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H P Clamann

Publications and source records attributed to H P Clamann.

At least 19 recordsLinked to original sources
PubMed1998

Synaptic differentiation of single descending fibers studied by triple intracellular recording in the frog spinal cord.

Evoked excitatory postsynaptic potentials (EPSPs) were simultaneously intracellularly recorded in two lumbar motoneurons located in spinal segments 8-10 in response to intraaxonal stimulation of a descending fiber. Their mean amplitudes, paired-pulse facilitation, and short- and long-term posttetanic potentiation were compared to reveal possible functional differences among synapses formed by one axon on different postsynaptic targets. The mean amplitudes of EPSPs recorded in two motoneurons were significantly different in most experiments. This amplitude difference was related to the location of motoneurons in that it was twofold larger in motoneurons separated by >1 mm than in motoneurons located within 200 micron m of one another and also that the amplitude of EPSPs recorded in motoneurons located in the tenth segment was regularly smaller than the amplitude recorded in the ninth segment. The estimation of binomial model parameters suggests that the difference in mean EPSP amplitude was due mostly to differences in the maximal number of quanta prepared for release (binomial parameter N) and in mean release probability rather than to differences in quantal size. The ability of connections formed by a single axon on different motoneurons to undergo use-dependent synaptic modulations was different on scales of milliseconds, seconds, and tens of minutes as revealed by the measurements of effects of paired-pulse and tetanic stimulation. The difference in magnitude of short-term posttetanic potentiation in connections formed by a single descending axon was significantly correlated with the difference in mean probability of release in these connections. Thus our data revealed a functional nonuniformity of synapses formed by individual descending fibers on widely separated motoneurons, most likely innervating different muscles. This process can be one of the mechanisms by which a fine descending control of recruitment of motoneuronal populations is achieved.

Animals↗
PubMed1997

Size principle and information theory.

The motor units of a skeletal muscle may be recruited according to different strategies. From all possible recruitment strategies nature selected the simplest one: in most actions of vertebrate skeletal muscles the recruitment of its motor units is by increasing size. This so-called size principle permits a high precision in muscle force generation since small muscle forces are produced exclusively by small motor units. Larger motor units are activated only if the total muscle force has already reached certain critical levels. We show that this recruitment by size is not only optimal in precision but also optimal in an information theoretical sense. We consider the motoneuron pool as an encoder generating a parallel binary code from a common input to that pool. The generated motoneuron code is sent down through the motoneuron axons to the muscle. We establish that an optimization of this motoneuron code with respect to its information content is equivalent to the recruitment of motor units by size. Moreover, maximal information content of the motoneuron code is equivalent to a minimal expected error in muscle force generation.

Animals↗
PubMed1996

Reliability of spike propagation in arborizations of dorsal root fibers studied by analysis of postsynaptic potentials mediated by electrotonic coupling in the frog spinal cord.

1. Postsynaptic potentials were recorded in lumbar motoneurons of the frog in response to electrical activation of dorsal roots. After chemical synaptic transmission was blocked by replacing Ca2+ with Mg2+ in the superfusion medium, it was confirmed that the remaining electrical excitatory postsynaptic potentials (EEPSPs) recorded in motoneurons consisted of potential changes-produced by electrical coupling between the motoneurons and the stimulated axons. The EEPSPs could then be used as an assay to study the reliability of spike propagation into presynaptic terminals. 2. EEPSPs typically consisted of three components. The first was a small positive deflection (prespike or presynaptic volley) that could also be recorded extracellularly. The second component was a spikelike fast positive component and the third was a slow positive component that followed the second but had a distinct maximum and a slow decay. The amplitude of the fast component did not correlate with that of either the prespike or the slow component. 3. 4-Aminopyridine (0.1 mM), which widens action potentials by blocking K+ channels, increased the amplitude and width of EEPSPs. Heptanol (1-4 mM), which is known to be a blocker of electrical coupling, could block EEPSPs. 4. The amplitudes of EEPSPs evoked by dorsal root stimulation were compared at different temperatures (7.5-19.5 degrees C). A slight decrease of the amplitude of the fast component with increasing temperature (Q10 = 0.8) was within limits predicted by resistance-capacitance filtering of the presynaptic spike at the different temperatures, suggesting that the temperature does not affect propagation of the spike in this synapse. 5. The amplitude of the fast component of EEPSPs evoked by single-pulse and paired-pulse stimulation did not fluctuate more than the baseline noise in 37 experiments in which the SD of baseline noise was < 100 microV. We conclude that electrical synaptic transmission does not fluctuate intermittently in this system, and that branch points conduct or fail to conduct for periods of time longer than the longest period in the analyzed experiments.

