Time Course Of Repolarization Research Articles

Transducer Characteristics of the Muscle Spindle as Revealed by its Receptor Potential

AbstractThe transducer properties of the sensory endings of the muscle spindle have been studied in isolated spindles of the frog's toe muscle. The depolarization of the sensory endings during dynamic stretch at constant velocity is a rectilinear function of the amount of lengthening of the spindle, for stretches up to about 30 % of its resting length. This suggests that the dynamic phase of the stimulus is transmitted to the endings with little distortion. When the spindle is stretched by more than 30 % of its resting length a saturation phase of constant depolarization is formed. The development of this phase may be explained by the assumption that parallel elastic elements take up the load or that the sensory membrane saturates beyond a given amount of distortion.After release of stretch the depolarization of the endings decreases exponentially with time. The time course of repolarization is considerably slower than the return of the spindle to resting length and relatively independent of the rate of release of stretch. This suggests that the repolarization is governed by the transduction process and related to the electrical proper ties of the sensory membrane.

Electrophysiological effects of diphenylhydantoin on canine purkinje fibers.

The effects of diphenylhydantoin (DPH) were studied on isolated, perfused Purkinje fibers over a range of concentrations from 10 -8 to 10 -4 M . The time course of repolarization of the transmembrane action potential shortened due to abbreviation of all phases of repolarization. The effective refractory period also shortened during exposure to DPH, but to a lesser extent than the action potential. As a result the earliest effective test stimulus elicited action potentials with greater amplitude and dv/dt of phase 0 than under control conditions. In driven fibers with normal action potentials, DPH had little effect on the amplitude or rate of rise (dv/dt) of phase 0 of the action potential. In driven fibers which were partially depolarized, or those with low dv/dt of phase 0 despite normal resting potentials, DPH caused an increase in the rate of rise of phase 0 of the action potential. DPH caused a decrease in the firing rate of normal automatic fibers by decreasing the slope of phase 4 depolarization. In automatic fibers which showed generalized diastolic depolarization and decreased maximum diastolic potential, DPH caused an increase in the latter as well as a decrease in the slope of phase 4 depolarization.

Effect of Cardiac Cycle Length upon Magnitude of Ventricular Gradient

The ventricular gradient of Wilson may be defined as the net electrical effect of differences in time course of repolarization of different parts of the ventricular muscle as this effect is projected onto the frontal plane of the body. In the electrocardiogram it is measured by the mean manifest net area of the QRS-T group of deflections. In previous papers2,3 it has been pointed out that the magnitude of the gradient varies inversely as the heart rate or directly as the average cardiac cycle length. The present report seeks to explain this relationship. Three main procedures were used. In one, the string galvanometer recorded the action current derived from the apex and base of the excised turtle ventricle, in air. Apical negativity yielded an upward deflection. Records were obtained when the ventricle was driven at various rates, as shown by the cycle lengths in Table I. In a second set of experiments the apex was cooled, though, as the irregularity of the results shows, no attempt was made to keep the temperature constant. Table I also shows the Q-T interval for each cycle length, and the net area of QRS-T in units of about 30 microvolt-seconds. The method of measuring the areas has been published elsewhere. Since the initial deflections were small, the larger part of the net QRS-T area is within the T wave, except when the cycles were short. It may be observed that the net areas are negative, that is, below the base line of the electrogram, when the apex was at the room temperature of 24°C. This finding indicates that in this heart the repolarization of the apex was more rapid than that of the base.