Neurons in the cochlear nuclei controlling the tensor tympani muscle in the rat: a study using pseudorabies virus

The middle ear muscle reflex has been implicated in modulation of auditory input and protection of the inner ear from acoustic trauma. However, the identification of neurons in the cochlear nuclei participating in this reflex has not been fully elucidated. In the present study, we injected the retrograde transynaptic tracer pseudorabies virus into single tensor tympani (TT) muscles, and identified transynaptically labeled cochlear nucleus neurons at multiple survival times. Motoneurons controlling TT were located ventral to the ipsilateral motor trigeminal nucleus and extended rostrally towards the medial aspect of the lateral lemniscus. Transynaptically labeled neurons were observed bilaterally in the dorsal and dorso-medial parts of ventral cochlear nuclei as early as 48 h after virus injection, and had morphological features of radiate multipolar cells. After >or=69 h, labeled cells of different types were observed in all cochlear nuclei. At those times, labeling was also detected bilaterally in the medial nucleus of the trapezoid body and periolivary cell groups in the superior olivary complex. Based on the temporal course of viral replication, our data strongly suggest the presence of a direct projection of neurons from the ventral cochlear nuclei bilaterally to the TT motoneuron pool in rats. The influence of neurons in the cochlear nuclei upon TT activity through direct and indirect pathways may account for multifunctional roles of this muscle in auditory functions.