| 1. |
- Edman, Paul, et al.
(författare)
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Synchronous oscillations of length and stiffness during loaded shortening of frog muscle fibres
- 2001
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Ingår i: Journal of Physiology. - : Wiley. - 1469-7793 .- 0022-3751. ; 534:2, s. 553-563
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Tidskriftsartikel (refereegranskat)abstract
- A study was made of the damped oscillations in fibre length that are observed when isolated muscle fibres from the frog are released during the plateau of an isometric tetanus to shorten against a constant load (force clamp recording) near the isometric level (temperature, 1.0-11.0 C; initial sarcomere length, 2.25 m). 2. The oscillatory length changes of the whole fibre were associated with similar length changes of marked consecutive segments along the fibre. The segmental length changes were initially in synchrony with the whole-fibre movements but became gradually more disordered. At the same time the length oscillation of the whole fibre was progressively damped. 3. The fast length step that normally occurs at the outset of the load-clamp manoeuvre was essential for initiating the oscillatory behaviour. Accordingly, no length oscillation occurred when the load clamp was arranged to start as soon as the selected tension level was reached during the rising phase of the tetanus. 4. The instantaneous stiffness was measured as the change in force that occurred in response to a high-frequency (2-4 kHz) length oscillation of the fibre. During the load-clamp manoeuvre, when the tension was kept constant, the stiffness underwent periodic changes that correlated well in time with the damped oscillatory changes in fibre length. However, there was a phase shift between the stiffness oscillation and the oscillation of shortening velocity, the latter being in the lead of the stiffness response by 21.4 0.8 ms (n = 19) at 1.8 0.1 C . 5. A mechanism is proposed to explain the oscillatory behaviour of the muscle fibre based on the idea that the quick length step at the outset of the load clamp leads to synchronous activity of the myosin cross-bridges along the length of the fibre.
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| 2. |
- Enevoldsen, L.H., et al.
(författare)
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The effect of exercise training on hormone-sensitive lipase in rat intra-abdominal adipose tissue and muscle
- 2001
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Ingår i: Journal of Physiology. - : Wiley. - 1469-7793 .- 0022-3751. ; 536:3, s. 871-877
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Tidskriftsartikel (refereegranskat)abstract
- 1. Adrenaline-stimulated lipolysis in adipose tissue may increase with training. The rate-limiting step in adipose tissue lipolysis is catalysed by the enzyme hormone-sensitive lipase (HSL). We studied the effect of exercise training on the activity of the total and the activated form of HSL, referred to as HSL (DG) and HSL (TG), respectively, and on the concentration of HSL protein in retroperitoneal (RE) and mesenteric (ME) adipose tissue, and in the extensor digitorum. longus (EDL) and soleus muscles in rats. 2. Rats (weighing 96 + 1 g, mean +/- S.E.M.) were either swim trained (T, 18 weeks, n = 12) or sedentary (S, n = 12). Then RE and ME adipose tissue and the EDL and soleus muscles were incubated for 20 min with 4.4 muM adrenaline. 3. HSL enzyme activities in adipose tissue were higher in T compared with S rats. Furthermore, in RE adipose tissue, training also doubled HSL protein concentration (P < 0.05). In ME adipose tissue, the HSL protein levels did not differ significantly between T and S rats. In muscle, HSL (TG) activity as well as HSL (TG)/HSL (DG) were. lower in T rats, whereas HSL (DG) activity did not differ between groups. Furthermore, HSL protein concentration in muscle did not differ between T and S rats (P > 0.05). 4. In conclusion, training increased the amount of HSL and the sensitivity of HSL to stimulation by adrenaline in intra-abdominal adipose tissue, the extent of the change differing between anatomical locations. In contrast, in skeletal muscle the amount of HSL was unchanged and its sensitivity to stimulation by adrenaline reduced after training.
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| 3. |
- Axelson, Hans W, et al.
(författare)
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Human motor control consequences of thixotropic changes in muscular short-range stiffness
- 2001
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Ingår i: Journal of Physiology. - : Wiley. - 0022-3751 .- 1469-7793. ; 535:Pt 1, s. 279-288
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Tidskriftsartikel (refereegranskat)abstract
- The primary aim of the present study was to explore whether in healthy subjects the muscle contractions required for unrestrained voluntary wrist dorsiflexions are adjusted in strength to thixotropy-dependent variations in the short-range stiffness encountered in measurements of passive torque resistance to imposed wrist dorsiflexions.After a period of rest, only the first movement in a series of passive wrist dorsiflexions of moderate amplitude exhibited clear signs of short-range stiffness in the torque response. During analogous types of voluntary movements, the extensor EMG during the first movement after rest showed a steep initial rise of activity, which apparently served to compensate for the short-range stiffness.The passive torque resistance to minute repetitive wrist dorsiflexions (within the range of short-range stiffness) was markedly reduced after various types of mechanical agitation. During analogous low-amplitude voluntary wrist dorsiflexions the extensor EMG signals were weaker after than before agitation.Mechanical agitation also led to enhancement of passive dorsiflexion movements induced by weak constant torque pulses. In an analogous way, the movement-generating capacity of weak voluntary extensor activations (as determined by EMG recordings) was greatly enhanced by mechanical agitation.The signals from a force transducer probe pressed against the wrist flexor tendons - during passive wrist dorsiflexions - revealed short-range stiffness responses which highly resembled those observed in the torque measurements, suggesting that the latter to a large extent emanated from the stretched, relaxed flexor muscles. During repetitive stereotyped voluntary wrist dorsiflexions, a close correspondence was observed between the degree of short-range stiffness as sensed by the wrist flexor tension transducer and the strength of the initial extensor activation required for movement generation.The results provide evidence that the central nervous system in its control of voluntary movements takes account of and compensates for the history-dependent degree of inherent short-range stiffness of the muscles antagonistic to the prime movers.
