Historical reflections on the afterhyperpolarization--firing rate relation of vertebrate spinal neurons.

In mammalian spinal motoneurons (MNs), the slow component of the afterhyperpolarization (AHP) that follows the spike of each action potential is a major but not the sole determinant of the cells' firing rate. In this brief historical review, we emphasize four points about the AHP-firing rate relation. (1) There is a relatively sparse literature across vertebrates that directly addresses this topic. (2) After the advent of intracellular recording in the early 1950s, there was evidence from mammals to the contrary of an idea that subsequently became prevalent: that the high-firing rates attainable by spinal interneurons (INs) and low-threshold MNs was attributable to their small AHP at rheobase. (3) Further work is needed to determine whether our present findings on the AHP-firing rate relation of turtle cells generalize to the spinal neurons of other vertebrate species. (4) Relevant to point 3, substantial in vivo and in vitro work is potentially available in raw data used in reports on several mammalian and non-mammalian vertebrates. In summary, the factors in addition to the slow AHP that help determine spinal INs and MN firing rate deserve further evaluation across vertebrates, with relevant data already potentially available in several laboratories.

Measurement and nature of firing rate adaptation in turtle spinal neurons.

There is sparse literature on the profile of action potential firing rate (spike-frequency) adaptation of vertebrate spinal motoneurons, with most of the work undertaken on cells of the adult cat and young rat. Here, we provide such information on adult turtle motoneurons and spinal ventral-horn interneurons. We compared adaptation in response to intracellular injection of 30-s, constant-current stimuli into high-threshold versus low-threshold motoneurons and spontaneously firing versus non-spontaneously-firing interneurons. The latter were shown to possess some adaptive properties that differed from those of motoneurons, including a delayed initial adaptation and more predominant reversal of adaptation attributable to plateau potentials. Issues were raised concerning the interpretation of changes in the action potentials' afterhyperpolarization shape parameters throughout spike-frequency adaptation. No important differences were demonstrated in the adaptation of the two motoneuron and two interneuron groups. Each of these groups, however, was modeled by its own unique combination of action potential shape parameters for the simulation of its 30-s duration of spike-frequency adaptation. Also, for a small sample of the very highest-threshold versus lowest-threshold motoneurons, the former group had significantly more adaptation than the latter. This finding was like that shown previously for cat motoneurons supplying fast- versus slow twitch motor units.

Afterhyperpolarization-firing rate relation of turtle spinal neurons.

This study addressed the afterhyperploarization-firing rate relationship of unanesthetized turtle spinal motoneurons and interneurons. The afterhyperploarization of their solitary action potential at rheobase was compared to that during the cells' minimum and maximum firing rates. Like previous mammalian findings, afterhyperpolarization duration and area at rheobase were 32 and 19% less for high- versus low-threshold motoneurons. Contrariwise, maximum firing rate was two times less for the high-threshold group. Other new findings were that for high- versus low-threshold interneurons, afterhyperpolarization duration and area were 25 and 95% less, and maximum firing rate 21% higher for the high-threshold group. For combined motoneurons versus interneurons, there were no differences in afterhyperpolarization duration and area at rheobase, whereas maximum firing rate was 265% higher for the interneurons. For high-threshold motoneurons alone, there were significant associations between minimum firing rate and afterhyperpolarization duration and area measured at rheobase. In summary, this study showed that (1) the afterhyperploarization values of both turtle spinal motoneurons and interneurons at rheobase provided little indication of their corresponding values at the cells' minimum and maximum firing states, and (2) the evolution of afterhyperploarization from rheobase to maximum firing state differed both qualitatively and quantitatively for motoneurons versus interneurons.

Associations between force and fatigue in fast-twitch motor units of a cat hindlimb muscle.

