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U Windhorst

Publications and source records attributed to U Windhorst.

At least 37 records · Page 2Linked to original sources
PubMed1994

Shaping static elbow torque-angle relationships by spinal cord circuits: a theoretical study.

Static torque-angle relationships (invariant characteristics) as measured by Feldman [Feldman A. G. (1980) Neuroscience 5, 81-90] at the human elbow joint for constant descending excitatory drive have a monotonic convex shape determining joint angle-dependent stiffness. In contrast, for constant activation of elbow flexors, the torque increases, peaks and decreases again with increasing angle because of related moment arm alterations [Hasan Z. and Enoka R. M. (1985) Expl Brain Res. 59, 441-450]. Conversion of such constant-excitation torque-angle shapes into an invariant characteristic might result from action of the stretch reflex which adds excitation with increasing joint angle. To test whether a simple linear model of the stretch reflex could convert constant excitation torque-angle relationships into invariant characteristics, the following assumptions were made. (1) Muscle fibre length increases linearly with joint angle. (2) Reflex muscle excitation (electromyogram) is linearly related to muscle (fibre) length. With these assumptions, invariant characteristic shape cannot be derived from constant excitation torque-angle relationships because it would be sigmoid at low and nearly straight at large joint angles, whilst real flexor invariant characteristics are more convex at large than small angles. It is suggested that recurrent inhibition via Renshaw cells contributes to bend the invariant characteristics into their right shape. Renshaw cells show a nonlinear saturating dependence on motor axon input rate and amount of excitation, i.e. number of active axon collateral synapses. These relationships can contribute to shape motoneuron output so as to yield convex invariant characteristics. Whilst it is not quite clear whether the gain of recurrent inhibition from and to skeleto-motoneurons is high enough to co-determine the invariant characteristic shape significantly, recurrent inhibition of Ia inhibitory interneurons mediating reciprocal inhibition between antagonists is supposed to be quite strong and may influence joint stiffness by interacting with reciprocal inhibition. The arguments presented here extend those of Feldman and co-workers concerning the role of recurrent inhibition and in addition provide a possible explanation for the functional role of mutual inhibition between Renshaw cells. Together with reflex feedback, recurrent inhibition thus contributes to fine-regulate force output and joint stiffness. To account for this cooperation and to make another step towards a general theory of spinal cord circuits, major traits of a new concept are briefly outlined.

Biomechanical Phenomena↗
PubMed1994

Waveform parameters of recurrent inhibitory postsynaptic potentials in cat motoneurons during time-varying activation patterns.

A considerable number of theoretical and experimental studies have been undertaken to establish quantitative relationships between the time course of postsynaptic potentials in a neuron and the change in firing probability thereby induced. Depending on background synaptic noise level, the time course of the postsynaptic potential per se as well as its time derivative are both of importance in varying proportion. We have recently begun to study recurrent inhibitory potentials in cat hindlimb motoneurons during rhythmically varying rates of stimulation of motor axons. The amplitude-rate relationship exhibits hysteresis in that amplitudes are usually larger during augmenting than decrementing rates in the cycle. We here report results on the other important variable, that is the slope of recurrent inhibitory potential development, which need not a priori be correlated with amplitude. We found that the slope has a relation to stimulus rate similar to amplitude, so that both parameters are correlated. In pentobarbitone anaesthetized or decerebrate cats, intracellular recordings were obtained from hindlimb skeleto-motoneurons. Various hindlimb muscle nerves were prepared for electrical stimulation to elicit recurrent inhibitory potentials, with dorsal roots cut. Test stimulus patterns consisted of repetitive pulse trains whose rates varied, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Modulation depths were either "full", with rates varying between a minimum of less than 10 and a maximum of around 50 pulses per s. Or they were about "half" this depth, with mean rates shifted into a "low", "medium" or "high" rate region. Recurrent inhibitory potentials were averaged with respect to stimuli occurring during different phases of the stimulation cycle. Most often when, throughout the cycle, the amplitude changed in a consistent way, so did the slopes of the inhibitory potentials. That is, when the amplitudes rhythmically declined with increasing and recovered with decreasing stimulus rate, the rate of hyperpolarization followed the same pattern. With prominent hysteresis in amplitude, a corresponding hysteresis appeared in slopes. Hence, amplitude and slopes were correlated, occasionally showing a hysteresis among themselves. To a certain extent, these results can be explained by Renshaw cell behaviour, the contribution of the Renshaw cell-motoneuron synapse being unknown and difficult to assess experimentally. For the inhibitory effect of Renshaw cells on motoneurons (and reciprocal Ia inhibitory interneurons), both its magnitude and its time course probably play an important role in determining the efficacy of counteracting local excitatory inputs. The change in slope of inhibitory potentials, and likely its underlying conductance, during cyclic motoneuron activation can be presumed to significantly contribute to the temporal pattern of discharge of motoneurons, in particular in relation to the prevention of synchronization leading to enhanced tremor.

