The origin of the anticholinesterase-induced repetitive activity of the phrenic nerve-diaphragm preparation of the rat in vitro.

1. Action potentials have been recorded in contracting muscle cells of the phrenic nerve-diaphragm preparation from rats. After the organophosphorous anticholinesterase, ecothiopate, some cells fired repetitive action potentials. In 0.1 mM [Mg2+]o the repetitive activity was generated presynaptically or postsynaptically, and in 1 mM [Mg2+]o, probably only postsynaptically. 2. The repetitive action potentials in muscle were generated ectopically about 0.2 mm away from the usual site. 3. In 1 mM [Mg2+]o, spontaneous action potentials in muscle were generated presynaptically. These were often followed by repetitive action potentials generated either presynaptically or postsynaptically. 4. The initiation of centripetal action potentials in the phrenic nerve was coincident with the repetitive firing in the muscle. 5. The results are discussed in relation to the manner of generation of repetitive activity in nerve and muscle after anticholinesterases.