The state of the contralateral motor center of the rat gastrocnemius muscle after unilateral damage of sciatic nerve

Effects of unilateral damage of the rat sciatic nerve on the spinal motor centre of contralateral gastrocnemius muscle were investigated. The state of contralateral gastrocnemius muscle centre was assessed using the method of paired monosynaptic testings and posttetanic potentiation of reflex response on days 1, 5 and 10 after nerve damage. The obtained data indicate changes in reflex activity of contralateral motor centre in these conditions. The activation of contralateral reflex systems after nerve damage seems to provide regenerative reorganisation of the motor control in the conditions of traumatic influence.     

Effects of local hyperthermia on the motor function of the rat sciatic nerve

The effect of local heat treatment of the sciatic nerve was assessed using the toe-spreading test, which mainly assesses the motor function of the sciatic nerve. A 5 mm long segment of the nerve was heated at temperatures from 42.0 to 45.0 degrees C in vivo using a brass thermode. Hyperthermia led to a decrease in spreading of the toes. Recovery from functional loss took place in all cases, and this recovery was completed in 4 weeks. A 50 per cent functional loss in 50 per cent of the treated animals was observed after 58, 32 and 12 min of heating at 43.0, 44.0 and 45.0 degrees C respectively.     

Unilateral immobilization affects contralateral rat gastrocnemius muscle architecture.

In order to study the effects of unilateral short length immobilization on the contralateral gastrocnemius muscle (GM), length measurements were conducted on photographs taken in the active condition (tetanic plateau). Comparison of geometry of experimental and control muscles was made at optimum muscle length. The results show that a process occurred in the muscle which can be ascribed predominantly in terms of atrophy. This atrophy did not reach a maximum after 4 weeks but gradually increased in time. The altered conditions imposed on the muscle changed its architecture. It was shown that variables of the contralateral GM muscle are not representative of those of normally used muscles and should therefore not be used as control muscles for the determination of immobilization effects.     

Effect of unilateral trauma of the sciatic nerve on characteristics of motor and reflex responses of paired gastrocnemius muscles

The influence of unilateral n. ischiadicus lesion of conductivity on the electromyographic characteristics of the rat gastrocnemius muscles was investigated. The lesion was made by n. ischiadicus crashing. Motor and reflex gastrocnemius muscles' responses caused by n. ischiadicus stimulation (single impulses of various intensity and 0.3 ms duration) were estimated on broken and symmetric hind-limb before and 1, 5, 10, 20 and 30 days after the nerve crashing. Considerable changes of threshold and amplitude of the registered gastrocnemius muscle responses were observed ipsi- and contralaterally. It is suggested that the central dependence of processes arising in the motor centres of symmetric gastrocnemius muscles, follows unilateral n. ischiadicus lesion.     

Effect of aging on properties of motor unit action potentials in the rat medial gastrocnemius muscle

The purpose of this study was to investigate whether age-related changes in motor unit (MU) contractile properties are reflected in parameters of motor unit action potentials (MUAPs). MUs of the medial gastrocnemius muscle were functionally isolated in anaesthetized Wistar rats. A control group of young animals (5–10 mo) was compared to two groups of old rats (24–25 mo and 28–30 mo). The basic contractile properties of MUs as well as the amplitude, total duration, peak-to-peak time, and number of turns within MUAPs were measured. Effects of aging were mainly observed for fast fatigable MUs (a prolongation of MUAPs and increased number of turns). The MUAP amplitude did not change significantly with aging in either MU type, but it correlated to the twitch or tetanic forces, which tended to increase with age, especially for slow MUs. We concluded that the prolongation of MUAPs and the greater incidence of signal turns was probably a result of a decrease in muscle fiber conduction velocity and/or an increase in their dispersion, and enlargement of MU territories – presumably caused by axonal sprouting of surviving motoneurons. The latter might also be responsible for the observed age-related tendency for a increase in MUAP amplitudes in slow MUs.     

