Fig. 2.

Anatomic arrangement of excitatory and inhibitory pathways in the avian brain stem auditory system. A: sound is transduced by hair cells, which synapse on primary afferents (nVIII). nVIII axons branch upon entering the brain to innervate two cochlear nuclei: the nucleus angularis (NA) and NM. In the NM, nVIII axons form large end bulb synapses. Each NM neuron projects bilaterally and forms the sole excitatory input to coincidence-detecting neurons in the NL. NM axons form delay lines in the contralateral projection. Both the NA and NL project to higher-order targets, including the superior olivary nucleus (SON), a major source of GABAergic inhibition in the circuit. B: the SON has two output pathways within the brain stem, a descending (feedback) pathway to the ipsilateral NA, NM, and NL and an ascending pathway that innervates the contralateral SON, forming a mutually inhibitory interaction. C: simple schematic model of this circuitry composed of two inhibitory feedback loops, which are negatively coupled to each other via the SON-SON connectivity. This arrangement suggests that elevated activity in one unilateral feedback loop will tend to disinhibit targets on the opposing side, bilaterally balancing overall activity.