Archivio di fisiologia最新文献

The relation between sarcomere length, tension and time course of tension development in twitch and tetanic contractions at 20 degrees C was determined for isolated fibres from the semitendinosus muscle of the frog (Rana esculenta). In twenty fibres at about 2.15 micron sarcomere length, the peak twitch tension, the maximum tetanic tension and the twitch/tetanus ratio ranged, respectively, from 0.22 to 1.6 kg/cm2, from 2.13 o 3.96 kg/cm2 an from 0.07 to 0.53. The peak twitch tension was found to be: i) directly correlated with the twitch/tetanus ratio and the time to the peak of the first derivative of the twitch tension, ii) inversely correlated with the time to the peak of the first derivative of tetanic tension. No significant correlation was found between the maximal tetanic tension and the peak twitch tension or the twitch/tetanus ratio. Peak twitch tension and twitch/tetanus ratio were not correlated with the fibre cross-sectional area which ranged from 1.052 to 6,283 micron2. Sarcomere length-tension curves for twitch and tetanic isometric contractions at 20 degrees C were determined in twelve fibres. Increases in sarcomere length from about 2.15 to 2.85 micron produced, depending on the peak twitch tension or the twitch/tetanus ratio at about 2.15 micron, either decrease and no change or increase in peak twitch tension, but constantly enhanced the twitch/tetanus ratio and the degree of this potentiation was inversely correlated with the twitch/tetanus ratio at 2.15 micron. Increase in sarcomere length above 2.15 micron did not alter the course of the early development of twitch and tetanic tensions, reduced considerably the variation in peak twitch tension and twitch/tetanus ratio, without altering that of tetanic tension and swamped the correlation between the peak twitch tension and the time to peak of the differentiated twitch tension. However, the peak twitch tension at about 2.85 micron resulted to be directly correlated with the peak twitch tension at about 2.15 micron and in addition the relative length-dependent change in the time of the peak of the first derivative of the twitch tension resulted to be directly correlated with the relative length-dependent change in the peak twitch tension. It is concluded that both the duration of the active state and the rate factors of activation contribute to the determining of the large variation in peak twitch tension at about 2.15 micron, whereas the length-dependent increase in twitch/tetanus ratio appears to be mainly determined by prolongation of the active state duration.

The arrangement of muscle spindles in m. ext. long. dig. IV has been examined by microdissection. It is confirmed that spindle systems generally appear to consist of individual receptors. Stimulation effects of fast motor fibres (conduction velocities greater than 12 m/sec) on the spindles of the same muscle were studied. Receptors were isolated with their nerves and the appropriate spinal roots, the latter ones were used for stimulating efferent fibres and recording sensory discharges. Single shocks to the ventral root filaments caused afferent responses ranging from a single action potential to a train of impulses. During repetitive stimulation (train of stimuli at frequency of 10 to 150/sec) a marked increase in afferent activity was found. Afferent activity could be driven by the frequency of stimuli ("driving") and the stimulus/action potentials ratio varied from 1:1 to 1:3 or more. The rate of sensory discharge depended on the frequency of stimuli: the maximum effect, was attained at 30 to 50 stimuli/sec and, in the most responsive receptors, up to 80 stimuli/sec. Slight increases of the initial lengths of the receptors caused facilitation of sensory responses to motor stimulation. Moreover, impairing effects, which appear during sustained or high-frequency stimulation, possibly related to fatigue in intrafusal neuromuscular transmission, could be relieved by increasing the initial length. The repetitive stimulation of fast fusimotor fibres increased both dynamic and static responses and also raised the afferent activity after a period of stretching, when usually a depression occurs; these effects varied according to the preparation, its initial tension and the frequency of stimulation. The main feature of the examined motor fibres, when stimulated, is the constant excitatory action on muscle spindle static response. Results are discussed. It is suggested that the different characteristics of intrafusal muscle fibres, the receptor initial tension and the frequency of motor units discharges, may together affect muscle spindles static or dynamic performance.

