Some of the most striking examples of the use of invertebrate models to investigate the neural mechanisms underlying behavior come from studies of fixed pattern generators, also known as central pattern generators. There are many examples of animals utilizing fixed patterns of movement, for example in cases where ongoing repetitive movements are utilized subconsciously (walking for example), or where a rapid but fixed response pattern is required (such as dodging an oncoming object that you don't see until the last second).
Crabs and lobsters have been widely used to study one example of repetitive subconscious movements. The stomatogastric ganglion in crustaceans such as these controls a stereotyped pattern of muscle contractions in the animals' digestive systems.
The muscle movement pattern is a highly synchronized, coordinated response to food ingestion that serves to provide the smooth movement of foodstuffs down the digestive tract. Many details of the neuronal circuitry and coordinated firing of individual neurons have been worked out for this system, along with an impressive dissection of the underlying cellular physiology.
In some cases the stereotyped behaviors can be quite elaborate, involving extended, multicomponent patterns of movement in the entire animal. One such example is a defensive escape response exhibited by the opisthobranch mollusc Tritonia (see Figures). Predatory starfish feed upon Tritonia, and a starfish touching Tritonia leads to the animal exhibiting a stereotyped response of defensive withdrawal and escape swimming.
BOX 1—cont'd CENTRAL PATTERN GENERATORS
Again, this pattern of behavior is mediated by the highly coordinated firing of an elaborate network of neurons in the animal's nervous system. Much of the circuitry and cellular physiology of this central pattern generator was worked out in the late Peter Getting's laboratory.
Why do I bring this up in the context of general theories of the chemistry of memory? Because these are classic examples of hard-wired behavioral responses. They are seemingly immutable in the absence of injury to the animal or its nervous system. Nevertheless, even highly stable behavioral patterns are mediated by neurons whose molecular constituents are undergoing constant turnover. Self-perpetuating chemical reactions, not anatomy, provide the constancy of behavioral output in these "fixed" patterns.
BOX 1 Escape swimming in Tritonia, a fixed-action pattern consisting of four stages. (A and B) Stages of Tritonia escape. (1) Contact and withdrawal—The relaxed animal with branchial tufts and rhinopores extended contacts a predator. After contact with a starfish the animal withdraws reflexly and bends ventrally. (2) Preparation for swimming—The animal elongates and enlarges the oral veil while bending slightly in the dorsal direction. (3) Swimming—The animal first makes vigorous ventral flexion and then vigorous dorsal flexion. This cycle is repeated several times (adapted from a figure by Tom Prentis). (4) Termination—After a final dorsal flexion the animal returns to an unflexed position with the extremities still withdrawn, oral veil and tail enlarged. One to five dorsal flexions occur before the animal regains its original relaxed posture. (C) Escape response (photograph by Bill Frost).
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