Type FF Research Articles

Size-dependent differences in mitochondrial volume density in phrenic motor neurons.

Neuromotor control of diaphragm muscle (DIAm) motor units is dependent on an orderly size-dependent recruitment of phrenic motor neurons (PhMNs). Slow (type S) and fast, fatigue resistant (type FR) DIAm motor units, which are frequently recruited to sustain ventilation, comprise smaller PhMNs that innervate type I and IIa DIAm fibers. More fatigable fast (type FF) motor units, which are infrequently recruited for higher force, expulsive behaviors, comprise larger PhMNs that innervate more type IIx/IIb DIAm fibers. We hypothesize that due to the more frequent activation and thus higher energy demand of type S and FR motor units, the mitochondrial volume density (MVD) of smaller PhMNs is greater compared with larger PhMNs. In eight adult (6 mo old) Fischer 344 rats, PhMNs were identified via intrapleural injection of Alexa488-conjugated cholera toxin B (CTB). Following retrograde CTB labeling, mitochondria in PhMNs were labeled by transdural infusion of MitoTracker Red. PhMNs and mitochondria were imaged using multichannel confocal microscopy using a ×60 oil objective. Following optical sectioning and three-dimensional (3-D) rendering, PhMNs and mitochondria were analyzed volumetrically using Nikon Elements software. Analysis of MVD in somal and dendritic compartments was stratified by PhMN somal surface area. Smaller PhMNs (likely S and FR units) had greater somal MVDs compared with larger PhMNs (likely FF units). By contrast, proximal dendrites or larger PhMNs had higher MVD compared with dendrites of smaller PhMNs. We conclude that more active smaller PhMNs have a higher mitochondrial volume density to support their higher energy demand in sustaining ventilation.NEW & NOTEWORTHY Type S and FR motor units, comprising smaller phrenic motor neurons (PhMNs) are regularly activated to perform indefatigable ventilatory requirements. By contrast, type FF motor units, comprising larger PhMNs, are infrequently activated to perform expulsive straining and airway defense maneuvers. This difference in activation history is mirrored in the mitochondrial volume density (MVD), with smaller PhMNs having higher MVD than larger PhMNs. In proximal dendrites, this trend was reversed, with larger PhMNs having higher MVD than smaller PhMNs, likely due to the maintenance requirements for the larger dendritic arbor of FF PhMNs.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons.

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

Heterogeneous glutamatergic receptor mRNA expression across phrenic motor neurons in rats.

The diaphragm muscle comprises various types of motor units that are recruited in an orderly fashion governed by the intrinsic electrophysiological properties (membrane capacitance as a function of somal surface area) of phrenic motor neurons (PhMNs). Glutamate is the main excitatory neurotransmitter at PhMNs and acts primarily via fast acting AMPA and N-methyl-D-aspartic acid (NMDA) receptors. Differences in receptor expression may also contribute to motor unit recruitment order. We used single cell, multiplex fluorescence in situ hybridization to determine glutamatergic receptor mRNA expression across PhMNs based on their somal surface area. In adult male and female rats (n=9) PhMNs were retrogradely labeled for analyses (n=453 neurons). Differences in the total number and density of mRNA transcripts were evident across PhMNs grouped into tertiles according to somal surface area. A~25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression was evident in PhMNs in the lower tertile compared to the upper tertile. These smaller PhMNs likely comprise type S motor units that are recruited first to accomplish lower force, ventilatory behaviors. In contrast, larger PhMNs with lower volume densities of AMPA and NMDA mRNA expression presumably comprise type FInt and FF motor units that are recruited during higher force, expulsive behaviors. Furthermore, there was a significantly higher cytosolic NMDA mRNA expression in small PhMNs suggesting a more important role for NMDA-mediated glutamatergic neurotransmission at smaller PhMNs. These results are consistent with the observed order of motor unit recruitment and suggest a role for glutamatergic receptors in support of this orderly recruitment. Cover Image for this issue: doi: 10.1111/jnc.14747.