Associations were quantified between the control force and fatigue-induced force decline in 22 single fast-twitch-fatigable motor units of 5 deeply anesthetized adult cats. The units were subjected to intermittent stimulation at 1 train/s for 360 s. Two stimulation patterns were delivered in a pseudo-random manner. The first was a 500-ms train with constant interpulse intervals. The second pattern had the same number of stimuli, mean stimulus rate, and stimulus duration, but the stimulus pulses were rearranged to increase the force produced by the units in the control (prefatigue) state. The associations among the control peak tetanic force of these units, 3 indices of fatigue, and total cumulative force during fatiguing contractions were dependent, in part, on the stimulation pattern used to produce fatigue. The associations were also dependent, albeit to a lesser extent, on the force measure (peak vs. integrated) and the fatigue index used to quantify fatigue. It is proposed that during high-force fatiguing contractions, neural mechanisms are potentially available to delay and reduce the fatigue of fast-twitch-fatigable units for brief, but functionally relevant, periods. In contrast, the fatigue of slow-twitch fatigue-resistant units seems more likely to be controlled largely, if not exclusively, by metabolic processes within their muscle cells.

Effects of fatigue on the catchlike property in a turtle hindlimb muscle.

The purpose of this report was to test for the possibility that a catchlike, force-enhancing property, attributable to a particular stimulation pattern, could be evoked in non-mammalian turtle muscle, just as it has been shown in mammalian muscle. We tested for the presence of this property in dynamic lengthening and shortening contractions, as well as in the more commonly studied isometric contractions. A second aim was to note the effects of fatigue on the catchlike property, if the latter was present. The force response of the external gastrocnemius muscle in the adult turtle, Pseudemys ( Trachemys) scripta elegans, was compared for a control, constant-frequency 10 Hz, 1-s duration stimulation pattern using 0.1-ms pulses vs. the same pattern, but with two additional pulses within the first 100-ms interspike interval of the control stimulus train. This latter train produced a pronounced and prolonged enhancement of muscle force, which was attributed to a catchlike effect. It was greatly increased when the muscle was in a fatigued state. The extent of this force enhancement was significantly different for the three contraction types, and generally in the order: isometric>lengthening>shortening contraction. These differences were greater in fatigued vs. fresh muscle. Comparative aspects and potential mechanisms underlying the catchlike effect are discussed.

Erk 1,2 phosphorylates p27(Kip1): Functional evidence for a role in high glucose-induced hypertrophy of mesangial cells.

AIMS/HYPOTHESIS: Mesangial cell hypertrophy is one of the earliest morphological abnormalities of diabetic nephropathy. We have previously shown that high glucose induces p27(Kip1) by a post-transcriptional mechanism and that mesangial cell hypertrophy depends on G(1)-phase arrest mediated by this CDK-inhibitor. However, it remains poorly understood how high glucose stimulates p27(Kip1) expression in mesangial cells. METHODS: Mesangial cells were isolated from p27(Kip1) +/+ and -/- mice and characterized by light microscopy and immunohistochemistry. It was tested by Western blotting and autoradiography whether high glucose medium activates Erk 1,2 and whether this activation phosphorylates p27(Kip1). The three consensus phosphorylation sites of p27(Kip1) were mutated and these constructs were expressed in p27(Kip1) -/- mesangial cells. Hypertrophy was assessed by different methods. RESULTS: High glucose stimulates phosphorylation of MAP kinases Erk 1,2 in p27(Kip1 )+/+ and -/- mesangial cells. Activation of Erk 1,2 leads to phosphorylation of p27(Kip1 )in vitro and in vivo. Mutations of serine(10) or threonine(187) still supported high glucose-induced hypertrophy. In contrast, a mutation of serine(178) converted the hypertrophic response into a proliferative phenotype. Mutation of serine(178) leads to the attenuated expression of p27(Kip1) protein in the presence of high glucose. CONCLUSIONS/INTERPRETATION: Our study shows that high glucose stimulates Erk 1,2 that phosphorylate p27(Kip1) at serine(178) increasing its expression. This is an important molecular mechanism of high glucose-induced hypertrophy of mesangial cells.