Animals↗
PubMed1992

Dynamic behaviour of alpha-motoneurons subjected to recurrent inhibition and reflex feedback via muscle spindles.

The dynamic transfer characteristics of mammalian spinal skeleto-motoneurons are determined by intrinsic properties and various sorts of feedback. Here, recurrent inhibition via Renshaw cells and reflex feedback via muscle units and muscle spindle (in particular Ia) afferents, in the cat, are considered. The dynamic properties of the motor axon-Renshaw cell and the motor unit-spindle afferent subsystems were experimentally determined by stimulating motor axons with pseudo-random patterns of electrical pulses at two mean rates (low: 9.5-13 pulses/s; high: 20-23 pulses/s) and recording discharges of the two output elements. Spectral analysis yielded frequency responses to which transfer functions were fitted. These transfer functions in conjunction with those previously derived for alpha-motoneurons were used to study the stability and input-output characteristics of motoneurons with regard to two issues: stability and input-output relations of the combined (recurrent plus reflex) system as compared with each subsystem alone, with (i) each feedback path consisting of a single loop at some moderate level of force production, and (ii) each pathway consisting of two loops related to two motoneuron subpopulations active at a higher level of recruitment. It is shown that Renshaw cells have frequency characteristics well suited to contribute to the stabilization of the reflex loop. They can do so at low gains of recurrent inhibition.

Animals↗
PubMed1992

Synaptic interaction between medullary respiratory neurones during apneusis induced by NMDA-receptor blockade in cat.

1. Termination of inspiration is an essential component of respiratory rhythm generation and its perturbation can result in apneusis, i.e. significant prolongation of mechanisms, we studied the postsynaptic events in respiratory neurones during apneustic respiratory periods, and compared them to normal respiratory cycles. 2. Experiments were performed in pentobarbitone-anaesthetized, paralysed, thoracotomized cats ventilated with a constant volume or a cycle-triggered constant pressure pump. Apneusis, separated by normal cycles, was induced as follows: the animal was ventilated by a cycle-triggered pump that normally inflated the lungs during the inspiratory burst of phrenic nerve discharge. The NMDA-receptor blocker MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-iminemaleate] (0.3-0.7 mg/kg) was administered intravenously, and, for designated breaths, inflation of the lungs was withheld during neural inspiration. 3. Membrane potential trajectories of forty-one late expiratory (E-2) and eight postinspiratory (PI) neurones of the caudal ventral respiratory group were analysed before and/or after MK-801 administration, during normal and apneustic periods. 4. Before MK-801 administration, withholding lung inflation caused modest (10-20%) lengthening of the inspiratory period; after MK-801 administration, withholding inflation caused apneusis. Provided that the lungs were inflated during the inspiratory phase, the temporal pattern of phrenic nerve, recurrent laryngeal nerve and membrane potential trajectories of E-2 and PI neurones were not significantly altered by MK-801. Apneusis following NMDA-receptor blockade produced consistent changes in the synaptic activation patterns of E-2 neurones. In particular, the slow late inspiratory-related depolarization pattern of E-2 neurones was consistently retarded during apneustic inspiratory phases when compared to normal inspiratory phases. This was due to continuation of Cl(-)-mediated synaptic inhibition of E-2 neurones. Superior laryngeal nerve stimulation stopped apneusis and sustained membrane hyperpolarization of E-2 neurones similar to lung inflation. 5. During the plateau phase of apneusis, correlated 10-20 Hz oscillations could be observed in the integrated phrenic and recurrent laryngeal nerve activities as well as in the membrane potential of E-2 neurones. 6. We conclude that: (i) the prolonged inhibition of E-2 neurones during apneusis is indicative of the process responsible for the prolongation of the inspiratory phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Interaction of recurrent inhibitory and muscle spindle afferent feedback during muscle fatigue.