Diaphragm neuromuscular transmission failure in aged rats.

In aging Fischer 344 rats, phrenic motor neuron loss, neuromuscular junction abnormalities, and diaphragm muscle (DIAm) sarcopenia are present by 24 mo of age, with larger fast-twitch fatigue-intermediate (type FInt) and fast-twitch fatigable (type FF) motor units particularly vulnerable. We hypothesize that in old rats, DIAm neuromuscular transmission deficits are specific to type FInt and/or FF units. In phrenic nerve/DIAm preparations from rats at 6 and 24 mo of age, the phrenic nerve was supramaximally stimulated at 10, 40, or 75 Hz. Every 15 s, the DIAm was directly stimulated, and the difference in forces evoked by nerve and muscle stimulation was used to estimate neuromuscular transmission failure. Neuromuscular transmission failure in the DIAm was observed at each stimulation frequency. In the initial stimulus trains, the forces evoked by phrenic nerve stimulation at 40 and 75 Hz were significantly less than those evoked by direct muscle stimulation, and this difference was markedly greater in 24-mo-old rats. During repetitive nerve stimulation, neuromuscular transmission failure at 40 and 75 Hz worsened to a greater extent in 24-mo-old rats compared with younger animals. Because type IIx and/or IIb DIAm fibers (type FInt and/or FF motor units) display greater susceptibility to neuromuscular transmission failure at higher frequencies of stimulation, these data suggest that the age-related loss of larger phrenic motor neurons impacts nerve conduction to muscle at higher frequencies and may contribute to DIAm sarcopenia in old rats. NEW & NOTEWORTHY Diaphragm muscle (DIAm) sarcopenia, phrenic motor neuron loss, and perturbations of neuromuscular junctions (NMJs) are well described in aged rodents and selectively affect FInt and FF motor units. Less attention has been paid to the motor unit-specific aspects of nerve-muscle conduction. In old rats, increased neuromuscular transmission failure occurred at stimulation frequencies where FInt and FF motor units exhibit conduction failures, along with decreased apposition of pre- and postsynaptic domains of DIAm NMJs of these units.

Aging effects on oxidative capacity in type‐identified diaphragm muscle fibers

The diaphragm muscle (DIAm) comprises four muscle fiber/ motor unit types: Fatigue resistant type S and FR motor units with type I and IIa fibers, respectively that are recruited for ventilatory behaviors; and more fatigable type FInt and FF motor units with type IIx and/or IIb fibers that are only recruited during high force expulsive/straining behaviors of the DIAm. With advanced aging in Fischer 344 rats (F344), type IIx and/or type IIb DIAm fibers are vulnerable to sarcopenia (reductions in specific force and cross‐sectional area, CSA), while type I and IIa fibers are resilient. This is consistent with the fact that ventilatory behaviors are conserved with aging, while airway clearance behaviors are compromised. Throughout life, breathing imposes a high duty cycle (>33 %) of DIAm activity and thus, a high energy demand for type I and IIa fibers that is met by a high oxidative capacity. However, the effect of advanced age on DIAm fiber oxidative capacity has not been examined. Accordingly, we hypothesize that the oxidative capacity in type I and IIa DIAm fibers is not affected by age, whereas concomitant with sarcopenia, the oxidative capacity of type IIx and/or IIb fibers is reduced.To assess DIAm fiber oxidative capacity, we measured succinate dehydrogenase (SDH) activity in type‐identified DIAm fibers from 6, 18 and 24‐month old male and female F344 rats. Following euthanasia, the DIAm was excised and 2–3 mm strips from the mid‐costal region were dissected, stretched to optimal length and rapidly frozen in melting isopentane. Serial transverse sections (6 μm) were cut to assess fiber type (based on myosin heavy chain, MyHC isoform), cross‐sectional area (CSA) and SDH activity. For SDH, sections were incubated at 25°C for 8 min in solutions containing 0 or 80 mM succinate and 1.5 mM nitro blue tetrazolium (reaction indicator), 5 mM EDTA, 0.2 mM PMS and 0.1 mM azide in 0.1 M phosphate buffer. Images were acquired every 5 s across a 15‐min period using a 20x objective on an Olympus IX71 inverted microscope. Linearity of the SDH reaction (change in optical density‐ OD) was confirmed across the 15‐min period for each fiber type.The presence of DIAm sarcopenia was confirmed in 24‐month old rats by a 25 % decrease in specific force and a 20% decrease in type IIx and/or IIb CSA. At all ages, the SDH activity of type I and IIa fibers was higher than type IIx/IIb fibers. No age‐related difference was observed in the SDH activity of type I or IIa fibers, whereas SDH activity of type IIx and/or IIb DIAm fibers was reduced at 24 months compared to younger animals.These results underscore the remarkable vulnerability of IIx and/or IIb fibers to sarcopenia, with profound resilience in I and IIa fibers. We propose that mitochondrial abnormalities may underlie the selective vulnerability of IIx and/or IIb fibers possibly due to changes at the phrenic motor neuron level.Support or Funding InformationSupported by NIH grant AG44615This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Prognostic Value of Vitamin D Insufficiency & Vitamin D Receptor Gene Polymorphism in Adult Acute Myeloid Leukemia Patients

