Measuring synaptic interactions.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
E E Fetz
Publications and source records attributed to E E Fetz.
Short-term synchronization of motor units in human extensor digitorum communis muscle: relation to contractile properties and voluntary control.
Synchronous activity was studied in relation to the contractile properties of pairs of motor units (MUs) recorded with independent microelectrodes in the right extensor digitorum communis muscle (EDC) of human subjects during isometric finger extension. MU contractile properties were characterized in terms of the rise time and amplitude of twitch tensions extracted by spike-triggered averages of the extension force. Synchronization of MU discharges appeared in the form of narrow central peaks in the cross-correlograms of 35 of 50 pairs of MUs, suggesting the contribution of common last-order neurons. Synchronization peaks tended to be briefer and higher among pairs of MUs with slower and smaller twitches than among pairs of MUs with faster and larger twitches. The higher peaks of slow-contracting MUs suggest a greater effectiveness of the common synaptic inputs. The broader peaks of fast-contracting MUs might reflect an additional synchronization of the inputs to fast MUs at high force levels. The areas of the cross-correlogram peaks were similar for both groups and suggest that under our conditions, about three motoneurons would discharge synchronously for a given motoneuron spike. To test whether the amount of MU synchronization could be altered voluntarily, four subjects attempted to increase or decrease synchrony, using as feedback clicks triggered by coincident firings of the recorded MUs. In nine of 15 conditioning sessions, the magnitudes of the synchronization peaks showed significant changes in the intended direction. These results imply that supraspinal centers can control the relative amount of inputs that contribute to the synchronization of motoneuron discharges during voluntary contraction of EDC.
Acoustic Stimulation↗
Cortical mechanisms controlling limb movement.
Significant advances have been made this past year toward understanding the anatomy and physiology of motor cortical regions. New anatomical tracing techniques have elucidated intrinsic cortical connections as well as inter-areal connectivity. Magnetic stimulation of human cortex has provided new insights about the pathways mediating movements in humans. Neural recording studies in animals have further explored the behavioral variables that may be coded in activity of single units and populations. Recent approaches to neural network modeling offer some hope of synthesizing this wealth of detail into working simulations of networks that mediate motor behavior.
Animals↗
Two modes of interspike interval shortening by brief transient depolarizations in cat neocortical neurons.
1. The effects of small, brief depolarizing pulses and excitatory postsynaptic potentials (EPSPs) on neuronal firing were examined in layer V neurons in slices of cat sensorimotor cortex. During intracellular recording, brief depolarizing current pulses (duration, 0.5-2.0 ms; amplitude, 0.1-4.0 nA) were injected in neurons to produce pulse potentials (PPs) with a near-linear rise to a peak (0.08-3.6 mV; rise time = pulse duration) followed by an exponential decay. These PPs resembled EPSPs evoked by electrical stimulation of adjacent sites. When injected in neurons that were induced to discharge tonically, the PPs shortened the interspike intervals (ISIs) in two ways, depending on their time of arrival in the ISI. 2. Toward the end of the ISI, the PPs crossed a time-varying firing level, thereby directly evoking action potentials and shortening the ISIs. These directly evoked spikes occurred during the rise or peak of the PPs. The absolute firing level increased with the membrane trajectory during the latter part of the ISI. 3. PPs that appeared earlier in the ISI did not cross firing level directly but could nevertheless shorten the ISI by a slow regenerative process. The indirectly evoked spikes occurred after the peak of the PPs, at latencies whose magnitude and variability increased as the PPs appeared at successively earlier times in the ISI. PPs that occurred during the initial portion (approximately the 1st 3rd) of the ISI did not affect ISI duration. 4. Stimulus-evoked EPSPs shortened the ISIs in a manner similar to that of PPs. Like PPs, EPSPs caused direct crossings late in the ISI and indirect crossings earlier. Comparison of the mean and maximum ISI shortenings and the range of delays in which the PPs and EPSPs consistently produced ISI shortenings revealed no systematic differences. These similarities suggest that PPs may be used to simulate the ISI shortenings caused by EPSPs. 5. To characterize possible mechanisms underlying the ISI shortening, we examined the PP shapes at different times in the ISI. PPs immediately following a spike were smaller and decayed more rapidly than those evoked by the same current at rest. Late in the ISI, when the membrane potential was > 5 mV above rest, the PP height exceeded that of the PP at rest. This amplitude increase may be due to activation of the persistent sodium current.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Effects of transient depolarizing potentials on the firing rate of cat neocortical neurons.
