Morphology and response properties of single olivocochlear fibers in the guinea pig

Cochlear efferent and afferent units were recorded from the spiral ganglion in anesthetized guinea pigs. Efferent units were identified by their regular discharge patterns and their long response latencies to tone bursts. In some cases these physiological criteria were confirmed by anatomical tracing of single axons labeled with horseradish peroxidase. Labeled efferent axons traveled in the vestibular nerve root and the intraganglionic spiral bundle, and crossed the tunnel to innervate up to 61 outer hair cells over longitudinal distances of 0.15 to 2.1 mm. Efferent units were subdivided into classes by their excitatory responses to monaural tone bursts. Fifty-seven percent responded only to ipsilateral-ear sound, 28% only to contralateral-ear sound, and 15% to sound in either ear. Tuning curves from efferent units were generally similar in shape to afferent units, often having equally low thresholds and equally high Q10s. Efferent Q10s were somewhat lower from preparations anesthetized with a Urethane/Innovar combination than from preparations anesthetized with a Nembutal/Innovar combination. Efferent units with spontaneous activity were uncommon at the start of the recording sessions but were more frequently encountered later in the experiments. Spontaneous activity could often be suppressed by tonal stimuli, even sometimes to the non-excitatory ear of Ipsi and Contra units.