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The speed of the myofilament lattice spacing response to rapid changes in load or length of single, intact muscle fibres of the frog, was investigated during isometric tetani. During ramp releases at close to Vmaxand during step length changes (completed within 250 μs), lattice spacing was calculated from the equatorial X-ray diffraction pattern (sampled at 250 μs time resolution using synchrotron radiation). Ramp releases (total shortening = 1.39 %) caused a spacing increase, described with an exponential function (α = 271 s−1, amplitude = 1.15 nm) plus an elastic component having the time course of discharge of axial tension (amplitude 0.28 nm). For a step release (amplitude = 0.87 %), lattice expansion could be described with an exponential (α = 1005 s−1, amplitude = 0.56 nm) plus an elastic component of 0.25 nm amplitude. Lattice compression was associated with a step stretch (amplitude = 0.62 %), and was also quasi-exponential (α = 367 s−1, amplitude = 0.74 nm), with an elastic component of 0.28 nm. The spacing change time course for length steps resembled that of the accompanying quick recovery of axial tension and the associated change in the meridional 14.5 nm reflection intensity, which are both believed to be determined by the kinetics of the molecular power stroke. Therefore, this shows that lattice spacing changes, arising from radial forces exerted by attached crossbridges, are fast enough to occur during the power stroke event.

1. The force‐velocity (P‐V) relation for normal or NO‐3 treated single fibres isolated from the semitendinosus muscle of the frog was determined at given times during the rise of tension and the plateau of isometric tetani. Experiments were made at about 2.25 micron sarcomere length and at constant temperatures, from 3 to 4.5 degrees C and from 19 to 21 degrees C. The controlled‐velocity release method was used. 2. During the rise of tension, at any initial tension higher than about 0.2 P0, the lowest release velocity required to drop the tension to zero was the same as at the tetanic plateau, independent of the temperature and the presence of NO‐3 ions in the bathing solution. 3. The degree of activation (measured by the steady force exerted at a given velocity of shortening lower than V0) increased with time, but attained its steady‐state level before isometric tension. 4. At about 20 degrees C, frog muscle fibres at about 2.2 micron sarcomere length were only partially activated after a single stimulus. 5. NO‐3 ions did not affect the steady‐state P‐V relation. At about 20 degrees C, NO‐3 ions increased the rate of development of activation. Potentiation of the twitch contraction was due at least in part to this mechanism. 6. The 'relative' P‐V relation appears to be independent of both the time after start of stimulation and the presence of NO‐3 ions in the bathing solution. 7. The results are discussed in terms of the sliding filament model of Huxley (1957), assuming that either the number of actin sites available for cross‐bridge formation, or the value of the rate constant for making of cross‐bridges, is time dependent.

1. The force‐velocity (P‐V) relation from a single fibres isolated from the semitendinosus muscle of the frog was determined at pre‐set times during the rise of tension and the plateau of isometric tetani. The controlled‐velocity release method was used. Experiments were performed at about 2.25 micrometers sarcomere length and at 3‐4 degrees C or at 19‐21 degrees C. 2. Replacing H2O with D2O resulted in a rapid large reduction of the peak twitch tension and of the speed of development of twitch and tetanic tensions. The tetanic tension (P0) was usually reduced, in certain fibres to as low as 5% of the value in H2O‐Ringer solution. 3. The depression of twitch and tetanus characteristics was followed by a recovery, the duration of which varied greatly in different fibres. During the recovery period previous conditioning activity potentiated the tetanus characteristics. 4. After the end of the recovery period in D2O‐Ringer solution both the peak twitch tension and the speed of development of tetanic tension was still greatly depressed, whereas the value of P0 was slightly greater than in H2O‐Ringer. The speed of rise of isometric tension after a quick release imposed at the tetanus plateau was reduced in D2O‐Ringer, usually to about 50% of the value in H2O‐Ringer. 5. D2O increased the development time of the P‐V relation and produced a conspicuous increase in the degree of its curvature. The value of V0 (the velocity of shortening at zero load) was not significantly depressed by D2O and it was the same independent both of the time after the beginning of stimulation and of the isometric tension at which the measurement was made. The P‐V relation attained its final characteristics before the isometric tension reached the plateau. During the recovery period in D2O‐Ringer, at the plateau of isometric tetani of different size, the relative force exerted at a given velocity of shortening was constant. 6. In D2O‐treated fibres, NO3‐ and caffeine (i) potentiated the peak twitch tension and the speed of development of tetanic tension without affecting significantly the speed of the redevelopment of tension after a quick release imposed at the tetanus plateau and (ii) reduced the development time of the P‐V relation, but did not affect either the degree of its curvature or the value of V0 and P0. 7. The results are discussed by assuming that the release of Ca2+ from the sarcoplasmic reticulum is a rate‐limiting process for the development of activation and in turn for the development of isometric tension. In terms of the cross‐bridge model of Huxley (1957), the time or Ca2+‐dependent factor of activation appears to be the recruitment of actin sites for cross‐bridge formation, whereas the value of the rate constants regulating the cross‐bridge kinetics appears to be time and Ca2+‐independent.

