Vestibular sympathetic nervous system in guinea pig.

The vestibular sympathetic fibers were examined in 20 guinea pigs by the immunohistochemical demonstration of tyrosine hydroxylase and dopamine B-hydroxylase. The vestibular sympathetics originated in the ipsilateral superior cervical ganglion and entered the internal auditory meatus along the labyrinthine artery. At the Schwann-glial border, some of the sympathetic fibers left the artery and went into the superior and inferior divisions of the vestibular nerve and formed a loose meshwork among the Scarpa's ganglion cells, while other fibers continued to follow the labyrinthine artery. Both groups of fibers entered the cristae ampullares and saccular and utricular maculas after several bifurcations in the cribrose areas and terminated either near the capillaries beneath the sensory epithelia or among the vestibular nerve fibers. These fibers traveled freely in the vestibular labyrinth without being restricted to following blood vessels or vestibular nerve fibers. Some sympathetic fibers made direct contact with the vestibular efferent fibers or the vestibular afferent fibers at the node of Ranvier. Sympathetic fibers were not observed in the sensory epithelia or semicircular canals, and were rarely found in the vicinity of the dark cells.