Electrophysiological properties of spinal motoneurons in the adult turtle.

The purpose of this study was to develop a scheme for classifying turtle motoneurons, such that their properties could be compared to those of other vertebrate species, including, in particular, the cat. A 130-cell sample of turtle motoneurons was provisionally classified into four groups (1-4) on the basis of a cluster analysis of the cells' intracellularly recorded input resistance, rheobase, and slope of their stimulus current-spike frequency relation. These measurements, using sharp microelectrodes and an in vitro spinal cord slice preparation, were particularly robust. It is argued that the cat counterpart of our turtle type 1, 2, and 3 motoneurons innervate slow-twitch muscle fibers, fast-twitch-oxidative fibers, and fast-twitch-glycolytic fibers, respectively. Our turtle type 4 motoneuron is thought analogous to a particularly high-threshold cat and human cell that innervates highly fatigable fast-twitch muscle fibers in both species. Our turtle type 1 category may include cells that innervate non-twitch muscle fibers, which are found in other non-mammalian vertebrates. To advance comparative spinal cord neurobiology, the present results invite comparison to the motoneurons of other vertebrate species, which have yet to be subjected to similar or other classification procedures.

Associations between the passive, transitional, and active properties of turtle motoneurons.

The literature on the associations between the properties of vertebrate spinal motoneurons (MNs) is dominated by studies on the deeply anesthetized cats, with the measurements made with low-resistance microelectrodes, and limited largely to passive (cell at rest) vs. transitional (rest-to-rheobase action potential) properties. There has been far less consideration of active (repetitive-firing) firing properties, like the parameters of the stimulus current-spike frequency (I-f) relation. The present study shows that several conventionally accepted associations among MN properties, including those between the determining and defining parameters of the I-f relation, are either absent or weak when the measurements are made with high-resistance microelectrodes, and using unanesthetized slices of the adult turtle's spinal cord. The strength of such associations is even further diminished when the MNs are exhibiting modulator-induced plateau potentials. These novel and potentially controversial findings invite consideration of their relation to literature on MN repetitive firing properties, as recorded with sharp microelectrodes in spinal and brainstem slices of several other vertebrate species, including neonatal and adult mammals.

Monocyte chemoattractant protein-1 and osteopontin differentially regulate monocytes recruitment in experimental glomerulonephritis.

BACKGROUND: This study evaluated the mechanisms of monocyte/macrophage (M/M) infiltration in a rat model of anti-glomerular basement membrane glomerulonephritis (GN). We focused on chemokines and osteopontin, which are known regulators of M/M recruitment. METHODS: Using immunohistology, in situ hybridization, and Northern blotting, the expression levels of chemokines and osteopontin were evaluated in isolated glomeruli and tubules 4, 10, and 20 days after the induction of GN. In vivo blocking experiments were performed by application of neutralizing antibodies against osteopontin and monocyte chemoattractant protein-1 (MCP-1). RESULTS: In nephritic animals, high glomerular MCP-1 and RANTES (regulated upon activation normal T cell expressed and secreted) expression levels were observed on days 4 and 10. The tubular expression of MCP-1, however, was only slightly enhanced. In contrast, tubular osteopontin production was maximally stimulated (day 10) and paralleled with peaks of albuminuria and tubulointerstitial M/M infiltration. Application of an anti-osteopontin antibody ameliorated tubulointerstitial and glomerular M/M recruitment, whereas treatment with an anti-MCP-1 antibody selectively reduced glomerular M/M recruitment. However, tubulointerstitial M/M infiltration remained unchanged. CONCLUSION: These studies show that chemokines and osteopontin are differentially expressed in glomeruli and tubules in this model of GN. Chemokines play a primary role in the glomeruli, whereas osteopontin has a predominant role in tubulointerstitial M/M recruitment. The roles of chemokines and osteopontin may thus be dependent on the renal compartment and on the disease model.