Background: Vitamin D regulates many aspects of cellular growth and differentiation in normal and cancer cells. There is growing evidence for both serum vitamin D level and VDR gene polymorphism as prognostic factors in hematologic malignancies. Aim of this work: Evaluation of vitamin D serum level and VDR FOKI polymorphism as prognostic factors in adult AML patients. Patients & Methods: Eighty subjects were included in this study, 50 adult patients with newly diagnosed AML and 30 apparently healthy controls matched for age and sex. Venous blood samples were withdrawn from all subjects for measurement of serum 25(OH) vitamin D using competitive photo chemiluminescence and molecular detection of VDR (FOKI) polymorphism, which was done by RFLP PCR. All patients received the standard induction chemotherapy regimen 3 & 7. Results: The rate of vitamin D insufficiency was significantly higher in AML patients compared to controls (58% vs 16%, p = 0.03). The mutant FOKI genotype (FF & Ff) was found in 52 % of patients compared to 23 % of controls (p = 0.02). Patient with sufficient vitamin D level showed a significantly higher complete response rate compared to those with insufficient level (90% vs 44%, p = 0.02), while none of the other clinical features showed significant relation. Patients with wild type FOKI polymorphism (FF) were more likely to have favorable cytogenetics, while patient with mutant FOKI polymorphism were more likely to have poor cytogenetics (p = 0.03). The CR rate was highest in the wild type FF group (87.5%) followed by the heterozygous Ff group (50%), while none of the patients in the homozygous ff group achieved CR (p = 0.04). Conclusion: VDR FOKI polymorphism and serum vitamin D level showed a significant impact on the treatment outcome of adult AML patients suggesting their potential role as prognostic factors in these patients. Longer follow up will be needed to study the impact on overall and disease free survival.

Inappropriate interpretation of surface EMG signals and muscle fiber characteristics impedes understanding of the control of neuromuscular function

as the final common pathway from the nervous system to muscle, the motor unit transmits an activation signal generated by the nervous system to engage the contractile proteins and produce the muscle forces needed for reflex responses, automatic behaviors, and voluntary actions ([11][1]). The net

