Changes in the activity level of cell SIN1 during a spontaneous swim e pisode. A. The membrane potential of cell SIN1 hyperpolarizes and its spikin\ activity ceases approximately 1.5 sec before the onset of a spontaneous swim e pisode. Spiking activity in cell SIN1 remains depressed throughout the durati\ n of the swim episode. B. Expanded trace of A. showing that the hyperpolariz\ tion produced in cell SIN1 before the initiation of swimming is caused by the i nput of inhibitory synaptic potentials to cell SIN1.