9514617 Brodfuehrer It is commonly observed that an animal when presented with the same stimulus on successive trials will often perform different behaviors. This means that an animal decides, often very rapidly, which behavior is appropriate for the circumstances at that moment. The concept of `motivation' is often used to explain this type of short-term behavioral variability observed under conditions where sensory input is constant. The underlying neuronal mechanism of motivation, however, has yet to be defined. Since an animal's survival depends upon it continually making the appropriate behavioral response to ever changing environmental conditions, understanding how an animal decides upon an appropriate motor response is integral to our understanding of animal behavior. In the medicinal leech, research on the neuronal basis of swimming has progressed to the point where the initiation of swimming following body wall stimulation can be traced from the sensory neurons that perceive the stimulus to the motor neurons that produce the swimming movements. However, stimulation of this pathway does not always elicit swimming. The probability that swimming occurs following a given stimulus trial is quite variable. The role that a pair of identified interneurons in the leech head ganglion plays in biasing the behavioral response of a leech toward swimming following a given sensory input will be addressed in this proposal. The results generated from this study will provide additional insights into the principles governing behavioral variability in the leech with respect to the initiation of swimming, and further enhance our understanding of the types of neuronal networks and mechanisms that can influence the motivational state of animals.
