Figure 1.
Photomicrographs of Nissl-stained AVCN.
(A) Section through rostral AVCN at low magnification. Granule cells (GC) are present along the lateral edge. (D) Section through caudal AVCN at low magnification. The lateral and dorsal edges of the nucleus are bounded by GCs, and the auditory nerve (AN) root is visible ventrally. Higher magnification images of large round and ovoid bushy cells of rostral AVCN (B, C) and caudal AVCN (D, E) demonstrate that cells display both centric and eccentric nuclei.
Figure 2.
Photomicrographs of Vglut1-positive auditory nerve endbulbs and bouton terminals contacting bushy cells of rostral AVCN (A, C) and caudal AVCN (B, D).
Some small Vglut1-positive terminals are also visible in the neuropil. The cells in A and B were counterstained with cresyl violet to visualize cell bodies and photographed using traditional transmitted light microscopy. The cell bodies in C and D were not labeled, but their locations are inferred on the basis of circular and ovoid rings of Vglut1-positive terminals.
Figure 3.
Electronmicrographs of bushy cell bodies and their synaptic contacts in the AVCN.
Bushy cells in both rostral AVCN (A) and caudal AVCN (B) receive numerous auditory nerve terminals (blue). Non-primary terminals originating from sources other than the auditory nerve (red) are more commonly observed contacting bushy cells in rostral AVCN compared to caudal AVCN. Three types of terminals form synapses with bushy cell bodies, characterized by (C) large round (LR) synaptic vesicles (SVs) and curved, asymmetric postsynaptic densities (PSDs); (D) small round (SR) SVs with flat, more symmetric PSDs, and pleomorphic (PL) SVs with symmetric PSDs of varying curvature. Terminals with LR SVs are VGLUT1-positive (F, G), consistent with their origin from the auditory nerve and are either bouton-like (F) or large endings with curved PSDs (asterisks), mitochondrial clusters, intermembramous cisternae (cis), and mitochondrial adherens complexes (MAC) characteristic of end bulb synapses (G).
Figure 4.
Large auditory nerve terminals, or endbulbs, ensheathed in glial processes (glia) contact bushy cell bodies (CB) in both rostral AVCN and caudal AVCN (A–E). These terminals contain mitochondria clusters (mit) and many LR SVs associated with curved, asymmetric PSDs (single asterisks). Mitochondrial adherens complexes (MAC), puncta adherentia (puncta), and intermembranous cisternae (cis) are often observed. Endbulbs sometimes form synapses (double asterisks) with nearby dendritic processes (d) presumed to originate from nearby bushy cells. Occasionally, very large endbulbs are observed in proximity to numerous dendritic processes (E). Multiple release sites are common (A–D), but are not always separated by intermembranous cisternae (A, E).
Figure 5.
Bushy cells labeled by microinjection of tracer into the trapezoid body.
Photomicrograph of trapezoid body injection site (A) and cells retrogradely labeled in caudal AVCN (B). Few cells were labeled in more rostral section (C).
Figure 6.
Examples of labeled bushy cells with varying dendritic morphology.
(A–C) Cells with short, primary dendritic trunks capped with relatively compact dendritic branches. (D–H) Cells with longer dendritic trunks that bifurcate and send off diffuse, thin dendritic processes. Dendrites are not oriented in a particular direction. Scale bars apply to all panels.
Figure 7.
Electronmicrographs showing ultrastructural detail of labeled bushy cells.
Synaptic terminals contacting two labeled bushy cells in caudal AVCN are highlighted in green (A, B). (C) High magnification image of a primary auditory nerve terminal forming a synapse with a labeled bushy cell. PSDs are marked with asterisks.
Figure 8.
Electronmicrographs of bushy cell dendrites in cross section.
(A) Multiple pieces of a labeled dendrite are visible in the neuropil surrounding bushy cells. (B) Bushy cell dendrites receive numerous bouton-like synaptic terminals (in green). This image shows terminals contacting the labeled dendrites in the boxed area in A. (C) Synaptic terminals (1–5) contacting an unlabeled dendrite (d) are shown so that ultrastructural details are visible. PSDs are marked with asterisks. Bushy cell (BC) body is indicated.