1. The effects of excitatory postsynaptic potentials (EPSPs) on interspike intervals (ISIs) of neocortical neurons can be mimicked by pulse potentials (PPs) produced by current injection. The present report documents the dependence of the ISI shortening on the amplitudes of PPs and EPSPs and on the firing rate of the affected neuron. 2. In rhythmically firing necortical neurons, the ISI shortenings caused by PPs arriving at specific times in the ISI can be described by a shortening-delay (S-D) curve. The S-D curve yields three measures of the PPs' ability to shorten the ISI: 1) the mean ISI shortening, S; 2) the maximum shortening, Smax; and 3) the effective interval, defined as the portion of the ISI in which the PP consistently shortens the ISI. For PPs ranging between 80 microV and 3.6 mV (and cells firing at 25 imp/s), the mean shortening increased with amplitude h as S (ms) = 1.2*h (mV)1.24 (r = 0.94; P < 0.01). Smax increased linearly with amplitude as 4.9 ms/mV (r = 0.86, P < 0.01). The effective interval (as a percentage of the ISI) increased slightly with PP amplitude and had a mean value of 65 +/- 21% (mean +/- SD). 3. S-D curves obtained with stimulus-evoked EPSPs varied with EPSP amplitude in a manner similar to those of PPs. The relations obtained for stimulus-evoked EPSPs were not statistically different from those obtained for PPs in the same cells. 4. To determine the effect of firing rate. PPs were applied while neurons fired at frequencies ranging from 8 to 71 imp/s. Both S and Smax were approximately inversely proportional to the baseline firing rate (fo) and could be described as: S or Smax = kfo-m. The mean value of the exponent m (+/- SD) was 0.96 +/- 0.25 for S and 1.2 +/- 0.4 for Smax. These values were not statistically different from a value of 1 (1 group, 2-tailed t test). The effective interval did not vary significantly with firing rate. 5. The dependence of S on PP amplitude and baseline firing rate was incorporated into an expression for the average change in firing rate (delta f) produced by PPs occurring at rate fs: delta f = 0.03 h1.24 fs. The delta f increased with PP amplitude but did not vary significantly with the baseline firing rate. The values of delta f calculated from the S-D curves matched the values that were computed directly from the spike trains.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Coherent 25- to 35-Hz oscillations in the sensorimotor cortex of awake behaving monkeys.
Synchronous 25- to 35-Hz oscillations were observed in local field potentials and unit activity in sensorimotor cortex of awake rhesus monkeys. The oscillatory episodes occurred often when the monkeys retrieved raisins from a Klüver board or from unseen locations using somatosensory feedback; they occurred less often during performance of repetitive wrist flexion and extension movements. The amplitude, duration, and frequency of oscillations were not directly related to movement parameters in behaviors studied so far. The occurrence of the oscillations was not consistently related to bursts of activity in forearm muscles, but cycle-triggered averages of electromyograms revealed synchronous modulation in flexor and extensor muscles. The phase of the oscillations changed continuously from the surface to the deeper layers of the cortex, reversing their polarity completely at depths exceeding 800 microns. The oscillations could become synchronized over a distance of 14 mm mediolaterally in precentral cortex. Coherent oscillations could also occur at pre- and postcentral sites separated by an estimated tangential intracortical distance of 20 mm. Activity of single units was commonly seen to burst in synchrony with field potential oscillations. These findings suggest that such oscillations may facilitate interactions between cells during exploratory and manipulative movements, requiring attention to sensorimotor integration.
Animals↗
A general diffusion model for analyzing the efficacy of synaptic input to threshold neurons.
