BOX 1contd 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.

Leech Swim Central Pattern Generator

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 (4) Termination—After a final dorsal flexion the animal returns to an unflexed position with the extremities still withdrawn, oral veil and tail enlarged.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).

Continued

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