DIFFERENTIAL CENTRAL PROJECTIONS OF PHYSIOLOGICALLY CHARACTERIZED HORIZONTAL SEMICIRCULAR CANAL VESTIBULAR NERVE AFFERENTS IN THE TOADFISH,OPSANUS-TAU

Anatomical and neurophysiological studies were undertaken to examine t he central projection pattern of physiologically characterized horizon tal semicircular canal vestibular nerve afferents in the toadfish, Ops anus tate. The variations in individual response characteristics of ve stibular nerve afferents to rotational stimulus provided a means of ty ping the afferents into descriptive classes; the afferents fell into a broad continuum across the spectrum from low-gain, velocity-sensitive to high-gain, acceleration-sensitive responses (Boyle and Highstein [ 1990b] J. Neurosci. 10:1557-1569; Boyle and Highstein [1990a] J. Neuro sci. 10:1570-1582). In the present study, each efferent was typed as a low-gain, high-gain, or acceleration fiber during rotational or mecha nical stimulation (Rabbitt et al. [1995] J. Neurophysiol. 73:2237-2260 ) and was then intracellularly injected with biocytin. The axons were reconstructed, and the morphology, synaptic boutons, and projection pa ttern of each axon were determined. The results indicated that the thr ee descriptive classes of vestibular nerve afferents have unique as we ll as overlapping central projection patterns and destinations in the vestibular nuclei, with intranuclear parcellation in the anterior octa vus, magnocellularis, tangentialis, posterior octavus, and descending octavus nuclei. In general, increased sensitivity and faster response dynamics were correlated with both a more extensive central projection and a progressive increase in morphological complexity. Low-gain, vel ocity-sensitive fibers were the simplest morphologically, with the few est number of branches (n = 17) and shortest length (4,282 mu m), and projections were confined to the middle portions of the vestibular nuc lei. High-gain, velocity-sensitive fibers were morphologically more di verse than low-gain fibers, with a greater number of branches (n = 26) , longer length (6,059 mu m), 29% greater volume, and a more widesprea d projection pattern with projections to both the anterior and the mid dle portions of the vestibular nuclei. Acceleration fibers were morpho logically distinct from low- and high-gain fibers, with more elaborate branching (n = 41), greatest overall length (17,370 mu m) and volume (16% greater than high gains), and displayed the most extensive centra l projection pattern, innervating all vestibular nuclei except tangent ialis. Thus, there are anatomically demonstrable differential central projections of canal afferents with different response dynamics within the vestibular complex of the fish. (C) 1997 Wiley-Liss, Inc.