, little is known about the intrinsic membrane properties of layer 6 cells, however and it is not clear whether cell morphology is related to electrophysiological properties in this layer. We used a combination of intracellular recording and dye-filling to study the electrophysiological and morphological characteristics of layer 6 cells of the rat sensorimotor cortex in vitro and compared their properties to those of large layer 5B pyramidal cells. Our sample of 24 filled and anatomically reconstructed cells in layer 6 confirms previous Golgi studies that showed them to be a morphologically diverse group consisting of regularly and irregularly oriented pyramidal cells and spiny nonpyramidal cells. Regular layer 6 pyramidal cells differed with respect to the length of their apical dendrites and extent of their axonal arborizations, while irregularly oriented pyramidal cells consisted of sideways or inverted pyramidal cells of variable size and morphology. Spiny nonpyramidal cells included bi-tufted and multipolar cell types that differed in size and extent of dendritic trees. Many layer 6 cells showed long horizontal axon collaterals in layer 6 and an oblique or vertical projection to layer 4. Stimulation with intracellular constant current pulses revealed that the morphological diversity was mirrored by a similar electrophysiological diversity. Most layer 6 cells were capable of firing trains of action potentials characterized by an initial doublet or triplet followed by a train of single spikes (phasic-tonic mode). The majority of layer 6 cells could fire in either a tonic (single spikes only) mode with low strength current input and a phasictonic pattern with higher current strengths. A minority fired either always phasic-tonic or tonic-only spike trains. The size and sequence of spike afterpotentials during low-rate repetitive firing was highly variable in layer 6 cells suggesting that the relative importance of ionic currents responsible for spike repolarization and afterpotentials varied from cell to cell. Subthreshold responses showed prominent inward rectification, while hyperpolarizing “sag” was present in most cells tested. In comparison, large layer 5B pyramidal cells fired either phasic-tonic only or both phasic-tonic and tonic patterns. A minority of cells were capable of firing repetitive bursts, while the remainder fired repetitive single spikes. Spike half-width was significantly shorter in large layer 5B pyramidal cells compared with layer 6 cells, but other membrane properties were not different. We were unable to establish a clear relationship between cell morphology and intrinsic electrical properties among layer 6 cells, possibly due to the large variety of cell types. Further studies will be needed to investigate specific morphological and electrophysiological subclasses of layer 6 cells and investigate how their properties relate to corticocortical and thalamocortical processing.

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 α-adrenergic) transmission.

Different cellular types in mesopontine cholinergic nuclei related to ponto-geniculo-occipital waves

The only mesopontine neurons previously described as involved in the transfer of ponto-geniculo-occipital (PGO) waves from the brain stem to the thalamus were termed PGO-on bursting cells. We have studied, in chronically implanted cats, neuronal activities in brain-stem peribrachial (PB) and laterodorsal tegmental (LDT) cholinergic nuclei in relation to PGO waves recorded from the lateral geniculate (LG) thalamic nucleus during rapid-eye-movement (REM) sleep. We constructed peri-PGO histograms of PB/LDT cells' discharges and analyzed the interspike interval distribution during the period of increased neuronal activity related to PGO waves. Six categories of PGO-related PB/LDT neurons with identified thalamic projections were found: 4 classes of PGO-on cells: PGO-off but REM-on cells: and post-PGO cells. The physiological characteristics of a given cell class were stable even during prolonged recordings. One of these cell classes (1) represents the previously described PGO-on bursting neurons, while the other five (2-6) are newly discovered neuronal types. (1) Some neurons (16% of PGO-related cells) discharged stereotyped low-frequency (120-180 Hz) spike bursts preceding the negative peak of the LG-PGO waves by 20-40 msec. These neurons had low firing rates (0.5-3.5 Hz) during all states. (2) A distinct cell class (22% of PGO-related neurons) fired high-frequency spike bursts (greater than 500 Hz) about 20-40 msec prior to the thalamic PGO wave. These bursts were preceded by a period (150-200 msec) of discharge acceleration on a background of tonically increased activity during REM sleep. (3) PGO-on tonic neurons (20% of PGO-related neurons) discharged trains of repetitive single spikes preceding the thalamic PGO waves by 100-150 msec, but never fired high-frequency spike bursts. (4) Other PGO-on neurons (10% of PGO-related neurons) discharged single spikes preceding thalamic PGO waves by 15-30 msec. On the basis of parallel intracellular recordings in acutely prepared, reserpine-treated animals, we concluded that the PGO-on single spikes arise from conventional excitatory postsynaptic potentials and do not reflect tiny postinhibitory rebounds. (5) A peculiar cellular class, termed PGO-off elements (8% of PGO-related neurons), consisted of neurons with tonic, high discharge rates (greater than 30 Hz) during REM sleep. These neurons stopped firing 100-200 msec before and during the thalamic PGO waves. (6) Finally, other neurons discharged spike bursts or tonic spike trains 100-300 msec after the initially negative peak of the thalamic PGO field potential (post-PGO elements, 23% of PGO-related neurons).(ABSTRACT TRUNCATED AT 400 WORDS)