We describe a general diffusion model for analyzing the efficacy of individual synaptic inputs to threshold neurons. A formal expression is obtained for the system propagator which, when given an arbitrary initial state for the cell, yields the conditional probability distribution for the state at all later times. The propagator for a cell with a finite threshold is written as a series expansion, such that each term in the series depends only on the infinite threshold propagator, which in the diffusion limit reduces to a Gaussian form. This procedure admits a graphical representation in terms of an infinite sequence of diagrams. To connect the theory to experiment, we construct an analytical expression for the primary correlation kernel (PCK) which profiles the change in the instantaneous firing rate produced by a single postsynaptic potential (PSP). Explicit solutions are obtained in the diffusion limit to first order in perturbation theory. Our approximate expression resembles the PCK obtained by computer simulation, with the accuracy depending strongly on the mode of firing. The theory is most accurate when the synaptic input drives the membrane potential to a mean level more than one standard deviation below the firing threshold, making such cells highly sensitive to synchronous synaptic input.
Computer Simulation↗
Response patterns and postspike effects of peripheral afferents in dorsal root ganglia of behaving monkeys.
1. The activity of single afferent units was recorded in cervical dorsal root ganglia (DRG) in two macaque monkeys as they generated alternating flexion and extension torques about the wrist during a step-tracking task. During these isometric and auxotonic muscle contractions, electromyographic (EMG) activity was recorded with electrode pairs in up to 12 independent forearm muscles. Spike-triggered averages (STAs) of rectified EMG activity were used to identify afferents that were associated with correlated facilitation of active muscles. 2. Our aim was to find peripheral afferents producing postspike effects in muscles and to compare their properties with those of corticomotoneuronal (CM) and rubromotoneuronal (RM) cells previously obtained under identical behavioral conditions. We documented the timing, magnitude and distribution of their postspike facilitation (PSF) of forearm muscles and investigated the response properties of task-related units. 3. Of 125 afferent units tested with STAs, 68 showed PSF of EMG activity in at least one muscle. Fifty-nine DRG units provided sufficiently long recordings to generate averages with greater than or equal to 2,000 triggers, the minimum number considered to demonstrate reliable effects. Of these 59 units, 29 (49%) were associated with facilitation of forearm muscle activity. 4. Many STAs showed a gradual increase in EMG activity starting before or near the afferent trigger spike; often superimposed on this broad facilitation was a sharply rising PSF starting at a longer latency. The earliest poststimulus facilitation evoked by single microstimuli delivered in DRG occurred in stimulus-triggered averages at a latency of 3.5 ms. In STAs the broad facilitation beginning at latencies shorter than the responses to electrical stimulation was attributed to synchronous discharges in other afferent units. The sharper postspike EMG increases occurring with latencies of greater than or equal to 3.5 ms were identified as PSF produced by the afferent. The PSF parameters documented in this study were measured after subtracting the effects of synchrony facilitation. 5. PSF of EMG activity began at a mean latency of 5.8 +/- 0.3 (SE) ms and peaked at a mean latency of 7.5 +/- 0.3 (SE) ms. In previous studies, the PSFs from CM and RM cells had mean onset latencies of 6.3 and 5.6 ms, respectively, and mean peak latencies of 10.2 and 9.1 ms. 6. A measure of the PSF amplitude is the mean percent increase (MPI), defined as the increase of the PSF above its base measured as a percentage of the prespike baseline mean.(ABSTRACT TRUNCATED AT 400 WORDS)
Afferent Pathways↗
Neural mechanisms underlying corticospinal and rubrospinal control of limb movements.
Explore the source record for details and available documents.
Action Potentials↗
Effects of synchrony between primate corticomotoneuronal cells on post-spike facilitation of muscles and motor units.
Cross-correlating the activity of neighboring motor cortex neurons recorded with independent electrodes in behaving monkeys has revealed synchronization peaks, largely due to common synaptic input. Corticomotoneuronal (CM) cells produced post-spike facilitation (PSF) of rectified forearm electromyograms (EMG); 15 cells synchronized with CM cells showed no PSF. Five pairs of CM cells with overlapping muscle fields exhibited similar synchrony peaks. The contribution of this synchrony to facilitation of common target muscles was assessed by two new methods: selective spike-triggered averaging and convolution. They showed that the PSF is composed predominantly of effects mediated by output of the triggering cell, but may include a broad, shallow component mediated by synchrony with other CM cells.