Mammalian skeletal motor units have differing properties including their different susceptibility to fatigue. The question discussed in this paper is whether and to what extent proprioceptive feedback via muscle spindles can contribute to shape the firing patterns of motor units so as to minimize their loss of force during fatiguing contraction. The firing of a skeleto-motoneuron dispatches signals which are fed back to the same and homonymous as well as synergistic motoneuron. Two feedback pathways are of concern here: one via the related muscle unit and muscle spindle afferents (proprioceptive path), and one via recurrent motor axon collaterals and Renshaw cells (recurrent inhibitory path). It is suggested that the contraction of a motor unit or a small group of adjacent ones is signalled to the homonymous alpha-motoneurons via proprioceptive afferents, the signal being filtered and enhanced by spinal recurrent inhibition. This is effected by timed correlation of the signals which are propagated through the two feedback loops. The effects of the correlation can be strengthened by (i) topographical order of the feedback connections, (ii) heterosynaptic interaction, and (iii) tendencies towards synchronous discharge between motoneurons. These mechanisms render the possibility more likely that information about the unfused contractions of a muscle unit (or a small group of them), mediated via proprioceptive afferents, play a role in shaping the precise discharge pattern of the innervating motoneuron(s). These mechanisms could be used to optimize the force output of large fatiguing motor units during long activation, during which their activation rates normally decrease (adapt) over time. Our results show that during adapting motoneuron firing Renshaw cells and muscle spindle afferents may show discharge patterns which at least in part are in keeping with such an hypothesis.

Animals↗
PubMed1991

Response of the medullary respiratory network of the cat to hypoxia.

1. The effect of systemic hypoxia was tested in anaesthetized, immobilized, thoracotomized and artificially ventilated cats with peripheral chemoreceptor afferents either intact or cut. Extracellular recordings from different types of medullary respiratory neurones and intracellular recordings from stage 2 expiratory neurones were made to determine the hypoxia-induced changes in neuronal discharge patterns and postsynaptic activity as an index for the disturbances of synaptic interaction within the network. 2. The general effect of systemic hypoxia was an initial augmentation of respiratory activity followed by a secondary depression. In chemoreceptor-denervated animals, secondary depression led to central apnoea. 3. The effects of systemic hypoxia were comparable with those of cerebral ischaemia following occlusion of carotid and vertebral arteries. 4. In chemoreceptor-denervated animals, all types of medullary respiratory neurones ceased spontaneous action potential discharge during hypoxia. 5. Reversal of inhibitory postsynaptic potentials (IPSPs) and/or blockade of IPSPs was seen after 2-3 min of hypoxia. 6. During hypoxia, the membrane potential of stage 2 expiratory neurones showed a slight depolarization to -45 to -55 mV and then remained stable. 7. The neurone input resistance increased initially and then decreased significantly during central apnoea. 8. Rhythmogenesis of respiration was greatly disturbed. This was due to blockade of IPSPs and, in some animals, to more complex disturbances of phase switching from inspiration to expiration. 9. Central apnoea occurred while respiratory neurones were still excitable as shown by stimulus-evoked orthodromic and antidromic action potentials. 10. The results indicate that the medullary respiratory network is directly affected by energy depletion. There is indication for a neurohumoral mechanism which blocks synaptic interaction between respiratory neurones in chemoreceptor-intact animals.

Action Potentials↗
PubMed1990

Higher-order non-linear phenomena in Renshaw cell responses to random motor axon stimulation.

Renshaw cell responses to random motor axon stimulation exhibit second-order non-linearities in that they depend on the occurrence of a preceding stimulus, although these non-linearities are not strong enough to significantly depress the coherence. However, higher-order non-linearities have not been checked for so far. This is carried out here. Lumbosacral Renshaw cells were recorded with micropipettes in anaesthetized cats. Their responses to random (pseudo-Poisson) stimulation of motor axons in peripheral nerves or ventral roots were quantified by calculating peristimulus-time histograms of various sorts, conventional and conditional. Conventional peristimulus-time histograms were computed with respect to all the stimuli in a train. Conditional peristimulus-time histograms were calculated with respect to "test" stimuli which were sorted out (by computer) from the original stimulus train when they were preceded by "conditional" stimuli at average intervals of delta 1 or delta 2 or both. These conditioned responses were compared with those to be expected from hypothetical linear superposition. Renshaw cell responses showed small third-order non-linearities to pairs of conditioning stimuli at small intervals (up to some tens of milliseconds before the test stimuli). These third-order effects were smaller than each of the second-order non-linearities elicited by any of the single-conditioning stimuli. Also, further higher-order non-linearities were apparent, but of little average significance. Hence, the non-linearities in Renshaw cell responses to random inputs are essentially of second-order and fairly small.