Burst generating and regular spiking layer 5 pyramidal neurons of rat neocortex have different morphological features

Intracellular recordings were obtained from pyramidal neurons in layer 5 of rat somatosensory and visual cortical slices maintained in vitro. When directly depolarized, one subclass of pyramidal neurons had the capacity to generate intrinsic burst discharges and another generated regular trains of single spikes. Burst responses were triggered in an all-or-none manner from depolarizing afterpotentials in most bursting neurons. Regular spiking cells responded to electrical stimulation of ascending afferents with a typical EPSP-IPSP sequence, whereas IPSPs were hard to detect in bursting cells. Orthodromic activation of the latter evoked a prominent voltage-dependent depolarization that could trigger a burst response. Intracellularly labelled bursting and regular spiking cells were located in layer 5b, but had distinctly different morphologies. Bursting neurons had a large pyramidal soma, a gradually emerging apical dendrite, and an extensive apical and basal dendritic tree. Their axonal collateral arborization was predominantly limited to layers 5/6. In contrast, regular spiking cells had a more rounded soma with abruptly emerging apical dendrite, a smaller dendritic arborization, and 2 to 8 ascending axonal collaterals that arborized widely in the supragranular layers. Both bursting and regular spiking cells had main axons that entered the subcortical white matter. These data show that some subgroups of pyramidal neurons within the deeper parts of layer 5 of rat cortex are morphologically and physiologically distinct and have different intracortical connections. Bursting cells presumably function to amplify and synchronize cortical outputs, whereas regular spiking output neurons provide excitatory feedback to neurons at all cortical levels and receive a more effective orthodromic inhibitory input. These data support the hypothesis that differences in gross neuronal structure, perhaps even the subtle differences that distinguish subclasses of neurons in a given lamina, are predictive of underlying differences in the type and distribution of ion channels in the nerve cell membrane and connections of cells within the cortical circuit.

Phasic activation of lateral geniculate and perigeniculate thalamic neurons during sleep with ponto-geniculo-occipital waves

