Spatial and morphological differentiation of trigger zones in afferent fibers to the teleost utricle.

A morphological correlate of the trigger site (the locus of action potential initiation) was identified in afferent axons of the utricle in the ear of two species of teleost fish. These sites were identified by the ferric-ferrocyanide (Prussian blue) cytochemical procedure and they were correlated with the geometries of afferent intraepithelial arbors as visualized by means of a silver stain. The intraepithelial arbors of afferent fibers show regional distributions that correlate with axon diameter and Prussian blue staining. Afferent axons with diameters greater than 4-5 microns only innervate the striola regions of the epithelium and terminate as one of two distinct types of intraepithelial arbors. Afferent axons with diameters smaller than 4 microns are ubiquitously distributed throughout the epithelium. Arbors that stained by Prussian blue within the utricular epithelium are restricted to the striolar regions. These arbors possess nodal-like membrane in different branches as postsynaptic membrane. Afferents that innervate hair cells in the extrastriolar epithelial regions stained with Prussian blue only at the extraepithelial terminal heminode. The postsynaptic membrane of these afferents is passive or dendritic-like.     

Heterogeneity of sensory hair cells in a fish ear.

The ultrastructure of sensory hair cells in the utricle of the cichlid fish, Astronotus ocellatus, the oscar, was studied by transmission electron microscopy of serial ultrathin sections from different regions of the epithelium. Two distinctly different types of hair cell were found, one located in the striolar region of the epithelium and the other in the extrastriolar region. Striolar hair cells have a well-defined perinuclear cisterna located just below the nucleus, and large perinuclear mitochondria. Synaptic bodies of striolar cells are small and located in clusters, while those in extrastriolar cells are relatively large and individually dispersed. The extrastriolar hair cell closely resembles the amniote type II hair cell. On the basis of these data, and consistent with earlier studies, it appears that the striolar hair cells closely resemble amniote type I hair cells in many significant ways. Thus we have called them type I-like cells. The extrastriolar hair cells appear to be typical of eighth nerve mechanoreceptors commonly described for fish and closely resemble the amniote type II hair cell.     

Correspondences between afferent innervation patterns and response dynamics in the bullfrog utricle and lagena

Otoconial afferents in the bullfrog were characterized as gravity or vibratory sensitive by their resting activity and their responses to head tilt and vibration. The responses of gravity afferents to head tilt were tonic, phasic-tonic, or phasic. A few afferents, termed vibratory/gravity afferents, had gravity as well as vibratory sensitivity. Functionally identified otoconial afferents were injected with Lucifer Yellow and subsequently traced to their peripheral arborizations. Morphological maps, previously constructed with the scanning electron microscope, were used to identify microstructural features of the sensory maculae associated with the peripheral arborizations of dye-filled afferents. The utricular and lagenar macula each is composed of a specialized central band surrounded by a peripheral field. The central bands are composed of densely packed medial rows and more sparsely packed lateral rows of hair cells. Hair cells exhibit a variety of surface topographies which correspond with their macular location. The response dynamics of afferents in the utricle and lagena correspond with the macular locations of their peripheral arborizations. Tonic afferents were traced to hair cells in the peripheral field. Phasic-tonic and phasic afferents innervated hair cells in the lateral rows of the central band, the former innervating hair cells at the edges of the central band and the latter innervating hair cells located more medially. Afferents with vibratory sensitivity were traced to hair cells in the medial rows of the lagenar central band. The response dynamics of afferents corresponded with the surface topography of their innervated hair cells. Tonic and phasic-tonic gravity afferents innervated hair cells with stereociliary arrays markedly shorter than their kinocilium (Lewis and Li types B and C) while phasic gravity and vibratory afferents innervated hair cells with stereociliary arrays nearly equal to their kinocilium (Lewis and Li types E and F). Vibratory sensitivity was uniquely associated with hair cells possessing bulbed kinocilium (Lewis and Li type E) while afferents sensitive to both gravity and vibration innervated hair cells from both of the above groups. We argue that afferent response dynamics are determined, at least in part, at the level of the sensory hair bundle and that morphological variations of the kinocilium and the otoconial membrane are dictated by specialization of sensitivity. We propose that morphological variations of the kinocilium reflect variations in its viscoelastic properties and that these properties determine the nature of the mechanical couple between the stereociliary array and the otoconial membrane.     