The dynamics of somatic input processing in spinal motoneurons in vivo

Uncovering how motoneurons utilize their voltage-sensitive conductances to systematically respond to a variety of inputs is paramount to understanding synaptic integration. In this study, we examine the input dynamics and frequency-dependent characteristics of active conductances in motoneurons as viewed from the soma in the decerebrate cat. We evaluated the somatic response of the motoneuron by superimposing a voltage sinus sweep (a sine wave in which frequency increases with time, which is often referred to as a zap or chirp) at a subset of membrane holding potentials during discontinuous, single-electrode, somatic voltage-clamp. Results from both experimental and modeling data indicate that ionic conductances can respond to a wide variety of input dynamics. Notably, it appears that there is a divergence between low input conductance type S and high input conductance type FF motoneurons in their response to input frequency. Type S motoneurons generate a larger response to lower frequency input dynamics (compared with their response to higher frequencies), whereas type FF generate a larger response to higher input frequency dynamics. Functionally, these results may indicate that motoneurons on the lower end of the motor pool (i.e., recruited first) may favor steady inputs, whereas motoneurons at the higher end (i.e., recruited later) may favor input transients in producing action potentials.

Fatigue of rat hindlimb motor units: biochemical-physiological associations.

Associations between fatigability and biochemical properties within motor unit (MU) types were explored in two hindlimb muscles of the adult rat. Type FF MUs in extensor digitorum longus and type S units in soleus were subjected either to a moderate (type FF) or severe (type S) 6-min, fatigue-inducing stimulation protocol. For both MU types, the range of values for their fatigability was considerably greater than the ranges in the activity levels of three enzymes in the units' constituent muscle fibers (MFs). These enzymes represented major energy-yielding pathways: adenylokinase, for high-energy phosphate metabolism; lactate dehydrogenase, for anaerobic glycolysis; and malate dehydrogenase, for oxidative metabolism. There were also relatively weak associations between the fatigue indices of the MUs and the activity levels of the three enzymes. Thus, this work supports previous conclusions that the force decline exhibited by MUs during electrically evoked contractions depends on both MF biochemistry and other intracellular mechanisms. Electromyographic measurements suggested that these other mechanisms are distal to the intramuscular branches of the axon in type FF units, and distal to excitation-contraction coupling in type S units.

Bestvina–Brady groups and the plus construction

A recent result of Bestvina and Brady [1, theorem 8·7], shows that one of two outstanding questions has a negative answer; either there exists a group of cohomological dimension 2 and geometric dimension 3 (a counterexample to the Eilenberg–Ganea Conjecture [4]), or there exists a nonaspherical subcomplex of an aspherical 2-complex (a counterexample to the Whitehead Conjecture [11]). More precisely, Bestvina and Brady construct a family of groups which are potential counterexamples to the Eilenberg–Ganea Conjecture, each of which has cohomological dimension 2. These are also examples of groups of type FP2 which are not finitely presented (see [1]). Dicks and Leary [3] give an explicit way of obtaining presentations (on finite generating sets) for these groups. For some of these examples, it is shown in [1] that any 2-dimensional classifying space would give rise to a counterexample to the Whitehead conjecture.We will refer to the examples cited above as Bestvina–Brady groups. These come equipped with natural, nonpositively curved cubical 3-dimensional classifying complexes, which we will call Bestvina–Brady complexes. In this short note, we show that these Bestvina–Brady complexes are (up to homotopy equivalence) formed by applying the Quillen plus construction to certain finite 2-complexes. From this, together with known facts about 2-complexes with aspherical plus constructions, we recover the result of Bestvina and Brady [1] that the Bestvina–Brady groups act freely on acyclic 2-complexes, and hence have cohomological dimension at most 2. It also follows that these groups have free relation modules of finite rank and so are of type FF. Finally, we use our construction to give an alternative proof of the cited theorem of Bestvina and Brady; at least one of the Eilenberg–Ganea and Whitehead conjectures is false.We do not use the full force of the Morse-theoretical techniques developed in [1], but will assume two results form that paper; the asphericity of the Bestvina–Brady complexes and the non-finite presentability of the Bestvina–Brady groups.

Horseradish peroxidase study of the spatial and electrotonic distribution of group Ia synapses on type-identified ankle extensor motoneurons in the cat.

