B21 is a sensory neuron in the buccal ganglion of Aplysia californica.
B21 excites B8 motor neuron in the lateral branch. Central depolarization of B21 determines whether the peripherally generated spikes will propagate to the lateral branch, allowing B21 to excite B8.
B4/5 inhibits B21, preventing B21 from exciting B8. The magnitude of the inhibition is spike frequency-dependent.