Distribution of the 275 kD hair cell antigen and cell surface specialisations on auditory and vestibular hair bundles in the chicken inner ear.

The 275 kD hair cell antigen (HCA) is a protein that is specifically associated with the apical surface of sensory hair cells in the chick inner ear. A comparative study of the vestibular and auditory organs of the inner ear, using both wholemounts and cryosections double labelled for the HCA and F-actin, reveals that two distinct types of hair cells can be distinguished on the basis of antibody staining in each of the vestibular epithelia. One type of hair cell has the HCA restricted to the base of the stereocilia bundle and is found in the striolae of the maculae and in a large, centrally located region of each ampulla. The other type of hair cell is found in the extrastriolar regions of the maculae and the peripheral regions of the ampullae and has the HCA distributed over the entire surface of the stereocilia bundle. In the basilar papilla, the auditory epithelium of the chick inner ear, the HCA is, as in the striolar regions of the maculae, restricted to the base of the hair bundles. In all sensory epithelia the HCA is also present on the apical, nonstereociliary surface of the hair cells. Ultrastructural examination of the basilar papilla and the striolar and the extrastriolar regions of the lagenar macula after staining with ruthenium red and tannic acid shows that there are four morphologically different types of interstereociliary connectors (oblique tip connectors, horizontal tip connectors, shaft connectors and basal connectors) associated with the hair bundles. Oblique tip connectors and basal connectors are found on hair cells from all regions and have a similar distribution. Horizontal tip connectors are seen only on hair cells in the basilar papilla and the striolar region of the lagenar macula. Shaft connectors extend all the way to the tips of extrastriolar hair cell bundles, but extend only a short way up the bundles of hair cells in the basilar papilla and striolar region of the lagenar macula. Immunogold labelling confirms the results obtained with immunofluorescence microscopy and demonstrates that the distribution of the HCA on the surface of adjacent stereocilia correlates closely with that of the shaft connectors; i.e., immunostaining is observed up to the tips of the extrastriolar hair cell bundles, but is restricted to the lower regions of hair cell bundles in the striolar region and basilar papilla.(ABSTRACT TRUNCATED AT 400 WORDS)     

Central projections of the utricular nerve in the gerbil

The central projections of primary afferent fibers in the utricular nerve, which convey linear head acceleration signals to neurons in the brainstem and cerebellum, are not completely defined. The purpose of this investigation was twofold: 1) to define the central projections of the gerbil utricular afferents by injecting horseradish peroxidase (HRP) and biotinylated dextran amine (BDA) into the utricular macula; and 2) to investigate the projections of individual utricular afferents by injecting HRP intracellularly into functionally identified utricular neurons. We found that utricular afferents in the gerbil projected to all divisions of the vestibular nuclear complex, except the dorsal lateral vestibular nucleus. In addition, terminals were observed in the interstitial nucleus of the eighth nerve, nucleus Y, external cuneate nucleus, and lobules I, IV, V, IX, and X of the cerebellar vermis. No projections appeared in the flocculus or paraflocculus. Fibers traversed the medial and intermediate cerebellar nuclei, but terminals appeared only occasionally. Individual utricular afferents collateralize extensively, projecting to much of the brainstem area innervated by the whole of the utricular nerve. This study did not produce complete filling of individual afferent collateral projections into the cerebellar cortex.     

Hair cell development in vivo and in vitro: analysis by using a monoclonal antibody specific to hair cells in the chick inner ear

