Innervation of regenerated spindles in muscle grafts of the rat

Features of the nerve supply and the encapsulated fibers of muscle spindles were assessed in grafted and normal extensor digitorum longus (EDL) muscles of rats by analysis of serial 10-microns frozen transverse sections stained for enzymes which delineated motor and sensory endings, oxidative capacity and muscle fiber type. The number of fibers was significantly more variable, and branched fibers were more frequently observed in regenerated spindles than in control spindles. Forty-eight percent of regenerated spindles received sensory innervation. Spindles reinnervated by afferents had a larger periaxial space than did spindles which were not reinnervated by afferents. Regenerated fibers innervated by afferents had small cross-sectional areas, equatorial regions with myofibrils restricted to the periphery of fibers, unpredictable patterns of nonuniform and nonreversible staining along the length of the fiber for 'myofibrillar' adenosine triphosphatase (mATPase) after acid and alkaline preincubation. In contrast, regenerated fibers devoid of sensory innervation resembled extrafusal fibers in that they usually exhibited myofibrils throughout the length of the fiber, no central aggregations of myonuclei, uniform staining for mATPase and a reversal of staining for mATPase after preincubation in an acid or alkaline medium. Approximately thirty percent of encapsulated fibers devoid of sensory innervation stained analogous to a type I extrafusal fiber, a pattern of staining never observed in intrafusal fibers of normal spindles. Groups of encapsulated fibers all exhibiting this pattern of staining reflect that either these fibers may have been innervated by collaterals of skeletomotor axons that originally innervated type I extrafusal fibers or that fibers innervated by only fusimotor neurons express patterns of staining for mATPase similar to extrafusal fibers in the absence of sensory innervation. Sensory innervation may also influence the reestablishment of multiple sites of motor endings on regenerated intrafusal fibers. Those regenerated fibers innervated by afferents had more motor endings than did regenerated fibers devoid of sensory innervation. Differences in size, morphology, and patterns of staining for mATPase and numbers of motor endings between fibers innervated by afferents and fibers devoid of sensory innervation reflect that afferents can influence the differentiation of muscle cells and the reestablishment of motor innervation other than during the late prenatal/early postnatal period when muscle spindles form and differentiate in rats.     

Innervation of regenerated spindles in muscle grafts of the rat

Summary Features of the nerve supply and the encapsulated fibers of muscle spindles were assessed in grafted and normal extensor digitorum longus (EDL) muscles of rats by analysis of serial 10-m frozen transverse sections stained for enzymes which delineated motor and sensory endings, oxidative capacity and muscle fiber type.The number of fibers was significantly more variable, and branched fibers were more frequently observed in regenerated spindles than in control spindles. Forty-eight percent of regenerated spindles received sensory innervation. Spindles reinnervated by afferents had a larger periaxial space than did spindles which were not reinnervated by afferents. Regenerated fibers innervated by afferents had small cross-sectional areas, equatorial regions with myofi-brils restricted to the periphery of fibers, unpredictable patterns of nonuniform and nonreversible staining along the length of the fiber for myofibrillar adenosine triphosphatase (mATPase) after acid and alkaline preincubation. In contrast, regenerated fibers devoid of sensory innervation resembled extrafusal fibers in that they usually exhibited myofibrils throughout the length of the fiber, no central aggregations of myonuclei, uniform staining for mATPase and a reversal of staining for mATPase after preincubation in an acid or alkaline medium. Approximately thirty percent of encapsulated fibers devoid of sensory innervation stained analogous to a type I extrafusal fiber, a pattern of staining never observed in intrafusal fibers of normal spindles. Groups of encapsulated fibers all exhibiting this pattern of staining reflect that either these fibers may have been innervated by collaterals of skeletomotor axons that originally innervated type I extrafusal fibers or that fibers innervated by only fusimotor neurons express patterns of staining for mATPase similar to extrafusal fibers in the absence of sensory innervation. Sensory innervation may also influence the reestablishment, of multiple sites of motor endings on regenerated intrafusal fibers. Those regenerated fibers innervated by afferents had more motor endings than did regenerated fibers devoid of sensory innervation.Differences in size, morphology, and patterns of staining for mATPase and numbers of motor endings between fibers innervated by afferents and fibers devoid of sensory innervation reflect that afferents can influence the differentiation of muscle cells and the reestablishment of motor innervation other than during the late prenatal/early postnatal period when muscle spindles form and differentiate in rats.     

