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
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