PubMed Health⌕ Search

Biomedical subjects

U Windhorst

Publications and source records attributed to U Windhorst.

At least 55 records · Page 3Linked to original sources
PubMed1987

Measurement of axonal conduction velocity in single mammalian motor axons.

In deeply anesthetized cats, determinations of motor-axonal conduction velocity (CV) were made using extracellular potentials recorded from single, functionally isolated motor axons innervating the muscle tibialis posterior. Axons were activated by suprathreshold electrical stimulation at the ventral-root level. Action potentials were recorded with 3 bipolar electrodes located on the muscle nerve at the level of the popliteal fossa. The most proximal and distal of the bipolar muscle-nerve electrodes were 16.4-22.0 mm apart. Estimates were made of CV from ventral root to muscle nerve (conventional CV) and between the proximal and the distal pairs of muscle-nerve electrodes (muscle-nerve CV). An evaluation was based on comparison of these CVs, estimates of uncertainties in time and distance measurements and simulations of the effects of recording conditions on CV estimates. The analysis indicated that the uncertainty in the conventional CV measurement of mammalian motor axons is at least +/- 2%. However, variability may be as great as 20% between muscle-nerve CV measurements from different experiments, probably due to such factors as regional variation in CV and differences in recording configuration.

Action Potentials↗
PubMed1987

Frequency response of spinal Renshaw cells activated by stochastic motor axon stimulation.

In anaesthetized or decerebrate cats, motor axons in lumbosacral ventral roots or hindlimb muscle nerves were stimulated with random trains of brief electrical pulses, and Renshaw cell spike sequences were recorded. Spectral analysis was used to determine the range of linear operation of Renshaw cells, via coherence computations, and to calculate their frequency-dependent gains and phases. The analysis showed that the dynamic behaviour of Renshaw cells was different for different strengths of their synaptic input from motor axons and for different mean stimulus rates. In general, the changes in dynamics associated with variation of these two input parameters followed a common trend. This can be related to the average response of Renshaw cells per stimulus, as assessed by peri-stimulus time histograms. For axons having a strong excitatory effect on a Renshaw cell (as judged from the size of early peri-stimulus time histogram peaks), and for low mean stimulus rates (10-23 pulses per second), the linear range of signal transmission (assessed by coherence computation) was usually very broad (from zero sometimes up to over 100 Hz, but mostly up to 50-100 Hz). Following an initial elevation in the range 2-15 Hz, the gain showed first a rapid decrease with frequency, down to a value which at 30-50 Hz could be a tenth of the gain at lower frequencies (2-15 Hz); it then continued to decline slowly. Otherwise the linear range was narrower and/or the coherence was generally lower; the gain was lower and showed little decline with frequency. The phase curves of Renshaw cells generally showed a low-frequency phase lead (up to roughly 10 Hz) and an increasing phase lag thereabove that was generated in part by the conduction delay. The results show that Renshaw cells can follow, particularly sensitively, inputs in a frequency range encompassing the steady firing rates of many alpha-motoneurons. This range of high gain also covers that of a component of physiological tremor (ca. 6-12 Hz), a basic mechanism of which is probably related to unfused contractions of newly recruited motor units firing in this range. It can therefore be expected that recurrent inhibition via Renshaw cells is especially powerful in this physiologically important range of alpha-motoneuron firing.

Action Potentials↗
PubMed1987

The measurement of single motor-axon recurrent inhibitory post-synaptic potentials in the cat.

