Adaptive motor control in crayfish

This article reviews the principles that rule the organization of motor com mands that have been described over the past five decades in crayfish. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sen sory feedback are presented in the order of increasing complexity. The simp lest circuits described are those involved in the control of a single joint during posture (negative feedback-resistance reflex) and movement (modulat ion of sensory feedback and reversal of the reflex into an assistance rifle r). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of diff erent limbs and different motor systems. In addition, beyond the question o f mechanical fitting, the motor command must be appropriate to the behavior al context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of n eural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permane ntly (such as plastic changes that modify the synaptic efficacy). Finally, the question of how "automatic" local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.