Multiple-site optical recording of membrane potential from a salivary gland. Interaction of synaptic and electrotonic excitation

The interaction between synaptic and electronic excitation of cells from the salivary gland of the snail Helisoma trivolvis was studied using a voltage-sensitive merocyanine dye. Linear and square photodiode matrix arrays were used to record simultaneously the response to neuronal stimulation of 15-25 separate regions of the gland. Laterally opposed acini exhibited highly synchronous electrical activity, which suggested a correspondingly high degree of electrical coupling. In the longitudinal direction, coupling appeared weaker. The onset of depolarization after neuronal stimulation was progressively delayed along the longitudinal gland axis, in agreement with the measured conduction velocity of the presynaptic nerve spike. In most instances, neuronal stimulation directly activated a regenerative gland response (action potential) at the junction between the anterior and central duct. Excitation of distal gland regions was usually mediated by electronic spread from active, more proximal gland regions. Occasionally, "collisions" between excitatory waves traveling in opposite directions were observed.