Electrophysiological and morphological properties of neurons in the ventral horn of the turtle spinal cord.

In this report, we present recent findings on the electrophysiological and morphological properties of spinal motoneurons (MNs) and interneurons (INs) of the adult turtle which were studied in slices of the spinal cord. The range of values for the measured electrophysiological parameters in 96 tested cells included: resting potential, -57 to -83 mV; input resistance, 2.5-344 M omega; time constant, 2.5-63 ms; rheobase current, 0.04-5.3 nA; after-hyperpolarization (AHP) duration, 72-426 ms; AHP half-decay time; 11-212 ms; and, slope of the stimulus current-spike frequency relationship, 3.4-235 Hz/nA. For another 20 cells, we made both morphological and electrophysiological measurements (the latter values within the above ranges). Their ranges in morphological properties included: soma diameter, 20-54 microm; soma surface area, 299-2045 microm2; soma volume, 2.3-45 microm3 x 10(4); rostro-caudal dendritic projection distance, 150-1200 microm; and, sum of dendritic lengths, 1.5-16 microm x 10(3). The emphasized findings include: 1) the quality and robustness of the intracellular recordings, which enabled accurate measurement of the action potential's shape parameters (spike, afterhyperpolarization [AHP]); 2) the substantial AHP of the INs' AP; 3) no single action-potential shape parameter (nor combination of parameters) being cardinal for its (or their combined) changes matching the profile of the initial and later phases of spike-frequency adaptation; 4) the utility and flexibility of a cluster analysis (using varying combinations of passive, transitional and active cell properties) for providing a provisional classification of low (like cat S) and high (like cat F) threshold MNs, and groups of INs with non-spontaneous versus spontaneous discharge; 5) the clear-cut morphological confirmation of the provisional classification strategy; 6) the basis for testing the possibility that one of the provisionally classified MN types innervates non-twitch muscle fibers; and 7) the heuristic value of comparing the properties of MNs versus INs across vertebrate species, with an emphasis on the lamprey, turtle, and cat.

Properties of spinal motoneurons and interneurons in the adult turtle: provisional classification by cluster analysis.

The purpose of the present study was to compare, in motoneurons (MNs) vs. interneurons (INs), selected passive, transitional, and active (firing) properties, as recorded in slices of lumbosacral spinal cord (SC) taken from the adult turtle. The cells were provisionally classified on the basis of (1) the presence (in selected INs) or absence (MNs and other INs) of spontaneous discharge, (2) a cluster analysis of selected properties of the nonspontaneously firing cells, (3) a comparison to previous data on turtle MNs and INs, and (4) a qualitative comparison of the results with those reported for other vertebrate species (lamprey, cat). The provisional nomenclature accommodated properties appropriate for solely MNs (Main MN group) vs. nonspontaneously firing INs (Main IN-N) vs. spontaneously firing INs (IN-S) and for neurons with two degrees of intermediacy between the Main MN and the Main IN-N groups (Overlap MN, Overlap MN/IN). Morphological reconstructions of additional cells, which had been injected with biocytin during the electrophysiological tests, were shown to provide clear-cut support for the provisional classification procedure. The values for the measured parameters in the 96 tested cells covered the spectrum reported previously across adult vertebrate species and were robust in measurements made on different SC slices up to 5 days after their removal from the host animal. The interspecies comparisons permitted the predictions that (1) our Main MN and Overlap MN cells would be analogous to two MN types that innervate fast-twitch and slow-twitch skeletomotor muscle fibers, respectively, in the cat, and (2) the MNs in our Overlap MN/IN group probably innervate slow (nontwitch, tonic) muscle fibers whose presence has recently been established in the turtle hindlimb. In summary, the results bring out the utility of the SC slice preparation of the turtle for study of spinal motor mechanisms in adult tetrapod vertebrates, particularly as an adjunct to the in vivo cat, because of the ease with which robust measurements can be made of the active properties of both MNs and INs.

