FUNCTIONAL ARCHITECTURE OF VESTIBULAR PRIMARY AFFERENTS FROM THE POSTERIOR SEMICIRCULAR CANAL OF A TURTLE, PSEUDEMYS (TRACHEMYS) SCRIPTA ELEGANS

Physiological studies in many vertebrates indicate that vestibular pri mary afferents are not a homogeneous population. Such data raise the q uestion of what structural mechanisms underlie these physiological dif ferences and what functional role is played by, afferents of each type . We have begun to answer these questions by characterizing the archit ecture of 110 afferents innervating the posterior canal of Pseudemys s cripta. We emphasize their spatial organization because experimental e vidence suggests that afferent physiological properties exhibit signif icant spatial heterogeneity. The sensory surface of the posterior cana l comprises paired, triangular hemicristae, which are innervated by tw o afferent types. Bouton afferents (66% of total afferents) are found over the entire sensory surface. They differ significantly in. the sha pe and size of their collecting areas, number of boutons, soma size, a nd axon diameter; this morphological variation is systematically relat ed to the afferent's spatial position. In addition, multivariate analy ses suggest that bouton afferents may comprise two;subtypes: a afferen ts have delicate processes and are found throughout the crista; beta a fferents are more robust and are concentrated preferentially toward th e canal center. Calyx-bearing afferents comprise two morphological sub types: dimorphs (13% of total afferents) bear calyceal and bouton endi ngs; calyceal afferents (21%) bear calyceal endings only. Both types o ccur exclusively in an elliptical region near the center of each hemic rista; their morphology varies with radial distance from the center of this elliptical region. Our data provide evidence that in Pseudemys: (1) the classical vestibular afferent types (bouton, calyx, dimorph) a re structurally heterogeneous, and (2) their spatial sampling characte ristics are highly structured and distinctive for each type. These spa tial patterns may shed light on regional differences in physiological profiles of vestibular afferents, and they raise questions about the r ole of this spatial heterogeneity in signaling head movement. (C) 1994 Wiley-Liss, Inc.