The purpose of this study was to establish a hair cell-specific marker and a convenient explant culture system for developing chick otocysts to facilitate in vivo and in vitro studies focusing on hair cell genesis in the inner ear. To achieve this, a hair cell-specific monoclonal antibody, 2A7, was generated by immunizing chick inner ear tissues to a mouse. Through the use of immunofluorescence and immunoelectron microscopy, it was shown that 2A7 immunoreactivity (2A7-IR) was primarily restricted to the apical region of inner ear hair cells, including stereocilia, kinocilia, apical membrane amongst the extending cilia, and superficial layer of the cuticular plate. Although the 2A7 antibody immunolabeled basically all of the hair cells in the posthatch chick inner ear, two different patterns of 2A7-IR were observed; hair cells located in the striolar region of the utricular macula, which consist of two distinct cell types identifiable on the basis of the type of nerve ending, Type I and II hair cells, showed labeling restricted to the basal end of the hair bundles. On the other hand, hair cells in the extrastriolar region, which are exclusively of Type II, showed labeling extending over virtually the entire length of the bundles. These findings raised the possibility that chick vestibular Type II hair cells, characterized by their bouton-type afferent nerve endings, can be divided into two subpopulations. Analysis of developing inner ear by using the 2A7 antibody revealed that this antibody also recognizes newly differentiated immature hair cells. Thus, the 2A7 antibody is able to recognize both immature and mature hair cells in vivo. The developmental potential of embryonic otocysts in vitro was then assessed by using explant cultures as a model. In this study, conventional otocyst explant cultures were modified by placing the tissues on floating polycarbonate filters on culture media, thereby allowing the easy manipulation of explants. In these cultures, 2A7-positive hair cells were differentiated from dividing precursor cells in vitro on the same schedule as in vivo. Furthermore, it was found that hair cells with both types of 2A7-IR were generated in culture as in vivo, indicating that a maturational process of hair cells also occurred. All these results as presented here suggest that the 2A7 monoclonal antibody as a hair cell-specific marker together with the culture system could be a potential tool in analysis of mechanisms underlying hair cell development.     

Central distribution of octavolateral afferents and efferents in a teleost (mormyridae)

The central distribution of afferents from individual eighth nerve branches (N VIII) and mechanical lateral line end organs in mormyrid fish are described. Afferents were labeled with horseradish peroxidase (HRP) placed on the cut ends of the different N VIII branches and the anterior and posterior lateral line nerves. Descending, tangential, and magnocellular nuclei receive input almost exclusively from the utriculus and canals. Nucleus octavius receives afferents from the lateral line nerves and all N VIII branches, with one part receiving exclusive and bilateral input from the sacculus. Afferents from both lateral line nerves and all N VIII branches, except the sacculus, end in eminentia granularis. Afferents from each of the two lateral line nerves and from each of the three otolith branches of N VIII end in different regions of the anterior lateral line lobe, with some areas of overlap. Behavioral studies in other families of fish indicate that the utriculus and canals are critical for postural control, whereas the sacculus and possibly the lagena are concerned with hearing. Such findings, together with the results of this study, suggest that mormyrids and perhaps other fish possess separate auditory and vestibular centers within the octavolateral area. The HRP method also shows the cell bodies and axons of octavolateral efferents. N VIII and lateral line efferents arise from a common nucleus, and the central course of their axons parallels that of facial motoneurons. Axons of efferent cells divide to supply two or more branches of N VIII and some axons supply both lateral line and N VIII end organs.     

The inner ear of the lungfish Protopterus

The sensory end organs of the inner ear of the lungfish, Protopterus, were examined using scanning and transmission electron microscopy. The utricle has a structure and hair cell orientation pattern that are typical for vertebrates, although the hair cells are unusually large. There are the typical three semicircular canals extending from the utricle, with the typical hair cell orientations, but the lateral canal sensory crista looks like the "hemicrista" of some amphibians and amniotes, lacking a saddle-shaped flare on one wall of the ampulla. Unlike most vertebrates that have the saccule and lagena as two separate pouches ventral to the utricle, the lungfish has a single large ventral pouch that contains a single large pasty otoconial mass. This mass covers two hair cell patches, each like a striola with prominent hair cell ciliary bundles, that are presumed to represent saccular and lagenar maculae. However, these two major sensory patches are not completely separate maculae because they lie within a less densely populated field of smaller hair cells, which forms an extrastriolar region that surrounds and fills the region between the two striolae of higher hair cell density. The more caudal lagenar striola is a vertically elongated stripe with hair cell orientation vectors facing antiparallel on either side of a midline drawn vertically along the macula, resembling the macula lagena of some bony fishes but not of tetrapods. The more rostral saccular striola is a curving band with hair cell orientation vectors facing away from its midline, but because this macula curves in three dimensions, the vectors at the rostral end of this striola are oriented mediolaterally, whereas the vectors on the caudal half of this striola are oriented dorsoventrally. The presence of a macula neglecta was confirmed near the posterior canal as a tiny single patch of a few dozen hair cells with all the cell orientations directed caudally. The ciliary bundles on the cells in the striolar-like regions of all of three otolithic organs average over 80 cilia, a number far greater than for any other fish studied to date. The features of the single sacculolagenar pouch with separate striolar-like regions, the cellular orientation in the otolith organs, and the large cells and ciliary bundles in Protopterus also were observed in specimens of the other extant lungfish genera, Lepidosiren and Neoceratodus.     