1. Signal averaging was used in forty experiments on low-spinal cats to measure and characterize the oligosynaptic responses of seventy-six motoneurons supplying the medial gastrocnemius muscle to the single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. In thirteen experiments on chloralose-urethane anaesthetized preparations, twelve (43%) of the tested twenty-eight motoneurones exhibited a single-axon recurrent inhibitory post-synaptic potential (recurrent i.p.s.p.), as compared to sixty-four (62%) of the 103 motoneurones tested in twenty-seven animals in the absence of anaesthetic after ischaemic decapitation. 3. Single-axon recurrent i.p.s.p.s most often consisted of a single, long-lasting hyperpolarization. Ten of the recurrent i.p.s.p.s contained a second late peak of hyperpolarization. In another eight of the i.p.s.p.s, a small late depolarization was evident. 4. The distinct profiles of the recurrent i.p.s.p.s were readily distinguished from the relatively flat profiles with low noise levels in the averages of the fifty-five 'no-response' cells. The transmembrane and post-synaptic nature of the i.p.s.p.s was confirmed by extracellular control recordings taken immediately outside seven of the cells with positive responses. In addition, ten cells with positive responses were subjected to current passage during the averaging procedure. In all cases, depolarization increased and hyperpolarization reduced the amplitude of their single-axon recurrent i.p.s.p.s. 5. The mean amplitude of the responses was 12.0 microV in chloralose-urethane preparations as compared to a peak-to-peak noise level less than 6.0 microV in the no-response averages. Corresponding values in ischaemic-decapitate preparations were 46.2 microV and less than 7.5 microV, respectively. 6. Latency, rise-time and half-width (i.e. duration at half-amplitude) values of the i.p.s.p.s were similar for chloralose-urethane and ischaemic-decapitate preparations. The average values in both preparations were 2.5, 5.6 and 19.3 ms, respectively. The latency values indicated both disynaptic and, perhaps, longer components in the recurrent i.p.s.p.s. The rise-time and half-width values were relatively similar to those reported or measured from published records for analogous composite recurrent i.p.s.p.s (i.e. responses to antidromic stimulation of the whole muscle nerve rather than single motor axons). A weak, but significant, correlation between rise-time and half-width was observed for the sixty-six single-axon recurrent i.p.s.p.s with a single negative-going peak.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1987

Distribution of single-axon recurrent inhibitory post-synaptic potentials in a single spinal motor nucleus in the cat.

1. The distribution of recurrent Renshaw inhibition was examined in the medial gastrocnemius motor nucleus of the cat by measurement of single-axon recurrent inhibitory post-synaptic potentials (recurrent i.p.s.p.s) in motoneurones supplying medial gastrocnemius produced by single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. Stimulated motor-axon-recorded-motoneurone pairs were classified as 'close' if both contributed their axons to the same portion of the ventral roots (L7 rostral, L7 caudal or S1). Alternatively, they were classified as 'distant' if their axons were in different portions of these ventral roots. 3. Both the incidence and amplitude of 'close-pair' recurrent i.p.s.p.s were shown to be significantly larger than the corresponding values for 'distant-pair' recurrent i.p.s.p.s. 4. The results provided evidence of a strong topographic effect on the distribution of recurrent Renshaw inhibition within a single spinal motor nucleus. This finding is discussed in relation to the roles of topographic and species specificity in determining the organization of recurrent inhibition and to the partitioned distribution of monosynaptic Ia excitation in single motor nuclei.

Action Potentials↗
PubMed1986

Amplitude reduction of motor unit twitches during repetitive activation is accompanied by relative increase of hyperpolarizing membrane potential trajectories in homonymous alpha-motoneurons.

In anaesthetized cats, medial gastrocnemius motor units (MUs) were electrically stimulated via their ventral-root axons with independent random patterns. Isometric muscle tension and homonymous alpha-motoneuron (MN) membrane-potential fluctuations in response to these stimuli were recorded simultaneously, usually for periods of about 2 min. The tension and membrane potential were averaged with respect to a stimulus train over two disjoint time intervals, one stretching 20-40 s at record beginning, and the other a similar duration at the end of recording. Whereas average MU twitch amplitudes usually decreased between these periods, average membrane potential trajectories did not do so, such that, when normalized to the change in twitch amplitude, the membrane potential trajectories usually increased in size. This suggests that the decline in the mechanical effect of MU activation was accompanied by an increase in the gain of the afferent pathway to homonymous MNs, which was confirmed by gain computations in the frequency domain. This compensation could be a mechanism to maintain the high quality of information about MU contractions transmitted to MNs in the course of MU fatigue.

