Nerve-mediated excitation of the taenia of the guinea-pig caecum.

1. A study was made of the responses of the isolated taenia of the guinea-pig caecum to stimulation of the intramural nerves.2. Four types of response are described, made up of a contraction and/or a relaxation occurring during stimulation and an after-contraction occurring when stimulation is stopped.3. A delayed relaxation which sometimes occurred at the end of stimulation is also described.4. Atropine usually abolished the contractions occurring during stimulation. After-contractions either appeared or were increased in amplitude in the presence of atropine, and delayed relaxations were abolished.5. The anti-cholinesterase drug neostigmine could convert the response during stimulation from a relaxation to a contraction. After-contractions were often abolished by neostigmine and delayed relaxations appeared.6. It is concluded that the contractions which occurred during stimulation were mediated by cholinergic nerves. The after-contraction appears to be a rebound phenomenon, following the hyperpolarization of the muscle caused by stimulation of the inhibitory nerves. It is suggested that the delayed relaxation is caused by the effects of persisting inhibitory transmitter substance on cells which do not undergo rebound excitation.