Tensor tympani motoneurons receive mostly excitatory synaptic inputs

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.