Animals↗
PubMed1986

The information carried by spindle afferents on motor unit activity as revealed by spectral analysis.

Spectral analysis was used to study the effects of motor unit activity on the discharge patterns of muscle spindle endings. Spindle afferents of hind-limb muscles of the cat were recorded during electrical stimulation of one or more motor units, and, for comparison, while the receptors discharged in the absence of induced extrafusal activity ('background discharge'). The stimulus sequences used were random, but had characteristic frequency components representing an underlying rhythm, similar to those of trains in real alpha-motoneuron output. The computed afferent spectra and coherences between stimulus and afferent trains indicate that the discharge patterns of muscle spindles carry information on the activity of particular subsets of motor units. The spectra also demonstrate a complex interaction of internal spindle (pacemaker) mechanisms and external (modulating) processes which determine the discharge patterns of primary and secondary endings. In addition, they reveal interesting differences between primaries and secondaries, possibly indicative of a particular role for each type of ending in motor control.

Action Potentials↗
PubMed1986

Spindle gain increase during muscle unit fatigue.

In anaesthetized cats, medial gastrocnemius motor units (MUs) were stimulated with random sequences (mean rates between 6 and 12 pps) of electrical pulses delivered to their axons in small ventral root filaments. Muscle tension was recorded under isometric conditions, and spike trains of muscle spindle afferents were recorded from small dorsal root filaments during prolonged MU activation. Time-domain (PSTH) and frequency-domain (gain) computations were performed to study the effects of fatiguing muscle unit contractions on the signal transmission from skeletomotor efferents to spindle afferents. In the course of muscle unit fatigue, during which the gain of the force-producing sub-system decreased, the gain of the sub-system transforming force to afferent discharge increased so that the overall gain between skeletomotor efferents and spindle afferents remained relatively high. This could be a mechanism that preserves a high quality of afferent information on MU contractions.

Animals↗
PubMed1986

Differential effects of stimulation of the cat's red nucleus on lumbar alpha motoneurones and their Renshaw cells.

The red nucleus region was stereotaxically stimulated with short trains of high-frequency alternating current pulses in anaesthetized cats. The effects were studied, in contralateral lumbar segments, on the responses of microrecorded individual Renshaw cells (RCs) to antidromic or orthodromic test shocks of ventral root or muscle nerve fibres. Monosynaptic reflexes (MRs) of their motoneurone pools were recorded from one of the cut lumbar ventral roots. Averages of 10-20 replicate test responses of the RC (converted into instantaneous frequency curves, IFCs) and of the MR shapes were computed and graphically displayed. 2. Orthodromic (afferent) test shocks induced simultaneously MRs as well as responses of a RC belonging to the same motor pool. From their paired records at systematically varied shock strengths, whole "linkage characteristics" of the relation between the two events could be obtained, representing the functional linkage from the motoraxon collaterals to the RC under study. The overall result of rubral conditioning was a change in the course of the characteristic, which indicated a reduction of this linkage (= relative inhibition of the RC against its recurrent input). 3. Sequential trials with test shocks of constant, submaximal strength were performed with 45 individual RCs. The clearest results were obtained with RC responses to antidromic ventral root shocks: 65% of the RCs were partially inhibited by rubral conditioning. Interposed minor facilitory subcomponents could be seen in the course of inhibited IFCs. Mixed sequences of manifest inhibitory/facilitory effects were observed in 11%; reversed sequences (facilitory/inhibitory) did not occur. A pure but weak facilitation was found in only one case, paralleled by an increase of the MR. RCs belonging to either extensor or flexor motor pools were affected about equally. A little over 20% of the tested RCs remained uninfluenced by rubral stimulation. 4. The MRs, induced by constant, submaximal, orthodromic test shocks, were usually enhanced with only few exceptions, by rubral stimulation. The effects on the orthodromic RC responses were mainly inhibitory, but could be more or less masked by the concurrent increase of the MR, providing a stronger recurrent input to the RC. Such inhibition could be uncovered, however, by observing the above described linkage change. 5. Variation of several parameters of rubral conditioning (train duration, timing of train with respect to test shock, strength of train) modified the inhibitory effects on antidromic RC responses to a certain extent without changing their principal character.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Linear and nonlinear effects in the interactions of motor units and muscle spindle afferents.