According to recent developments the atrial pacemaker area and the right atrium show a peculiar morpho-functional organization, i.e.: 1) The pacemaker area is formed of clusters of cells containing relatively few myofibrils and showing embryologic characteristics. Such cells are known as nodal cells and between these and the atrial muscles are in general situated transitional cells. Each cluster is separated from the other by collagenous boundaries. The resistance of the membranes to the current flow seems to be relatively low between the cells of the same cluster but the collagenous boundaries are, according to TRAUTWEIN e UCHIZONO (1963), very poor conductors. The pacemaker activity seems to originate inside the various clusters. 2) The functional relationships between the sinoatrial node and the atrioventricular node as well as the interatrial relationship would take place through preferential pathways. These pathways corresponding approximately to the tracts described by JAMES (1966) (anterior, posterior and middle internodal tracts) and to the interatrial or Bachmann bundle, seems to show a higher velocity conduction. In general the fibres of which the tracts are composed are neither morphologically nor functionally isolated from the atrial muscle. The functional consequences of the above mentioned nodal and atrial organization seems to be: a) The possible conditioning of the pacemaker functions by the various clusters activity i.e. the dominance of one cluster over another. b) The shifting of the pacemaker activity from one cluster to anothr due to the arrival of nervous stimuli or chemical substances, etc. According to some Authors as a consequence of the shift the pacemaker area can sometimes move out side the nodal tissue and settle inside an area belonging to the internodal pathways. c) Another consequence of the shift can be the different involvement of the conducting pathways which can lead to a change in the dynamics of the atrial invasion by the excitement.

The mode of action of acetylcholine (ACh) and succinylcholine (SCh) on the isolated frog's muscle spindle has been studied. Receptor afferent nervous supply was maintained; the appropriate spinal roots were dissected for stimulating motor axons and recording from sensory fibres. Excitatory effects on the afferent activity, when the receptor was held still and during stretching, were found with ACh or SCh concentrations of 10(-8) to 10(-3); 10(-6) g/ml being usually effective. These effects are similar to those obtained by stimulating fusimotor nerve fibres. The contractile activity of intrafusal muscle fibres which occurred during these effects was observed. Seldom, and only for high concentrations of ACh and SCh, a decrease in afferent activity following the excitatory effects was found. Tubocurarine chloride (10(-5)-10(04) g/ml) in the bath prevented both motor fibres and drugs effects. Sometimes slight transient excitation occurred at very high concentrations of the two tested substances; however, this effect was prevented by stronger curarization. The observed blocking effects were always reversed by removing tubocurarine from the bath. No more excitatory effects by motor fibres stimulation and by ACh and SCh action could be found after destruction of intrafusal muscle fibres, by pinching them as close as possible to the ends of the spindle. It is suggested that ACh and SCh act indirectly by causing mechanical changes in intrafusal muscle fibres, and that a direct action on sensory nerve endings, if any, cannot, by itself, increase the afferent activity of the receptor.

During a respiratory effort against a closed airway the afferent activity of vagal fibres from pulmonary stretch receptors does not appreciably increase during the inspiratory phase because the lung is prevented from expanding. The possibility to perform occlusions at different levels of the airways allows the localization of pulmonary stretch receptors in the tracheo-bronchial tree. 100 fibres from pulmonary stretch receptors of the left and right sides of the tracheo-bronchial tree have been studied in 3 cats and their localization found as follows: 10% in the higher half of the intrathoracic trachea, 22% in the lower half of the intrathoracic trachea and the carina, 7% in the main bronchus and 61% in the intrapulmonary airways. Knowing the surface area of the tracheo-bronchial tree at different levels and assuming total of 1200 stretch receptors from each side their average concentration resulted as follows: 50.0 receptors/cm2 in the higher half of the intrathoracic trachea, 108.0/cm2 in the lower half of the intrathoracic trachea and the carina, 213.0/cm2 in the main bronchus and 1.3/cm2 in the intrapulmonary airways.