Short latency cutaneous reflex responses of γ-efferents in the decerebrate cat

Summary The effect of single shock stimulation, up to 20 × threshold (T), of the sural nerve on the discharges of triceps surae -efferents was investigated in decerebrate cats. Units were classified as static (12) or dynamic (7) on the basis of their resting discharge rates (Murphy et al. 1984). All neurones were excited at short latency by sural nerve stimulation and response size was graded with stimulus intensity. Short latency mixed or inhibitory responses were not evident. Although reflex effects first occurred at low stimulus strengths (<-1.5T) in both types of efferent, most responses appeared at higher intensities (> 1.5T). The estimated central delays of the responses of static (3.0 ±1.1 ms, mean± SD) and dynamic (3.4 ± 1.0 ms) -motoneurones were not significantly different and are consistent with spinal oligosynaptic pathways. The present results differ from those of the only previous study (Johansson and Sojka 1985) of the short latency responses of triceps surae static and dynamic -motoneurones to sural nerve stimulation, in which mixed and inhibitory effects were common in anaesthetised cats. Although differences in recording techniques and sampling may account for the apparent disparity between these studies, it is also feasible that a difference in the setting of interneuronal pathways in the two types of preparation is responsible. The results are discussed in relation to the control of -motoneurones with particular reference to the final common input hypothesis (Johansson 1981; Appelberg et al. 1983).     

Blood flow in the rabbit tenuissimus muscle Influence of preparative procedures for intravital microscopic observation

LINDBOM, L., TUMA, R. F. & ARFORS, K.-E.: Blood flow in the rabbit tenuissimus muscle: Influence of preparative procedures for intravital microscopic observation. Acta Physiol Scand 1982, 114 :121–127. Received 21 April 1981. ISSN 0001–6772. Department of Experimental Medicine, Pharmacia AB, Uppsala, Sweden. The tenuissimus muscle in the rabbit and the cat is a suitable tissue for intravital microscopic investigation of skeletal muscle blood flow. In this study the influence of surgical procedures necessary for direct microscopic observation on the physiological state of the rabbit tenuissimus muscle was assessed by means of blood flow measurements. Mean resting blood flow was 2.8±0.8 (mean ± S.D.) ml. min^-1. 100 g^-1 in the left tenuissimus muscle when prepared for microscopic observation as determined by the radioactive microsphere method. This value was not significantly different from that in the intact unexposed muscle in the contralateral leg, 3.3 ±1.1 ml. min^-1. 100 g^-1. Exposure of the muscle to atmospheric oxygen tension resulted in a reduction of blood flow to 0.7±0.4 ml. min^-l. 100g^-l, suggesting that local metabolic control mechanisms were active. The normal range of vascular control seemed to be maintained, as demonstrated by an increase in blood flow to 64.2± 18.8 ml. min-¹. 100 g^-l during “maximal” vasodilation induced by topical application of PGE₁. The tenuissimus muscle showed a marked sensitivity to mechanical stimulation. Slight stretching of the muscle, similar to what may occur during surgical preparation, resulted in an increase in blood flow to 17.5±5.7 ml. min^-1. 100 g^-1. Flow values calculated from data obtained by direct microscopic measurements in the tenuissimus muscle agreed well with those obtained by the microsphere method.     

Mechanical and energy characteristics during shortening in isolated type-1 muscle fibres from Xenopus laevis studied at maximal and submaximal activation

 The mechanical and energy characteristics of isolated fast-twitch muscle fibres (type 1) of Xenopus laevis in isometric- and isovelocity contractions were measured at 20°C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of activation. The high stimulation frequency gave fused contractions, while at the low stimulation frequency tension fluctuated. When maximum isometric force had been reached, the fibres were shortened by 10% of the fibre length at different velocities. At 60 Hz stimulation during shortening the rate of heat production increased above the isometric rate of heat production. At 20 Hz stimulation during shortening, however, the rate of heat production was not different from the isometric rate of heat production. Mechanical efficiency was the same at the high and low level of activation. The actomyosin efficiency (i.e. the mechanical efficiency corrected for ”activation heat”) was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activated muscle. From a comparison of the present results with those obtained from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow-twitch muscle fibres. Received after revision: 13 May 1997 / Accepted: 1 July 1997     

Recruitment of fibre types and quadriceps muscle portions during repeated, intense knee-extensor exercise in humans

To investigate recruitment of slow-twitch (ST) and fast-twitch (FT) muscle fibres, as well as the involvement of the various quadriceps femoris muscle portions during repeated, intense, one-legged knee-extensor exercise, 12 healthy male subjects performed two 3-min exercise bouts at ~110% maximum thigh O₂ consumption (EX1 and EX2) separated by 6 min rest. Single-fibre metabolites were determined in successive muscle biopsies obtained from the vastus lateralis muscle (n=6) and intra-muscular temperatures were continuously measured at six quadriceps muscle sites (n=6). Creatine phosphate (CP) had decreased (P<0.05) by 27, 73 and 88% in ST fibres and 25, 71 and 89% in FT fibres after 15 and 180 s of EX1 and after 180 s of EX2, respectively. CP was below resting mean–1 SD in 15, 46, 84 and 100% of the ST fibres and 9, 48, 85 and 100% of the FT fibres at rest, after 15 and 180 s of EX1 and after 180 s of EX2, respectively. A significant muscle temperature increase (T_m) occurred within 2–4 s at all quadriceps muscle sites. T_m varied less than 10% between sites during EX1, but was 23% higher (P<0.05) in the vastus lateralis than in the rectus femoris muscle during EX2. T_m in the vastus lateralis was 101 and 109% of the mean quadriceps value during EX1 and EX2, respectively. We conclude that both fibre types and all quadriceps muscle portions are recruited at the onset of intense knee-extensor exercise, that essentially all quadriceps muscle fibres are activated during repeated intense exercise and that metabolic measurements in the vastus lateralis muscle provide a good indication of the whole-quadriceps muscle metabolism during repeated, intense, one-legged knee-extensor exercise.     

