Single Fiber Level Research Articles

Endogenous MLC2 phosphorylation and Ca(2+)-activated force in mechanically skinned skeletal muscle fibres of the rat.

A method has been developed for measuring the level of phosphorylation of myosin regulatory light chains (MLC2) by the endogenous myosin light chain kinase in mechanically skinned skeletal muscle fibres. The method was used to characterize the endogeous MLC2 phosphorylation capacity of single fast-twitch fibres from the rat and to investigate the relationship between the endogenous MLC2 phosphorylation and the Ca(2+)-activated force. The results show that (1) about 50% of MLC2 were 32P-phosphorylated after activation of the skinned fibre preparation by 30 microM [Ca2+] for longer than 30 s, but that there was variability between fibres; (2) most of the endogenous phosphorylating system diffused out of the skinned fibre preparation after 5 min exposure to an aqueous solution; (3) the MLC2 phosphorylation by the endogenous phosphorylating system followed with a delay of the order of 1-2 s after the sudden rise in [Ca2+] from below 10 nM to 30 microM; and (4) the sensitivity of the contractile apparatus to Ca2+ was markedly increased when the MLC2 were phosphorylated by the endogenous phosphorylating system following a rise in [Ca2+]. The Kd for MgATP of the endogenous MLC2 phosphorylating system was estimated to be less than 300 microM. These results unequivocally demonstrate that prolonged activation of the fast-twitch muscle fibre leads to increased Ca2+ sensitivity of the contractile apparatus and that mechanically skinned fibres can be successfully used to study the regulation of the endogenous MLC2 phosphorylation capacity at single muscle fibre level.

Detection of "deleted" mitochondrial genomes in cytochrome-c oxidase-deficient muscle fibers of a patient with Kearns-Sayre syndrome.

Using in situ hybridization and immunocytochemistry, we studied a muscle biopsy sample from a patient with Kearns-Sayre syndrome (KSS) who had a deletion of mitochondrial DNA (mtDNA) and partial deficiency of cytochrome-c oxidase (COX; EC 1.9.3.1). We sought a relationship between COX deficiency and abnormalities of mtDNA at the single-fiber level. COX deficiency clearly correlated with a decrease of normal mtDNA and, conversely, deleted mtDNA was more abundant in COX-deficient fibers, especially ragged-red fibers. The distribution of mtRNA had a similar pattern, suggesting that deleted mtDNA is transcribed. Immunocytochemistry showed that the nuclear DNA-encoded subunit IV of COX was present but that the mtDNA-encoded subunit II was markedly diminished in COX-deficient ragged-red fibers. Because the mtDNA deletion in this patient did not comprise the gene encoding COX subunit II, COX deficiency may have resulted from lack of translation of mtRNA encoding all three mtDNA-encoded subunits of COX.

Single-fiber EMG study of the flexor carpi radialis H reflex.

Late responses were studied in the flexor carpi radialis muscle by surface-recording and single-fiber electromyography. By single-fiber studies we were able to distinguish 2 distinct components. One was an H reflex obtained without a preceding M response and with a stimulus response jitter of about 100 musec; its latency was shortened and jitter decreased with the Jendrassik maneuver. The other was an F wave always preceded by an M response and with a stimulus response jitter of under 50 musec; its jitter and latency are unaffected by the Jendrassik maneuver. At the single-fiber level it was possible to determine the rate of occurrence of individual H reflexes and F responses for a given number of surface-applied stimuli. The H reflex had a very high rate of occurrence (up to 97%), whereas the F wave occurred very infrequently (less than 2% of the time). Using the H reflex rate of occurrence in conjunction with the maximal surface H/M amplitude ratios allowed us to determine the proportion of alpha motor neurons which participate in the H-reflex generation. The study of H-reflex jitter gives an estimate of the central synaptic jitter and effectiveness of the spatial and temporal summation of type 1a fiber inputs on the motor neurons.

Fatigue crack propagation under step variation of stress frequency in orthotropic material: Sakamoto, H. and Takezono, S.Eng. Fract. Mech. 1988, 31, (3), 463–474

Civil structures subjected to extreme loading conditions, e.g., earthquakes, undergo large deformations and fracture due to ultra-low cycle fatigue (ULCF). Although several efforts have been made to understand and to model monotonic damage and low-cycle fatigue, so far ULCF is neither sufficiently investigated nor understood. In the present work, ULCF of fibre-reinforced concrete (FRC) beams subjected to bending is discussed in the framework of Fracture Mechanics by means of the Updated Bridged Crack Model (UBCM). The model focuses on the evolution of the fracturing process at the critical cross-section of the beam, taking into account the loading reversal. The concrete matrix is assumed to be linear-elastic and perfectly-brittle. On the other hand, the toughening contribution of the reinforcing fibres is described by a constitutive law that defines the hysteretic behaviour at the single fibre level. The suitability of the hysteretic constitutive law is discussed on the basis of experimental results recently reported in the current scientific literature, in which ULCF flexural tests were carried out on FRC specimens.