Summation of motor unit forces in rat medial gastrocnemius muscle

The summation of contractile forces of motor units (MUs) was analyzed by comparing the recorded force during parallel stimulation of two and four individual MUs or four groups of MUs to the algebraic sum of their individual forces. Contractions of functionally-isolated single MUs of the medial gastrocnemius muscle were evoked by electrical stimulation of thin filaments of the split L5 or L4 ventral roots of spinal nerves. Additionally, contractions of large groups of MUs were evoked by stimuli delivered to four parts of the divided L5 ventral root. Single twitches, 40 Hz unfused tetani, and 150 Hz fused maximum tetani were recorded. In these experimental situations the summation was more effective for unfused tetani than for twitches or maximum tetani. The results obtained for pairs of MUs were highly variable (more- or less-than-linear summation), but coactivation of more units led to progressively weaker effects of summation, which were usually less-than-linear in comparison to the algebraic sums of the individual forces. The variability of the results highlights the importance of the structure of the muscle and the architecture of its MUs. Moreover, the simultaneous activity of fast and slow MUs was considerably more effective than that of two fast units.     

Comparative proteomic profile of rat sciatic nerve and gastrocnemius muscle tissues in ageing by 2‐D DIGE

Ageing induces a progressive morphological change and functional decline in muscles and in nerves. Light and electron microscopy, 2‐D DIGE and MS, were applied to profile the qualitative and quantitative differences in the proteome and morphology of rat gastrocnemius muscle and sciatic nerve, in healthy 22‐month‐old rats. At muscle level, morphological changes are associated to fibre atrophy accompanied by myofibrillar loss and degeneration, disappearance of sarcomeres and sarcoplasmic reticulum dilatation, internal migration of nuclei, longitudinal fibre splitting, increment of subsarcolemmal mitochondria aggregates and increment of lipofuscin granules. Sciatic nerve shows myelin abnormalities like enfoldings, invaginations, onion bulbs, breakdowns and side axonal atrophy. Proteomic analysis identified changes correlated to morphological abnormalities in metabolic, contractile and cytoskeletal proteins, deregulation of iron homeostasis, change of Ca²⁺ balance and stress response proteins, accompanied by a deregulation of myelin membrane adhesion protein and proteins regulating the neuronal caliber. By comparing proteomic results from the two tissues, 16 protein isoforms showed the same up and down regulation trend suggesting that there are changes implying a general process which may act as a signal event of degeneration. Only β enolase and tropomyosin 1α were differentially expressed in the tissues.     

Adaptation of rat lateral gastrocnemius muscle motor units during hindlimb unloading

Contractile and fatigue-resistance properties of 71 lateral gastrocnemius muscle (LG) motor units (MU) following 14 days of hindlimb unloading (HU) were compared to those of 60 LG MU from control rats. The MU properties were assessed from isolated and stimulated individual motor axons. The MU were classified using standard criteria (shape of unfused tetani and fatigue resistance). The HU did not affect LG MU composition, but diminished the maximal tetanic tension (Po) of all MU types: P0 was significantly reduced by about 40% for the slow and fast-resistant MU, and by 18% for the fast-fatigable ones. The speed-related properties of fast-resistant MU became more similar to those of slower MU. The fatigue properties of MU were evaluated during a 5-min exercise test, using two fatigue indexes, FI2 and FI5, which expressed the relative capacity of MU to generate tension after 2 and 5 min, respectively. Results showed that 14 days of HU did not change the fatigue sensitivity of the LG MU. However, when F15 was compared to FI2, a greater decrease was observed after HU than in control conditions for the fast-resistant and fast-intermediate MU. It was concluded that a prolonged fatigue test may show changes in metabolic properties of muscle fibres during 14 days of HU. Specific adaptations of LG MU as well as comparisons with those of the soleus muscle under the same conditions are discussed.     

