Tuning of single fibers in the cochlear nerve of the alligator lizard: relation to receptor morphology.

(1) The general anatomy of the peripheral portion of the cochlear nerve in the alligator lizard is described. (2) Spike discharges of single units were recorded with micropipets placed in the peripheral portion of the cochlear nerve of anesthetized lizards. (3) In response to tone bursts, each unit is maximally sensitive to a charactertistic frequency (CF). There are two distinct populations of units having different CFs: a low CF population (CF in the range 0.2-0.8 kHz) recorded in the portion of the nerve that enters the apical region of the basilar papilla and a high CF population (CF in the range 0.9-4.0 kHz) recorded in the portion of the nerve that enters the basal region. The low CF units are more sharply tuned than the high CF units. (4) Comparison of cochlear nerve units of the alligator lizard with those of mammals shows that the tuning of low CF units resembles that of mammalian units of the same CF. The tuning of high CF lizard units differs significantly from mammalian units. (5) The distinct differences in tuning of low and high CF units are correlated with distinct differences in the structure of the basilar papilla in the apical and basal regions rather than with differences in the width of the basilar membrane.