Histological study of motor innervation to long nuclear chain intrafusal fibers in the muscle spindle of the cat

Several muscle spindles of the cat tenuissimus muscle were cut in serial, 1-micron thick transverse sections and stained with toluidine blue in search for long nuclear chain intrafusal muscle fibers. Five complete poles of the long chain fibers were located. Each fiber pole displayed one plate-type motor ending situated in the extracapsular fiber region. The endings were supplied by myelinated motor axons that originated from intramuscular nerve fascicles containing motor axons to extrafusal muscle fibers. One of the endings was innervated by a collateral from a motor axon that supplied an extrafusal end-plate. Ultrastructurally, the long chain endings resembled extrafusal end-plates. They were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other intrafusal ending present in the spindles. The motor endings of the long chain fibers were assumed to be the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat muscle spindles.     

The role of sensory input in motor neuron sprouting control

Primary sensory neurons project to motor neurons directly or through interneurons and affect their activity. In our previous paper we showed that intramuscular sprouting can be affected by changing the sensory synaptic input to motor neurons. In this work, motor axon sprouting within a peripheral nerve (extramuscular sprouting) was induced by nerve injury at such a distance from muscle so as not to allow nerve-muscle trophic interactions. Two different procedures were carried out: (1) sciatic nerve crush and (2) sciatic nerve crush with homosegmental ipsilateral L3-L5 dorsal rhizotomy. The number of regenerating motor axons innervating extensor digitorum longus muscle was determined by in vivo muscle tension recordings and an index of their individual conduction rate was obtained by in vitro intracellular recordings of excitatory postsynaptic end-plate potentials in muscle fibers. The main findings were: (1) there are more regenerated axons distally from the lesion than parent axons proximally to the lesion (sprouting at the lesion); (2) sprouting at the lesion was negatively affected by homosegmental ipsilateral dorsal rhizotomy; (3) the number of motor axons innervating extensor digitorum longus muscle extrafusal fibers counted proximally to the lesion increased following nerve injury and regeneration but this did not occur when sensory input was lost. A transient innervation of extrafusal fibers by gamma motor neurons may explain the increase of motor axons counted proximally to the lesion.     

The role of sensory input in motor neuron sprouting control

Primary sensory neurons project to motor neurons directly or through interneurons and affect their activity. In our previous paper we showed that intramuscular sprouting can be affected by changing the sensory synaptic input to motor neurons. In this work, motor axon sprouting within a peripheral nerve (extramuscular sprouting) was induced by nerve injury at such a distance from muscle so as not to allow nerve-muscle trophic interactions. Two different procedures were carried out: (1) sciatic nerve crush and (2) sciatic nerve crush with homosegmental ipsilateral L3-L5 dorsal rhizotomy. The number of regenerating motor axons innervating extensor digitorum longus muscle was determined by in vivo muscle tension recordings and an index of their individual conduction rate was obtained by in vitro intracellular recordings of excitatory postsynaptic end-plate potentials in muscle fibers. The main findings were: (1) there are more regenerated axons distally from the lesion than parent axons proximally to the lesion (sprouting at the lesion); (2) sprouting at the lesion was negatively affected by homosegmental ipsilateral dorsal rhizotomy; (3) the number of motor axons innervating extensor digitorum longus muscle extrafusal fibers counted proximally to the lesion increased following nerve injury and regeneration but this did not occur when sensory input was lost. A transient innervation of extrafusal fibers by &#110 motor neurons may explain the increase of motor axons counted proximally to the lesion.     

Origin and nature of correlations in the Ia feedback pathway of the muscle control system

Mammalian primary muscle spindle endings receive two inputs from fusimotor fibers and length changes of their extrafusal environment. Both inputs are also powerful noise sources disturbing the discharge patterns of the receptors. In this paper emphasis is laid on the extrafusal component. It is shown that extrafusal muscle activity can be viewed as a stochastic process which, acting as a common source, can correlate discharge patterns of adjacent muscle spindles. Some quantitative characteristics of these correlations and their relation to the underlying mechanical events are investigated in some detail in order to provide data for more theoretical considerations on their possible physiological importance.     