The inner ear of the common rhea (Rhea americana L.)

The morphology of the inner ear in rheas was examined by light and electron microscopy. The shape is typically bird-like with very long semicircular canals. The anterior and posterior cristae have small septa cruciata. The vestibular sensory epithelia contain two main types of hair cell innervation; bouton-innervated hair cells and calyceal hair cells characterized by a surrounding nerve calyx. The utricular macula has a single zone of calyceal hair cells, while all other previously examined birds, except the mute swan, have 2 zones. The height of the tallest sensory hairs of the cristae is 20-30 microns. In the utricular and lagenar macula, the hairs are 5-7 microns in the striola and 10-20 microns in the main parts of the sense organs. Along the edges of the maculae the longest hairs may reach 20-30 microns. The number of stereovilli on mature vestibular hair cells is 40-60. The sensory hairs of the hearing organ, the basilar papilla, are generally shorter but more numerous than the vestibular sensory hairs. In the proximal end, the tallest of the 175-200 stereovilli are 2.8-3.7 microns; in the distal end of the papilla, the number of stereovilli decrease to 65-100, and their height increases gradually to 7.3-8.7 microns. The neural sensory hairs are generally taller than those of the abneural side.     

Peripheral and central aspects of the acoustic and lateral line system of a bottom dwelling catfish, Ancistrus sp.

The topographical relationship between the swim bladder, the inner ear, and the otic lateral line was studied in the bottom dwelling catfish, Ancistrus sp. In addition, afferent and efferent subcomponents of the eighth and lateral line nerves were labelled with horseradish peroxidase (HRP) or with differently fluorescing dextran amines. The swim bladder of Ancistrus consists of two separate, transversely oriented parts of each of which is connected to the sinus impar of the inner ears via two Weberian ossicles and the perilymphatic sac. The osseous capsula of the ear has two foramina other than the nerve foramina. One is for the sinus impar. The other foramen, which also separates two fluid-filled spaces, exits where the horizontal canal of the ear contacts the otic lateral line. Both the otic and the postotic lateral line canal run deep below the epidermis. Each canal contains a neuromast that is innervated by the middle lateral line nerve. Further caudally, the otic lateral line canal gives rise to the postotic and finally to the truck canal whose nonossified anterior part travels through an ossified chamber that surrounds the swim bladder. Thus the anterior part of each trunk lateral line canal is in contact with a bipartite sound pressure receiver, the swim bladder. Anterior and posterior lateral line afferents terminate ipsilaterally throughout the neuropil of the electroreceptive lateral line nucleus and the mechanoreceptive nuclei medialis and caudalis of the medulla. Middle lateral line afferents terminate between the projection sites of anterior and posterior lateral line afferents. Some primary mechanosensory anterior lateral line nerve fibers continue into the ipsilateral eminentia granularis and the valvula cerebelli. In the electroreceptive lateral line projection, anterior lateral line fibers terminate more medially and posterior fibers more laterally. This somatotopy is not as clear-cut in the mechanosensory lateral line. Afferents of the sacculus and the lagena terminate predominantly in the saccular nucleus. Afferents of the utriculus, the horizontal canal, and the anterior vertical canal terminate in the magnocellular vestibular nucleus and in the medial octavolateral nucleus. The projection sites of the anterior part and the posterior part of the eighth nerve show little overlap. Eighth nerve projections to the valvula cerebelli are less prominent than the projections from the lateral line. Eighth nerve and lateral line nerve efferents arise from a common nucleus, the octavolateralis efferent nucleus. Axons of efferent cells may divide to supply two or more branches of the eighth nerve and some axons supply both lateral line and eighth nerve endorgans.(ABSTRACT TRUNCATED AT 400 WORDS)     

Morphophysiology of synaptic transmission between type I hair cells and vestibular primary afferents. An intracellular study employing horseradish peroxidase in the lizard, Calotes versicolor.

