Reciprocal synapses between hair cells and first order afferent dendrites in the crista ampullaris of the bullfrog

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Abstract
Reciprocal synapses were found between afferent dendrites and hair cells of the crista ampullaris in the vestibular system of the bullfrog. The reciprocal synapse consisted of two components forming a dual synaptic complex between a single hair cell and a single dendritic process such that the receptor cell was both presynaptic and postsynaptic to the afferent dendrite. The first component, a typical afferent synaptic complex, consisted of a synaptic sphere surrounded by a single layer of clear vesicles, and was located within the hair cell. In addition, 3–4 arciform structures were located between the sphere and the plasma membrane of the hair cell. The second component of the reciprocal synapse, located approximately 1.5 μm from the first, was recognized by a localized accumulation of clear membrane vesicles within the neuronal process and a short membrane profile within the cytoplasm of the adjacent hair cell. Additional characteristic features included continuity of the arciform structures with the plasma membrane of the hair cell, and within the neuronal process, a greatly reduced number of vesicles when compared with the efferent axonal terminal. This latter was important in distinguishing a first-order afferent dendritic process from an efferent neuronal process. The possible functional significance of these reciprocal synapses has been discussed in terms of the two components having an opposite effect on the membrane polarization of the hair cell.