Successive motor unit (MU) twitches often do not sum linearly. Also, muscle spindle (MS) afferents may react nonlinearly to MU contractions occurring at short intervals. Little data is presently available on the interactions between two (or more) MUs regarding their effects on tension output and MS responses. We have studied these effects in cat Mm. gastrocnemius medialis (MG), soleus and semitendinosus. In adult anaesthetized cats, MUs of the muscle under study were electrically stimulated via their ventral root axons with random sequences of brief pulses having mean rates between 6 and 12 pulses per second. Isometric tension fluctuations were recorded from the muscle under study, and discharge patterns of MS afferents (Ia and group II) were recorded from dorsal root filaments. A cross-correlation analysis was performed to display linear and nonlinear effects evoked by selected time constellations of MU activations. 18 (67%) of 27 MG MUs showed marked potentiation of the second of two twitches in response to pairs of stimuli separated by 5 to about 25 ms. The remainder of these and 16 of the soleus MUs did not exhibit conspicuous nonlinearities. MS responses to such pairs of MU activations usually showed a prolonged spindle pause. About 28% of 36 pairs of MG MUs produced twitch tension less than expected for linear summation if activated nearly simultaneously. If two MUs both produced a spindle pause and possibly a relaxation discharge in an MS afferent, the near-synchronous activation of the units produced respective discharge variations that were less than expected for linear summation. If one MU produced an early discharge, contraction of another MU would often prevent it. These results are discussed in regard to mechanisms of tremor suppression.

Animals↗
PubMed1985

Stimulus-related correlations between medial gastrocnemius muscle tension and homonymous motoneuron membrane potential result from non-linearities.

In anesthetized cats, medial gastrocnemius motor units (MUs) were electrically stimulated via their ventral-root axons with independent random patterns. Muscle tension and homonymous alpha-motoneuron (alpha-MN) membrane potential fluctuations in response to these stimuli were recorded simultaneously. Cross-correlating these two signals in temporal relation to the stimuli showed the covariance of the two signals around their respective means. With moderate to strong MU-MN couplings indicated by clear and large average membrane potential changes (PSP trajectories), the correlation coefficients tended to be positive at times when the PSP hyperpolarized, and vice versa. These relations are probably caused by the non-linear behavior of MUs, muscle receptors and afferent pathways. Thus, the signal transfer from motor efferents back to homonymous alpha-MNs depended upon the level of background tension, upon which MU twitches were superimposed.

Animals↗
PubMed1985

Responses of the spinal alpha-motoneurone-Renshaw cell system to various differentially distributed segmental afferent and descending inputs.

A previously presented multi-loop model of the mammalian spinal alpha-motoneurone-Renshaw cell system was extended to incorporate different physiological input patterns: Ia fibres from primary muscle spindle endings, spinal input systems descending in the ventral quadrant and from the nucleus ruber. The main goal of the computer simulation calculations was to present a number of dynamic input-output relations between these inputs which are distributed inhomogeneously to different types of alpha-MNs (that is, S-, FR-, and FF-type MNs) and the outputs of pools of the latter, for the purpose of experimental testing. The main outcome was that the phase relations of the outputs of the different types of MNs depend very much on the overall strength of recurrent inhibition, such that small changes of this strength, which appears to be small anyway, can significantly alter these phase relations. Since this strength is alterable through descending and segmental afferent inputs, this provides a physiological means of phase-decorrelation although it is unlikely to put the discharges of different MN types totally out of phase (by about 180 degrees). Also, the inhomogeneity of recurrent inhibition would help to prevent a strong phase separation of this kind. Yet a decorrelation at the microscopic level could help suppress physiological tremor.