Ponto-geniculo-occipital (PGO) waves are spiky field potentials generated in cholinergic nuclei of the dorsolateral mesopontine tegmentum just prior to and during rapid-eye-movement (REM) sleep and transferred toward thalamic nuclei. These events are commonly regarded as physiological correlates of oneiric behavior. We have examined the PGO-related discharges of physiologically identified neurons located in the dorsal lateral geniculate (LG) nucleus and perigeniculate (PG) sector of the reticular thalamic complex in chronically implanted, naturally sleeping cats. PGO focal waves and associated unit discharges were simultaneously recorded by the same microelectrode. PGO waves herald the other signs of REM sleep (EEG desynchronization and muscular atonia), appearing 30-90 sec before REM sleep over the EEG-synchronized activity of slow-wave sleep (pre-REM epoch). (1) Most PG neurons discharged bursts of action potentials in relation to PGO waves during both pre-REM and REM sleep. (2) The PGO-related activity of LG neurons was quite different. During the pre-REM stage, PGO waves correlated with a short (7-15 msec), high-frequency (300-500 Hz) spike burst of LG neurons, followed by a long (0.2-0.4 sec) train of single spikes, whereas during REM sleep, the PGO-related activity lacked the initial burst and consisted of a spike train that only slightly exceeded the tonically increased background firing of LG cells. The stereotyped characteristics of the PGO-related spike bursts during the pre-REM epoch suggest that they are the extracellular reflection of a low-threshold spike deinactivated by the tonic membrane hyperpolarization of LG cells associated with the EEG-synchronized sleep state. Such bursts are inactivated during the tonic depolarization of LG cells that occurs in REM sleep. The synchronous spike bursts discharged by LG cells in relation with the PGO waves of the pre-REM epoch probably underlie the much larger amplitude of the PGO waves of the pre-REM epoch as compared with those of the REM-sleep state. Since LG neurons have relatively low spontaneous firing rates during the EEG-synchronized pre-REM epoch, the PGO-related activity of this transitional stage leads to a higher signal-to-noise ratio in the visual thalamocortical channel than during REM sleep. We suggest that the PGO-related activity during the pre-REM epoch is related to vivid imagery during this stage of sleep.

Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex

Intracellular recordings were made from neurons of the mouse somatosensory cortex isolated in vitro. Two physiologically distinct classes of pyramidal cells were observed: regular-spiking cells were the majority, and generated accommodating trains of single spikes; bursting cells generated clusters of 2–5 action potentials, and clusters were separated by prolonged afterhyperpolarizations. The bursting cells of the mouse neocortex were unusual in producing repetitive bursting during sustained current pulses, and in being localized to a laminar zone straddling layers V and VI.

Spontaneous neuronal firing patterns in fetal rat cortical networks during development in vitro: a quantitative analysis.

The development of spontaneous bioelectric activity (SBA) was studied in dissociated occipital cortex cultures prepared from 19 day old rat fetuses. All cultures, recorded one per diem from 5 to 33 days in vitro (div), showed SBA. Computer analysis of 76 extracellularly recorded single unit spike trains was carried out after selection on the basis of stationarity criteria. Statistically significant developmental trends were found in (i) interspike interval dependencies and (ii) fluctuations in mean firing rate, on the order of a minute or longer. The highly dependent firing patterns, including stereotyped bursting, were present mostly in the 9-12 div group, whereas minute-to-minute fluctuations in the intensity of firing were considerably more pronounced in the oldest group (22-33 div) than in the younger cultures. In addition, firing categories defined on the basis of factor-analysis revealed that such fluctuations were almost exclusively to be found in neurons which fired in a pronounced 'burst', rather than a relatively continuous fashion. Only a few mature appearing synaptic structures were observed electron microscopically prior to 12 div, but increased steadily in number thereafter. No cultures prior to 14 div, but all cultures older than this, stained positively for the presence of glutamic acid decarboxylase. An extensive immunoreactive, putative GABAergic, network was present by three weeks in vitro.

Motor unit-muscle spindle interactions in active muscles of decerebrate cats

Single muscle spindle afferent and motor unit EMG spike trains have been recorded simultaneously during periods of spontaneous motor activity in triceps surae muscles of decerebrate cats. The approximate time course and magnitude of the motor unit contractions were extracted from the whole muscle force record by spike-triggered averaging, and the functional interactions between motor unit contractions and spindle discharge were assessed by cross-correlating their respective spike trains. We have found that both spindle group Ia and II afferents are responsive to the contractions of single motor units in the presence of spontaneous motor activity, being strongly coupled to the activity of some motor units and indifferent to the contractions of others. Moreover, the cross-correlation analysis revealed modulation of a single motor unit's discharge pattern by the input of a single Ia afferent.

