At least three sequential steps are involved in the tetanus toxin-induced block of neuromuscular transmission.

Tetanus toxin causes a block of the neuromuscular transmission. The kinetic aspects of the block were studied in vitro on the mouse phrenic nerve-hemidiaphragm exposed to toxin (1 microgram/ml). 1. The toxin action on the nerve ending involves three sequential steps: binding, "translocation" and paralysis. 2. Diffusion and binding of tetanus toxin molecules to the presynaptic membrane is complete in about 60 min. The binding step is irreversible, independent of transmitter release and of the temperature. Tetanus antitoxin, however, inactivates the bound toxin molecules. 3. After a second step which is probably due to a "translocation" of the toxin molecules into or through the presynaptic membrane the antitoxin molecules are now ineffective to prevent the toxin-induced inhibition of transmitter release. This so called "translocation" step requires transmitter release and therefore depends strongly on the frequency of nerve stimulation. 4. The paralytic step does not depend on the transmitter release. It, however, depends strongly on temperature with a break in the Arrhenius-plot around 33 degrees C which suggests the involvement of a phase transition rather than of an enzymatic activity of the toxin.