Ultrastructural study of calycine synaptic endings of colossal vestibular fibers in the cristae ampullares of the developing chick

Striated organelle, a cytoskeletal structure positioned to modulate hair-cell transduction

The striated organelle (SO), a cytoskeletal structure located in the apical region of cochlear and vestibular hair cells, consists of alternating, cross-linked, thick and thin filamentous bundles. In the vestibular periphery, the SO is well developed in both type I and type II hair cells. We studied the 3D structure of the SO with intermediate-voltage electron microscopy and electron microscope tomography. We also used antibodies to α-2 spectrin, one protein component, to trace development of the SO in vestibular hair cells over the first postnatal week. In type I cells, the SO forms an inverted open-ended cone attached to the cell membrane along both its upper and lower circumferences and separated from the cuticular plate by a dense cluster of exceptionally large mitochondria. In addition to contacts with the membrane and adjacent mitochondria, the SO is connected both directly and indirectly, via microtubules, to some stereociliary rootlets. The overall architecture of the apical region in type I hair cells--a striated structure restricting a cluster of large mitochondria between its filaments, the cuticular plate, and plasma membrane--suggests that the SO might serve two functions: to maintain hair-cell shape and to alter transduction by changing the geometry and mechanical properties of hair bundles.

Sensory cells determine afferent terminal morphology in cross-innervated electroreceptor organs: implications for hair cells

Type I and type II hair cells of the vestibular system are innervated by afferents that form calyceal and bouton terminals, respectively. These cannot be experimentally cross-innervated in the inner ear to determine how they influence each other. However, analogous organs are accessible for transplantation and cross-innervation in the brown ghost electric fish. These fish possess three types of electroreceptor organs. Of these, the sensory receptors of the type I tuberous organ are S-100- and parvalbumin-positive with a calbindin-positive afferent that forms a large calyx around the organ. Neither the sensory receptors nor the afferents of the ampullary organs label with these antibodies, and the afferent branches form a single large bouton beneath each receptor cell. In controls, when cut ampullary afferents reinnervate transplanted ampullary organs, they have characteristic calbindin-negative terminals with large boutons. When type I tuberous afferents reinnervate ampullary organs, receptor cells remain S-100- and parvalbumin-negative, and the tuberous afferents still express calbindin. The nerve terminals, however, make large ampullary-like boutons on the receptor cells. These results suggest that (1) afferent terminal morphology is dictated by the receptor organ; (2) expression of calbindin by the afferent is not suppressed by innervation of the incorrect end organ; (3) ampullary organs generate ampullary receptor cells although innervated by tuberous afferents; and (4) ampullary receptor cells can be trophically supported by tuberous afferents.

Synaptic bodies and vesicles in the calix type synapse of chicken semicircular canal ampullae

Calix afferent fibers generate depolarizing DC (direct current) and superimposed AC (alternating current) potentials in response to a vibrating stimulation of the hair bundle in an isolated preparation of a chicken semicircular canal ampulla. The wave form of the postsynaptic potential appears similar to the transduction potential of hair cells, which suggests electrical transmission in the calix type synapse. However, synaptic bodies and vesicles were found by EM observation instead of gap junctions in the hair cell presynaptic to the calix afferent. The number of synaptic body was 11-12/hair cell. These structures support chemical transmission in the calix type synapse.

Horseradish peroxidase labeling of the efferent and afferent pathways of the avian tangential vestibular nucleus

The efferent and afferent pathways of the chick tangential nucleus were studied by using horseradish peroxidase (HRP: Sigma type VI) to label nerve cell bodies and fibers. Depositions of HRP into the tangential nucleus, as well as into the second cervical level of the spinal cord, show that the axons of tangential neurons on leaving the nucleus form an anteriorly coursing tract that passes through the ventrolateral vestibular nucleus without branching and then to the contralateral medial longitudinal fasciculus (MLF). Within the MLF, the tangential axons course posteriorly, forming collaterals that innervate the abducens nucleus, and then proceed to the cervical spinal cord. This pathway was demonstrated for the axons of the two main neurons, the principal and elongate cells, in 1-day, 1-week, and 7-week-old animals. In addition, we propose the existence of an unidentified, ipsilateral pathway to the spinal cord for the tangential axons, since HRP injections into one side of the spinal cord resulted in the bilateral labeling of tangential neurons. No labeled cells were found in the tangential nucleus following HRP depositions into the uvula, flocculus, pontine reticular formation, nucleus piriformis, nucleus jumeaux, vestibulocerebellar nucleus, retrotangential nucleus, or the dorsomedial part of the medial vestibular nucleus. The tangential nucleus receives afferents from the colossal vestibular fibers (spoon endings), small collaterals of fine vestibular ampullary fibers, flocculus, and high cervical levels of the spinal cord. From our small sample, it appears that the spinal cord fibers form most of the afferent terminals in the tangential nucleus in 1-day, 1-week, and 7-week-old animals.

Synaptic responses to mechanical stimulation in calyceal and bouton type vestibular afferents studied in an isolated preparation of semicircular canal ampullae of chicken

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.