Differential axonal transport of isotubulins in the motor axons of the rat sciatic nerve

The axonal transport of the diverse isotubulins in the motor axons of the rat sciatic nerve was studied by two-dimensional polyacrylamide gel electrophoresis after intraspinal injection of [35S]methionine. 3 wk after injection, the nerve segments carrying the labeled axonal proteins of the slow components a (SCa) and b (SCb) of axonal transport were homogenized in a cytoskeleton-stabilizing buffer and two distinct fractions, cytoskeletal (pellet, insoluble) and soluble (supernatant), were obtained by centrifugation. About two-thirds of the transported-labeled tubulin moved with SCa, the remainder with SCb. In both waves, tubulin was found to be associated mainly with the cytoskeletal fraction. The same isoforms of tubulin were transported with SCa and SCb; however, the level of a neuron-specific beta-tubulin subcomponent, termed beta', composed of two related isotubulins beta'1 and beta'2, was significantly greater in SCb than in SCa, relative to the other tubulin isoforms. In addition, certain specific isotubulins were unequally distributed between the cytoskeletal and the soluble fractions. In SCa as well as in SCb, alpha'-isotubulins were completely soluble in the motor axons. By contrast, alpha' and beta'2-isotubulins, both posttranslationally modified isoforms, were always recovered in the cytoskeletal fraction and thus may represent isotubulins restricted to microtubule polymers. The different distribution of isotubulins suggests that a recruitment of tubulin isoforms, including specific posttranslational modifications of defined isoforms (such as, at least, phosphorylation of beta' and acetylation of alpha'), might be involved in the assembly of distinct subsets of axonal microtubules displaying differential properties of stability, velocity and perhaps of function.     

Effect of adenosine receptor blockade on pial arteriolar dilation during sciatic nerve stimulation

In the present study, we report the effects of adenosine receptor antagonists on pial vasodilatation during contralateral sciatic nerve stimulation (SNS). The pial circulation was observed through a closed cranial window in alpha-chloralose-anesthetized rats. In artificial cerebrospinal fluid (CSF), SNS resulted in a 30.5 +/- 13.2% increase in pial arteriolar diameter in the hindlimb somatosensory cortex. Systemic administration of the selective adenosine A2A receptor antagonist, 4-(2-[7-amino-2-[2-furyl][3,2,4]triazolol[2,3-a][1,3,5]triazin-5-yl-amino] ethyl)phenol (ZM-241385), significantly (P < 0.05, n = 6) attenuated the SNS-induced vasodilatation. Systemic administration of 8-(p-sulfophenyl)theophylline (8SPT), a nonselective antagonist that is blood-brain barrier (BBB) impermeable, had no effect on vasodilatation to SNS. In contrast, systemic theophylline, which readily penetrates the BBB, nearly abolished the SNS-induced vasodilatation (P < 0.01; n = 7). Topical superfusion of 8SPT significantly (P < 0.01; n = 6) attenuated vasodilatation during SNS. Topical superfusion of 8- cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, significantly potentiated SNS-induced vasodilatation (P < 0.01; n > or = 5). Hypercarbic vasodilatation and somatosensory-evoked potentials were not affected by any of the compounds tested. Our findings suggest that luminal endothelial adenosine receptors are not involved in the arteriolar response to SNS, as demonstrated by a lack of effect with systemic 8SPT. Furthermore, the adenosine A2A receptor subtype appears to be involved in the dilator response to SNS. Finally, the neuromodulatory action of adenosine, via the A1 receptor subtype, significantly influences SNS-induced vasodilatation. Thus the present study provides further evidence for a role of adenosine in the regulation of CBF during somatosensory stimulation.     

Effect of thyroidectomy on fast and slow muscle fibres of rat gastrocnemius muscle

A combined histochemical, biochemical and electrophoretic study with respect to the enzymes succnic dehydrogenase(SDH), myofibrillar adenosine triphosphatase (m-ATPase), lactate dehydrogenase (LDH) isozymes and myosin light chains was carried out to investigate the response of rat gastrocnemius muscle (medial head). Twelve weeks after thyroidectomy, the results indicated a shift from fast to slow type pattern of LDH isozymes, fibre type transformation from Type II to Type I and a decrease in SDH and m-ATPase activity. The results suggest, possible thyroidal involvement in determining the phenotypic properties of skeletal muscle.     