Action Potentials↗
Control of forelimb muscle activity by populations of corticomotoneuronal and rubromotoneuronal cells.
We review and synthesize evidence on the activity of corticomotoneuronal (CM) and rubromotoneuronal (RM) cells and single motor units in forearm muscles in monkeys performing alternating wrist movements. The CM and RM cells were identified by post-spike facilitation of rectified forelimb EMG activity. RM cells facilitated more muscles per cell (mean: 3.0 of 6 synergist muscles) than CM cells (2.4/6). Both groups had "reciprocal" cells which also suppressed antagonists of their facilitated target muscles. Unlike CM cells, some RM cells cofacilitated flexor and extensor muscles (5.8 or 12 muscles). During performance of a standard ramp-and-hold force tracking task the firing patterns of CM and RM cells, as well as single motor units, fell into distinct response types. Each population had phasic-tonic and tonic cells. Unique to the CM population were cells whose discharge increased during the static hold period; unique to RM cells were bidirectionally responsive and unmodulated neurons. Many motor units showed decrementing discharge. To estimate the ensemble activities of these populations the response histograms of different cells were summed (with force ramps aligned) in proportion to the relative frequency of each cell type. The population response histogram of CM cells was phasic-tonic, consistent with the predominant response type. The population response of RM cells was also phasic-tonic, but showed a shallower phasic modulation relative to discharge that was sustained during both directions of movement. The population histogram of motor units of a muscle was proportional to the average of rectified multiunit EMG, and typically exhibited decrementing activity during the static hold. The effects of excitatory postsynaptic potentials (EPSPs) on firing probability of motoneurons previously documented in intracellular studies are combined with the mean firing rates in the population histograms and the known amplitudes of CM-EPSPs and RM-EPSPs to infer the relative contributions of the supraspinal cells to tonic discharge of active motoneurons. This analysis suggests that for intermediate levels of force, the CM cells would increment motoneuron discharge by about 9 impulses/second (i.p.s.) and RM cells by about 2.4 i.p.s. The analysis also reveals differences in the population activity of CM and RM cells compared to their target motoneurons, which may be due to other input cells and to recruitment properties of motoneurons.
Animals↗
Intracortical connectivity revealed by spike-triggered averaging in slice preparations of cat visual cortex.
Intracortical connectivity was studied in slice preparations of cat's visual cortex by spike-triggered averaging. The experiments documented the unitary postsynaptic potentials underlying the inhibitory and excitatory connections from layer III-IV border cells to supragranular cells, as demonstrated previously by cross-correlation studies. In addition the analysis demonstrated the existence of two excitatory connections, between supragranular and layer V cells, that were not detectable in previous cross-correlation studies.
Animals↗
Sustained excitatory synaptic input to motor cortex neurons in awake animals revealed by intracellular recording of membrane potentials.
1. Most of the intracellular electrophysiological data on cortical neurons has been obtained in anesthetized or reduced preparations, and differs from observations in awake, intact animals. To determine whether these differences are due to experimental techniques or physiological factors, we recorded membrane potentials intracellularly from motor cortex neurons in chronically prepared cats and monkeys under Nembutal-anesthetized, Halothane-anesthetized, and unanesthetized conditions, or during transitions between anesthetized and awake conditions. 2. Resting membrane potentials were found to depend on the anesthetic state of the animal. Membrane potentials of neurons recorded in awake animals were more depolarized than those recorded in the anesthetized state. In the awake state membrane potentials were all less than -65 mV. 3. The input resistance of neurons recorded in awake animals were significantly smaller than those measured in the anesthetized state. Action potentials recorded in awake animals typically showed an undershoot (i.e. negative values at peak), implying that voltage-dependent conductances may be altered. Undershoot of the action potential was more prominent in pyramidal tract neurons (PTNs) than non-PTNs. 4. These data suggested that in awake animals motor cortex neurons, especially PTNs, receive sustained excitatory synaptic input or neuro-modulatory activities.
