Vibration of the patellar tendon recruits motor units in the knee extensors via excitation of muscle spindles and subsequent
Ia afferent input to the α-motoneuron pool. Our first purpose was to determine if the recruitment threshold and firing rate
of the same motor unit differed when recruited involuntarily via reflex or voluntarily via descending spinal pathways. Although
Ia input is excitatory to the α-motoneuron pool, it has also been shown paradoxically to inhibit itself. Our second purpose
was to determine if vibration of the patellar tendon during a voluntary knee extension causes a change in the firing rate
of already recruited motor units. In the first protocol, 10 subjects voluntarily reproduced the same isometric force profile
of the knee extensors that was elicited by vibration of the patellar tendon. Single motor unit recordings from the vastus
lateralis (VL) were obtained with tungsten microelectrodes and unitary behaviour was examined during both reflex and voluntary
knee extensions. Recordings from 135 single motor units showed that both recruitment thresholds and firing rates were lower
during reflex contractions. In the second protocol, 7 subjects maintained a voluntary knee extension at 30 N for approximately
40–45 s. Three bursts of patellar tendon vibration were superimposed at regular intervals throughout the contraction and changes
in the firing rate of already recruited motor units were examined. A total of 35 motor units were recorded and each burst
of superimposed vibration caused a momentary reduction in the firing rates and recruitment of additional units. Our data provide
evidence that Ia input modulates the recruitment thresholds and firing rates of motor units providing more flexibility within
the neuromuscular system to grade force at low levels of force production.
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The effect of extracellular magnesium concentration (Mgo) on the upstroke of the action potential was studied in guinea pig ventricular muscle under various K+ concentrations (2.7–19mM). Increased Mgo shifted the steady state inactivation curve of the fast Na channel in the depolarizing direction and this effect was concentration-dependent (0–20mM). Such an effect could explain the Mg-induced increase in maximum rate of rise of the action potential which Späh and Fleckenstein (1979) proposed to be due to a Mg channel. 相似文献
When subjects repetitively lift an object, the grip force they select is influenced by the mechanical object properties of the preceding lift. Similar effects on grip force scaling are observed whether the subsequent lift is performed with the same hand or the hand contralateral to the preceding lift. Here we demonstrate that passive vibration of the hand muscles involved in the generation of grip force in the interval between two blocks of lifting trials interferes with predictive grip force scaling. Following ten trials in which subjects lifted an object with constant mechanical properties with the dominant hand, muscle vibration was given to the first interosseus and adductor pollicis muscles of the dominant hand during a 10-min rest period. Compared with the last lift preceding vibration, peak rates of grip force increase and peak grip forces were scaled too high during the first lift following vibration whether the lift was made with the dominant or non-dominant hand. Subjects scaled grip force accurately to the object properties within three lifts following vibration. If subjects rested for 10 min after the first ten trials and received no vibration, then there was no significant difference in the peak grip force or its rate of increase between the last lift preceding rest and the first lift following it. We suggest that vibration impairs the memory processes responsible for predictive grip force scaling. Our data are consistent with the recent suggestion that these memory processes are neither specific for a certain motor action nor do they reflect internal representations of mechanical object properties. 相似文献
Summary We studied suppression of oculomotor and perceptual after-responses by post-rotary head movements in high (1.8 G), low (0 G), and normal (1 G) gravitoinertial force backgrounds in parabolic flight. Our aim was to identify what sensory and motor signals are critical for triggering suppression. In a prior experiment (DiZio and Lackner 1988), we found suppression using 40° post-rotary head tilts in 1 G and 1.8 G but not 0 G force backgrounds. However, in free fall even without head tilts there was a significant suppression of nystagmus relative to 1 G and 1.8 G force backgrounds, thus potentially masking an effect of head tilt on suppression in 0 G. We have retested four of the original subjects with 90° head tilts to maximize the likelihood of detecting suppression in 0 G. Although nystagmus and illusory afterrotation were suppressed by post-rotary head tilts in normal and high gravitoinertial force environments, there was still no evidence of suppression in free fall. We present evidence that the lack of suppression in 0 G is not attributable to post-rotary responses already being at a basement level, but rather that suppression depends on the registration of a change in head position relative to a significant level of gravitoinertial force. 相似文献
Epithelial cells lose their usual polarization during carcinogenesis. Although most malignant tumours are of epithelial origin little is known about ion channels in carcinoma cells. Previously, we observed that migration of transformed Madin-Darby canine kidney (MDCK-F) cells depended on oscillating K+ channel activity. In the present study we examined whether periodic K+ channel activity may cause changes of cell volume, and whether K+ channel activity is distributed in a uniform way in MDCK-F cells. After determining the average volume of MDCK-F cells (2013±270 m3; n=8) by means of atomic force microscopy we deduced volume changes by calculating the K+ efflux during bursts of K+ channel activity. Therefore, we measured the membrane conductance of MDCK-F cells which periodically rose by 22.3±2.5 nS from a resting level of 6.5±1.4 nS (n=12), and we measured the membrane potential which hyperpolarized in parallel from –35.4±1.2 mV to –71.6±1.8 mV (n=11). The distribution of K+ channel activity was assessed by locally superfusing the front or rear end of migrating MDCK-F cells with the K+ channel blocker charybdotoxin (CTX). Only exposure of the rear end to CTX inhibited migration providing evidence for horizontal polarization of K+ channel activity in transformed MDCK-F cells. This is in contrast to the vertical polarization in parent MDCK cells. We propose that the asymmetrical distribution of K+ channel activity is a prerequisite for migration of MDCK-F cells. 相似文献
Direct visualization of crystal growth in poly(L ‐lactide) thin films was carried out by using a temperature‐controlled atomic force microscopy (AFM). At the initial stage of crystallization, edge‐on lamellar crystals have nucleated and elongated. Subsequently, the edge‐on lamellar crystals showed S‐shaped morphology and changed their orientation from edge‐on manner to flat‐on one. The curvature of edge‐on lamellar crystal has been discussed in terms of inclination and distortion of polymer chains in the crystal. In addition, mechanism on the formation of flat‐on crystal from edge‐on lamellae was proposed as derivative growth on the basis of in situ AFM observation of crystal growth and enzymatic degradation.
Summary The effects of long-term fatigue upon maximal force and peak rate of tension development (PRTD) (dF/dt max) are studied in man (elbow flexors), in the rat (pseudo-isolated gastrocnemius muscle) and in the frog (isolated sartorius muscle). The muscles are fatigued by voluntary anisometric anisotonic contractions against an elastic resistance in man, and by maximal tetanic contractions in the frog and the rat. In man, the excitation level of the muscle is controlled by the integrated surface EMG of the biceps brachii. In the animals, the muscles are stimulated by a neurostimulator. The PRTD and the maximal isometric force are measured during fatigue tests.In man, frog and rat, the maximal voluntary isometric torque or the maximal force and the PRTD decrease initially more or less rapidly according to the power developed during the fatigue process, and then less rapidly. The relationship between PRTD and maximal force is linear in the animals and curvilinear in man.The variations of maximal force and PRTD are discussed in relation to the level of excitation of the muscles and of the composition in different motor units types and their spatio-temporal recruitment. From a biomechanical point of view, it seems necessary to study the behavior of the series elastic component during the evolution of long term fatigue. 相似文献