Eight functionally identified group Ia muscle afferents from triceps surae or plantaris muscles were labeled intraaxonally with horseradish peroxidase (HRP) in seven adult cats. Subsequently, HRP was injected into two to six homonymous or heteronymous alpha-motoneurons per animal (total = 22), each identified by motor unit type and located near the site of afferent injection. The complete trajectories of labeled afferents were reconstructed, and putative synaptic contacts on HRP-labeled motoneurons were identified at high magnification. Dendritic paths from each contact were also mapped and measured. A total of 24 contact systems (the combination of a group Ia afferent and a postsynaptic motoneuron) were reconstructed, of which 17 were homonymous, and seven were heteronymous. Overall, homonymous contact systems had an average of 9.6 boutons, whereas heteronymous contact systems had an average of 5.9 boutons. The average number of boutons found on type S motoneurons in homonymous contact systems was smaller (6.4, range 3-17) than in systems involving types FF or FR motoneurons (FF: 10.4, range 4-18; FR: 11.3, range 4-32). Neither of these differences were statistically significant. In contrast to earlier reports, a majority (15/24) of contact systems included more than one collateral from the same Ia afferent. The complexity (number of branch points) in the arborization pathway leading to each contact (overall mean 8.4 +/- 3.3) was virtually identical in all contact systems, irrespective of the type of postsynaptic motoneuron. The three-dimensional distribution of group Ia contacts was not coextensive with the radially organized dendrites of motoneurons: Dendrites oriented in the ventromedial to dorsolateral axis had the fewest (8%) contacts, whereas rostrocaudal dendrites had the most (63%) contacts. Nevertheless, contacts were widely distributed on the motoneuron surface, with few on and near the soma (< or = 200 microns radial distance from the soma) or on the most distal parts of the tree (> or = 1,000 microns). The boutons in individual contact systems also showed wide spatial and estimated electrotonic distributions; only 3/24 systems had all contact located within a restricted spatial/electrotonic region. The relations between these anatomical results and existing electrophysiological data on group Ia synaptic potentials are discussed.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. I. Motor-unit recruitment.

1. The recruitment order of 64 pairs of motor units, comprising 21 type-identified units, was studied during centrally evoked muscle contractions of the cat medial gastrocnemius (MG) muscle in an unanesthetized, high decerebrate preparation. Motor units were functionally isolated within the MG nerve by intra-axonal (or intramyelin) penetration with conventional glass microelectrodes. 2. Graded stimulation of the mesencephalic locomotor region (MLR) was used to evoke smoothly graded contractions, which under favorable conditions was estimated to reach 40% of maximum tetanic tension of the MG muscle. With this method of activation, 100% of slow twitch (type S) units, 95% of fast twitch, fatigue-resistant (type FR) units, 86% of fast twitch, fatigue-intermediate (type FI) units, and 49% of fast twitch, fatigable (type FF) units studied were recruited. 3. Motoneuron size as estimated by axonal conduction velocity (CV) was correlated with muscle-unit size as estimated by maximum tetanic tension (Po). Although the correlation between these properties was significant among type S and FR units, no significant correlation was found for these properties among type FI and FF units. 4. Motor-unit recruitment was ordered by physiological type (S > F, 100% of pairs; S > FR > FI > FF, 93% of pairs). Although none of the motor-unit properties studied predicted recruitment order perfectly, motor-unit recruitment was found to proceed by increasing Po (89% of pairs), decreasing contraction time (79% of pairs), decreasing fatigue index (80% of pairs), and increasing CV (76% of pairs). These percentages were significantly different from random (i.e., 50%). Statistically, all four motor-unit properties were equivalent in predicting recruitment order. These results are similar to those reported by other investigators for motor-unit recruitment order evoked from other supraspinal centers, as well as from peripheral sites. 5. When, however, motor-unit recruitment within pairs of motor units containing two fast-twitch (type F) units was examined, Po was a significantly better predictor of recruitment order than CV (85% vs. 52% of pairs). One explanation for this observation is that the correlation between Po and CV is high among type S, type FR units, and possibly among the lower-tension type FF units, but not among the remaining higher-tension type FF units. 6. The reproducibility of recruitment order in multiple contractions was investigated in 16 motor-unit pairs. Recruitment order was found to be variable in only three motor-unit pairs, all of which contained units of similar physiological type and recruitment threshold. 7. Analysis of recruitment order by pair-wise testing confirms the general conclusion reached in human studies that the muscle force level at recruitment for a motor unit is highly correlated with its strength. As an additional confirmation, the whole-muscle force level at recruitment for 41 units was measured in a series of contractions in which the rate of rise of muscle tension was limited to rates < 1,000 g/s. For these contractions, a significant correlation was found between muscle tension at recruitment and motor-unit Po.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation.