Reduced motor unit activation of muscle spindles and tendon organs in the immobilized cat hindlimb.

Six weeks of limb immobilization of a healthy muscle (cat tibialis posterior) at a short length resulted in a significant reduction of mean fiber area for all fiber types (I, 71% of control; IIa, 77% of control; IIb, 79% of control), whereas fiber type proportions were unchanged. For motor units, there was a reduction in peak tetanic force (type slow > fast fatigue resistant > fast fatigable); an increase in the twitch-to-tetanus ratio for fast fatigue-resistant and slow units; and no effect on the twitch force, twitch time course, or fatigability. The reduction in peak force was greater than expected because of fiber atrophy in slow units. Immobilization had a minimal effect on muscle spindle afferent (Ia and spindle group II) responses to a ramp-and-hold stretch of the passive muscle. Tendon organ (Ib) afferents had an increased responsiveness to stretch after immobilization but only when the muscle was stretched from a short resting length. However, immobilization reduced the modulation of muscle afferent discharge in response to tetanic contractions of single motor units. The decline in responsiveness of spindles was a result of the reduced tetanic force of motor units. In contrast, tendon organs in immobilized muscle were twice as likely to convey no information on the contraction of a single motor unit and were more likely to be unloaded, suggesting that immobilization caused the functional denervation of some muscle fibers. Thus the responses of muscle spindles and tendon organs in immobilized muscle reflected atrophic changes in extrafusal fibers but did not provide evidence for substantial disturbance of receptor function.

Impaired response of activated mononuclear phagocytes to interleukin 4 in inflammatory bowel disease.

BACKGROUND/AIMS: In inflammatory bowel disease (IBD), peripheral monocytes and intestinal macrophages show an increased state of priming and activation. The aim of this study was to test the hypothesis that the response of IBD mononuclear phagocytes to the contrainflammatory cytokine interleukin (IL) 4 may be altered. METHODS: The in vitro secretion of proinflammatory cytokines (IL-1 beta, tumor necrosis factor alpha [TNF-alpha], and IL-1-receptor antagonist [IL-1ra]) by peripheral monocytes and by intestinal lamina propria mononuclear cells (LPMNCs) was assessed by enzyme-linked immunosorbent assay. In parallel, superoxide anion release, macrophage mannose receptor, and IL-4 receptor expression were investigated. RESULTS: IBD peripheral monocytes and intestinal LPMNCs in vitro secrete increased amounts of proinflammatory cytokines (IL-1 beta and TNF-alpha) with decreased IL-1ra/IL-1 beta ratios. IL-4 down-regulates proinflammatory cytokine (IL-1 beta and TNF-alpha) and superoxide anion secretion in a dose-dependent manner. In contrast to normal and disease-specific controls, IBD peripheral monocytes and IBD intestinal LPMNCs show a diminished responsiveness to the inhibitory effect of IL-4. The IL-1ra/IL-1 beta ratios in normal monocytes are increased by IL-4, whereas in IBD monocytes low IL-1ra/IL-1 beta ratios persist after IL-4 treatment. IL-4-induced expression of macrophage mannose receptor, which is a molecule pivotal to macrophage-mediated host defense, again appeared to be impaired in IBD monocytes. CONCLUSIONS: IL-4-mediated regulation of mononuclear phagocyte effector functions is disturbed in IBD.

Adaptation of cat motoneurons to sustained and intermittent extracellular activation.

