The Action of Tetrodotoxin on the Frog's Isolated Muscle Spindle

Tetrodotoxin in concentrations of 0.8–1.0 × 10 ‐7 g/ml completely blocked conducted activity of the afferent nerve of isolated frog muscle spindle without affecting the receptor potential. Development of block was characterized by an increase in threshold of the nerve, a gradual delay of the onset of the impulse response and a reduction and decomposition of the individual action potentials into smaller spikes. The spike decomposition appeared to be due to a de‐synchronization of the conducted activity in the branched afferent terminals. After block by tetrodotoxin for up to 20 min the normal response could be restored by washing the preparation with Ringer's solution. The blocking action of tetrodotoxin was delayed and reduced by increasing calcium; removal of calcium had the opposite effect. Tetrodotoxin (1000 ng/ml) had no significant effect on the receptor potential. With low sodium plus tetrodotoxin the receptor potential was reduced by about 50 %; this reduction was reversed by addition of normal sodium. It is suggested that the sodium carrying system of the generator potential differs from that underlying the regenerative activity.