Diverse Synaptic Terminals on Rat Stapedius Motoneurons

Abstract

Stapedius motoneurons (SMN) mediate the contraction of the stapedius muscle, which protects the inner ear from injury and reduces the masking effects of background noise. A variety of inputs to SMNs are known to exist, but their terminal ultrastructure has not been investigated. We characterized the synaptic terminals on retrogradely labeled SMNs found just ventromedial to the facial motor nucleus. About 80% of the terminals contained round synaptic vesicles. One type (Sm Rnd) had small, round vesicles filling the terminal with occasional dense core vesicles and formed an asymmetric synapse. Sm Rnd terminals were small with lengths of apposition to the SMN less than 3 μm. Partial reconstructions from serial sections demonstrated that these terminals formed up to three synapses per terminal. Another terminal type (Lg Rnd) had large, round vesicles and asymmetric synapses. Most Lg Rnd terminals were small but some were extensive, e.g., abutting the SMN for up to 10 μm. One of these terminals formed at least seven synapses. Another terminal type (Pleo) had pleomorphic vesicles and symmetric active zones that, in some cases, were invaginated by spines from the SMN. A fourth uncommon terminal type (Het Rnd) had round vesicles of heterogeneous sizes and asymmetric synapses. A fifth rare terminal type (Cist) had large, round vesicles and an accompanying subsurface cistern in the SMN. These were generally the same kinds of terminals found on other motoneurons, but the high proportion of round vesicle synapses indicate that SMNs receive mostly excitatory inputs.

Ultrastructure of spines and associated terminals on brainstem neurons controlling auditory input

“…6. The data fit our visual impression and previous reports (Benson and Brown, 2006; Benson et al, 2013; Lee et al, 2008) that there are three types of terminals with non-overlapping mean values. Using the k -means analysis (Fig.…”

“…1). Two rare types of terminals on motoneurons, those with vesicles of heterogenous sizes or with a postsynaptic cistern, were not part of this analysis (Benson et al, 2013; Lee et al, 2008). …”

“…Previous studies of SMNs and MOC neurons have separated the synaptic vesicles by visual inspection (Benson and Brown, 2006; Helfert et al, 1988; Lee et al, 2008; Spangler et al, 1986; White, 1984). The new data show in a quantitative way that TTMN and SMN spines are only associated with terminals containing Pleo synaptic vesicles, which are the terminals that form symmetric synapses.…”

“…A shared property of these auditory brainstem neurons is that they are sparsely spined (Benson and Brown, 2006; Benson et al, 2013; Lee et al, 2008; Mulders and Robertson, 2000a), but the spine documentation and comparisons of their associated synapses are incomplete. There is extensive documentation of spines on the dendrites of pyramidal cells of the cerebral cortex and neurons in the hippocampus (reviewed by Alvarez and Sabatini, 2007; Bourne and Harris, 2008).…”

Oromotor Nuclei

“…These neurons, together with projections from cochlear root neurons in the pons, play an important role in the acoustic pinna reflex ( Horta-Junior et al, 2008). Neurons in this region, as well as the trapezoid nuclei, also project to stapedius motoneurons (Rouiller et al, 1989;Lee et al, 2008).…”