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)

Muller A, Chabbert C. TRPV4 channels mediate the infrared laserevoked response in sensory neurons.

Sound-evoked compound action potential (CAP), which captures the synchronous activation of the auditory nerve fibers (ANFs), is commonly used to probe deafness in experimental and clinical settings. All ANFs are believed to contribute to CAP threshold and amplitude: low sound pressure levels activate the high-spontaneous rate (SR) fibers, and increasing levels gradually recruit medium- and then low-SR fibers. In this study, we quantitatively analyze the contribution of the ANFs to CAP 6 days after 30-min infusion of ouabain into the round window niche. Anatomic examination showed a progressive ablation of ANFs following increasing concentration of ouabain. CAP amplitude and threshold plotted against loss of ANFs revealed three ANF pools: 1) a highly ouabain-sensitive pool, which does not participate in either CAP threshold or amplitude, 2) a less sensitive pool, which only encoded CAP amplitude, and 3) a ouabain-resistant pool, required for CAP threshold and amplitude. Remarkably, distribution of the three pools was similar to the SR-based ANF distribution (low-, medium-, and high-SR fibers), suggesting that the low-SR fiber loss leaves the CAP unaffected. Single-unit recordings from the auditory nerve confirmed this hypothesis and further showed that it is due to the delayed and broad first spike latency distribution of low-SR fibers. In addition to unraveling the neural mechanisms that encode CAP, our computational simulation of an assembly of guinea pig ANFs generalizes and extends our experimental findings to different species of mammals. Altogether, our data demonstrate that substantial ANF loss can coexist with normal hearing threshold and even unchanged CAP amplitude.