Intracellular records with glass microelectrodes filled with horseradish peroxidase (HRP) were taken from primary afferents of the horizontal semicircular canal in the lizard, Calotes versicolor. A coefficient of variation (CV) of the interspike intervals of spontaneous action potentials (APs) was calculated and correlated with the terminal morphologies of afferents within the canal crista. Irregular fibers with CV greater than 0.4 always correlated with a nerve chalice or calyx afferent terminal expansion surrounding one or more type I hair cells; more regular fibers with CV less than 0.4 always correlated with a dimorphic or bouton only terminal expansion of afferents. Afferents with a CV greater than 0.4 demonstrated miniature excitatory postsynaptic potentials (mEPSPs) that summated to initiate APs. APs were blocked by tetrodotoxin and mEPSP frequency was modulated by caloric stimulation. Cobalt application reversibly blocked mEPSPs. Electron microscopic examination of physiologically studied afferents with CV greater than 0.4 revealed synaptic profiles consisting of typical synaptic bodies and synaptic vesicles in the type I hair cell presynaptic to the nerve chalice. Examples of the interspike baseline in regular and irregular afferents suggest differential modes of impulse initiation in these two fiber types.     

Analysis of temporal and spatial patterns of rat vestibular hair cell differentiation by tritiated thymidine radioautography

The location in time and space of the terminal mitoses of type I and type II sensory hair cells (HCI and HCII) of the developing crista ampullaris in rat lateral semicircular canal and macula utriculi was determined by radioautographs of specimens exposed to tritiated thymidine from the 13th to the 20th day of gestation. Qualitative analysis and statistical treatment of the percentages of labeled HCI and HCII show that the terminal mitoses occur first in the macula utriculi with a maximum percentage of the 14th day of gestation, for the HCI, and on the 15th day of gestation, for the HCII. In the lateral crista, the maximum percentage of labeled HCI occurs on the 17th and 18th day of gestation and on the 19th day of gestation for the HCII. A spatial distribution of this labeling activity is also described: the older cells are located at the top of the crista and at the level of the striola of the macula utriculi while the younger cells are found at the bottom of the crista and on the sides of the utricle. A study of the vestibular receptors in the fetuses shows that synaptic contacts already exist on the 18th day of gestation in the macula utriculi at the level of the striola and on the 19th day at the top of the crista; the cells situated on the periphery are still immature. The first hair cells to undergo their terminal mitoses are, therefore, connected first. These results also suggest that the two types of cells are genetically programmed and that the HCI start functioning first during the development of the labyrinth.     

Morphology and somatotopic organization of the central terminals of hindlimb hair follicle afferents in the rat lumbar spinal cord

The morphology of the central collateral arborizations of 24 A-beta hair follicle afferents (HFAs) innervating different regions of the skin of the hindlimb were studied by the intra-axonal injection of horseradish peroxidase (HRP) in adult rats. A total of 236 collaterals were recovered. These fell into three classes--complex, simple, and blind-ending--based on numbers of boutons and terminal branch patterns. The morphology of the HFA central arbors innervating the lateral and medial leg and dorsum of the foot was flame-shaped. Afferents with receptive fields on the glabrous-hairy skin border consistently had extra terminal branches running ventromedially into laminae IV/V. Differences in the width of terminal arbors were found. HFA terminals innervating the lateral leg formed narrower sheets than those innervating the dorsum of the foot and toes. The somatotopic organization of the collaterals and terminal arborizations of individual afferents were analyzed both by considering all the collaterals along an axon's rostrocaudal extent and by only examining arbors with boutons (the complex and simple arbors). Thirty-seven percent of blind-ending and 18% of simple collaterals were found to overlap in the rostrocaudal direction with the complex arborizations of afferents whose receptive fields were in a different cutaneous nerve territory. There was no overlap between complex arborizations of afferents from different nerve territories. However, the complex arbors of afferents with receptive fields within a particular nerve territory showed considerable terminal overlap even if they had nonadjacent peripheral receptive fields. The topographical organization of the central terminals of HFAs, forms a coarse somatotopic map of overlapping terminals whereby a particular region of dorsal horn has a maximal, but not exclusive, input from a particular area of skin.     

