Circuit architecture and axonal branching in the efferent auditory system

The auditory efferent system has been implicated in nearly all aspects of hearing, including adaptations to aging and response to damage or disease. The system encompasses a collection of descending neural pathways that are widespread, supporting modulation of auditory processing from the cochlea to the cortex. To understand efferent function at a cellular and circuit level, it is necessary to determine which of the inputs to a region contact which of the various output pathways. For both inputs and the outputs, the presence or absence of axonal branching has important implications for how these circuits function. Lack of branching can allow projections to two targets to be modulated independently. The presence of branching can allow for efficient delivery of information and coordinated neuronal processing in multiple targets. The present review considers the merits of several methods that have been used to assess branching. These methods reveal that axonal branching is prominent in some efferent pathways and minimal or absent in others.