Morphological identification of physiologically characterized afferents innervating the turtle posterior crista

The turtle posterior crista consists of two hemicristae. Each hemicrista ex tends from the planum semilunatum to the nonsensory torus and includes a ce ntral zone (CZ) surrounded by a peripheral zone (PZ). Type I and type II: h air cells are found in the CZ and are innervated by calyx, dimorphic and bo uton afferents. Only type II hair cells and bouton fibers are found In the PZ. Units were intraaxonally labeled in a half-head preparation. Bouton (B) units could be near the planum (BP), near the torus (BT), or in midportion s of a hemicrista, including the PZ and CZ. Discharge properties of B units vary with longitudinal position in a hemicrista but not with morphological features of their peripheral terminations. BP units are regularly discharg ing and have small gains and small phase leads re angular head velocity. BT units are irregular and have large gains and large phase leads. BM units h ave intermediate properties. Calyx (C) and dimorphic (D) units have similar discharge properties and were placed into a single calyx-bearing (CD) cate gory. While having an irregular discharge resembling BT units, CD units hav e gains and phases similar to those of BM units. Rather than any single dis charge property, it is the relation between discharge regularity and either gain or phase that makes CD units distinctive. Multivariate statistical fo rmulas were developed to infer a unit's morphological class (B or CD) and l ongitudinal position solely from its discharge properties. To verify the us e of the formulas, discharge properties were compared for units recorded in traaxonally or extracellularly in the half-head or extracellularly in intac t animals. Most B units have background rates of 10-30 spikes/s. The CD cat egory was separated into CD-high and CD-low units with background rates abo ve or below 5 spikes/s, respectively. CD-low units have lower gains and pha ses and are located nearer the planum than CD-high units. In their response dynamics over a frequency range from 0.01-3 Hz, BP units conform to an ove rdamped torsion-pendulum model. Other units show departures from the model, including high-frequency gain increases and phase leads. The longitudinal gradient in the physiology of turtle B units resembles a similar gradient i n the anamniote crista. In many respects, turtle CD units have discharge pr operties resembling those of calyx-bearing units in the mammalian central z one.