Morphology and histochemistry of the ambiens muscle of the red-eared turtle (Pseudemys scripta)

Six fiber types have been described in the ambiens muscle of red-eared turtles. These include one slow oxidative type, two fast oxidative types, two fast oxidative and glycolytic types, and one fast glycolytic type. Fiber types are non-randomly distributed throughout cross sections of the muscle. There is a decreasing gradient of oxidative staining and an increasing gradient of glycolytic staining along an axis from the superficial to deep regions of the muscle. The slow oxidative fibers are predominantly located within one or two fascicles of the superficial surface of the muscle. The fast glycolytic fibers are predominant in deep fascicles. In contrast to previous reports of histochemically monotypic intrafusal fibers in turtle muscle, ambiens muscle spindles have been observed containing one to eleven intrafusal fibers, including two fiber types. Fiber diameter and area are consistently smaller than observed in most extrafusal fibers. Spindles are predominantly located in superficial and cranial fascicles of the ambiens muscle and are located in regions characterized by extrafusal fibers with high oxidative activity.     

Ultrastructure of Type 2A Extrafusal Muscle Fibers and Muscle Spindle Intrafusal Fibers of Adult Rats after Prolonged Swimming

We compared the ultrastructure of type 2A extrafusal muscle fibers, the nuclear chain, and other intrafusal fibers of muscle spindle (muscle stretch mechanoreceptor) in adult rats after prolonged swimming (5–10 h/day, 10 days). The Golgi apparatus was expressed moderately in type 2A extrafusal fibers and hypertrophied in the motor B zone of nuclear chain intrafusal fibers. Intense development of the Golgi apparatus in the nuclear chain intrafusal fiber appears to be related to glycogenolysis in the autophagous vacuoles, involvement in the lysosome activity, and plasma membrane renewal. Recapitulation of the mechanism of glycogen autophagy, which is observed in newborn rats during mobilization of glycogen from the liver and muscle, was demonstrated in adult rats under the influence of physical load in the nuclear chain and nuclear bag₂ intrafusal fibers and in type 2A extrafusal fibers. This is accounted for by a weak differentiation of the intrafusal muscle fibers: structurally, they are similar to myotubes and have specific features of blood supply and innervation. Individual features of experimental adult rats may also play a certain role.     

Occurrence and distribution of muscle spindles in masticatory and suprahyoid muscles of the rat.

The occurrence and distribution of muscle spindles was studied in histochemically and conventionally stained serial cross sections of 6-week-old and adult rat masticatory and suprahyoid muscles. Spindles were present in moderate to large numbers in jaw closers, but they were absent in jaw openers and two of four muscles of an accessory suprahyoid group. In jaw closers, 67% or more of the total spindle population was concentrated relatively distant from the temporomandibular joint, in muscle portions which contained large numbers of extrafusal fibers reacting strongly for oxidative enzymes. Because of their location, spindles in these portions should be stretched more and, subsequently, should respond with a greater afferent discharge at any given muscle length than spindles situated nearer to the joint. Spindles in jaw closers, especially the medial pterygoid and deep masseter, often occurred in clusters and complex forms near the terminal branching of intramuscular nerve trunks. No such concentrations were seen in the two muscles of the accessory suprahyoid group that had spindles. The association in jaw closers of spindles with extrafusal fibers high in oxidative enzyme activity is consistent with the view that spindles are the sensory component of a reflex system that recruits these fibers for finely-graded contractions in response to small internal length-changes of the muscle (Botterman et al., '78); however, in jaw openers and two muscles of the accessory suprahyoid group, the absence of spindles, coupled with the presence of large populations of extrafusal fibers high in oxidative enzyme activity, is not easily reconciled with this concept.     

Superior laryngeal and hypoglossal afferents tonically influence upper airway motor excitability in anesthetized rats.

Upper airway (UA) muscle activity is stimulated by changes in UA transmural pressure and by asphyxia. These responses are reduced by muscle relaxation. We hypothesized that this is due to a change in afferent feedback in the ansa hypoglossi and/or superior laryngeal nerve (SLN). We examined 1) the glossopharyngeal motor responses to UA transmural pressure and asphyxia and 2) how these responses were changed by muscle relaxation in animals where one or both of these afferent pathways had been sectioned bilaterally. Experiments were performed in 24 anesthetized, thoracotomized, artificially ventilated rats. Baseline glossopharyngeal activity and its response to UA transmural pressure and asphyxia were moderately reduced after bilateral section of the ansa hypoglossi (P < 0.05). Conversely, bilateral SLN section increased baseline glossopharyngeal activity, augmented the response to asphyxia, and abolished the response to UA transmural pressure. Muscle relaxation reduced resting glossopharyngeal activity and the response to asphyxia (P < 0.001). This occurred whether or not the ansa hypoglossi, the SLN, or both afferent pathways had been interrupted. We conclude that ansa hypoglossi afferents tonically excite and SLN afferents tonically inhibit UA motor activity. Muscle relaxation depressed UA motor activity after section of the ansa hypoglossi and SLN. This suggests that some or all of the response to muscle relaxation is mediated by alterations in the activity of afferent fibers other than those in the ansa hypoglossi or SLN.     

