Neurodynamic response analysis of anterior semicircular canal afferents in the pigeon

1. The neurodynamic responses to sinusoidal and pulse angular accelerations were studied in anterior semicircular canal afferents in the barbiturate-anesthetized pigeon. 2. The resting discharge frequency, aS, varied from 7.4 to 149.0 impulses/s. For most units, aS remained fairly constant for long periods of time during the experiment. 3. The neural-response harmonic distortion, resulting from stimulation by sinusoidal angular accelerations, varied in different units. Percent distortions from as low as 3% to those as high as 57% were determined. 4. Intensity-function plots of peak first harmonic neural response as a function of the peak sinusoidal angular acceleration (with frequency, f, as a parameter) are of two types: one has a linear relationship between the variables; the other demonstrates pronounced nonlinearities ("saturation," particularly for low values of f). In saturation-type units, the data of which fit a power law function, the exponent of the function is frequency dependent, becoming closer and closer to unity with increasing f. 5. Data for all units fit the transfer function, G'(s) = Csk/(tauLS + 1), where G'(s) relates the unit response to angular acceleration, C is a gain constant, 0 < k < 1, and tauL is the so-called long time constant of the classical torsion pendulum model. tauL varied from 4.45 to 22.17 S (mean +/- SE = 10.24 +/- 1.20 S). This may be interpreted as an indication of a regional distribution of tauL'S within the neuroepithelium. Arguments are advanced to show that this is consistent with our present understanding of the ampullary end organ. 6. The degree of regularity of the spontaneous discharge (as determined by the coefficient of variation, CV) was significantly correlated with the parameter k in G'(S). The larger the CV, the larger is the corresponding k. Further work indicated that the larger the value of k, the more adaptation a unit exhibited (k varied from 0.017 to 0.66). 7. The time-domain response of G(S) = G'(S)/(tauSS + 1) to different durations of pulse angular acceleration stimuli agreed well with the neural response to these stimuli (tauS = 2.27 ms is the short time constant of the torsion pendulum model). 8. The term Sk was decomposed into an expression containing a series of polynomials in S in the numerator and denominator. The first term in this expansion K0tau1S/(tau1S + 1), has previously been shown to describe so-called adaptation properties in the dynamics of the semicircular canals. A mean (+/-SE) tau1 = 71.56 (+/-10.01) S was determined. Evidence is presented that Sk probably represents a relaxation phenomenon comprised of a time-varying intracellular Na+/K+-transport process, components of which are summed with the generator potential in the afferent terminal(S) of the receptor hair cell.