Hair cells in mammalian utricles.
Two morphological classes of mechanosensory cells have been described in the vestibular organs of mammals, birds, and reptiles: type I and type II hair cells. Type II hair cells resemble hair cells in other organs in that they receive bouton terminals from primary afferent neurons. In contrast, type I hair cells are enveloped by large cuplike afferent terminals called calyces. Type I and II cells differ in other morphological respects: cell shape, hair bundle properties, and more subtle ultrastructural features. Understanding the functional significance of these strikingly different morphological features has proved to be a challenge. Experiments that correlated the response properties of primary vestibular afferents with the morphologies of their afferent terminals suggested that the synapse between the type I hair cell and calyx ending is lower gain than that between a type II hair cell and a bouton ending. Recently, patch-clamp experiments on isolated hair cells have revealed that type I hair cells from diverse species have a large potassium conductance that is activated at the resting potential. As a consequence, the voltage responses generated by the type I hair cells in response to injected currents are smaller than those generated by type II hair cells. This may contribute to the lower gain of type I inputs to primary afferent neurons. Studies of neonatal mouse utricles show that the type I-specific potassium conductance is not present at birth but emerges during the first postnatal week, a period of morphological differentiation of type I and type II hair cells.
Eatock
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Bobby R. Alford Department of Otorhinolaryngology and Communicative Sciences, Baylor College of Medicine, Houston, Texas 77030, USA.
Rüsch
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Lysakowski
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Saeki
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D000818:Animals; D018069:Hair Cells, Vestibular / Q000033:anatomy & histology / Q000502:physiology*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000502:physiology; D051379:Mice; D009431:Neural Conduction; D009475:Neurons, Afferent / Q000166:cytology / Q000502:physiology; D009476:Neurons, Efferent / Q000166:cytology / Q000502:physiology; D051381:Rats; D012444:Saccule and Utricle / Q000033:anatomy & histology / Q000294:innervation*
D007473:Ion Channels