Action Potentials↗
Functional relations between primate motor cortex cells and muscles: fixed and flexible.
In behaving monkeys the effects of motor cortex cells on muscles are inferred from two quite different types of 'correlational' evidence: their coactivation and cross-correlation. Many precentral cells are coactivated with limb muscles, suggesting that they make a proportional contribution to muscle activity; however, such coactivation is typically quite flexible, and can be changed by operantly conditioning the dissociation of cell and muscle activity. Cross-correlating cells and muscles by spike-triggered averaging of the electromyogram (EMG) shows that certain cells produce short-latency post-spike facilitation of EMG; this correlational linkage is relatively fixed under different behavioural conditions and its time course suggests it is mediated by a corticomotoneuronal (CM) synaptic connection. CM cells typically facilitate a set of coactivated agonist muscles, and some also inhibit their antagonists. The firing patterns of CM cells can differ significantly from those of their target muscles. During ramp-and-hold wrist responses most CM cells discharge a phasic burst that precedes target muscle onset and that contributes to changes in muscle activity. At low force levels many CM cells are activated without their target motor units. Conversely, many CM cells are paradoxically inactive during rapid forceful movements that vigorously activate their target muscles; they appear to be preferentially active during finely controlled movements. Thus CM cells, with a fixed correlational linkage to their target muscles, may be recruited without their target muscles, and vice versa.
Animals↗
Activity-related changes in electrical thresholds of pyramidal tract axons in the behaving monkey.
In monkeys generating torques about the wrist we investigated changes in the excitability of pyramidal tract (PT) axons, measured as the probability of evoked antidromic responses in motor cortex with constant juxtathreshold stimuli delivered in the brain stem. When PT stimuli were delivered 2-20 ms after an orthodromic action potential in the PT neuron, the excitability of axons was elevated, with a characteristic post-spike time course. Excitability peaked at a post-pike delay of 7.0 +/- 2.7 ms (n = 33). Axonal thresholds typically dropped to 80-90% of the unconditioned values (obtained for stimuli with no preceding spike). Controlling for such post-spike threshold changes by delivering stimuli at fixed post-spike delays, we found that excitability of many PT axons also fluctuated with the wrist responses, being slightly higher during flexion or extension. The place of movement in which excitability increased had no consistent relation to the phase of movement in which the PTN fired. Task-related threshold changes were also seen in PTNs whose discharge was not modulated with the wrist response. Delivering a subthreshold conditioning stimulus also increased the excitability of most PT axons to a subsequent test stimulus. Such post-stimulus changes may be mediated by the effects of adjacent fibers activated by the conditioning stimuli. The post-spike and post-stimulus changed added in a nonlinear way. All three types of threshold change may be mediated by a common mechanism: changes in the ionic environment of the axon produced by activity of the axon itself or its neighbors.(ABSTRACT TRUNCATED AT 250 WORDS)
Action Potentials↗
Cross-correlation assessment of synaptic strength of single Ia fibre connections with triceps surae motoneurones in cats.
