Tensor Tympani Motoneurons Receive Mostly Excitatory Synaptic Inputs

Abstract

The tensor tympani is a middle ear muscle that contracts in two different situations: in response to sound or during voluntary movements. To gain insight into the inputs and neural regulation of the tensor tympani, we examined the ultrastructure of synaptic terminals on labeled tensor tympani motoneurons (TTMNs) using transmission electron microscopy. Our sample of six TTMNs received 79 synaptic terminals that formed 126 synpases. Two types of synapses are associated with round vesicles and form asymmetric junctions (excitatory morphology). One of these types has vesicles that are large and round (Lg Rnd) and the other has vesicles that are smaller and round (Sm Rnd) and also contains at least one dense core vesicle. A third synapse type has inhibitory morphology because it forms symmetric synapses with pleomorphic vesicles (Pleo). These synaptic terminals can be associated with TTMN spines. Two other types of synapse are found on TTMNs but they are uncommon. Synaptic terminals of all types form multiple synapses but those from a single terminal are always the same type. Terminals with Lg Rnd vesicles formed the most synpases per terminal (avg. 2.73). Together, the synaptic terminals with Lg Rnd and Sm Rnd vesicles account for 62% of the terminals on TTMNs, and they likely represent the pathways driving the contractions in response to sound or during voluntary movements. Having a high proportion of excitatory inputs, the TTMN innervation is like that of stapedius motoneurons but proportionately different from other types of motoneurons. Anat Rec,

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.…”

“…Morphometry of vesicles was previously used to separate synaptic terminals on TTMNs (Benson et al, 2013), and here that type of analysis is extended to terminals on SMNs and MOC neurons. The approach is shown for one TTMN terminal in 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). …”

“…The material for this study was compiled from our previous studies of brainstem neurons (Benson and Brown, 2006; Benson et al, 2013; Lee et al, 2008). TTMN and SMN material was from adult Sprague–Dawley rats (a species that has a robust middle ear muscle reflex to sound (Relkin et al, 2005) and have been the subject of previous retrograde labeling studies (Billig et al, 2007; Reuss et al, 2008; Reuss et al, 2009; Rouiller et al, 1986; Rouiller et al, 1989; Spangler et al, 1982).…”

“…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).…”