The vestibular nerve of the chinchilla. II. Relation between afferent response properties and peripheral innervation patterns in the semicircular canals

Abstract

1. The relation between the response properties of semicircular canal afferents and their peripheral innervation patterns was studied by the use of intra-axonal labeling techniques. Fifty physiologically characterized units were injected with horseradish peroxidase (HRP) or Lucifer yellow CH (LY) and their processes were traced to the crista. The resting discharge, discharge regularity, and responses to both externally applied galvanic currents and sinusoidal head rotations were determined for most neurons. Terminal fields were reconstructed and, as in the preceding paper, the fibers were classified as calyx, bouton, or dimorphic units. 2. To determine if the intra-axonal sample was representative, the physiological properties of the labeled units were compared with those of a sample of extracellularly recorded units. A comparison was also made between the morphology of the intra-axonal units and those labeled by extracellular injection of HRP into the vestibular nerve Most of the discrepancies between the intra-axonal and the two extracellular samples can be explained by assuming that small-diameter fibers are underrepresented in the former sample. 3. A normalized coefficient of variation (CV*), independent of discharge rate, was used to classify units as regular, intermediate, or irregular. The CV* ranged from 0.020 to 0.60. Regular units (CV* less than or equal to 0.10) outnumbered irregular units (CV* greater than or equal to 0.20) by an approximately 3:1 ratio and had higher resting discharges. 4. Calyx units were invariably irregular. The one recovered bouton unit was regular. The discharge regularity of dimorphic units was related to their epithelial location, with those found in the periphery of the crista having a more regular discharge than those located more centrally. Dimorphic units, even those with quite similar morphology, can differ in their discharge regularity. Calyx and dimorphic units, which differ in their morphology, can both be irregular. These observations imply that discharge regularity is not determined by the branching pattern of a fiber or the number and types of hair cells it contacts. 5. The galvanic sensitivity (beta*) of an afferent, irrespective of its peripheral innervation pattern, was strongly correlated with CV*. This is consistent with the notion that discharge regularity and galvanic sensitivity are causally related, both being determined by postspike recovery mechanisms of the afferent nerve terminal.(ABSTRACT TRUNCATED AT 400 WORDS)

Efferent Actions in the Chinchilla Vestibular Labyrinth

“…As indicated by the straight line in Figure 2, calculated from the present sample, the rotational gains of noncalyx units were related to the cv* by a power law, gain = a AE cv* b with a = 0.69 ± 0.09 spikes s )1 /deg s )1 and b = 0.94 ± 0.07. The power law is statistically indistinguishable from a linear relation or from the relation previously obtained by Baird et al (1988). Calyx units in our study had a mean cv* of 0.38 ± 0.02.…”

“…Responses, averaged for a 5-s, 50-l A cathodal current, are plotted against cv* in Figure 10E. As had been found previously (Goldberg et al 1984;Baird et al 1988), galvanic sensitivity was related to cv* by a power law. The exponent, b = 0.74 ± 0.09, was nearly identical to the pooled exponent, b = 0.68 ± 0.05, for the relations between efferent response magnitude and cv* (Fig.…”

“…Nonlinear regressions were calculated in Igor Pro using the MarquardtLevenberg algorithm. The quadratic discriminant function of Figure 2 was calculated from the dye-filled units in Baird et al (1988). Based on their morphology, the units were assigned to the calyx (C) or noncalyx (NC, bouton plus dimorphic) class.…”

“…By the intra-axonal labeling of physiologically characterized afferents, Baird et al (1988) showed that calyx units innervating the semicircular canals could be recognized by their irregular discharge and low rotational gains. Using the same population of labeled canal units, we derived a quadratic disciminant function based on the normalized coefficient of variation (cv*) and the rotational gain (Fig.…”

“…Studies of the responses to externally applied galvanic currents (Goldberg et al 1984(Goldberg et al , a,1990bBaird et al 1988), as well as modeling studies (Smith and Goldberg 1986), imply that encoder sensitivity is related to discharge regularity. As shown in the present study, galvanic sensitivity and efferent response magnitude grow almost identically with cv* suggesting that both reflect variations in encoder sensitivity.…”