Pyramidal influences on interneurons of spinal segmentary reflector arcs in cat

Microelectrode discharges of potentials have been realized from segmentary interneurons of the dorsal horn and intermediate nucleus of the spinal cord in cat at the L₆–L₇ level by electrical stimulation of the sensorimotor region of the brain cortex. It has been established that corticifugal influences on segmentary interneurons of the system of the flexor reflex and on neurons activated by high threshold muscle afferents (groups Ib, II, and III), or high threshold cutaneous afferents are predominantly excitatory. Interneurons activated by muscle afferents of group Ia or by the lowest threshold cutaneous fibers are weakly subjected to pyramidal influences. The mean latencies of excitatory postsynaptic potentials (EPSP's) and discharges evoked under the influence of pyramidal volley, for the neurons under study in the system of afferents of the flexor reflex are equal to 11.8±2.6 and 20.1±1.8 msec, respectively; for interneurons, excited only by high threshold muscle afferents, they are equal to 15.5±3.6 and 16.3±2.2 msec, respectively; and for interneurons, excited by high threshold cutaneous fibers they are equal to 11.8±2.6 and 18.3±1.4 msec, respectively. Possible pathways of activating segmentary interneurons from the lateral sensorimotor region of the brain cortex have been discussed.The A. A. Bogomolets Institute of Physiology, Academy of Sciences, Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 17–25, January–February, 1970.     

Role of nerve and muscle factors in the development of rat muscle spindles

The soleus muscles of fetal rats were examined by electron microscopy to determine whether the early differentiation of muscle spindles is dependent upon sensory innervation, motor innervation, or both. Simple unencapsulated afferent-muscle contacts were observed on the primary myotubes at 17 and 18 days of gestation. Spindles, encapsulations of muscle fibers innervated by afferents, could be recognized early on day 18 of gestation. The full complement of spindles in the soleus muscle was present at day 19, in the region of the neuromuscular hilum. More afferents innervated spindles at days 18 and 19 of gestation than at subsequent developmental stages, or in adult rats; hence, competition for available myotubes may exist among afferents early in development. Some of the myotubes that gave rise to the first intrafusal (bag2) fiber had been innervated by skeletomotor (alpha) axons prior to their incorporation into spindles. However, encapsulated intrafusal fibers received no motor innervation until fusimotor (gamma) axons innervated spindles 3 days after the arrival of afferents and formation of spindles, at day 20. The second (bag1) intrafusal fiber was already formed when gamma axons arrived. Thus, the assembly of bag1 and bag2 intrafusal fibers occurs in the presence of sensory but not gamma motor innervation. However, transient innervation of future bag2 fibers by alpha axons suggests that both sensory and alpha motor neurons may influence the initial stages of bag2 fiber assembly. The confinement of nascent spindles to a localized region of the developing muscle and the limited number of spindles in developing muscles in spite of an abundance of afferents raise the possibility that afferents interact with a special population of undifferentiated myotubes to form intrafusal fibers.     

Effect of the tendon reflex on motoneurons of the antagonist muscle in man

The knee jerk was elicited during regular firing of relatively low-threshold motor units of the biceps femoris muscle (during weak voluntary contraction). Besides the reflex response of the rectus femoris muscle, synchronous discharges of motor units of the biceps femoris muscle and activation of new motor units also were observed. Poststimulus histograms and statistical analysis of interspike intervals of motor units of the biceps femoris muscle revealed well-marked excitatory influences synchronous with the reflex response of the rectus femoris. This result can be explained by the presence of excitatory inputs of Ia afferents on motoneurons of the antagonist muscle. In the knee jerk, excitation of motoneurons of the antagonist was followed by later inhibitory influences which evidently correspond to the "silent period" of motoneurons of the agonist muscle during the elicitation of its tendon reflex.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 6, pp. 624–632, November–December, 1976.     