1. The relation between sarcomere length and tetanic tension was determined at 10‐12 degrees C for 70‐80 microns long segments of single fibres isolated from the tibialis anterior and semitendinosus muscles of the frog. Measurements of segment striation spacings were performed during fixed‐end or length‐clamp contractions by means of a laser light diffractometer. 2. At sarcomere lengths of around 2.10 microns tetanic tension rose promptly to a steady plateau, independent of the recording conditions. At greater sarcomere lengths under fixed‐end conditions the tension rise occurred in two distinct stages: an initial rapid rise followed by a much slower creep. The tension creep was entirely abolished in length‐clamp contractions. 3. The sarcomere length‐tension diagram of length‐clamped segments of tibialis anterior fibres exhibited a definite flat region between about 1.96 and 2.16 microns where tension varied by less than 1.5%. The highly linear descending limb reached zero tension at about 3.53 microns. The shift to the left by about 0.10 microns, with respect to the length‐tension diagram of length‐clamped segments of semitendinosus fibres, may be tentatively explained by assuming that thin filament lengths vary in different muscles. 4. The results are in agreement with those of a previous work by Gordon, Huxley & Julian (1966) and support the hypothesis (Huxley, 1957, 1980) that muscle tension is produced by simultaneous action of independent force generators, in proportion to the number of myosin bridges overlapped by actin filaments.

The internal friction and ΔEeffect of NiZn and MnZn ferrites are measured along the magnetization curve starting from states either thermally or a.c. demagnetized. The samples demagnetized by an alternating field show a higher internal friction and a lower ΔEeffect than those thermally demagnetized. The results are explained by supposing that the two different demagnetization methods change the number of Bloch walls and their interactions with internal stresses in the samples. Sono stati misurati l’attrito interno e l’effetto ΔEin ferriti NiZn e MnZn lungo la curva di prima magnetizzazione partendo da stati di smagnetizzazione ottenuti in due differenti modi. Nel primo modo, il campione era smagnetizzato mediante riscaldamento fino ad una temperatura superiore al punto di Curie e quindi raffreddato lentamente fino alla temperatura ambiente; nel secondo modo, la smagnetizzazione era ottenuta sottoponendo il campione ad un campo magnetico alternato di ampiezza decrescente progressivamente fino a zero. L’attrito interno risulta minore dopo il primo trattamento che non dopo il secondo, mentre l’effetto ΔEsi comporta, generalmente, in modo opposto. I risultati sono stati interpretati in base al diverso numero totale di pareti di Bloch presenti nel campione ed alla diversa percentuale di pareti che interagiscono con gli sforzi locali e che si ottengono con i due diversi metodi di smagnetizzazione. Измеряются вутреннее трение и ΔEэффект для ферритов NiZn и MnZn вдоль кривой намагничивания. Образцы, размагниченные с помощью переменного поля, обнаруживают большее внутреннее трение и меньший ΔEэффект, чем образцы, размагниченные термически. Полученные рерультаты обьясняются, предполагая, что два различных метода размагничивания изменяют число блоховских стенок и их взаимодействия с внутренними напряжениями в образцах.