1. The main purpose of this study was to quantify the adaptation of spinal motoneurons to sustained and intermittent activation, using an extracellular route of stimulating current application to single test cells, in contrast to an intracellular route, as has been used previously. In addition, associations were tested between firing rate properties of the tested cells and other type (size)-related properties of these cells and their motor units. 2. Motoneurons supplying the medial gastrocnemius muscle of the deeply anaesthetized cat were stimulated for 240 s with microelectrodes which passed sustained extracellular current at 1.25 times the threshold for repetitive firing. Many cells were also tested following a rest period with intermittent 1 s current pulses (duration 600 ms) at the same relative stimulus strength. Cell discharge was assessed from the EMG of the motor unit innervated by the test neuron. The motoneurons and their motor units were assigned to four categories (i.e. types FF, FR, S and F; where F = FF + FR) based on conventional criteria. In all, twenty F (16 FF, 4 FR) and fourteen S cells were studied with sustained stimulation. Thirty of these cells (17 F, 13 S) and an additional two cells (1 F, 1 S) were studied with intermittent stimulation. 3. The mean threshold current required for sustained firing for a period of > or = 2 s was not significantly different for F and S cells. However, most of the other measured parameters of motoneuron firing differed significantly for these two cell groups. For example, at 1.25 times the threshold current for repetitive firing, the mean firing duration in response to 240 s of sustained activation was 123 +/- 88 s (+/- S.D.) for F cells vs. 233 +/- 19 s for S cells. These values were significantly longer than those from a comparable, previously reported study that employed intracellular stimulation. With intermittent stimulation, the firing durations of F and S cells were not significantly different from each other. 4. All cells exhibited a delay from the onset of current to the first spike, followed by a brief accelerating discharge that was followed by a slower drop in firing rate. Some cells (21 of 34 with sustained activation; 20 of 32 with intermittent) exhibited doublet discharges (interspike intervals < or = 10 ms) that were intermingled with the more predominant singlet discharges. Doublets were more common in the S cell type.(ABSTRACT TRUNCATED AT 400 WORDS)

Prolonged depression of force developed by single motor units after their intermittent activation in adult cats.

The fatigue of fast-twitch, glycolytic mammalian motor units [i.e., type FF; nomenclature of (3)] is dependent, in part, on the stimulation regimen (total number of stimuli, frequency, duty cycle, temporal patterning of stimuli, etc.) used to induce fatigue. To study the effect of the temporal pattern of the stimulus train on the rate and extend of fatigue in single FF units, one theoretically acceptable approach would be to use each motor unit as its own control: i.e., a sequential testing with two fatigue tests that differ only in the temporal organization of their stimuli. The purpose of this communication is to provide evidence that such an approach is not feasible when studying FF units, due to the delayed recovery of force following their repetitive activation. It was shown that 1/s activation of single FF units for only 15 or 45 s with intermittent 40-Hz, 300-ms duration trains significantly reduced their force response to a double-pulse shock for several hours. This finding suggests that in studies designed to test for the effects of different stimulation patterns on the fatigue of single motor units, deeply anaesthetized, reduced animal preparations are not appropriate models for the sequential application of different stimulation regimens to fast-twitch, glycolytic, mammalian motor units.

The effect of the stimulation pattern on the fatigue of single motor units in adult cats.

