Signal processing at olivocochlear neuron

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The cochlea recieves an efferent innervation from olovocochlear neurons(OC)located in the superior olivary complex OC neurons have been anatomically classified into two groups by their origins and targets ; the lateral and medial OC (LOC and MOC,respectively) neurons. The majority of OC neurons is known to be cholinergic from histochemical investigations. Both groups of OC neurons recieves excitatory voley from afferent auditory tract. Thus, OC neurons form reflex arch to regurate peripheral auditory signal processing Because of the difficulty of their identfication in slice preparation, there were few direct electrophysiological study of OC neurons in spite of their physiological significance. We succeeded to stain living OC neurons by retrograde labeling. Stained OC neurons were identified under a fluorescence microscope and they were sujected to whole-cell recording. LOC and MOC neurons had different electrophysiological properties. When the membrane was slightly hyperpolarized (-72 to -76 mV), LOC neurons showed spike trains with along first interspike interval, whereas MOC neurons showed spike trains with a long latency to the first spike. Under voltage clamp, LOC neurons revealed two transient outward current with different inactivation kinetics. In MOC neurons, there was only one transient outward current. The different expression of these transient outward current between LOC and MOC neurons seems to be responsible for the difference in firing properties of these two groups of neurons.