Effect of electromagnetic radiation of millimeter range on restoration of motor activity after impairment of the sciatic nerve in the rat

The effect of low-intensity (10–12 mW/cm²) amplitude-modulated electromagnetic radiation (EMR) with a wavelength of 8.1 mm on restoration of motor activity of hindlimb after the sciatic nerve had been crushed was studied in rats (only the injured area was radiated). The recovery was found to be significantly accelerated by EMR: after a seven-week-long treatment by EMR, the mean amplitude of the extensor force developed by the hindlimb of the EMR-treated animals reached 62.3% of its initial value (recorded before the nerve had been crushed), while in the control group of animals it was equal to near 40%.Neirofiziologiya/Neurophysiology, Vol. 28, No. 1, pp. 3–6, January–February, 1996.     

Interspecies differences of motor units properties in the medial gastrocnemius muscle of cat and rat

The purpose of the study was to analyze the interspecies differences of motor unit contractile properties in two most frequently studied mammals: cats and rats. A total sample of 166 motor units (79 in cats and 85 in rats) was investigated in the medial gastrocnemius muscle. Considerable differences were found in composition of the studied muscle. In cats, fast fatigable, fast resistant and slow units formed 68, 18 and 14% of the investigated population, whereas in rats 36, 52 and 12%, respectively. The contraction and relaxation times of motor units in the cat muscle were evidently longer than in the rat and the border values for fast/slow motor units division in these species were 44 and 20 ms, respectively. The mean values of twitch and tetanic forces appeared to be 7-8 times lower in rats, for fast, while 2-5 times for slow motor units. Also variability between the strongest and the weakest units within each type revealed differences 10-60 times in cats, whereas only 3.5-14 times in rats. The summation of twitches into tetanus for fast units was comparable in both species, but for S units was evidently more effective in the cat. In fast motor units' tetanic contractions evident interspecies differences concerned sag appearance and profiles of unfused tetani of FF and FR units. Differences in contractile properties described in the study may depend on the size, number and innervation ratio of motor units in the muscle of cat and rat, as well as their biochemical variability. Differences in composition of motor unit types and uneven mechanisms of force development may reflect biological adaptation to variable behaviour of cats and rats.     

A conditioning lesion promotes in vivo nerve regeneration in the contralateral sciatic nerve of rats

A conditioning lesion in the sciatic nerve increases in vivo axonal regeneration in the nerve after a second transection. We studied whether this increased regeneration also occurs in the contralateral nerve. The left sciatic nerve was transected and sutured in Wistar rats; the nerve was exposed but not transected in controls. After 5 days, the right sciatic nerves of all rats were transected and sutured. Neuronal regeneration was measured at 0, 1, 3, 5, and 7 days with the pinch test and histological staining. IL-1beta and TGF-beta1 expression was also measured. The initial delay in the experimental group was significantly shorter, but the regeneration rates were the same. The expression of IL-1beta and TGF-beta1 in the right dorsal root ganglia was significantly higher in the experimental group. Nerve injury enhances cytokine expression in the contralateral dorsal root ganglion and promotes contralateral nerve regeneration in vivo by shortening the initial delay.     