1. The relation between excitatory post-synaptic potentials (e.p.s.p.s) by single I a fibres and the resultant cross-correlograms in triceps surae motoneurones was investigated in barbiturate-anaesthetized cats. The e.p.s.p.s. were documented first, using the discharge of single Ia fibres evoked by muscle stretch to compile spike-triggered averages of motoneurone membrane potential. Subsequently, Ia fibre action potentials were cross-correlated with rhythmic discharge of the same motoneurones induced by intracellular injection of current. 2. Primary correlogram peaks were statistically significant for thirty-one of forty-nine single Ia fibre-motoneurone connections. Cumulative sums of correlograms were used to identify the onset and duration of peaks. For twenty cases involving more than 2000 trigger spikes, thirteen showed significant correlogram peaks. For these thirteen, the mean percentage increase (m.p.i.) in motoneurone firing probability, defined as the mean height of the correlogram peak above base line, ranged from 29 to 138%. The k values (maximum height divided by base line) ranged from 2.1 to 5.2. Peak duration varied from 1.8 to 3.2 ms. In the remaining seven cases the Ia e.p.s.p.s produced no significant correlogram peak (i.e. P greater than 0.05). 3. A significant positive relationship (r = 0.76; P less than 0.005) was found between m.p.i. in motoneurone firing probability and e.p.s.p. amplitude (n = 13), with a mean slope of 0.30%/microV. The k values were more weakly related to e.p.s.p. amplitude (r = 0.67; P less than 0.01). The correlogram parameter most strongly related to e.p.s.p. amplitude (r = 0.80) was correlogram peak area (number of spikes above base line per excitatory post-synaptic potential). E.p.s.p. rate of rise was not significantly related (P greater than 0.10) to either m.p.i. in firing probability (r = 0.28) or peak area (r = 0.36). 4. The shapes of the primary correlogram peaks could be accounted for largely by a function proportional to the e.p.s.p. derivative (after temporal alignment). Subtracting a function proportional to the e.p.s.p. derivative from the correlogram peak left either a negligible remainder or a remainder term whose duration was shorter than the e.p.s.p. 5. To investigate properties of single-fibre Ia e.p.s.p.s occurring near motoneurone threshold during repetitive firing, e.p.s.p.s were selectively averaged using Ia spikes occurring near the end of the depolarizing ramp in membrane potential. These 'ramp e.p.s.p.s' tended to be somewhat smaller (by ca.8%) than the 'rest e.p.s.p.s' produced at the same connections with the motoneurone at rest.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Activity of forelimb motor units and corticomotoneuronal cells during ramp-and-hold torque responses: comparisons with oculomotor cells.
Explore the source record for details and available documents.
Animals↗
Comparable patterns of muscle facilitation evoked by individual corticomotoneuronal (CM) cells and by single intracortical microstimuli in primates: evidence for functional groups of CM cells.
We compared the averaged responses of forelimb muscles to action potentials of single motor cortex cells and to single intracortical microstimuli (S-ICMS). Activity of precentral neurons and 12 identified forelimb muscles (6 flexors and 6 extensors of wrist and fingers) was recorded in macaques while they performed alternating ramp-and-hold wrist movements. Action potentials of cells that covaried reliably with wrist flexion or extension were used to compile spike-triggered averages (spike-TAs) of rectified electromyographic (EMG) activity of six synergistically coactivated muscles. Cells whose spikes were followed by a clear postspike facilitation (PSF) of rectified muscle activity were designated corticomotoneuronal (CM) cells. CM cells typically facilitated a subset of the coactivated muscles called the cell's target muscles. The relative strength of the PSF in different target muscles ranged from clear increases above base-line fluctuations to weak but significant effects. For each CM cell we characterized the "PSF profile" of facilitation across different muscles, defined as the relative strength of PSF in each of the coactivated agonist muscles. After identifying the CM cell's target muscles, we delivered S-ICMS through the microelectrode at the same site. Biphasic stimuli were delivered during the same wrist movements in which the recorded CM cell had been active. Stimulus intensities were too weak (typically 5-10 microA) and their repetition rate too slow (5-15 Hz) to evoke muscle excitation evident in the raw EMG record. However, stimulus-triggered averages (stimulus-TAs) of the rectified EMGs of coactivated muscles revealed consistent patterns of poststimulus facilitation (PStimF). In most cases the muscles facilitated by the CM cell in spike-TAs (n = 60) were also facilitated by S-ICMS in stimulus-TAs. At sites of CM cells the threshold stimulus intensities for evoking a statistically significant effect were between 0.5 and 2 microA. S-ICMS of 5 microA evoked PStimF that was, on the average, six times stronger than the PSF of the CM cell. The height of the facilitation peak relative to base-line fluctuations was 5-60 times greater for the stimuli than the spikes of the CM cell. The average onset latency of PStimF (8.0 +/- 1.2 ms) was 1.3 ms longer than the mean latency of PSF (6.7 +/- 1.4 ms). At two-thirds of the cortical sites where both spike- and stimulus-TAs were computed (n = 30), the PStimF profile exactly matched the PSF profile.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