Comparative aspects of neural circuits for inking behavior and gill withdrawal in Aplysia californica.

1. Defensive inking behavior and gill withdrawal in ApIysia offer simple test systems in which the cellular and biophysial determinants of elementary behavioral acts can be examined. Since a good deal is known regarding both the behaviors and their underlying neural circuits, it is possible to compare directly these two defensive reflexes at the cellular level. 2. Both the ink and gill motor neurons are activated by tactile or electrical stimulation of the mantle region. At least part of the sensory input is mediated by previously identified LE, RE, and RF cluster sensory neurons (4, 6). Some sensory neurons make direct monosynaptic connections to both gill and ink motor neurons. 3. In addition to the monosynaptic pathway, both types of motor neurons receive polysynaptic input. R 18 is an example of one excitatory interneuron that produces fast excitatory postsynaptic potentials (EPSPs) in both ink motor neuron L14 and gill motor neuron L7. 4. Tactile stimulation of the skin also excites a cluster of at least three interneurons, one of which has been identified as L31. L31 produces a slow decreased conductance EPSP in ink motor neuron L14 but appears to make no connection to gill motor neuron L7. 5. In addition to the excitatory input to ink motor neurons, there is a pronounced slow inhibitory input from a cluster of at least five cells, one of which has been identified as L32. In contrast to the predominant slow inhibition of ink motor neurons, L32 produces predominant fast EPSPs in gill motor neuron L7. 6. Tactile stimulation of the skin inhibits L32 and at least part of the inhibition is mediated by the previously identified interneuron L16 ( 18). L16 is weakly electrically coupled to ink motor neuron L14 but not to gill motor neuron L7. 7. The two circuits utilize common sensory neurons and interneurons and are mediated by both monoand polysynaptic pathways. However, some neurons (L32) have different synaptic actions on each type of motor neuron, others (L3 1) appear to be used for one circuit and not the other, while one neuron (L16) is electrically coupled to ink motor neuron L14 but not to gill motor neuron L7. In addition, the ink motor neurons are electrically coupled while the gill motor neurons are not (9, 11). Furthermore, the ink motor neurons have intrinsic biophysical properties, which contribute to their firing pattern (2, 9, 12). Thus, while some general features of organization of these reflexes are common, specific differences have developed that contribute to differences in the expression of the two behaviors (11, 26).