Correlations between somatosensory thalamic activity and cortical rhythms

(1) Extracellular single neuron and associated focal slow potential recordings were made in the thalamic nucleus ventralis posterior (VP), together with somatosensory cortical ECoGs, from cats with lesions of the dorsal column nuclei and adjacent structures, and from cats without such lesions. Rhythmic activity was measured by computing autocorrelation and cross-correlation functions and by averaging slow potentials, when patterns varied spontaneously or when they were altered by stimulating the midbrain reticular formation or by injecting small amounts of sodium thiopental. The most prominent rhythmic pattern was an essentially continuous, typically 80 msec (type A) periodic modulation of slow potentials and of single VP projection neuron spike trains in both lesion and non-lesion cats. (2) Activity with periods shorter than 65 msec (type B) was sometimes observed, and was accompanied by arrhythmic, fast activity in adjacent non-primary cortex. Mean firing rates of VP projection neurons were higher than during type A activity, but periodic modulation of focal slow potentials and neuronal spike trains in VP was of low amplitude or was not detectable. (3) Cross-correlations demonstrated that type A rhythmicity was substantially coherent between pairs of neighboring neurons, between different somatosensory ECoG recording sites, and between ECoG and VP slow potentials. The data suggest that A periodicity represents relatively weak, but widespread and coherent, centrally arising oscillations of activity at thalamic and cortical somatosensory levels. (4) Most of the innervated VP neurons showing coherent type A periodicity gave rapidly adapting responses to movement of hairs. Neurons innervated by slowly adapting cutaneous receptors and which also showed the central periodicity were uncommon. (5) Stimulation of the midbrain reticular formation usually transformed type A into type B ECoG patterns and concurrently increased the mean firing rates of VP neurons, but occasionally evoked type A activity in initially arrhythmic VP and PS recordings. (6) Administration of small amounts of thiopental transformed type B into type A activity with concurrent reductions in firing rates of VP neurons. Administration of thiopental during type A activity led to discontinuous, spindle-like activity and lower firing rates of individual neurons, with a small increase in length of the period. The results suggest that the period of type B activity varies with the level of activity in convergent pathways and is probably controlled at a site external to VP. In contrast, the period of type A activity is relatively stable and is evidently determined by the mechanism which also times spindle activity in VP. However, sustained type A rhythmic activity probably requires activity in convergent extrathalamic pathways.

Statistical Analysis of Motor Unit Firing Patterns in a Human Skeletal Muscle

A statistical analysis of the firing pattern of single motor units in the human brachial biceps muscle is presented. Single motor unit spike trains are recorded and analyzed. The statistical treatment of these spike trains is as stochastic point processes, the theory of which is briefly discussed. Evidence is presented that motor unit spike trains may be modelled by a renewal process with an underlying gaussian probability density. Statistical independence of successive interspike intervals is shown using scatter diagrams; the hypothesis of a gaussian distribution is accepted at the 99th percentile confidence limit, chi-square test, in 90% of the units tested. A functional relationship between the mean and standard deviation is shown and discussed; its implications in obtaining sample size are presented in an appendix.The results of higher order analysis in the form of autocorrelograms and grouped interval histograms are presented. Grouped interval histograms are discussed in the context of motor unit data, and used to confirm the hypothesis that a stable probability density function does not represent a good model of the data at this level of analysis.

Neuronal Spike Trains and Stochastic Point Processes: I. The Single Spike Train

In a growing class of neurophysiological experiments, the train of impulses ("spikes") produced by a nerve cell is subjected to statistical treatment involving the time intervals between spikes. The statistical techniques available for the analysis of single spike trains are described and related to the underlying mathematical theory, that of stochastic point processes, i.e., of stochastic processes whose realizations may be described as series of point events occurring in time, separated by random intervals. For single stationary spike trains, several orders of complexity of statistical treatment are described; the major distinction is that between statistical measures that depend in an essential way on the serial order of interspike intervals and those that are order-independent. The interrelations among the several types of calculations are shown, and an attempt is made to ameliorate the current nomenclatural confusion in this field. Applications, interpretations, and potential difficulties of the statistical techniques are discussed, with special reference to types of spike trains encountered experimentally. Next, the related types of analysis are described for experiments which involve repeated presentations of a brief, isolated stimulus. Finally, the effects of nonstationarity, e.g. long-term changes in firing rate, on the various statistical measures are discussed. Several commonly observed patterns of spike activity are shown to be differentially sensitive to such changes. A companion paper covers the analysis of simultaneously observed spike trains.