Differential central projections of vestibular afferents in pigeons

The question of whether a differential distribution of vestibular afferent information to central nuclear neurons is present in pigeons was studied using neural tracer compounds. Discrete tracing of afferent fibers innervating the individual semicircular canal and otolith organs was produced by sectioning individual branches of the vestibular nerve that innervate the different receptor organs and applying crystals of horseradish peroxidase, or a horseradish peroxidase/cholera toxin mixture, or a biocytin compound for neuronal uptake and transport. Afferent fibers and their terminal distributions within the brainstem and cerebellum were visualized subsequently. Discrete areas in the pigeon central nervous system that receive primary vestibular input include the superior, dorsal lateral, ventral lateral, medial, descending, and tangential vestibular nuclei; the A and B groups; the intermediate, medial, and lateral cerebellar nuclei; and the nodulus, the uvula, and the paraflocculus. Generally, the vertical canal afferents projected heavily to medial regions in the superior and descending vestibular nuclei as well as the A group. Vertical canal projections to the medial and lateral vestibular nuclei were observed but were less prominent. Horizontal canal projections to the superior and descending vestibular nuclei were much more centrally located than those of the vertical canals. A more substantial projection to the medial and lateral vestibular nuclei was seen with horizontal canal afferents compared to vertical canal fibers. Afferents innervating the utricle and saccule terminated generally in the lateral regions of all vestibular nuclei in areas that were separate from the projections of the semicircular canals. In addition, utricular fibers projected to regions in the vestibular nuclei that overlapped with the horizontal semicircular canal terminal fields, whereas saccular afferents projected to regions that received vertical canal fiber terminations. Lagenar afferents projected throughout the cochlear nuclei, to the dorsolateral regions of the cerebellar nuclei, and to lateral regions of the superior, lateral, medial, and descending vestibular nuclei. © 1996 Wiley-Liss, Inc.     

The effects of experimental denervation and reinnervation on skeletal muscle fiber type and intramuscular innervation.

Porcine muscle has a unique grouped arrangement with 3-70 Type I fibers occurring in clusters surrounded by Type II fibers. The deep medial portion of the porcine semitendinosus exhibits a regular pattern of extensive Type I grouping whereas the superficial or lateral portion of the muscle exhibits Type II predominance. Mean values for terminal innervation ratios were 1.00 +/- 0.01 and 1.02 +/- 0.01 in normal superficial and deep semitendinosus respectively. Subterminal axons, therefore, do not branch intramuscularly and innervate only one muscle fiber in normal porcine muscle. Following nerve crush and subsequent reinnervation, fiber type conversion occurred which resulted in a fiber type grouping pattern dissimilar to the normal grouped arrangement. Significantly (P less than 0.01) elevated terminal innervation ratios (1.45-2.15) were measured in reinnervated muscle as a result of extensive branching of subterminal axons, but the percentage of Type I and Type II muscle fibers was unchanged. It was concluded that: (1) nerve crush causes distal nerve degeneration, loss of the normal fiber type pattern, and extensive collateral ramification of subterminal axons; (2) collateral reinnervation imposes a neuronal influence on muscle fibers which dictates transformation of all muscle fibers innervated by a single subterminal axon to a uniform histochemical profile; and (3) the type grouping observed in normal porcine muscle is not a result of neuronal influence mediated by collateral branching of subterminal axons.     

Afferent and efferent connections of the primary octaval nuclei in the clearnose skate, Raja eglanteria

Horseradish peroxidase techniques were employed to trace the central projections of afferents from the individual endorgans of the membranous labyrinth and to delineate the efferent projections from the primary octaval nuclei to the spinal cord and midbrain octavolateralis area in the clearnose skate, Raja eglanteria. First-order octaval afferents project ipsilaterally to five primary octaval nuclei, namely: magnocellular, descending, posterior, anterior, and periventricular. Octaval afferents also terminate in the reticular formation, nucleus intermedius (primary mechanoreceptive lateral-line nucleus), and vestibulolateral lobe of the cerebellum. Each primary octaval nucleus receives afferent input from each labyrinthine endorgan, with the possible exception of macula neglecta input to the magnacellular nucleus. Within the anterior, descending, and to a lesser extent posterior and magnocellular nuclei, this input is largely nonoverlapping. Semicircular canal cristae afferents terminate ventrally, saccular and lagenar afferents dorsally, utricular afferents laterally, and macular neglecta afferents course ventrally but terminate largely dorsally within these nuclei. In the vestibulolateral lobe of the cerebellum, cristae afferents project primarily to the pars medialis, whereas macular endorgan afferents terminate in the pars lateralis. Primary afferent input to the reticular formation is predominantly from the horizontal canal crista. The densest projections to nucleus intermedius are from the utriculus and sacculus. Vestibulospinal projections originate primarily from the magnocellular and descending nuclei. Second-order auditory neurons are most likely located in dorsomedial parts of the descending and anterior nuclei. Cells in these nuclei project directly to the auditory area of the midbrain octavolateralis complex, but projections to this area originate predominantly from nuclei C1 and C2, which are possible superior olivary homologues.     

