Neural Mechanisms of Leg Motor Control in Crayfish - Advancing Artificial Intelligence Through Biological Process Applications - page 30

wiring of the sensory-motor resistance pathway
alone allows for the sequential recruitment of Dep
MNs during postural correction (Le Ray et al. ,
1997a,b). However, the switch between postural
and locomotor modes involves the engagement of
the CPG that not only cyclically selects parallel
sensory-motor pathways, but also plays an impor-
tant role in shaping the properties and efficacies
of all the synapses involved.

Automatic Gain Control Achieved by
Postsynaptic Motoneurons

From Resting to Active State:
'Awakening' of the Negative
Sensory-Motor Feedback

A crayfish may spend a large part of its life in
a more or less totally immobile state. During
such a resting state, the negative sensory-mo-
tor feedback controlling leg movements must
be profoundly depressed. However, the synapse
involved is capable of rapid functional restoration
and capable of a long-lasting transformation to
a high level of synaptic transmission (Le Ray &
Cattaert, 1999). The underlying mechanisms have
been deciphered in experiments in which paired
intracellular recordings were made from a CBCO
sensory terminal and a postsynaptic motoneuron
(Figure 4A). After a long period of motoneuronal
inactivity, sensory spikes evoked small unitary
EPSPs (<0.2 mV) in the motoneuron (left inset
in Figure 4B). At this point, the postsynaptic
motoneuron was intracellularly stimulated with
depolarizing current pulses that each evoked a
few spikes (circle inset) over several minutes in
the absence of sensory stimulation. Within ten
minutes after such motoneuron activation, the
unitary EPSPs were potentiated up to 200% of
their control amplitude (right inset) for at least
three hours.
Presynaptic activation of the sensory terminal
alone (up to 100 Hz) never induced any monosyn-
aptic EPSP enhancement in the absence of post-
synaptic activity. Thus, the motoneuron activation
was necessary and sufficient to obtain specific
long-term potentiation (LTP) of its sensory-motor
synapses. During activation, glutamate (the moto-
neuronal excitatory neurotransmitter in crayfish
- Van Harreveld, 1980) is released and activates
metabotropic receptors located on the presynaptic
sensory terminals (Figure 4C). A quantal analysis
of the EPSP amplitude distribution before and

PLASTICITy AND MODULATION OF
SENSORy-MOTOR INTEGRATION

The sensory-motor circuits controlling the stretch
reflex of vertebrates, or the resistance reflex of
arthropods, operate like automatic controllers
(Clarac et al. , 2000). Therefore, mechanisms
should allow these circuits to adapt to the animal's
ongoing behavior. In the crayfish, the CBCO
terminal-to-motoneuron synapse involved in
the resistance reflex is monosynaptic and should
therefore, be constantly active. This feature
has two consequences: (1) the strength of the
synapse, which directly controls the gain of the
reflex, should vary to match the activation level
of postsynaptic motoneurons. If not, the gain
would be too high in resting animals and insuf-
ficient in very active ones; (2) depending on the
activity engaged, a monosynaptic negative feed-
back control may be inappropriate (see above).
During the past fifteen years, several adaptive
functional mechanisms of this sensory-motor
circuit have been highlighted (Cattaert et al. ,
1990; El Manira et al. , 1991; Cattaert et al. , 1992;
El Manira et al. , 1993; Cattaert & Le Ray, 1998;
Cattaert & El Manira, 1999; Le Ray & Cattaert,
1999; Cattaert et al. , 2001; Cattaert & Bévengut,
2002; Le Bon-Jego et al. , 2004; Le Ray et al. ,
2005; Le Bon-Jego et al. , 2006; and for reviews,
see Clarac et al. , 2000; Cattaert & Le Ray, 2001;
Cattaert et al. , 2002), and we now present three
classes of such mechanisms.