Longitudinally orientated smooth muscle cells in rabbit arteries

Intima formation in vessels, spontaneous or experimentally induced, is generally characterized by the presence of longitudinally orientated smooth muscle cells (LSMC). During an experiment of neo-intima induction in carotid arteries in rabbits, by application of a nonconstrictive silastic cuff, a study was performed to investigate the presence of LSMC in the systemic and pulmonary circulations, in both elastic and muscular arteries. Three patterns could be distinguished: intimai cushions in muscular arteries, single or small groups of LSMC in the intima in elastic and larger muscular arteries, and intra-medially located layers or columns of LSMC in the aorta, the pulmonary artery, at the bifurcation of the aorta and around orifices of branches. In order to understand this peculiar orientation a biomechanical approach was used: this showed that near the lumen the circumferential stress is 4.5 times higher than the longitudinal. Because the cell surface of the smooth muscle cells exposed to this stress per unit vessel length is much less in the longitudinal than in the circular direction we conclude that the LSMC align in the direction which allows them to cope most effectively with the mechanical stresses.     

Use of the iso-inertial force mass relationship in the prediction of dynamic human performance

The purpose of this investigation was to develop a new test of muscle function, termed the isoinertial force-mass relationship, and to determine its relationship to dynamic physical performance in comparison to an isometric test. A group of 13 trained subjects performed an isometric, and a series of iso-inertial maximal upper body tests, in a bench press movement at loads of 30%, 60%, 100% (concentric) and 100%, 130% and 150% (eccentric) of maximum. Vertical forces exerted throughout the movement were recorded by a force plate. In addition, the subjects performed the following three performance tests: a maximal bench press, a seated shotput, and two drop bench-press throws from a height of 0.25 m, with loads of 10 kg and 30% of maximum. Correlation analysis demonstrated that in each instance the iso-inertial force mass tests were the best predictors of performance (r=0.78–0.88) with both contraction type and mass specific effects apparent. Maximal isometric force and rate of force development were significantly related to some performance variables (r=0.22–0.78). However, for all the performance movements assessed, the iso-inertial test modality recorded the highest relationship to performance. The difference in the relationship between performance and iso-inertial and isometric test modalities was particularly evident in the light load dynamic performance of the seated shotput (r=0.86 vsr=0.38, respectively). These results are explained in part by the neural and mechanical differences between iso-inertial and isometric muscle actions and their respective specificity to dynamic physical performance.     

Lactate dehydrogenase activity and isoform distribution in normal and hypertrophic smooth muscle tissue from the rat

The lactate dehydrogenase (LDH) activity and isoform distribution of LDH were investigated in tissue samples from the rat portal vein, aorta and urinary bladder. In addition, samples were obtained from hypertrophic urinary bladder. The total LDH activity per unit smooth muscle volume was higher in the urinary bladder compared to that in portal vein and aorta. Five LDH isoforms, reflecting different combinations of the two polypeptide chains denoted H and M, could be separated by agarose gel electrophoresis. The aorta contained more of the H form compared to the portal vein and urinary bladder. This difference suggests that the aorta, which is a slow smooth muscle, is more adapted for aerobic metabolism than the faster muscles of portal vein and urinary bladder. In the hypertrophic urinary bladder a shift in LDH isoform pattern towards less of the H form was found, which correlates with a better maintenance of contraction in anoxia in this type of hypertrophic smooth muscle.     

Changes of energy metabolism,myosin light chain composition,lactate dehydrogenase isozyme pattern and fibre type distribution of denervated fast-twitch muscle from rabbit after low frequency stimulation

The influence of low frequency (8–10 Hz) electrical stimulation on denervated fast-twitch muscle from rabbit was investigated. Prolonged direct stimulation of denervated muscle resulted in higher oxidative enzyme activities. Furthermore, single fibre analyses for succinate dehydrogenase showed a more uniform distribution of activity in stimulated-denervated muscle when compared to normal muscle. As was also the case following stimulation of innervated muscle, glycolytic enzymes were decreased in activity and the LDH-isozyme pattern also shifted towards heart type. No change of the myosin light chain pattern could be observed after 56 days of stimulation.This study was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 138, and by grant NI 204-2-2. H. R. is grateful to Dr. Dirk Pette for his guidance and encouragement during this work, which was done in Dr. Pette's laboratory at the University of Konstanz     

Skinned smooth muscle: calcium-calmodulin activation independent of myosin phosphorylation

In chemically skinned chicken gizzard smooth muscle fibers investigated shortly after preparation, a contraction may be induced by calcium and calmodulin which is independent of myosin phosphorylation at intermediate Ca²⁺-concentrations. However, fibers stored for a prolonged period also contract in the absence of exogenous calmodulin and exhibit a close relationship between force development and myosin phosphorylation.