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

Distribution of vestibulospinal synaptic input to cat triceps surae motoneurons.

We applied supramaximal, repetitive stimulation to the lateral vestibular nucleus (Deiters' nucleus, DN) at 200 Hz to evoke stead-state synaptic potentials in ipsilateral triceps surae motoneurons of the cat. The effective synaptic currents underlying these potentials were measured using a modified voltage-clamp technique. The steady-state effective synaptic currents evoked by activating DN were generally small and depolarizing (mean 2.5 +/- 2.6 nA). DN stimulation generated hyperpolarizing synaptic currents in 2 of the 34 triceps motoneurons studied. The effective synaptic currents from DN tended to be larger in putative type F motoneurons than in putative type S cells (type F mean 3.0 +/- 3.1 nA; type S mean 1.8 +/- 1.0 nA). There was a statistically significant difference between the inputs to putative type FF and putative type S motoneurons (mean difference 2.8 nA, t = 2.87, P < 0.01). The synaptic input from DN to medial gastrocnemius motoneurons had approximately the same amplitude as that from homonymous Ia afferent fibers. However, the distribution of DN input with respect to putative motor unit type was the opposite of that previously reported for Ia afferent input. Thus, the synaptic input from DN might act to compress the range of recruitment thresholds within the motoneuron pool and thereby increase the gain of its input-output function.

Changes of intracellular electrolyte contents in rat skeletal muscle during body suspension.

The CNS-mediated inhibition of active Na(+)-K+ transport in both "type S" muscle, soleus (SOL), and "type FF" muscle, extensor digitorum longus (EDL) was investigated in rats suspended horizontally. Plasma Na+ and K+ contents did not change during the suspension period. The relative wet weight of SOL decreased more than that of EDL by suspension. There was significant intracellular Na+ accumulation and K+ loss in both SOL and diaphragm of suspended rats. However, cerebrum, cerebellum, medulla oblongate, ventricle, liver, pancreas, kidney, intestine, aorta and EDL were spared from the intracellular Na+ accumulation and K+ loss. Sciatic nerve sectioning or cervical spinal cord transection recovered the Na+ and K+ contents in the SOL of suspended rats. The results indicate the existence of neural inhibition of the active Na(+)-K+ transport in skeletal muscle of the suspended rats.

Determination of fatigue in the electrically stimulated quadriceps muscle and relative effect of ischaemia

