Repetitive actions

Twitch is a muscle contraction caused by a single action potential, whereas tetanus is a sustained muscle contraction caused by a series of repetitive action potentials. The amplitude of tetanus contraction is larger than that of twitch, due to mechanical summation. 

Figure 4.5 Influence of oxidant stress on action potentials recorded In an isolated rabbit ventricular myocyte, (a) Control action potential, (b) Action potential recorded 3 min after exposure to oxidant stress induced by the photoactivation of rose bengal (50 nu). (c) Spontaneous and repetitive action potential discharges induced 6.5 min after exposure to rose bengal. Action potentials were recorded via a 2.5 MQ suction electrode and a current-clamp amplifier. The cell was stimulated at 0.1 Hz with a 2 ms suprathreshold current pulse and, when the cell showed automaticity (after 6 min), stimulation was stopped. Redrawn from Matsuura and Shattock (1991b).

In the absence of drug, a series of high-frequency repetitive action potentials filled the entire duration of the current pulse. Phenytoin, carbamazepine, and sodium valproate all markedly reduced the number of action potentials elicited by the current pulses. (Modified and reproduced, with permission, from Macdonald RL, Meldrum BS Principles of antiepileptic drug action. In Levy RH, et al [editors]. Antiepileptic Drugs, 4th ed. Raven Press, 1995.)... 

The mechanism of phenytoin s action probably involves a combination of actions at several levels. At therapeutic concentrations, the major action of phenytoin is to block sodium channels and inhibit the generation of repetitive action potentials. 

Tibbs, G. R., Barrie, A. P., Van-Mieghen, F., McMahon, H. T., and Nicholls, D. G. (1989). Repetitive action potentials in isolated nerve terminals in the presence of 4-aminopyridine Effects on cytosolic free Ca2+ and glutamate release. J. Neurochem. 53, 1693-1699. 

Actions Carbamazepine [kar ba MAZ a peen] reduces the propagation of abnormal impulses in the brain by blocking sodium channels, thereby inhibiting the generation of repetitive action potentials in the epileptic focus. 

Movement will be limited when the limb muscles are affected, making any repetitive action, for example in lifting, walking, running and climbing stairs, difficult to sustain. Vision will also be affected. 

Figure 7.8 The effect of DDT on action potential in single nerve fibers. (A) Control. (B) Action potential with negative after-potential, 65 min after application of 5 x 10 t5 M DDT. (C, D) Repetitive action potentials (repetitive discharges) 85 min and 135 min after application of DDT, respectively. (From van den Bercken, European J. Pharmacol20, 205,1972. With permission.)...

Phenytoin is a hydantoin derivative like dantrolene and the oldest non-sedative anticonvulsant drug known. It alters sodium, potassium and calcium conductance across cell membranes thereby altering membrane potentials and amino acid and neurotransmitter concentrations (i.e. norepinephrine (noradrenaline), acetylcholine and GABA). Its major mode of action appears to be the blockade of sodium channels and e inhibition of the generation of repetitive action potentials (membrane stabilization) (see Chs 9 and 12). 

Netzer R, Bigalke H 1990 Memantine reduces repetitive action potential firing in spinal cord nerve cell cultures. Eur J Pharmacol 186 149—155... 

Compulsion repetitive action shields person from thoughts, action fixes bad thought... 

C. Mechanisms of Action The general effect of antiseizure drugs is to suppress repetitive action potentials in epileptic foci in the brain. Different mechanisms are involved in achieving this effect. In the case of some drugs, multiple mechanisms may contribute to their antiseizure activities. Some of the recognized mechanisms are described below. 

The pyrethrins and pyrethrolds (permethrin) are nerve membrane sodium channel toxins that do not affect potassium channels. The compounds bind to specific sodium channel proteins and slow the rate of Inactivation of the sodium current elicited by membrane depolarization and, as a result, prolong the open time of the sodium channel. At low ooncentratlons, the pyrethrolds produce repetitive action potentials and neuron firing at high concentrations, the nerve membrane is depolarized completely and excitation blocked. 

Catecholamines are necessary for the repetitive action potentials generated by heart pacemaker cells. Binding of catecholamines in the cell could be the mechanism by which the action potential is initiated. 

Repetitive actions-. Anytime someone is asked to repeat short duration tasks repetitively, it is an opportunity for improvement. 

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