Motor innervation of the eustachian tube muscles in the guinea pig

The brainstem location of motoneurons innervating eustachian tube-associated muscles in the adult guinea pig was determined using intramuscular injections of the neural tracer horseradish peroxidase (HRP). Following HRP injections into the tensor veli palatini and the eustachian tube belly of the medial pterygoid muscle, an ipsilateral column of HRP-labeled motoneurons was present medial to the dorsolateral division of the trigeminal motor nucleus. Following HRP injection into the levator veli palatini, labeled motoneurons were present in the ipsilateral dorsal division of nucleus ambiguus. The locations of the tensor veli palatini and levator veli palatini motoneurons are similar to those found in studies of other animals. A distinct eustachian tube belly of the medial pterygoid muscle was also identified. This sub-belly had a motoneuron pool distinct from the main medial pterygoid muscle group. The authors have provided the gross anatomical and neuroanatomical substrates upon which future studies of eustachian tube function in the guinea pig may be based.