Fig. 4. KA-induced excitation of neuron B19 is due to electrotonic coupling with neuron B27. (A) Electrotonic coupling between neurons B19 and B27 in the buccal ganglia of Helisoma is shown during simultaneous intracellular recordings of neurons B19 and B27. DC hyperpolarization of neuron B19 produced a hyperpolarization in the membrane potential of neuron B27. A coupling coefficient could not be quantified due to the presence of a slight bridge imbalance. (B) Application of 10 µmol l^–1 KA (arrowhead) had no apparent effect on the membrane potential of an isolated B19 neuron (top trace). In the same neuron, 1 mmol l^–1 glutamate (arrowhead) produced a pronounced hyperpolarization of the membrane potential. Horizontal calibration bar indicates 10 s in the top trace and 20 s in the bottom trace. HS, Helisoma saline.