The effects of hypercapnia and hypoxia on single hypoglossal nerve fiber activity

The respiratory related modulation of hypoglossal nerve activity has been studied at the single fiber level in cats under hyperoxic hypercapnia and hypoxic conditions and their conduction velocities determined. Changes in fiber activity wer compared to simultaneous changes occurring in phrenic activity. Three different kinds of discharge patterns were observed: (a) inspiratory, (b) phasic activity during both inspiration and expiration, and (c) continuous random with no respiratory modulation. These fibers could be grouped into three categories according to their pattern of discharge during CO 2 breathing. Type I fibers, mean conduction velocity of 30.0 m/sec, exhibited only an inspiratory phasic discharge during 100% O 2 breathing. Their discharge frequency increased rapidly with higher levesl of CO 2 and hypoxia. Type II fibers, mean conduction volocity of 36.7 m/sec, had three different kinds of inspiratory-expiratory discharge patterns during 100% O 2 breathing. With increasing hypercapnia of hypoxia fibers of this group discharged phasically during inspiration and discharged at low frequency during expiration. Type III fibers has a non phasic discharge pattern at 100% O 2 breathing and at all levels of CO 2 tested (up to 10%). Discharge frequency rose during CO 2 rebreathing and hypoxia, but the rate of increase was much less than Type I and Type II fibers. Their mean conduction velocity was 41.3 m/sec. The inspiratory activity of Type I and II fibers increased their activity more than the phrenic during hypercapnia and hypoxia. Type II and Type III fibers are responsible at least in part for the tonic activity of the nerve.

Modelling compound action potentials of peripheral nerves in situ. II. A study of the influence of temperature

Compound nerve action potentials of sural nerves of healthy volunteers have been measured at different temperatures. In the range from 21 to 39°C a linear temperature dependence of the propagation velocity, related to the fastest fibres, is found (slope 1.9 m/sec/°C). The causes of amplitude and shape alterations in the compounds action potentials (CAPs) with changing temperature are analysed with the help of a model. It appears that the changes in the measured CAPs are well explained by known temperature effects on the single fibre level. Both the considerable increase in CAP duration after cooling and the relative constancy of the CAP amplitude were reproduced by the model. Close fits to the actually measured CAPs can be obtained when some crucial parameter values are optimized. The results of this procedure are presented.

Optimizing filter fiber diameter

A quality factor by which various filters can be compared is the negative reciprocal of the pressure drop times the logarithm of the penetration, q = ( − 1 ΔP ) ln(Pn) . This factor is extended to the single-fiber level and used to study the problem of finding an optimal fiber diameter, one minimizing penetration for a given pressure drop. The analysis assumes no change in pressure drop or collection efficiency due to particle capture and negligible interaction among the collectors. By analyzing the individual collection mechanism quality factors, it is shown to be optimal for minimizing pressure drop to have impaction parameters near 2 if impaction predominates, to use fibers as small as practicable where interception predominates, or to use fibers as large as practicable where gravitational sedimentation dominates. High-viscosity situations (such as high-temperature filtration) are seen to be inherently less than optimal, in energy usage (except perhaps where diffusion dominates); high-velocity filtration is also inherently less than optimal in energy usage.

THERMAL SENSIBILITY AND THERMORECEPTORS

The sensing of changes in skin temperature is important (a) in contributing to the total sensory information about an object making contact with the skin, and (b) in assessing body heat loss or gain, that occurs over a large part of the body surface. These functions depend on (a) resolution of small, local, shifts in skin temperature, and (b) effective spatial summation by the central nervous system of thermal stimulus information. Aδ cold fiber populations excited uniquely by cooling the skin and unmyelinated warm fibers excited only by warming the skin constitute the labeled lines that signal all the immediate information available to the brain about changes in skin temperature. We have correlated the capacity of human subjects to identify incremental differences within pairs of warming or cooling temperature pulses and the neural events evoked by similar stimuli in the cutaneous nerve fiber populations of monkeys ( Macaca nemestrina ). The conclusions are that human discrimination of incremental changes in skin temperature depends on (a) information coded at the single fiber level as cumulative impulse count or rate over an integration interval of about 2 see, (b) the brain using information signaled by all the thermoreceptive fibers engaged by the stimulus, (c) the dependent variability among fibers of the responding population being slight, and (d) the brain integrating this information by some method of weighted averaging, which biases the process to favor these fibers that most effectively relay stimulus information.