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| 4. |
- Edin, Benoni B
(författare)
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Cutaneous afferents provide information about knee joint movements in humans.
- 2001
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Ingår i: Journal of Physiology. - : Wiley. - 0022-3751 .- 1469-7793. ; 531:Pt 1, s. 289-297
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Tidskriftsartikel (refereegranskat)abstract
- 1. Neurophysiological evidence that afferent information from skin receptors is important for proprioception has been gathered mainly in experiments relating to the human hand and finger joints. To investigate if proprioceptive information is also provided by skin mechanoreceptor afferents from skin areas related to large joints of postural importance, microneurography recordings were obtained in humans from skin afferents in the lateral cutaneous femoral nerve to study their responses to knee joint movements. 2. Data were collected from 60 sequentially recorded afferents from slowly (n = 23) and fast (n = 6) adapting low-threshold mechanoreceptors, hair follicle receptors (n = 24), field receptors (n = 1) and C mechanoreceptors (n = 6). Fascicular recordings showed that the lateral cutaneous femoral nerve supplies extensive areas of the thigh: from 5-10 cm below the inguinal ligament down to below and lateral to the knee joint; accordingly, the afferents originated in receptors located in wide areas of the human thigh. 3. All afferents from fast and slowly adapting low-threshold mechanoreceptors, as well as C mechanoreceptors, responded to manually applied skin stretch. In contrast, the same stimulus elicited, at most, feeble responses in hair follicle receptors. 4. Qualitative and quantitative analyses of the responses of a subset of afferents revealed that in particular slowly adapting afferents effectively encode both static and dynamic aspects of passively imposed knee joint movements. 5. It was concluded that receptors in the hairy skin of humans can provide high-fidelity information about knee joint movements. A previously undefined type of slowly adapting receptor (SA III) seemed particularly suited for this task whereas this does not seem to be the case for either hair follicle receptors or C mechanoreceptors.
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| 5. |
- Gladden, M. H., et al.
(författare)
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Coupling between serotoninergic and noradrenergic neurones and γ-motoneurones in the cat
- 2000
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Ingår i: Journal of Physiology. - : Wiley. - 0022-3751 .- 1469-7793. ; 527, s. 213-223
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Tidskriftsartikel (refereegranskat)abstract
- 1. Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to γ-motoneurones, and serotonin (5-HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of γ-motoneurones. 2. Eight γ-motoneurones were labelled with rhodamine-dextran, and 50 μm thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine β-hydroxylase (DBH) and 5-HT. All the cells were directly and/or indirectly excited by muscle group II afferents from the muscle they innervated and/or other muscles. 3. Appositions between monoaminergic fibres and the labelled somata and dendrites were located with three-colour confocal laser scanning microscopy by examining series of optical sections at 1 or 0·5 μm intervals. 4. DBH and 5-HT varicosities formed appositions with the somata and dendrites of all the γ-motoneurones. The mean packing densities for 5-HT (1·12 ± 0·11 appositions per 100 μm2 for somata and 0·91 ± 0·07 per 100 μm2 for dendrites) were similar to the densities of contacts reported for α-motoneurones. Monoaminergic varicosities in apposition to dendrites greatly outnumbered those on the somata. 5. The density of DBH appositions was consistently lower - corresponding means were 53% and 62% of those for 5-HT on the somata and dendrites, respectively. 6. It is concluded from an analysis of the distribution and density of varicosities in apposition to the γ-motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of γ-motoneurones.
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| 8. |
- Mekjavic, I.B., et al.
(författare)
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Motion sickness potentiates core cooling during immersion in humans
- 2001
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Ingår i: Journal of Physiology. - : Wiley. - 0022-3751 .- 1469-7793. ; 535:2, s. 619-623
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Tidskriftsartikel (refereegranskat)abstract
- 1. The present study tested the hypothesis that motion sickness affects thermoregulatory responses to cooling in humans. 2. Ten healthy male volunteers underwent three separate head-out immersions in 28 degrees C water after different preparatory procedures. In the 'control' procedure immersion was preceded by a rest period. In the 'motion sickness' procedure immersion was preceded by provocation of motion sickness in a human centrifuge. This comprised rapid and repeated alterations of the gravitational (G-) stress in the head-to-foot direction, plus a standardized regimen of head movements at increased G-stress. In the 'G-control' procedure, the subjects were exposed to similar G-stress, but without the motion sickness provocation. 3. During immersion mean skin temperature, rectal temperature, the difference in temperature between the forearm and 3rd digit of the right hand (DeltaT(forearm-fingertip)), oxygen uptake and heart rate were recorded. Subjects provided ratings of temperature perception, thermal comfort and level of motion sickness discomfort at regular intervals. 4. No differences were observed in any of the variables between control and G-control procedures. In the motion sickness procedure, the DeltaT(forearm-fingertip) response was significantly attenuated, indicating a blunted vasoconstrictor response, and rectal temperature decreased at a faster rate. No other differences were observed. 5. Motion sickness attenuates the vasoconstrictor response to skin and core cooling, thereby enhancing heat loss and the magnitude of the fall in deep body temperature. Motion sickness may predispose individuals to hypothermia, and have significant implications for survival time in maritime accidents.
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