Animals↗
PubMed1995

Comparison of the topology and growth rules of motoneuronal dendrites.

The complexity, shape, and branching modes of the dendrites of spinal motoneurons were compared in cat, rat, and frog using topological analysis and growth models. The complexity of motoneuronal dendrites, measured as the mean number of terminal segments, varied significantly among samples and was related to contractile properties of innervated motor units. Despite this variation, all mature motoneurons having a mean number of terminal segments per dendrite greater than ten (up to 24.3) exhibited a narrow range of values of coefficients describing the symmetry of tree shapes (0.42-0.47). This implies low variability in the topological shape of motoneuronal dendrites of different animals. This similarity of tree shapes proved to be a result of the similarity of growth rules. The growth of the dendrites could be described to a first approximation by a two-parameter (Q and S) model called the QS model and by a multitype Markovian model. The estimation of parameters of the QS model, in which parameter Q is related to the probability of branching of intermediate segments, revealed that Q was equal or close to 0, implying that branching of dendrites is restricted to terminal segments. The estimates of the parameter S, which describes whether the probability of branching increases (S < 0) or decreases (S > 0) exponentially with segment order, were positive. This was in agreement with the results of estimation of probabilities of branching provided by the Markovian model, which showed that the branching probabilities decreased with segment order in an exponential manner in most of the neurons studied. The QS and Markovian models involve different assumptions about the sequence and timing of branching events, and selection of the best model can provide insight into details of dendritic outgrowth. Extensive simulation of tree outgrowth using a Markovian model revealed significant differences between stimulated trees and real dendrites, particularly with regard to variability of the number of terminals and to symmetry. In contrast, the QS model provided a good fit to the mean values and standard deviations of basic topological parameters. This model is adequate to describe the shape of mature motoneuronal dendrites. It implies that dendritic branches have many opportunities to bifurcate during the whole time of development and that bifurcating potency of a branch is a function of the number and position of other branches of that dendrite. Combined with analysis of metrical properties such as lengths of segments, the QS model can assist in a quantitative analysis of development and plasticity.

Animals↗
PubMed1994

Practical guidance for testing the accuracy of deconvolution results from quantal analysis.

A Monte Carlo study was carried out to test the reliability of the Maximum Likelihood Estimator (MLE) approach for quantal analysis. This widely used statistical method was applied to extract a finite mixture of Gaussian distributions from simulated data. The data were generated by convolving a distribution of discrete amplitude steps (multiples of a unitary step Q) with Gaussian noise of various standard deviations (sigma n). Our results offer practical guidance on when to use the MLE, taking into account the determining parameters: signal to noise ratio (Q/sigma n, the most important parameter), number of samples collected and the number of components (k). For a given set of parameters the algorithm always converged to the "true" values, never converged to the "true" values or converged in only a fraction of cases to the "true" values. The behavior of the fitting routine in the parameter space is displayed in contour plots. These contour plots can be used as a guide to test the accuracy of deconvolution results.

Computer Simulation↗
PubMed1994

Quantal analysis based on spectral methods.

A method for calculating quantal size based on spectral analysis of postsynaptic potential (PSP) amplitude distributions was developed and tested by Monte-Carlo techniques. It was combined with a test to reveal the bias of the estimate of quantal size and to determine whether the peaks in amplitude distributions reflect quantal release or a sampling artifact. Spectral density was computed using fast Fourier transformation after subtraction of a fitted polynomial from the probability density function. The method overestimated quantal size for values less than two standard deviations of noise, indicating that those estimates as well as estimates of quantal size computed for examples of non-quantal distributions are not reliable. This was the case for 34 of 36 sets of sensorimotor excitatory PSPs of the frog, suggesting that most values of the quanta in synapses between primary fibres and lumbar motoneurons are smaller than 70-90 microV.

Animals↗
PubMed1993

Limits of quantal analysis reliability: quantal and unimodal constraints and setting of confidence intervals for quantal size.

An accurate objective method for determining the reliability of estimates of quantal size (Q) at central synapses was developed. To do this, distributions of amplitudes of postsynaptic responses were simulated by convolving a number of discrete amplitudes separated by equal increments Q with gaussian noise, after which the value of Q was estimated by the maximum likelihood method under different constraints on the discrete distribution. It was shown that the likelihood function (LF) had several local maxima under the quantal constraint, and, if the value of the ratio between Q and the standard deviation of the noise (sigma) was less than 3, the global maximum of the LF corresponded to a biased estimate of Q lying in a range of values less than 1.5 sigma. The best estimates of Q were obtained when unimodal discrete distributions of amplitudes resulting from the maximum likelihood method were selected. However, this method also gave biased estimates when Q/sigma was less than 1.5-2.3. The limit of reliability depended on the number of discrete components and the sample size. To calculate confidence intervals for the quantal size, different numbers and weights of components were used to simulate amplitude histograms with different values of Q/sigma. Three data sets were used to illustrate the procedure.

