Organization of the Auditory Brainstem in a Lizard, Gekko gecko. II. Afferent and Efferent Projections of Nuclei of the Lateral Lemniscus and the Torus Semicircularis.

In reptiles, two taxa, lepidosaurs and archosaurs, have ears with thin tympanic membranes that permit sensitive hearing of high-frequency sounds in air. The organization of their central auditory systems may reflect this increased sensitivity to sound. To understand auditory processing in lepidosaurs further, we used tract tracing techniques to examine the organization and connections of the lemniscal and midbrain auditory nuclei in the Tokay gecko. The nuclei of the lateral lemniscus (LLD) consist of anterior (LLDa), medial (LLDm), and posterior (LLDp) subdivisions and receive ascending projections from the contralateral cochlear nuclei (nucleus angularis and nucleus laminaris) and the ipsilateral dorsal superior olive (SOd). LLD projects to the ventral division of the ipsilateral torus semicircularis (TS). In the Tokay gecko, the central nucleus of the torus (TSC) is large, with two major subdivisions, ventral and dorsal. Caudally, the TSC is fused at the midline, with the dorsal divisions adjoining their contralateral homolog. More rostrally, the TSC bifurcates to form two wings below the tectal ventricles. The ventral division of the TSC receives ascending input from NA/NL, SOd, and the lateral lemniscal complex, while both the dorsal and ventral divisions send descending projections to the lateral lemniscal complex. The TSC projects to the auditory thalamic nucleus medialis through the tractus opticus lateralis. Physiological recordings from TSCv injection sites revealed a patchy distribution of best frequencies, while recordings in TSCd were characterized by broad frequency tuning. The tract tracing results revealed conserved anatomical patterns of ascending auditory connections in lizards and archosaurs.