Perioral synaptic connections and their possible role in the feeding behavior of Hydra

Electron microscopic observations of serially sectioned perioral neurons revealed a complex synaptic organization in which reciprocal synapses were observed for the first time in Hydra. Sensory cells had reciprocal synapses with each other and with ganglion cells, which in turn had reciprocal synapses with each other. A two-way chemical synapse with vesicles on both sides of the paramembranous densities was observed between ganglion cells; none was found between sensory cells. Ganglion cell axons participated in serial axo-axo-epitheliomuscular synapses. Two-cell pathways formed by direct sensory cell-nematocyte or neuromuscular synapses and three-cell pathways forming indirect sensory cell-ganglion cell-nematocyte or neuromuscular synaptic interconnections were found. It is possible that either simple direct changes in or direct effects on threshold stimuli could trigger both nematocyst discharge and/or muscular contraction and effect more complex intermediate pathways modulating feeding behavior. Each large epitheliomuscular cell enveloped from one to four sensory cells in the perioral region. The concentration of sensory cells around the mouth and their complex synaptic connections with each other and with ganglion and effector cells support our hypothesis for neural control of feeding behavior in Hydra.