Animals↗
PubMed1993

Motor unit recruitment and the gradation of muscle force.

The capabilities of the different types of motor units are reviewed, and their properties in a variety of muscles are discussed. Because the tension-generating capacities of motor units are so different, the order in which they are recruited will have a strong influence on the way force output of the whole muscle is graded. Activation of motor units in a random order produces a roughly linear force increase with progressive recruitment, whereas recruitment of motor units in order of increasing force produces an approximately exponential force increase as the number of active motor units increases. The latter scheme allows fine control of weak movements and rapid production of powerful movements. Motor units are shown to be well adapted to the tasks they must perform, and a "compromise" motor unit will not fulfill all the tasks demanded of it. Finally, changes in motor unit properties produced by different activity patterns and by muscle reinnervation are reviewed, and the implications for rehabilitation are discussed.

Adaptation, Physiological↗
PubMed1991

The influence of noise on quantal EPSP size obtained by deconvolution in spinal motoneurons in the cat.

1. The amplitudes of quantal components that make up single-fiber excitatory postsynaptic potentials (EPSPS) were determined by a deconvolution technique and by simulation studies and were compared with the background noise. 2. A strong correlation was found between the sizes of EPSP quantal components and the standard deviation of the noise from which the data were extracted by deconvolution. A similar correlation was then shown in published data from several other laboratories. 3. EPSPS having amplitudes less than 100 microV were recorded that had little or no variance in their amplitudes. Most of these EPSPS showed a much smaller peak variance than would be expected if they fluctuated among amplitudes in steps of approximately 100 microV--the proposed mean value for the amplitude of the quantal EPSP. 4. Deconvolution of simulated data with the maximum likelihood algorithm resulted in the suppression of components less than 1.5 SD of the background noise. The remaining components were approximately equally spaced. No way was found to detect this error, and rejection of deconvolved data with components less than 1.5 noise SD did not eliminate it. The resulting erroneous data showed a strong correlation between the amplitudes of the components obtained and the noise standard deviation. 5. It is concluded that at least some EPSPS generated by single Ia-afferents on motoneurons are composed of quantal components significantly less than 100 microV and that deconvolution procedures are not capable of detecting such small components.

Action Potentials↗
PubMed1989

Force output of cat motor units stimulated with trains of linearly varying frequency.

1. A relation between stimulation frequency and muscle force is usually determined with stimulus trains of constant frequency and described as a single-valued sigmoid curve. This relationship fails to explain a number of features of rate coding. 2. Single motor units were isolated in medial gastrocnemius or soleus muscles of cats deeply anesthetized with pentobarbital sodium. Motor units were classified as fast or slow. Each unit was stimulated with a train whose frequency varied linearly from less than 3 pulses per second (pps) to 20% above the unit's fusion frequency and back to about 3 pps with a period of 5 s. 3. All motor units showed a marked hysteresis during frequency-varying stimulation. A greater force was produced when frequency was decreasing than when it was increasing. The force output of each unit remained nearly maximal as stimulus frequency declined from its maximum to about one-half of the unit's fusion frequency; force rapidly declined with further decreases in frequency. The force-frequency relation could change with each trial as frequency increased but was highly reproducible when frequency decreased. This suggested a strategy by which central nervous system (CNS) control could maximize the force at any discharge rate and produce a predictable force-frequency relation. 4. Posttetanic potentiation, motor unit slowing, and a preload which causes a motor unit to operate on the negatively sloping portion of the length-tension curve may each contribute to the observed hysteresis under certain circumstances. None can explain why hysteresis was consistently seen in all motor units. A time-dependent rate of tension development and decay together with a catchlike property can account for all of the properties of hysteresis and appeared to be the primary cause of hysteresis in fully potentiated motor units.

Animals↗
PubMed1989

Variance analysis of excitatory postsynaptic potentials in cat spinal motoneurons during posttetanic potentiation.