The creation of muscle fatigue using surface electrical stimulation represents a highly reproducible phenomenon in spinal cord injured patients. The torque output was recorded as a function of time. The fatigue curves recorded over 110s exhibited three main parts: first, a plateau of short duration, followed by a more or less steep slope and then a second plateau which was maintained for a long time. This phenomenon was fitted using an exponential equation which had been developed and four parameters introduced that outlined to the muscle behaviour. A set of fatigue indices was defined to characterize the asymptotic value, the slope, the coordinates of the inflexion point, the time constant and the vertical amplitude of the curve recorded. Two populations were studied; a group of 11 thoracic level of injury paraplegic patients and a group of 10 able-bodied control subjects. The computed coefficients of determination, r 2, were of very high values (0.99). Therefore, fatigue indices gave reliable information. Torque output did not differ between the two populations until 25 s had elapsed, but from 30s onwards it was markedly lower in paraplegics. The residual torque output was 21.1 ± 10.6% in the paraplegic group while it was 58.5 ± 8.9% in the control group. The effect of blood supply in the production of fatigue was also studied by repeating the same test using a tourniquet at the groin level. The residual torque became 14.7 ± 2.3% in the paraplegic group and 42.9 ± 6.3% in the control group. This test permitted the determination at a gross level of the different metabolic phases and therefore the recruitment of the different populations of muscle fibres within the quadriceps. Fast fatiguable fibres (type FF) produced the maximum output during the first phase, then fast resistant (type FR) explained the slope, and slow fibres (type S) were responsible for the second plateau. The effects of the tourniquet were obvious on the fatigue indices in both populations. In the spinal cord injured patients, there appeared to be a lack of slow fibres, probably due to disuse. Fatigue appeared to be independent from blood flow in paraplegics.

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.

Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle

1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.

Motor unit function during evolution of proximal axonal swellings

ß,ß′-Iminodipropionitrile (IDPN) impairs axonal transport of neurofilaments; their accumulation leads to the formation of proximal swellings in motor axons. Similar proxaml swellings are a feature of some cases of motor neuron disease such as amyotrophic lateral sclerosis (ALS). Motor units in IDPN-treated animals were assessed to determine their relative susceptibilities to impaired function and whether the functional changes resulting from proximal axonal swellings share certain electromyographic features with ALS. Intrinsic properties of medial gastrocnemius motoneurones (MN) and contractile responses of their motor units were examined during the evolution of proximal axonal swellings in cats administered IDPN (50 mg/kg once weekly) for 7, 14 or 35 days. While conduction velocities were significantly decreased in all motor unit types by 35 days, the conduction slowing was greater in fast fatigable (types FF and FI) motor units than in fatigue resistant (types FR and S) motor units. Normal correlations between axonal conduction velocity and MN input resistance ( R in) and the inverse relationship between R in and rheobase were lost with progression of the neuropathy. Twitch and maximum tetanic tension developed by fast-fatigable motor units declined early in the neuropathy, whereas fatigue-resistant units did not show similar changes until later stages of the intoxication. In some motor units, irregular and abnormal tetanic tensions were elicited by repetitive MN discharge. At 14 and 35 days, a novel, intermediate motor unit response classified as slow and fatigable (SF) was observed. Conduction block, characterized by repetitive MN firing without a corresponding contractile response, was observed in some type FF and S units by 35 days. Morphometric analysis of muscle fiber types showed significant atrophy, particularly in the type I fibers at 14–35 days; the atrophy reversed following cessation of IDPN administration. The influence of proximal axonal swellings on motor unit function in IDPN neuropathy is discussed in terms of reported electrophysiological alterations in motoneurone disease.

Genome specificity of rDNA spacer fragments from Oryza sativa L.

The intergenic spacer derived from a cloned rDNA unit from a cultivated rice was dissected into several subclones, which were used as probes to analyze sequence homologies between rDNA spacers from wild rice belonging to genome types AA, BB, CC, EE, FF, BBCC, and CCDD. This analysis allowed us to detect several regions with different degrees of homology. A series of 250-260 bp repeats is located in the central part of the AA spacer. This sequence cross-hybridizes with the BB, CC, BBCC, and FF genomes, but is absent in the EE and CCDD genomes. Regions proximal to 25S and 18S sequences are well conserved in all genomes. Finally, two adjacent sequences of 61 bp and 94 bp, located downstream from the repeats, have been found to have a narrow genomic specificity, restricted respectively to AA and FF genomes for the first one and to AA for the second. These data provide new information on the evolution of the ribosomal RNA gene spacer within a complex of related species, and add to our knowledge on the relationship between the various rice genomes.