Neuronal mechanisms for bilateral coordination of salivary gland activity in Helisoma

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Abstract
The salivary neuroeffector system of Helisoma consists of the paired salivary glands and buccal ganglia. Previous work demonstrated that neuronal control was required for coordination of activity in the two salivary glands. This neuronal control is provided by a pair of identified buccal ganglion neurons, 4R and 4L. This study examines the organization of this neuronal control and addresses the questions of monosynaptic vs. polysynaptic pathways as well as the bilateral effects of each neuron 4. Action potentials in neuron 4 elicit one-for-one EPSPs in a subpopulation of the salivary cells. These EPSPs can, in some cases, be increased by TEA injection into a neuron 4 and are unaffected by the addition of six-times normal calcium. These data coupled with the constancy of synaptic transmission, as well as morphological evidence, further indicate the monosynaptic nature of the connection between neurons 4 and salivary secretory cells. Three different mechanisms exist to insure that activity in 4R and 4L result in coordinated activation of the salivary glands: (1) Lucifer Yellow injection and direct intracellular recording and stimulation demonstrate that both 4R and 4L can send axons to and innervate both salivary glands; (2) both 4R and 4L receive virtually identical synaptic input from higher-order buccal ganglion neurons; and (3) 4R and 4L are electrically coupled. Thus, the system is organized with a high degree of redundancy, and bilateral synchrony of glandular activity is assured by mechanisms at various levels of neuronal organization.