1. Fluctuations in the peak amplitudes of composite excitatory postsynaptic potentials (EPSPs) in cat spinal motoneurons were analyzed during posttetanic potentiation (PTP). Each of a series of identical tetanic stimulus trains delivered to a muscle nerve was followed by 45 test stimuli applied at 2-s intervals. The mean peak amplitude and mean peak variance were calculated for EPSPs evoked by all those stimuli following a tetanus with the same time interval. It was assumed that the variance arises primarily from the probabilistic all-or-none behavior of single synaptic boutons and background noise due to spontaneous synaptic activity and thermal noise in the recording system. The variance was corrected for the contribution from additive Gaussian background noise. 2. If it is assumed that individual synaptic boutons behave independently, corrected mean peak variance and mean peak amplitude are related by a parabolic function. The expected parabolic relationship was seen in 9 of 31 cases studied, and the parameters of the best parabolic fit to the data allowed estimation of some synaptic properties. From these parameters, the mean amplitude of the unit EPSP (v) was estimated to be 102.1 +/- 57.4 (SD) microV. An average of 3.7 boutons comprised each Ia-motoneuron contact system. 3. On average, only 27% of all synaptic boutons given off by the stimulated Ia fibers to one motoneuron were active and releasing transmitter during unpotentiated reflex transmission. The remaining 73% of the synapse population was intermittently silent. The population of boutons which took part in synaptic transmission could be divided into two subpopulations, one with a release probability P = 1 and a second with a mean release probability P = 0.13 +/- 0.086. 4. We conclude that synaptic boutons connecting Ia afferents to motoneurons exist in two populations, one having a high and one a low probability of transmitter release. Transmitter release is quantal, resulting in a unit EPSP of approximately 100 microV measured at the motoneuron soma.

Analysis of Variance↗
PubMed1988

Effects of glucocorticoids on motor units in cat hindlimb muscles.

The purpose of this study was to examine the effects of glucocorticoid treatment on the contractile, electrical and fatigue properties of isolated motor units of identified type. Although it is known that glucocorticoid administration induces atrophy and weakness most strongly in fast, pale muscles and to a lesser extent in red muscle, the relationship between steroid effects and motor unit type is not known. Properties of medial gastrocnemius (MG) and soleus (SOL) motor units were studied in normal cats and in cats treated with triamcinolone acetonide (3-4 mg/kg body weight for 10-16 days). Glucocorticoid treatment produced weakness preferentially in fast-twitch motor units. This suggests that catabolic steroids cause a reduction in the amount of contractile protein and hence contractile strength of motor units in inverse proportion to their relative activity or degree of use.

Animals↗
PubMed1988

Nonlinear force addition of newly recruited motor units in the cat hindlimb.

The present experiments were designed to examine the interaction of simultaneously active motor units. Pairs of medial gastrocnemius (MG) or soleus (Sol) units were stimulated individually and then together with constant frequency trains of 5-40 pulses per second. Stimulating two units asynchronously produced a smoother contraction than synchronous stimulation, but rarely a force increase. This contrasts with similar experiments on whole muscle bundles. A force increase may require that adjacent muscle fibers be active. The combined force of two motor units exceeded the algebraic sum of their separate forces by 12% in MG and 5% in Sol on average. The force a unit could sustain after a second unit fell silent was greater than the force the unit produced alone (21% in MG and 8% in Sol). We conclude that motor units produce more force when interacting than alone. During derecruitment the units remaining active produce more force than when recruited.

Animals↗
PubMed1987

Changes in contractile speed of cat motor units during activity.

Experiments were conducted to measure the extent of contractile changes during phasic activity of different motor units. Motor units of cat medial gastrocnemius were isolated and classified by their mechanical properties as fast and fatigable (FF), fast and fatigue resistant (FR), or slow (S). Single stimuli interpolated between stimulus trains of the fatigue test produced twitches whose shapes were measured at different times during this test. After 30 seconds of fatigue testing, twitch contraction times of 33% of FF units fell into the slow range, i.e., 5 of 17 units had become slower than the fastest slow units. Mean twitch contraction time of FF units increased by 11.8 msec, whereas that of S units decreased by 16.2 msec. We conclude that the mechanical properties of rested motor units change markedly with use and are a poor index for determining the contractile speed of active muscles.

Animals↗
PubMed1985

A comparison of electromyographic and mechanical fatigue properties in motor units of the cat hindlimb.

