Synaptic responses to mechanical stimulation in calyceal and bouton type vestibular afferents studied in an isolated preparation of semicircular canal ampullae of chicken.
ABSTRACT Relationships between the response patterns of semicircular canal afferents to mechanical stimulation and the morphologies of their peripheral endings were investigated in an isolated preparation of the anterior semicircular canal ampulla of chicken, using a combination of electrical recording with intracellular injections of Lucifer Yellow CH. The hair bundle mechanical stimulus was applied in a diffuse manner by a glass rod vibrating in the nearby bathing medium. Two types of spike discharge patterns and postsynaptic potentials were recorded. One type was found exclusively in the bouton type afferent and demonstrated a phasic increase of firing frequency and transient depolarizing postsynaptic potentials at the beginning of mechanical stimulation. These synaptic potentials were also observed spontaneously and their amplitudes were increased by membrane hyperpolarization. The other type was found exclusively in afferents with calyceal endings and showed a tonic increase of spiking frequency and depolarizing DC postsynaptic potentials with superimposing AC responses at the frequency of the mechanical stimulation. Amplitudes of postsynaptic potentials were increased by hyperpolarization. Hair cells generated depolarizing DC transduction potentials superimposed with AC potentials at frequency of the mechanical stimulation. The spontaneous spike discharging patterns of afferent nerve fibres were classified either as a regular type (CV less than 0.10) or as an irregular type (CV greater than 0.25) on the basis of coefficient of variation (CV) of interspike intervals. The spontaneous firing rate of regular units was higher than that of irregular units. Several membrane characteristics are different between these two types of afferent fibers; irregular units had short membrane time constants and fast spikes associated with clear spike-afterhyperpolarization. These features fit with the fact that irregular units tend to have phasic responses to mechanical stimulation while regular units typically have tonic responses. Irregular units had bouton endings with an average axonal diameter thicker than the regular units which had calix endings.
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ABSTRACT: Embryonic hair cells in chicks and mammals have functional transduction channels and voltage-gated outwardly rectifying potassium (K+) channels, fast inwardly rectifying channels, and voltage-gated sodium (Na+) and calcium (Ca2+) channels. Together these channels may participate in spiking by the immature hair cells, which may drive rhythmic or bursting activity of neurons at higher levels of the auditory pathway. The electrical activity of immature hair cells may influence afferent synaptogenesis and differentiation indirectly by promoting neurotrophin release or more directly by glutamatergic transmission. With maturation, a number of changes tend to reduce hair cell spiking: Na+, Ca2+, and fast inwardly rectifying channels may become less numerous, whereas outwardly rectifying K+ channels become more numerous and diverse. These changes signal the transformation from a developing epithelium with active formation of synaptic contacts to a sensing epithelium where receptor potentials represent the mechanical input in a graded fashion. The composition of the late-arriving outwardly rectifying K+ channels is specific to the hair cell's type and location in the sensory epithelium and confers specialized properties on the receptor potentials. Fast, Ca2+-gated channels serve high-quality electrical tuning in certain tall hair cells of the chick cochlea. In rodent cochlear hair cells and type I hair cells of chick and rodent vestibular organs, large outwardly rectifying conductances lower the input resistance, enhancing the speed and linearity of voltage responses.Current Topics in Developmental Biology 02/2003; 57:389-448. · 6.91 Impact Factor
- Tribology and Interface Engineering Series 01/2002; 40:467-474.
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ABSTRACT: With the development of intraaxonal labeling methods, it has become possible to relate the discharge properties of a vestibular afferent with its peripheral innervation patterns. In this chapter, we review the results of such morphophysiological studies. To provide a context for the review, we first consider the morphology of the vestibular organs from their gross anatomy to their ultrastructure. Because the species used in the morphophysiological studies have ranged from fish to mammals, we adopt a comparative approach to the morphology. A second perspective is provided by considering general features of afferent physiology. We next summarize results for each of the four species in which intraaxonal labeling has been used. In a final section, we describe general comparative trends that have emerged, consider the strengths and weaknesses of a morphophysiological approach, and speculate about the relation between the diversity of afferent physiology and the several stages of vestibular transduction.04/2006: pages 57-152;