Changes in contralateral protein metabolism following unilateral sciatic nerve section

Changes in nerve biochemistry, anatomy, and function following injuries to the contralateral nerve have been repeatedly reported, though their significance is unknown. The most likely mechanisms for their development are either substances carried by axoplasmic flow or electrically transmitted signals. This study analyzes which mechanism underlies the development of a contralateral change in protein metabolism. The incorporation of labelled amino acids (AA) into proteins of both sciatic nerves was assessed by liquid scintillation after an unilateral section. AA were offered locally for 30 min to the distal stump of the sectioned nerves and at homologous levels of the intact contralateral nerves. At various times, from 1 to 24 h, both sciatic nerves were removed and the proteins extracted with trichloroacetic acid (TCA). An increase in incorporation was found in both nerves 14–24 h after section. No difference existed between sectioned and intact nerves, which is consistent with the contralateral effect. Lidocaine, but not colchicine, when applied previously to the nerves midway between the sectioning site and the spinal cord, inhibited the contralateral increase in AA incorporation. It is concluded that electrical signals, crossing through the spinal cord, are responsible for the development of the contralateral effect. Both the nature of the proteins and the significance of the contralateral effect are matters for speculation.     

Analysis of the unfused tetanus course in fast motor units of the rat medial gastrocnemius muscle

The course of unfused tetani with the sag effect in fast motor units of rat medial gastrocnemius was studied. The analysis of the course of successive contractions within these tetani showed that the high peak force at the beginning of tetanus before the sag resulted from temporary, very efficient sum mation of contractions at this phase, both in FF (fast fatigable) and FR (fast resistant to fatigue) units. The process of summation developed in spite of parallel shortening of the contraction and relaxation. The peak of tetanus force was visible on the average at the 2nd contraction in FF units and at the 5th contraction in FR units. After the tetanus peak the process of the efficient summation was completed and the force decreased what was visible as a sag. In the following part of the tetanus, mainly in FF units, the potentiation occurred and the force of successive contractions increased. The rise of force was visible in spite of shortening of the contraction time and was due to prolongation of the relaxation in this part of the tetanus. These observations indicated that the processes of the summation of successive contractions before sag and during the potentiation underwent different mechanisms discussed in this paper. Considerable release of Ca2+ ions from the sarcoplasmic reticulum was proposed as a possible mechanism responsible for a very efficient summation at the beginning of the tetanus whereas phosphorylation of regulatory light chain of myosin (RLC) in muscle fibers was considered as the reason of potentiation. Moreover, the present analysis revealed that previously found differences in tetani profiles between FF and FR units resulted from faster development of described changes in the course of contractions summating into the tetanus in FF motor units.     

Effect of synchronization of firings of different motor unit types on the force variability in a model of the rat medial gastrocnemius muscle

The synchronized firings of active motor units (MUs) increase the oscillations of muscle force, observed as physiological tremor. This study aimed to investigate the effects of synchronizing the firings within three types of MUs (slow—S, fast resistant to fatigue–FR, and fast fatigable–FF) on the muscle force production using a mathematical model of the rat medial gastrocnemius muscle. The model was designed based on the actual proportion and physiological properties of MUs and motoneurons innervating the muscle. The isometric muscle and MU forces were simulated by a model predicting non-synchronized firing of a pool of 57 MUs (including 8 S, 23 FR, and 26 FF) to ascertain a maximum excitatory signal when all MUs were recruited into the contraction. The mean firing frequency of each MU depended upon the twitch contraction time, whereas the recruitment order was determined according to increasing forces (the size principle). The synchronization of firings of individual MUs was simulated using four different modes and inducing the synchronization of firings within three time windows (± 2, ± 4, and ± 6 ms) for four different combinations of MUs. The synchronization was estimated using two parameters, the correlation coefficient and the cross-interval synchronization index. The four scenarios of synchronization increased the values of the root-mean-square, range, and maximum force in correlation with the increase of the time window. Greater synchronization index values resulted in higher root-mean-square, range, and maximum of force outcomes for all MU types as well as for the whole muscle output; however, the mean spectral frequency of the forces decreased, whereas the mean force remained nearly unchanged. The range of variability and the root-mean-square of forces were higher for fast MUs than for slow MUs; meanwhile, the relative values of these parameters were highest for slow MUs, indicating their important contribution to muscle tremor, especially during weak contractions.