1. The main purpose of this study was to examine the effects of two subtly different stimulus patterns on the force developed by fast-twitch, fatiguable motor units in a cat hindlimb muscle during control (pre-fatigue) and fatiguing contractions. 2. The peak force and the force-time integral responses of nineteen high fatigue (FF) and three intermediate fatigue (FI) motor units of the tibialis posterior muscle in five deeply anaesthetized adult cats were measured at selected times during the course of a 360-s fatigue test. 3. The fatigue test involved a pseudo-random alternation of two patterns of stimulation. One pattern (regular) was composed of a train of stimuli with constant interpulse intervals, set at 1.8 x the twitch contraction time of each unit (interval range, 27-51 ms), and delivered for 500 (or 400) ms. For the total (FF + FI) motor-unit sample, the mean (+/- S.D.) stimulation frequency was 26 +/- 4 Hz (range, 19-37 Hz). The other stimulus pattern (optimized) consisted of three initial stimuli with short (10 ms) interpulse intervals, followed by a constant interpulse-interval train that was adjusted (interval range, 29-62 ms; frequency, 23 +/- 5 Hz; frequency range, 16-36 Hz) such that the total train had the same number of pulses, and the same average frequency and duration as the regular train. 4. The stimulus trains were delivered at 1 s-1 for 360 s, using three-train sequences of each pattern, randomly alternating with one another. The response of the third train in each sequence was selected for the force measurements. The force profile obtained from the fatigue test was subsequently decomposed into two profiles: one attributable to regular and one to optimized stimulation. 5. During the initial responses to the fatigue test, the optimized stimulus pattern produced significantly more force than the regular stimulus pattern. For FF units, the mean increase in peak force (141%) was significantly greater than the increase in the force-time integral (59%). 6. All motor units exhibited an initial potentiation of peak force with the regular stimulation pattern, whereas peak force declined monotonically with the optimized pattern. In contrast, the force-time integral potentiated in the first 30 s for both regular and optimized stimulus patterns. 7. Each motor unit maintained an increased force response to optimized stimulation during the fatigue test, with the greatest relative increase occurring about 120 s into the test, well after the potentiation effect had subsided.(ABSTRACT TRUNCATED AT 400 WORDS)

Fatigue-related changes in motor unit action potentials of adult cats.

The purpose of this study was to quantify the changes in motor-unit action potentials (MUAP) and force during a standard motor-unit fatigue test. MUAP waveforms were characterized by the measurement of amplitude, duration, area, and shape (as reflected in a coefficient of proportionality). Fatigue-resistant motor units exhibited small, but statistically significant, changes in MUAP amplitude and area during the fatigue test, whereas fatigable motor units displayed variable changes in MUAP amplitude, duration, and area. For all motor-unit types, the coefficient of proportionality did not change, and hence the change in MUAP area was proportional to the combined changes in amplitude and duration. The between- and within-train changes in MUAP were also distinct for the fatigue-resistant and fatigable motor units. Although several mechanisms could be responsible for the changes in the MUAP as the fatigue test proceeded, the dissociation of the time courses for MUAP and force indicated that these MUAP changes were not the principal reason for the decline in force under these conditions.

Measurement systems calibration: microcomputer implementation.

Measurement systems used in the collection and processing of laboratory data must be calibrated periodically to obtain accurate results. Because calibration factors can change over time or may be reset to optimize measurements for specific tests, care must be taken to assure that calibration factors and data are aligned correctly. Users should be able to process current data or re-process older data using appropriate calibration factors. The alignment of calibration factors and data should occur in a simple, automatic and transparent way. This document describes one approach to calibration procedures and computer programs used to collect, process, document, measure and display laboratory data. The examples are from our neurophysiology laboratory, where investigators study the mammalian spinal cord and peripheral neuromuscular system. Typical calibration problems, some workable solutions, and computer programs (described in pseudocode) are presented.

Triggering module for waveform digitization.

A full circuit description is provided for a triggering module used to assist a small laboratory computer in digitizing muscle force- and EMG waveforms. During the stimulation of individual motor units using a standard fatigue test, a train of 13 pulses are delivered at a rate of 40 pps either intracellularly to a motor neuron, or extracellularly to functionally isolated single motor axons from among divided ventral-root nerve filaments. Trains are delivered at a rate of 1/s for the duration of the test, which may range from 120 to 3600 s. Both the force and EMG profiles undergo changes during such tests and the quantification of parameters associated with their waveforms are of interest to neurobiologists. The triggering module allows a typical small laboratory computer to capture user-selected waveforms and thereby reduces the programming problems, timing constraints, storage requirements and analysis time associated with obtaining these parameters. The versatile circuit may be easily adapted to solve similar data-acquisition problems. The method was implemented on an Apple Macintosh II computer but can also be applied to other systems equipped with appropriate software and a data-acquisition card.