Electromechanical Coupling in Potentiation of Muscular Contraction

SUMMARY1. Unloaded shortening velocity, stiffness, and the effects of potentiators were studied to understand the basis for the shallow ascending limb (1'65-2'0jm sarcomere length) of the sarcomere length-tension diagram of frog single fibres.2. The velocity of externally unloaded shortening was found to be constant over most of this range. It is therefore unlikely that this part of the sarcomere length-tension diagram results from an internal force opposing shortening.3. Stiffness was found not to vary in proportion with tension between sarcomere lengths 1-65 and 2'0,sm, nor to be constant between 2-0 and 2'2,um, where tension is constant. By assuming a small filament compliance, the observations could be adequately modelled on the hypothesis that the variation in tension in the range of sarcomere lengths 1-65-2 0,m was caused by variations in the number of attached cross-bridges. 4. The twitch potentiators Zn2 , tetraethylammonium (TEA), nitrate and caffeine were found not to change the shape of the sarcomere length-tension diagram.Potentiation in a tetanus was less than 3 % in all experiments.5. Contractures induced by raised [K+] in the bathing solution were found to produce more tension than a tetanus beyond optimum length, insignificantly different tension near optimum length, and less tension at sarcomere lengths near 1'7,sm. An explanation is proposed for these results in terms of inhomogeneous activation and internal motion.6. It is concluded that there is no evidence from this work that a tetanized fibre is other than maximally activated over the range of sarcomere lengths spanned by the shallow ascending limb.

supporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Tension, stiffness, unloaded shortening speed and potentiation of frog muscle fibres at sarcomere lengths below optimum.

Julian¹,

Morgan²

1981

“…A direct effect of pentobarbitone on the muscle contractile mechanism seems unlikely, although a number of other chemical agents are known to potentiate twitch tension responses (see Sandow, 1964). The small unit tension outputs in the present series of observations did not appear to be due to a failure of the contractile mechanism, since the general condition of the animals was good and the gross MG twitch was checked periodically during the course of each experiment.…”

Section: Methodsmentioning

confidence: 52%

Firing patterns of gastrocnemius motor units in the decerebrate cat

Burke¹

1968

155

SUMMARY1. The patterns of medial gastrocnemius (MG) motor unit firing in response to MG muscle stretch have been studied in decerebrate cats, using intracellular recording techniques. In most of the units, the motoneurone axonal conduction velocity and cell input resistance were measured, and the maximum amplitude and wave form of the group Ia composite EPSP evoked by MG nerve stimulation were determined. The mechanical properties of the muscle unit portion of each motor unit were also studied.2. Motor unit firing patterns were classified into two groups, 'tonic' and 'phasic'. With few exceptions, tonic motor units showed sustained firing throughout MG stretches of varying duration, while most phasic units did not fire at all to the same stimulus. Tetanization of the afferents did not convert any of the previously phasic units to tonic firing.3. MG motor units in this study were divided into three groups on the basis of the muscle unit twitch properties. Units with short twitch time to peak values (< 35 msec) were subdivided into two groups according to twitch tension output: (a) type F, with twitch tension > 1*5 g, and (b) type F*, with twitch tension < 1U5 g. Units with slow twitch time to peak (> 35 msec) were classified as type S.4. The presence or absence of tonic firing during sustained MG stretch was found to be significantly related to the following factors: (a) the motor unit twitch type, in that tonic firing was observed in 100 % of type S, 70 % of type F*, and only 10 % of type F units; (b) the apparent motoneurone size, in that tonic units tended to have higher cell input resistance values and slower axonal conduction velocities than phasic units; and (c) the density and spatial organization of the group Ia synaptic input, in that the MG monosynaptic EPSPs found in tonic motor units tended to be both larger in amplitude and longer in duration than those found in phasic units.5. The intrinsic properties of MG motor units, the characteristics of the group I a synaptic input to the motoneurones and the unit firing patterns elicited by MG muscle stretch appear to be mutually interrelated, forming a pattern of motor units within the MG pool which is analogous to the pattern thought to characterize the motor units belonging to 'slow' and 'fast' muscles. In most respects this pattern of MG motor units appears to be a continuum without clearly separable subgroups.

“…Caffeine (Sandow, Taylor, Isaacson & Seguin, 1964); quinine (Benoit, Carpeni & Przybyslawski, 1964); NO-and SCN- (Hodgkin & Horowicz, 1960) are all known to lower the mechanical threshold, and all increase the stretch response to a greater or lesser extent. Imidazole (Sandow, Isaacson & Preiser, 1964), and raised extracellular K+ (Kahn & Sandow, 1952), also increase the duration of the active state plateau though their effect on the mechanical threshold is unknown.…”

Section: Resultsmentioning

confidence: 99%

On the increase in rate of heat production caused by stretch in frog's skeletal muscle

Clinch¹

1968

SUMMARY1. The increase in rate of heat production caused by stretch in the unstimulated frog's sartorius (stretch response) has been measured using a conventional thermopile technique.2. The rate of heat production was found constant between l0 (the distance in vivo between the tendons when the legs were in a straight line) and 1-2 1l, and rose rapidly above this length to reach 3-5 times the basal rate at 1-3 lo. Stretching to greater lengths appeared to damage the muscles.3. The stretch response is increased by several substances which increase the duration of the active state.4. Unlike the rate of heat production at lo, the stretch response is increased by procaine; while the presence of C02 greatly reduces it.5. Evidence is presented supporting the hypothesis that the stretch response is associated with the appearance of tension in the sarcolemma.