Fig. 6. Intracellular recordings from subumbrellar muscle cells approximately 2–2.5 mm from the nerve ring. (A) A double recording from a radial muscle cell (top trace) and a circular muscle cell (bottom trace). An initial spontaneous swim produced a full action potential (with initial junctional potential) in the circular muscle cell, but only a `field effect' in the radial cell. When electrical stimuli were delivered to the segment with a suction electrode, both a junctional potential (circular muscle cell) and an action potential (radial muscle cell) were triggered. The small `blip' before each event is the stimulus artifact. (B) Variability in the shape and amplitude of circular muscle action potentials. Each event is initiated by a single junctional potential. All were spontaneously generated in a reduced preparation that showed varying contractions. This recording was from a small medusa, 3 cm bell diameter, so the action potentials are of a shorter duration. (C) Complex electrical event from a circular muscle cell in a segment in which a radial response was triggered by a mechanical stimulus (light touch with a glass micropipette electrode) delivered to the velum in the region of that segment. The contraction in that segment was weaker than in the adjacent segments on each side.