Presence of embryonic myosin in normal postural muscles of the adult rat

Indirect immunofluorescence was used to localize embryonic myosin heavy chains in soleus, adductor longus, tibialis anterior, plantaris, and extensor digitorum longus muscles of 6-month-old rats. A monoclonal antibody (2B6), specifically recognizing rat embryonic myosin, was applied to unfixed, transverse, frozen sections. The number of embryonic myosin-positive (EMP) extrafusal fibers was expressed as a percentage of the total number of fibers. EMP extrafusal fibers were only seen in the soleus and adductor longus muscles, both postural muscles. Approximately 1% of the soleus muscle fibers appeared positively stained for embryonic myosin. The majority of such fibers had a small diameter (<500 ), appeared intensely fluorescent, and typically contained central nuclei. Re-expression of embryonic myosin due to spontaneous fiber denervation is not a likely factor in this study, since alpha-bungarotoxin and N-CAM localization were restricted to the motor end-plate region of EMP fibers. Since embryonic myosin was shown to disappear in all normal-sized myofibers by 2 to 3 months of age, the results suggest that the EMP extrafusal fibers seen in postural muscles of 6 to 12-month-old animals are regenerating myofibers. We speculate that a small number of muscle fibers may be regenerating in normal, adult postural muscles, in response to fiber damage possibly caused by excessive recruitment or overloading.     

Origin of intrafusal muscle fibers in the rat

The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag2 fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag1 fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag2 and bag1 intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.     

Study of motor cortex excitability in the load holding task

Changes in the amplitude of hand muscle responses to a series of ten stimuli applied to the motor cortex has been studied in subjects holding a small load for 3 min. The amplitude of muscle responses and the background activity decreased with time as compared to the initial level. Regression analysis showed that the muscle response amplitude decreased with the number of stimuli to a greater extent than the background activity. Comparison of the parameters of hand muscle activity during load holding in the stable and unstable equilibrium positions showed that the decrease in the muscle response to motor cortex stimulation during load holding in a state of unstable equilibrium is less pronounced than during load holding in a state of stable equilibrium. For the forearm muscles, the muscle response amplitude and background activity decreased less with the number of stimuli, and this decrease did not depend on the stability of the load position. It may be supposed that the evoked responses decreased more rapidly than the background activity because the motor cortex is involved in the adjustment of the level of muscle activity at the stage of the development of the program for the performance of motor tasks and then transfers the control to subcortical structures.     

Magnetic stimulation of the human motor cortex evokes skin sympathetic nerve activity.

Single-pulse magnetic coil stimulation (Cadwell MES 10) over the cranium induces without pain an electric pulse in the underlying cerebral cortex. Stimulation over the motor cortex can elicit a muscle twitch. In 10 subjects, we tested whether motor cortical stimulation could also elicit skin sympathetic nerve activity (SSNA; n = 8) and muscle sympathetic nerve activity (MSNA; n = 5) in the peroneal nerve. Focal motor cortical stimulation predictably elicited bursts of SSNA but not MSNA; with successive stimuli, the SSNA responses did not readily extinguish (94% of discharges to the motor cortex evoked SSNA responses) and had predictable latencies [739 +/- 33 (SE) to 895 +/- 13 ms]. The SSNA responses were similar after stimulation of dominant and nondominant sides. Focal stimulation posterior to the motor cortex elicited extinguishable SSNA responses. In three of six subjects, anterior cortical stimulation evoked SSNA responses similar to those seen with motor cortex stimulation but without detectable movement; in the other subjects, anterior stimulation evoked less SSNA discharge than that seen with motor cortex stimulation. Contrasting with motor cortical stimulation, evoked SSNA responses were more readily extinguished with 1) peripheral stimulation that directly elicited forearm muscle activation accompanied by electromyograms similar to those with motor cortical stimulation; 2) auditory stimulation by the click of the energized coil when off the head; and 3) in preliminary experiments, finger afferent stimulation sufficient to cause tingling. Our findings are consistent with the hypothesis that motor cortex stimulation can cause activation of both alpha-motoneurons and SSNA.     