Animals↗
PubMed1990

Frequency characteristics and nonlinear features of responses of cat dorsal horn neurons to random stimulation of cutaneous afferents.

The system between cutaneous (suralis) afferents and dorsal horn neurons was studied for comparison with studies previously performed on the motor axon-Renshaw cell system, using the same methods. In anaesthetized or decerebrated cats, 27 dorsal horn neurons of segments L5 to S1 were recorded extracellularly in depths of 1-2.3 mm from cord dorsum. Cutaneous afferents in branches of the ipsilateral suralis nerve were stimulated with sequences of randomly occurring electrical pulses at two levels of mean rate. The responses of the dorsal horn neurons to the stimuli were evaluated in the frequency and time domain. Calculation of coherence, gain and phase functions (via spectral analysis) showed that the frequency response depended on the precise pattern on cell discharge and could vary from broad-band to low-pass or occasionally band-pass characteristics. There were minor differences in these characteristics with those of Renshaw cells. A special type of nonlinear analysis, using conditional peristimulus-time histograms, showed that the responses to test stimuli were facilitated, depressed or both by conditioning stimuli occurring some tens to a few hundred milliseconds before. Early and late response components could be conditioned individually and differently. Exponential fits to such conditioning curves yielded two time constants for depression (means of 21 and 94 ms) and one for facilitation (14 ms). Similar conditioning effects and time constants were previously found for the motor axon-Renshaw cell system although a few differences were apparent. By analogy, it is suggested that part of the long-lasting conditioning effects (with long time constants) are probably due to presynaptic mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

A method to estimate the effects of parallel inputs on neuronal discharge probability.

We here present a method to study the interaction of parallel neural input channels regarding their effects on a neurone. In particular, the method allows to disclose the effects of oligosynaptic pathways that may exist in parallel to direct monosynaptic connections to the cell. Two (or more) inputs (nerves) are stimulated with random patterns of stimuli. The response of the cell to these patterns is evaluated by the computation of peristimulus-time histograms (PSTHs). One of the two stimulus trains is selected as the one to yield reference events for the PSTH computation. From this stimulus train are selected those stimuli as reference events which are preceded, at defined mean intervals, by stimuli in the same or a parallel channel. These "conditioning" stimuli are determined (1) separately from each single stimulus train and (2) concomitantly from the two trains as events occurring simultaneously in both. The effects exerted by these various conditioning events on the effects of the "test" pulses on the cell response yield insights into the interactions between the two (or more) inputs. These methods are demonstrated on spinal Renshaw cells activated by independent random stimulation of two muscle nerves and on dorsal horn neurones responding to cutaneous nerve stimulation.

Animals↗
PubMed1989

The relationship between coherence and nonlinear characteristics in Renshaw cell responses to random motor axon stimulation.

Cat spinal Renshaw cells were activated by stimulating muscle nerves or ventral roots with random (pseudo-Poisson) patterns of brief electrical stimuli. This input pattern is optimal for a comparative study in both the frequency- and time-domain. The frequency-dependent variable of particular interest in this study was the coherence as a measure of the degree to which signal transmission is linear and noise-free; it was estimated via spectral analysis. Time-domain analysis consisted of calculating peri-stimulus time histograms in order to estimate the amount of nonlinearity in the cell responses to pairs of stimuli. The main result was that the amount of nonlinearity measured in this way did not profoundly depress the coherence. Two types of peri-stimulus time histogram were calculated: the "conventional" peri-stimulus time histogram (as a reference) computed with respect to all the stimuli in a train, and the "conditional" peri-stimulus time histogram computed with respect to the second in pairs of stimuli which were separated from each other by varied intervals delta. The latter type of peri-stimulus time histogram showed that Renshaw cell responses to stimuli were conditioned by preceding stimuli, which could facilitate (at small delta s) and/or more often depress (up to several hundreds of milliseconds) the subsequent responses in a nonlinear manner. The objective of this study was to test the hypothesis that nonlinear characteristics contribute significantly to depress the coherence from its optimal value (1).(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1989

Early and late components in cat Renshaw cell responses to random stimulation of motor axons: their differential sensitivity to preceding activation.