Animals↗
PubMed1984

Contractions of single motor units are reflected in membrane potential changes of homonymous alpha-motoneurons.

In anesthetized cats 3 medial gastrocnemius (MG) motor units (MUs) were electrically activated with independent pseudorandom patterns at physiological mean rates. Recordings of isometric MG tension (T) and membrane potential changes (PSPs) in MG alpha-motoneurons (MNs) were averaged with respect to each stimulus train. The resulting T and PSP trajectories displayed the responses of each MN to the 3 average MU twitches. These responses could vary from no response to a sequence of depolarizing and hyperpolarizing waves in a PSP trajectory. The results are consistent with the known behavior of muscle stretch receptors to muscle and MU twitches and the receptors' connections to homonymous MNs.

Afferent Pathways↗
PubMed1984

Linear and non-linear summation of alpha-motoneuron potential changes elicited by contractions of homonymous motor units in cat medial gastrocnemius.

An analysis is made of linear and non-linear summation of the motor unit (MU) tension and motoneuron (MN) membrane potential (PSP) trajectories evoked by stimulation of MUs as previously reported. In most cases, summation of both averaged responses was linear. Seldom, PSPs summed non-linearly, then usually coinciding with non-linear summation of the responsible MU twitches. The results suggest that the afferent pathways mediating the mechanical events in muscle to MNs behave rather linearly, at least in the small-signal range of interest here.

Afferent Pathways↗
PubMed1984

Neural activity states in different forms of physiological tremor. Facts and hypotheses.

This paper attempts to combine (and thereby briefly review) various sets of physiological data in order to outline a qualitative model of the different states of stochastic neural activity underlying different forms of physiological tremor. Particular emphasis in put on spatial distributions of the properties of neural elements and their interconnexions , and on discharge characteristics of motor units and muscle spindle afferents including so-called "early discharges" and nonlinearities. It is argued that the wide variety of internal anatomical and functional structures of skeletal muscles and of their reflex organization must be considered when dealing with stability problems. Computer simulations of stochastic population models of the involved neuromuscular elements are advocated as means to investigate the relative importance of the many factors possibly contributing to stabilizing or de-stabilizing neuromuscular systems.

Animals↗
PubMed1984

The use of frequency domain techniques in the study of signal transmission in skeletal muscle.

Spectral analysis provides a description of the moments of random signals and enables the characterization of the behaviour of systems in terms of input-output relations. The merits of such an approach in the study of signal transmission in skeletal muscle are described in this paper. The representation of neural spike trains as impulse sequences and the subsequent treatment appropriate for this kind of analysis are discussed together with some practical problems. Spectral analysis of muscle afferent signals is applied to data obtained from cat experiments, and the use of the related frequency-domain techniques is demonstrated on a subsystem of the stretch reflex.

Action Potentials↗
PubMed1983

Analysis of nonlinear physiological systems with single or multiple spike inputs and analogue or spike outputs.

This paper outlines an extension of Krausz' (1975) approach to nonlinear analysis of physiological systems with random impulse train inputs. This extension concerns multiple (here two) inputs and spike train outputs in addition to analogue outputs. The respective theoretical formula are given, and the approach is illustrated by experimental data relating to the nonlinear interaction of cat motor units activated with random pulse trains. As outputs are considered muscle tension (as an example for an analogue signal) and discharge frequency of a Ia fibre (as an example for a spike output).

Animals↗