Involvement of Dopamine Neurons in the Regulation of β-Adrenergic Receptor Sensitivity in Rat Prefrontal Cortex

The contribution of dopamine (DA) afferents to the regulation of beta-adrenergic receptor sensitivity (isoproterenol-stimulated adenylate cyclase activity) in the rat prefrontal cortex was investigated by comparing the effects of lesions affecting either both DA and noradrenaline (NA) or NA fibers alone. Bilateral 6-hydroxydopamine (6-OHDA) lesions made in the ventral tegmental area destroyed ascending DA and to a variable extent ascending NA fibers innervating the prefrontal cortex. Two opposite effects were observed depending on the extent of cortical NA denervation: (a) When NA denervation was complete (less than 4% of controls), a marked increase in the isoproterenol-sensitive adenylate cyclase activity (+78%) was found. The amplitude of this denervation supersensitivity was similar to that occurring following complete and selective destruction of NA innervation induced by bilateral 6-OHDA injections made into the pedunculus cerebellaris superior. (b) When 6-OHDA injections into the ventral tegmental area led to a partial destruction of cortical NA afferents (10-40% of control values), a hyposensitivity of the isoproterenol-induced adenylate cyclase activity (-30%) was observed. This effect contrasted with the moderate supersensitivity seen in rats with partial, but selective, destruction of NA innervation (pedunculus cerebellaris superior lesions). The hyposensitivity of beta-adrenergic receptors obtained in rats with partial lesions of cortical NA fibers, but devoid of cortical DA innervation, suggests that DA neurons may regulate, under certain conditions, the denervation supersensitivity of beta-adrenergic receptors.     

Oxidative capacity and capillary density of diaphragm motor units

Motor units in the cat diaphragm (DIA) were isolated in situ by microdissection and stimulation of C5 ventral root filaments. Motor units were classified based on their isometric contractile force responses and fatigue indexes (FI). The muscle fibers belonging to individual units (i.e., the muscle unit) were identified using the glycogen-depletion method. Fibers were classified as type I or II based on histochemical staining for myofibrillar adenosine triphosphatase (ATPase) after alkaline preincubation. The rate of succinate dehydrogenase (SDH) activity of each fiber was determined using a microphotometric procedure. The location of capillaries was determined from muscle cross sections stained for ATPase after acid (pH = 4.2) preincubation. The capillarity of muscle unit fibers was determined by counting the number of capillaries surrounding fibers and by calculating the number of capillaries per fiber area. A significant correlation was found between the fatigue resistance of DIA units and the mean SDH activity of muscle unit fibers. A significant correlation was also observed between DIA unit fatigue resistance and both indexes of muscle unit fiber capillarity. The mean SDH activity and mean capillary density of muscle unit fibers were also correlated. We conclude that DIA motor unit fatigue resistance depends, at least in part, on the oxidative capacity and capillary density of muscle unit fibers.     

Experimental muscle pain reduces initial motor unit discharge rates during sustained submaximal contractions.

The aim of this human study was to investigate the effect of experimentally induced muscle pain on the modifications of motor unit discharge rate during sustained, constant-force contractions. Intramuscular and multichannel surface electromyographic (EMG) signals were collected from the right and left tibialis anterior muscle of 11 volunteers. The subjects performed two 4-min-long isometric contractions at 25% of the maximal dorsiflexion torque, separated by a 20-min rest. Before the beginning of the second contraction, hypertonic (painful; right leg) or isotonic (nonpainful; left leg) saline was injected into the tibialis anterior. Pain intensity scores did not change significantly in the first 150 s of the painful contraction. Exerted torque and its coefficient of variation were the same for the painful and nonpainful contractions. Motor unit discharge rate was higher in the beginning of the nonpainful contraction than the painful contraction on the right side [means +/- SE, 11.3 +/- 0.2 vs. 10.6 +/- 0.2 pulses/s (pps); P < 0.01] whereas it was the same for the two contractions on the left side (11.6 +/- 0.2 vs. 11.5 +/- 0.2 pps). The decrease in discharge rate in 4 min was smaller for the painful (0.4 +/- 0.1 pps) than for the control contractions (1.3 +/- 0.1 pps). Initial value and decrease in motor unit conduction velocity were not different in the four contractions (right leg, 4.0 +/- 0.1 m/s with decrease of 0.6 +/- 0.1 m/s in 4 min; left leg, 4.1 +/- 0.1 m/s with 0.7 +/- 0.1 m/s decrease). In conclusion, stimulation of nociceptive afferents by injection of hypertonic saline did not alter motor unit conduction velocity but reduced the initial motor unit discharge rates and the difference between initial and final discharge rates during sustained contraction.