Lumbosacral Renshaw cells were activated by random stimulation of motor axons in muscle nerves or ventral roots. The stimulus patterns had mean rates of 9.5-13 or 20-23 pulses per second. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms. "Conventional" peristimulus-time histograms were calculated by averaging the cell discharge with respect to all the stimuli in a train. "Conditional" peristimulus-time histograms were determined by averaging the cell discharge with respect to the second ("test") stimulus in pairs of stimuli which were separated by varied intervals. The effects of the conditioning stimuli were evaluated after correcting for the effect of linear superposition of the conditioning and test stimuli. The conventional peristimulus-time histograms showed an excitatory response which often consisted of two distinct components: a narrow and high "early" peak and a broad and low "late" elevation of firing probability. The early and late excitatory components were conditioned in different ways. Whereas the late component was virtually always depressed, the early component showed three patterns: (1) uniform depression; (2) uniform facilitation; (3) a mixture of depression and facilitation. Frequency responses (coherence and gain estimates) were also calculated separately for the cell discharges underlying either the early or the late components. The estimates for the "late spikes" showed a stronger decline with increasing frequency than those for the "early spikes". The origin of the different conditioning effects probably lies in a combination of pre and postsynaptic factors. They may play a role in tremor mechanisms.

Action Potentials↗
PubMed1989

Do Renshaw cells tell spinal neurones how to interpret muscle spindle signals?

In vertebrates many alpha motoneurone pools are subjected both to recurrent inhibition via Renshaw cells and to proprioceptive feedback via muscle fibres and proprioceptors, particularly spindles. In these cases, the two feedback loops have a common input (alpha motoneurone output) and a common target (alpha motoneurones). This implies that the target alpha motoneurones receive a compound information dispatched by the source alpha motoneurones, but processed in different ways via the two feedback paths. Since the Renshaw cells monitor the input to skeletal muscle, and the spindles (and Golgi tendon organs) monitor certain aspects of muscle output, both feedback paths in conjunction contain information about the mechanical state of skeletal muscle. Based on these interrelationships the following hypothesis is discussed. At a micro-level, muscle spindles might provide information about motor unit contractions to the homonymous alpha motoneurones. This information is filtered and enhanced by recurrent inhibition via Renshaw cells. This is effected by correlation of the signals which are propagated through the two feedback loops after having been initiated by firing of the same alpha motoneurone(s). The effects of the correlation can be strengthened by (a) topographical order in the feedback connections, (b) heterosynaptic modulation, and (c) tendencies towards synchronous discharge between motoneurones. The information about the unfused contractions of a muscle unit (or a small group of them), thus retrieved from the barrage of signals delivered by proprioceptive afferents, could then play a role in shaping the precise discharge pattern of the innervating motoneurone. This in turn may be of importance for mechanisms of optimal force production during muscle fatigue.

Animals↗
PubMed1988

Relations between time-and frequency-domain measures of signal transmission from cutaneous afferents to dorsal horn neurons.

In pentobarbitone-anesthetized cats, the spike sequences of dorsal horn neurons were recorded in response to random stimulation of branches of the suralis nerve. Combined frequency- and time-domain analysis was performed on the stimulus and spike trains. Coherence function estimates computed by spectral analysis were compared with peristimulus time histograms (PSTHs). The cell responses were divided into 4 main types: PSTHs with a single high and narrow peak were associated with broad-range high coherence; PSTHs with two (or sometimes 3) distinct peaks concurred with a coherence which was high a low frequencies, low at intermediate ones and higher again at high frequencies; broad unstructured PSTH peaks of varying height concurred with coherence declining from high values at low frequencies to lower values at higher ones; and small and broad PSTH peaks were associated with generally low coherence. Thus, the variation of coherence with frequency depends on the precise pattern of cell discharge.

Action Potentials↗
PubMed1988

Time constants of facilitation and depression in Renshaw cell responses to random stimulation of motor axons.

In 9 adult anaesthetized cats, 22 lumbosacral Renshaw cells recorded with NaCl-filled micropipettes were activated by random stimulation of ventral roots or peripheral nerves. The stimulus patterns had mean rates of 9.5-13 or 20-23 or 45 pulses per second and were pseudo-Poisson; short intervals below ca. 5 ms (except in two cases) were excluded. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms (PSTHs). "Conventional" PSTHs were calculated by averaging the Renshaw cell discharge with respect to all the stimuli in a train. These PSTHs showed an early excitatory response which was often followed by a longer-lasting slight reduction of the discharge probability. These two response components were positively correlated. "Conditional" PSTHs were determined by averaging the Renshaw cell discharge with respect to the second ("test") stimulus in pairs of stimuli which were separated by varied intervals, delta. The direct effect of the first "conditional" response was subtracted from the excitation following the second ("test") stimulus so as to isolate the effect caused by the second stimulus per se. After such a correction, the effect of the first "conditioning" stimulus showed pure depression, pure facilitation or mixed facilitation/depression. Analysis of such conditioning curves yielded two time constants of facilitation (ranges: ca. 4-35 ms and 93-102 ms) and two of depression (ranges: ca. 7-25 ms and 50-161 ms). It is concluded that these time constants are compatible with processes of short-term synaptic plasticity known from other synapses. Other processes such as afterhyperpolarization and mutual inhibition probably are of less importance.

