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Replicas of the apical surface of hair cells of the inner ear (vestibular organ) were examined after quick-freeze, deep-etch and rotary shadowing. With this technique we demonstrate how actin filaments are organized and associated with the plasma membrane in the stereocilia and cuticular plate as well as inside the junctional complex. In each stereocilium there are thread-like connectors running from the actin filament bundle to the limiting membrane. Many of the actin filaments in the cuticular plate are connected to the apical cell membrane by tiny branched connecting units like a 'crow's foot'. Where these 'feet' contact the membrane there is a small swelling. These branched 'feet' extend mainly from the ends of the actin filaments but some connect the lateral surfaces of the actin filaments as well. Actin filaments in the cuticular plate are also connected to each other by finer filaments, 3 nm in thickness and 74 +/- 14 nm in length. These 3 nm filaments (which measure 4 nm in replicas) connect actin filaments, not only of the same polarity, but of opposite polarities as documented by examining replicas of the cuticular plate which had been decorated with subfragment 1 (S1) of myosin. At the apicolateral margins of the cell we find two populations of actin filaments, one just beneath the tight junction as a network, the other at the level of the intermediate junction as a ring. The latter is composed of actin filaments that run parallel to each other; adjacent filaments often show opposite polarities, as evidenced by S1 decoration. Because of the polarity of the filaments this ring may be a 'contractile' ring.

The receptor organs of the vestibular apparatus of rats flown for 7 days on Cosmos-1667 were examined. Serial sections were examined by light microscopy, some utriculus sections by electron microscopy, and otolith membranes by scanning electron microscopy. The fixation method used revealed a distinct structural heterogeneity of the receptor epithelium. In the striola area of the utriculus and sacculus as well as in the central apical area of cristae there are receptor cells surrounded by enlarged cup-like nerve endings. The nerve endings occupy over 70% of the cup-receptor cell complex. The area incorporating the enlarged nerve endings differs in size from animal to animal and from left to right ear in the same animal. The flown rat that was the first to be killed after recovery showed a very well pronounced asymmetry: in the right ear enlarged cups were seen all over the epithelium while in the left ear they were located in distinct spots. Since such changes were not identified in the remaining flown and control rats, it is concluded that they were produced by space flight effects but remained reversible and disappeared after recovery. This paper describes the causes responsible of the changes and their structural and functional relevances as well as other structural modifications that should be considered during vestibular studies.

The projection site in the central nervous system of individual end-organ branches of regenerated vestibular nerves was documented in bullfrogs. The eighth cranial nerve was surgically sectioned in 12 bullfrogs (Rana catesbeiana) and allowed to regenerate. Horseradish peroxidase was used to label the primary vestibular afferent fibers from the whole nerve (one frog) and the whole anterior branch (two frogs), and the individual receptor end-organ branches to the anterior semicircular canal (three frogs), the horizontal semicircular canal (three frogs), and the sacculus (three frogs). A sufficient number of fibers from the individual receptors were labeled in nine specimens to allow a description of their central projection pattern. The overall pattern of central projections of the regenerated nerves was similar to the pattern of normal nerves. In the eighth nerve root proper and in the area proximal to the vestibular nuclei, fibers from the anterior semicircular canal were ventral, fibers from the sacculus and posterior branch were dorsal, and fibers from the horizontal semicircular canal were in an intermediate position. In ventral, superior, and descending vestibular nuclei, a projection pattern was also identifiable in some specimens. The fibers of the anterior semicircular canal projected ventral to the fibers of the horizontal semicircular canal, with fibers from the posterior branch more dorsal. The saccular fibers projected to the dorsal aspect of the ventral nucleus and the ventral aspect of the dorsal nucleus. For the most part, projections in the regenerated specimens, as verified histologically, were comparable to individual end-organ projections previously identified in normal frogs. The only major discrepancy between the findings from the regenerated specimens and those from normal frogs was in the position of the vestibular tract and the innervation to the medial nucleus. The regenerated vestibular tract was shifted laterally in the brain stem. Regenerated thick and thin fibers were intermixed in the tract and projected to the medial nucleus. Normally, the thin fibers are lateral to the thick fibers in the tract. Therefore, the regenerated afferent fibers of the vestibular nerve selectively reinnervated the vestibular nuclei, and the fibers to the individual end-organs selectively reinnervated particular parts of the nuclei. In addition, the thick and thin regenerated fibers preserved the type of bouton endings normally found. The regenerated thick fibers had rare boutons en passant and a great number of collaterals, while the thin fibers had numerous boutons en passant and only a few collaterals.(ABSTRACT TRUNCATED AT 250 WORDS)

1. In two series of cats all vestibular afferents except those innervating the saccular or utricular macula were transected and allowed to degenerate. Subsequent anatomical and physiological verification showed that the saccular nerve had been selectively spared in six cats. The utricular nerve was wholly or partly spared in five cats, but there was some possibility of contamination from other vestibular afferents. 2. The connections of the remaining nerve with dorsal neck motoneurons were studied 27-179 days after the initial surgery in preparations anesthetized with chloralose. 3. Stimulation of the saccular nerve usually evoked IPSPs in contralateral, EPSPs in ipsilateral neck motoneurons. Some of the potentials were disynaptic, many were later and probably trisynaptic. 4. The effects of utricular nerve stimulation were more complex, perhaps because of contamination by other vestibular, particularly canal, afferents. The predominant pattern consisted of ipsilateral inhibition and contralateral excitation, opposite to the effects of saccular nerve stimulation. Many potentials were disynaptic. 5. Various factors that may complicate interpretation of the results, including plastic changes following denervation, are considered. It is concluded that they are not a significant factor. 6. Our main result is a demonstration that the sacculus can contribute to vestibulospinal reflexes acting on the head.

Responses to head tilt were recorded from eighth nerve axons in barbiturate anesthetized cats. The maximally excitatory head tilt (polarization vector), a zero-force discharge rate, and tilt sensitivity were measured for each cell. In one population of afferents, the maximum discharge frequency was obtained by aligning the saccular plane with gravity. The response properties of these saccular afferents were compared with a second population arising from the utriculus. Both the resting discharge rate and the response sensitivity were lower for saccular than utricular afferents in the cat. The average resting discharge was about 20% lower and the sensitivity about 15% higher in the cat than in the squirrel monkey.

Electrical stimulation of the utricle in C-1 transected cats (whose labyrinths remained sensitive to natural stimulation) induced bilateral pupillary dilatation and bilateral 0.4 to 3.0 diopter lens change in an anti-accommodative direction. Both effects could still be induced two weeks after cutting the ipsilateral cervical sympathetic trunk. It is concluded that these effects take place by inhibiting the parasympathetic pathway originating in the Edinger-Westphal nucleus. The mediating pathways from the vestibular complex to the Edinger-Westphal nucleus pass through the brain stem in the region outside the medial longitudinal fasciculi.