Single motor units were isolated in medial gastrocnemius (MG) or soleus (Sol) muscles of the cat. Single shocks delivered to the motor axon elicited EMG waveforms which were recorded in the muscle. The amplitude of each individual EMG waveform as well as the area under the full-wave rectified waveform were measured. Mechanical properties of the motor units were then measured so that each unit could be classified as FF, FR or S according to the criteria of Burke. Finally, each unit was stimulated continuously at 80 pulses per second (pps), and changes in electrical and mechanical responses were recorded and compared. IEMG was positively correlated with maximum tetanic tension of motor units in MG. The relationship could be fitted with a parabola showing that 'large' motor units produce relatively more electrical activity than do 'small' motor units. Two types of electrical changes were seen during continuous stimulation of motor units at 80 pps: a smooth continuous decline in IEMG was attributed to electrical changes occurring at a site or sites distal to the neuromuscular junction; and random, abrupt changes in unit-EMG waveform and hence IEMG amplitude, were attributed to failure of the action potential to propagate past axonal branch points, resulting in the intermittent failure of groups of muscle fibers to respond to stimuli. The rates of decline of electrical and mechanical activity were compared for motor units. It was found that in fast fatiguing motor units, tetanic tension declined more rapidly than did IEMG, while in slowly fatiguing motor units, IEMG declined more rapidly than did force. We conclude that fatigue of motor units induced by continuous 80 pps stimulation can occur at different sites in the motor unit, and that the site of fatigue depends on motor unit properties or motor unit type.

Animals↗
PubMed1985

Structural and topographical influences on functional connectivity in spinal monosynaptic reflex arcs in the cat.

A greatly expanded version of spike-triggered averaging (Mendell & Henneman, 1971), performed off-line on tape-recorded signals, was utilized to determine the presence or absence of functional connexions between stretch-afferent fibres and homonymous motoneurones. As many as 264 possible connexions between eleven Ia or spindle group II fibres and twenty-four motoneurones were studied in each single, acute experiment. Morphological and topographical factors influencing functional connectivity were analysed with the aid of wiring diagrams and connectivity matrices. In all experiments the greater the conduction velocity (i.e. diameter) of a Ia or group II fibre, the higher was the probability of its having functional connexions with homonymous motoneurones. The greater the longitudinal distance between the spinal entry points of Ia fibres and the location of a motoneurone, the less was the same probability. The influence of axonal conduction velocity of motoneurones on functional connectivity was apparent in some experiments, but not in others. In pooled data large motoneurones received functional connexions from a higher percentage of group II fibres than did small cells. The projection percentage reached 100 only when both Ia fibres and motoneurones were large, suggesting that motoneurone size influences the probability of functional connexions from group Ia as well as group II fibres. On a cell-to-cell level, connectivity apparently does not follow strict, deterministic rules. The results raise the question of how probabilistic connexions between afferent fibres and motoneurones give rise to deterministic outputs from the whole pool.

Action Potentials↗
PubMed1983

Motor pool organization in monosynaptic reflexes: responses in three different muscles.

Recruitment order of motoneurons was measured as a function of their conduction velocities in the presence of monosynaptic reflexes evoked by dorsal root stimulation. Motoneurons were studied in three cat hindlimb muscles: medial gastrocnemius (MG), plantaris (Pl), and tibialis anterior (TA). A relationship between recruitment order and unit conduction velocity (CV) was clearly seen in all three muscles. The correlation between these two variables was lower than that found in previous studies. A CV-dependent recruitment order was most clearly sen in tibialis anterior motoneuron pool; the relationship was poorest in plantaris. Recruitment order of MG motoneurons was measured and related to their conduction velocities in response to monosynaptic reflexes evoked by L7 + S1 dorsal root stimulation. Recruitment order was then retested in the presence of rostral root stimulation (largely heteronymous) alone and again in the presence of caudal root stimulation (largely homonymous) alone. Changing the composition of the afferent input changed the critical firing levels (rank order) of some motoneurons by as much as 40% and that of others, not at all. Some motoneurons became harder to recruit and others easier; changes in recruitment order were not related to conduction velocities of the units. 1a-afferent inputs are not uniformly distributed to all the motoneurons of a pool. It appears that this nonuniformity is a determining factor in establishing a recruitment order. TA, which receives the most uniformly distributed monosynaptic input, also has the most nearly size-dependent recruitment order. Fractionation of input ca induce additional nonuniformity, and results in recruitment-order changes in some motor units that are independent of their conduction velocity. It is concluded that nonuniformity of afferent inputs, whether present or induced by experiment, can produce large recruitment-order changes among individual motoneurons in a pool and that these individual motoneurons need not share a common property such as conduction velocity or recruitment threshold in response to a control input. Therefore, arguments based on reversals in recruitment order of pairs of motor units or even changes in rank order of individual motor units do not present sufficient evidence for the presence of input specifically directed to motor units sharing a particular property.

Animals↗