Neural Circuit for Inking Behavior in Aplysia calz$omica

1, Inking behavior is a stereotyped response, which can be utilized as a model system to study the neuronal mechanisms underlying simple neuroglandular behaviors. While much is known about the features of the behavior and its motor neurons (2, 811, 20), little is known regarding the sensory neurons and interneurons that drive the motor cells. The present study was undertaken to examine these additional elements of the neural circuit. 2. Strong tactile stimuli presented to the mantle region excite the ink motor neurons. At least part of the sensory input is mediated by previously identified LE and RE cluster mechanoreceptor neurons as well as by a newly identified cluster (RF) of mechanoreceptor sensory neurons, The LE and RE cells innervate the siphon and mantle shelf, while the RF cells partially overlap these regions and also innervate the gill. 3. Some of the sensory neurons produce monosynaptic excitatory postsynaptic potentials (EPSPs) in the L14 ink motor neurons, others produce EPSPs that have both monoand polysynaptic components, while still others produce EPSPs that have only polysynaptic components. 4. There are at least two types of polysynaptic excitatory pathways. Sensory neurons make monosynaptic connections onto an interneuron, identified as cell R18, which in turn makes a monosynaptic increasedconductance fast EPSP to the ink motor neurons. Tactile stimulation of the skin also excites a cluster of at least three interneurons, one of which has been identified as L31. L3 1 makes a slow decreased-conductance EPSP to the L14 ink motor neurons.