Segregation of coarse and fine glossopharyngeal axons in the visceral nucleus of the tractus solitarius of the cat

The projections of coarse and fine axons of the glossopharyngeal (IX) nerve upon the caudal two thirds of the nucleus of the tractus solitarius (NTS) were studied in the cat. These afferents convey the chemo- and baroreceptor activities from the carotid receptors. We applied the Fink-Heimer method on brainstem sections, at different survival times, after a petrosal ganglionectomy. A segregation of fine and coarse fibered components was observed. Degeneration of coarse axons was mostly found in the lateral NTS, while fine fiber degeneration was predominant in regions of the medial and commissural NTS. The injection of WGA-HRP in the different NTS divisions demonstrated that the lateral NTS was mainly innervated by the set of largest neurons of the petrosal ganglion and that the medial and the commissural NTS were innervated by the set of smaller neurons of the ganglia. These results were discussed in relation to cytoarchitecture, myeloarchitecture, distribution of normal axons, and known central connectivity of the different NTS divisions. We concluded that coarse and fine visceral afferents of the IX nerve, which includes the afferents of the carotid body and the carotid sinus, represent different afferent populations that project to particular divisions of the NTS and connect to different central pathways.     

Vestibular afferent responses to microrotational stimuli

Intracellular microelectrode recording/labelling techniques were used to investigate vestibular afferent responses in the bullfrog, to very small amplitude (less than 0.5 degree p-p) sinusoidal rotations in the vertical plane over the frequency range of 0.063-4 Hz. The axis of rotation was congruent with the axis of the anterior semicircular canal. Robust responses to peak accelerations as low as 0.031 degree/S2 were obtained from units subsequently traced to either the central portion of the anterior canal crista or the striolar region of the utricle. All of these microrotationally sensitive afferent neurons had irregular resting discharge rates and the majority had transfer ratios (relative to rotational velocity) of 1-40 spikes/s per degree/s. Individual utricular afferent velocity transfer ratios were nearly constant over the frequency range of 0.125-4 Hz. Canal units generally displayed decreasing response transfer ratios as stimulus frequencies increased. These findings indicate that although utricular striolar and central crista afferent velocity transfer ratios to microrotations were very similar, utricular striolar afferent neurons were more faithful sensors of very small amplitude rotational velocity in the vertical plane.     

The sensura neglecta in the pigeon: a scanning electron and light microscope study

Both scanning electron and light microscopy have been used to observe the tiny sensura neglecta which is located in the inferior utricular sinus of the vestibular labyrinth in the pigeon. The following new observations were made. The end organ, which occupies one-third of the floor of the inferior utricular sinus, is ovoid, measuring 200 μm in length × 80 μm in width and 30 μm in height. The neuroepithelium of the neglecta is uniformly innervated by the neglectar nerve which contains about 60 to 90 myelinated nerve fibers that fasciculate from the posterior ampullary nerve. The fiber spectrum is unimodal with a mode diameter of 1–2 μm. From 150 to 200 Type I and Type II hair cells are found on the surface of the neglecta. The surface features of the hair cells, i.e., the pipe organ patterns of the stereocilia are identical to that of the other vestibular sensory regions. No hair cells are found on the sides of the neglecta and all hair cells project their tufts upward from the top surface. The sensory region of the neglecta is covered by a gelatinous membrane. In no instance are statoconia observed on this covering. The non-sensory epithelium contiguous to the sensura neglecta is comprised only of “light cells” (Dohlman, '67). The above observations indicate that the sensura neglecta in the pigeon has both crista- and macula-like features. It is hypothesized that this is reasonable from an evolutionary and teleological point of view. It is further suggested that regardless of its similarities to the statolithic and canalicular organs, the mode of stimulation of the neglecta can best be described by analogy to the culmen type of system associated with the auditory papilla of the lizard (Wever, '71).