Responses to efferent activation and excitatory response-intensity relations of turtle posterior-crista afferents

Multivariate statistical formulas were: used to infer the morphological typ e and longitudinal position of extracellularly recorded efferents. Efferent fibers were stimulated electrically in the nerve branch interconnecting th e anterior and posterior VIIIth nerves. Responses of bouton (B) units depen ded on their inferred position: BP units (near the planum semi-lunatum) sho wed small excitatory responses; BT units (near the torus) were inhibited; B M units (in an intermediate position) had a mixed response, including an in itial inhibition and a delayed excitation. Calyx-bearing (CD-high) units wi th an appreciable background discharge showed large per-train excitatory re sponses followed by smaller post-train responses that could outlast the sho ck train by 100 s. Excitatory responses were smaller in calyx-bearing (CD-l ow) units having Little or no background activity than in CD-high units. Ex citatory response-intensity functions, derived from the discharge during 2- s angular-velocity ramps varying in intensity, were fit by empirical functi ons that gave estimates of the maximal response (r(MAX)), a threshold veloc ity (nu(T)), and the velocity producing a half-maximal response (nu(1/2)). Linear gain is equal to r(MAX)/nu(S), nu(S) = nu(1/2) - nu(T). nu(S) provid es a measure of the velocity range over which the response is nearly linear . For B units, r(MAX) declines by as much as twofold over the 2-s ramp, whe reas for CD units, r(MAX) increases by 15% during the same time period. At the end of the ramp, r(MAX) is on average twice as high in CD as in B units . Thresholds are negligible in most spontaneously active units, including a lmost all B and CD-high units. Silent CD-low units typically have threshold s of 10-100 deg/s. BT units have very high linear gains and nu(S) < 10 deg/ s. Linear gains are considerably lower in BP units and nu(S) > 150 deg/s. C D-high units have intermediate gains and near 100 deg/s nu(S) values. CD-lo w units have low gains and nu(S) values ranging from 150 to more than 300 d eg/s. The results suggest that BT units are designed to measure the small h ead movements involved in postural control, whereas BP and CD units are mor e appropriate for monitoring large volitional head movements. The former un its are silenced by efferent activation, whereas the latter units are excit ed. This suggests that the efferent system switches the turtle posterior cr ista from a "postural" to a "volitional" mode.