Animals↗
PubMed1988

Facilitation and depression in the responses of spinal Renshaw cells to random stimulation of motor axons.

1. We investigated the responses of cat lumbosacral Renshaw cells to pseudo-Poison stimulus sequences (of three different mean rates) delivered to motor axons in ventral roots or various muscle nerves. The Renshaw cell responses were evaluated by computation of peristimulus time histograms (PSTHs). 2. PSTHs computed with respect to all the stimuli showed, before the reference time, near-constant bin contents corresponding to the mean firing probability (rate), and an initial excitatory component (increase in discharge probability) after the reference time, followed by a small but longer-lasting reduction of firing rate. These two response components were strongly correlated linearly. It is suggested that the postexcitatory rate reduction is predominantly due to afterhyperpolarization. 3. In general, Renshaw cell responses to any stimulus in a stimulus train depended upon the stimulation history. In the averaged record, the response to the second of a pair of stimuli was affected by the first stimulus independently of intervening (random) stimuli. Very often, the second response showed a long-lasting depression (from 25 to greater than 250 ms). In a number of cases a briefer facilitating effect preceded the depression. 4. These conditioning effects were largely homosynaptic, i.e., confined to the particular input channel that was stimulated. This was shown by stimulating two different nerves (or nerve branches) with independent random patterns of similar mean rates and determining the cross-conditioning exerted by one input channel on the excitatory effects of the other. At small intervals between conditioning and test stimuli of some tens of milliseconds, a facilitatory effect could often be seen, which almost certainly reflected spatial summation. However, the subsequent depressant effect was largely accounted for by the postexcitatory rate reduction consequent to the conditioning stimulus in the parallel channel. Autoconditioning was still present. 5. The amount of facilitation and depression as well as their balance depended on the average Renshaw cell response. This in turn depended, at each mean stimulus rate, on the strength of synaptic coupling between an input channel and the cell, and on the mean stimulus rate, declining with an increase in mean rate. That is, the facilitation increased and the depression decreased with decreasing synaptic coupling and increasing mean stimulus rate. 6. Several factors may contribute to facilitation and depression; these are discussed with respect to their relative quantitative significance.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

After-effects of stochastic synaptic Renshaw cell excitation on their discharge probability.

We have studied Renshaw cell (RC) responses to pseudo-Poisson stimulus sequences (small intervals below 5 ms excluded; mean rates between 10 and 45 pulses/s) delivered to motor axons in ventral roots or muscle nerves. Average RC responses to stimuli in a single stimulus channel depended upon the preceding stimulation history. The responses to pairs of stimuli separated by variable intervals (irrespective of intervening stimuli) generally showed a long depressant effect (from 25 to more than 250 ms) of the first ('conditioning') stimulus; in a number of cases a briefer facilitating effect preceded the depressant action. Several possible causes of these conditioning effects are discussed.

Action Potentials↗
PubMed1987

Event-related cross-correlations between spike trains illustrated on interactions between motor units and muscle spindle afferents.

A method is presented for computing correlation coefficients of two (or more) output spike trains in temporal relation to one (or more) input even trains. These event-related correlation functions are computed by convolving the output spike trains, represented as point processes, with rectangular pulses of selectable width, and by then calculating linear correlation coefficients for the pairs of amplitude values obtained from the two convolved processes in temporal relation to the input events. The merits of this technique are illustrated on stimulus trains delivered to motor units (MUs) and output spike trains recorded from muscle spindle afferents of the same cat hindlimb muscle. The correlation functions obtained show the temporal course of the correlated firings of the two afferents (mostly Ia afferents from primary muscle spindle endings) as a function of time from MU activation; they are compared with the conventional cross-correlation histograms (CCHs) between afferents and with peri-stimulus time histograms (PSTHs) between stimulus and afferent firing patterns. Stimulus-related cross-correlation functions as displayed here can be calculated for any three spike trains. Possible extensions of the method to larger numbers of input and output channels are also discussed.

Action Potentials↗