Abstract
Lubinska 1 has described a type of peripheral summation or facilitation in cats. Following the administration of sufficient MgSO4 intraperitoneally, the muscular response to single stimuli applied to a motor nerve is lost. Repetitive stimuli, however, evoke a tetanus, and temporarily thereafter single stimuli are again effective.
It seems to us that such effects might be due to local accumulation of some decurarizing chemical agent released at the nerve endings. Acetyl choline 2 and potassium 3 are temporarily effective antagonists to curare, and it has been inferred that both substances are released at the endings of cholinergic nerves. 4 5
We have used cats and dogs, anesthetized with nembutal. Contractions were recorded from the tibialis anticus muscle, the lever allowing 1.1 mm. of shortening per kilogram of tension. The sciatic trunk was sectioned and shielded electrodes placed on the peroneal branch. Single test stimuli, from a neon-tube circuit, were applied at intervals of 4 to 6 seconds. For tetanizing we used a photocell stimulator with a rotating disc interrupter. Both circuits were adjusted so that the shocks were barely strong enough, to give a maximum normal twitch. MgSO4 (7.7% solution of the heptahydrated salt) was administered intravenously in small repeated doses.
When response to single shocks had failed, it was always restored following a period of tetanic stimulation of the nerve. The higher the frequency of tetanic stimuli (up to 120 per second) the greater the degree of recovery after a short tetanus. The “physiological’ frequency of 27 per second, used by Lubinska, is therefore not an optimum for this effect. The duration of the recovery varies both with the frequency and with the duration of the preceding tetanus.
Get full access to this article
View all access options for this article.