Potassium currents and excitability in second-order auditory and vestibular neurons

Potassium channels are involved in the control of neuronal excitability by fixing the membrane potential, shaping the action potential, and setting firing rates. Recently, attention has been focused on identifying the factors influencing excitability in second-order auditory and vestibular neurons. Located in the brainstem, second-order auditory and vestibular neurons are sites for convergence of inputs from first-order auditory or vestibular ganglionic cells with other sensory systems and also motor areas. Typically, second-order auditory neurons exhibit two distinct firing patterns in response to depolarization: tonic, with a repetitive firing of action potentials, and phasic, characterized by only one or a few action potentials. In contrast, all mature vestibular second-order neurons fire tonically on depolarization. Already, certain fundamental roles have emerged for potassium currents in these neurons. In mature auditory and vestibular neurons, I(K), the delayed rectifier, is required for the fast repolarization of action potentials. In tonically firing auditory neurons, I(A), the transient outward rectifier, defines the discharge pattern. I(DS), a delayed rectifier-like current distinguished by its low threshold of activation, is found in phasically firing auditory and some developing vestibular neurons where it limits firing to one or a few spikes, and also may contribute to forming short-duration excitatory postsynaptic potential (EPSPs). Also, I(DS) sets the threshold for action potential generation rather high, which may prevent spontaneous discharge in phasically firing cells. During development, there is a gradual acquisition and loss of some potassium conductances, suggesting developmental regulation. As there are similarities in membrane properties of second-order auditory and vestibular neurons, investigations on firing pattern and its underlying mechanisms in one system should help to uncover fundamental properties of the other.