Differences In Action Potential Duration Research Articles

Concealed conduction.

The existence of concealed conduction of atrial beats has been demonstrated. Concealed conduction within the atrioventricular node has been studied by recording the transmembrane potential of single nodal fibers. Conduction block in this part of the specialized conducting system depends more upon decremental conduction within certain nodal fibers than upon differences in action-potential duration. Also, the extreme delay of activation that results from such decremental conduction explains certain changes in atrioventricular delay associated with premature atrial beats as well as a prolonged refractoriness of atrioventricular transmission after concealed nodal conduction of an atrial extrasystole. In the in situ dog heart, local bipolar electrograms recorded from various parts of the bundle of His, bundle branches, and peripheral Purkinje fibers have shown that con cealed conduction may extend through the atrioventricular node and the bundle of His and that block may occur between common bundle and bundle branches or between the latter and the peripheral Purkinje fiber. Block in these locations, which results from local differences in the action-potential duration, has been compared to decremental conduction within the atrioventricular node.

Physiologic basis of disturbances of cardiac rhythm and conduction

Recent studies of the electrophysiology of cardiac muscle and in particular the fibers of the specialized tissues of the heart have provided information on several physiologic mechanisms which can be responsible for the production of disturbances of rhythm and conduction. Intrinsic rhythmicity is a property of all of the specialized fibers; in some, slow diastolic depolarization is always present, and thus these fibers are latent pacemakers which can and will generate ectopic activity at any time that the frequency of the normal pacemaker is depressed or at any time that their own intrinsic frequency is enchanced. Other fibers may show little evidence of intrinsic rhythmicity under normal conditions but, when influenced by a wide variety of factors (hypoxia, sympathetic activity, stretch, local changes in concentration of K, Ca ++ , pH and others) will develop rapid pacemaker activity. In addition to the property of intrinsic rhythmicity which is present in the specialized fibers, the differences in action potential duration and conduction velocity of these same cells enhances the likelihood of complex arrhythmias. Studies of single fibers must now be supplemented by records from the specialized conducting system of the intact heart during the clinically important arrhythmias to determine the exact manner in which each disturbance of rate and rhythm is brought about.

Innervation zone and propagation velocity in human muscle.

Summary.The innervation of the brachial biceps was found to be localised to a narrow band in the proximal third of the short head and in the distal third of the long head. Counts of terminal nerve endings showed that 67 per cent of all endings were situated within 10 per cent of the muscle length.The action potential recorded with an electrode placed within the zone of innervation had an abrupt onset, and with two or more electrodes placed in the innervation zone near fibres of the same motor unit, the initial deflection of the action potential was simultaneous within 0.5 msec. This was seen up to electrode distances of 40 mm, along the longitudinal axis of the muscle, and indicates that the end plates of the same unit are scattered over this area. It excludes furthermore that propagation over the thin terminal nerve branches contributes to the action potential duration.When care was taken to place the electrodes distal to the site of innervation and an identical threshold to electrical stimulation had ensured that the electrodes led off from the same muscle fibres, the propagation velocity of the spike of the action potential released by voluntary effort varied over a narrow range. The mean value was 4.7 m/sec. with a maximal variation of 1.3 m/sec. within the same muscle (36.57° C). The small variation in velocity was unexpected in view of the threefold variation in diameter, but could be accounted for by the influence of the external resistance which was shown to be localised mainly in the interspaces between inactive fibres in situ.The propagation velocity during voluntary effort exceeded the velocity of electrically activated fibres by 0.7 m/sec.Differences in action potential duration during voluntary effort can mainly be attributed to the spatial dispersion of the site of innervation for the different fibres of the motor unit.