Nerve fibers supplying the utricular macula of the chinchilla were labeled by extracellular injection of horseradish peroxidase into the vestibular nerve. The peripheral terminations of individual fibers were reconstructed and related to the regions of the end organ they innervated and to the sizes of their parent axons. The macula is divided into medial and lateral parts by the striola, a narrow zone that runs for almost the entire length of the sensory epithelium. The striola can be distinguished from the extrastriolar regions to either side of it by the wider spacing of its hair cells. Calyx endings in the striola have especially thick walls, and, unlike similar endings in the extrastriola, many of them innervate more than one hair cell. The striola occupies 10% of the sensory epithelium; the lateral extrastriola, 50%; and the medial extrastriola, 40%. The utricular nerve penetrates the bony labyrinth anterior to the end organ. Axons reaching the anterior part of the sensory epithelium run directly through the connective tissue stroma. Those supplying more posterior regions first enter a fiber layer located at the bottom of the stroma. Approximately one-third of the axons bifurcate below the epithelium, usually within 5-20 μm of the basement membrane. Bifurcations are more common in fibers destined for the extrastriola than for the striola. Both calyx and bouton endings were labeled. Calyces can be simple or complex. Simple calyces innervate individual hair cells, whereas complex calyces supply 2-4 adjacent hair cells. Complex endings are more heavily concentrated in the striola than in the extrastriola. Simple alyces and boutons are found in all parts of the epithelium. Calyces emerge from the parent axon or one of its thick branches. Boutons, whether en passant or terminal, are located on thin collaterals. Fibers can be classified into calyx, bouton, or dimorphic categories. The first type only has calyx endings; the second, only bouton endings; and the third, both kinds of endings. Calyx units make up 6% of the labeled fibers, bouton units <2%, and dimorphic units > 92%. The three fiber types differ in the macular zones they supply and in the diameters of their parent axons. Calyx units were restricted to the striola. The few bouton units were found in the extrastriola. Dimorphic units innervated all parts of the epithelium. Axon diameters are largest for calyx units and smallest for bouton units. Dimorphic axons show a regional variation: they are thickest in the anterolateral extrastriola, thinner in the striola, and thinnest in the rest of the extrastriola. Calyx units were almost always unbranched and ended as complex calyces or, less often, as simple calyces. The terminal arbors of bouton units consisted of fine processes containing 15-64 endings. Dimorphic units varied in complexity from fibers with a single calyx and one bouton to those with 1-7 calyces and > 100 boutons. Striolar and extrastriolar dimorphic units differed, the former having more compact terminal trees with fewer calyx and bouton endings. A juxtastriolar zone, immediately surrounding the striola, had dimorphic innervation patterns that were transitional between those in the striola and in more peripheral parts of the extrastriola. On this basis three zones can be recognized. The striola is supplied with calyx and dimorphic fibers, the juxtastriola with dimorphic fibers, and the peripheral extrastriola with dimorphic and bouton fibers. There is a sharp morphological boundary between the striola and juxtastriola, and few afferents supply branches to both zones. No clear boundary exists between the juxtastriola and peripheral extrastriola. An attempt was made to reconstruct the afferent innervation of the mucala. Data from the present study were used together with previously published hair-cell counts in the guinea pig. The proportions of labeled calyx, bouton, and dimorphic units were corrected for biases involved in the labeling of thin afferents. The juxtastriola and peripheral extrastriola were considered together. The calculations indicate that calyx units make up 2%, bouton units 12%, and dimorphic units 86% of the fiber population. The striola receives 9% of the afferent supply; striolar dimorphic units outnumber calyx units by a 3:1 ratio. The remaining afferents, the large majority of which are dimorphic units, supply the extrastriola.
