Neural Mechanisms of Leg Motor Control in Crayfish - Advancing Artificial Intelligence Through Biological Process Applications - page 37

sponsiveness to electrical stimulation of the cut
sensory nerve (mechanical stimulation being no
longer possible). In control animals, this evokes a
typical biphasic response through the recruitment
of both monosynaptic EPSPs and oligosynaptic
PSPs (see above). Within 21 days following pro-
prioceptor ablation, no changes were observed in
the motoneuronal response. Thereafter, the mono-
synaptic EPSPs disappear, but the oligosynaptic
IPSPs persist for up to six months more. Beyond
this period, no functional synaptic connection
seems to remain (Figure 6A1). However, apply-
ing in vitro a tonic electrical stimulation to the
sensory nerve for several minutes was sufficient
to progressively restore both components in a
characteristic sequence in which IPSPs reappeared
first, then EPSPs (Figure 6A2). Furthermore,
chronic stimulation in vivo of the proximal end
of the cut sensory nerve prevented the loss of
synaptic transmission (Le Bon-Jego et al. , in
prep). Thus, as shown in many networks (see:
Davis & Bezprozvanny, 2001; Perez-Otaño &
Ehlers, 2005) activity is necessary to preserve
the full functionality of synapses. However, it
also appears that in the absence of activity, some
compensatory mechanisms exist, mainly to assure
the inhibitory pathway onto Dep MNs. Because
the leg is kept elevated when the CBCO nerve is
cut, we may hypothesize that the reverse effects
are produced in the CBCO-to-Lev MNs pathway,
including the 'protection' of excitatory connec-
tions. Although the functional significance of this
latter process remains unclear, it may represent
a control mechanism that would prevent any
over-activation of the depressor muscle in case of
sensory (CBCO) injury, which would cause the
leg to trail continuously on the ground.

status (Issa et al. , 1999). In crayfish, social status
is established by fighting, the victor becoming
dominant and the vanquished a subordinate.
Because both social states are characterized by
distinct postures, we analyzed the central effects
of serotonin on the CBCO sensory-motor system
in vitro (Le Bon-Jego et al. , 2004). Serotonin was
found to enhance resistance reflex responses in
a subclass of animals (Figure 6B1), correspond-
ing to the typical elevated posture of dominant
animals. We demonstrated that this enhance-
ment is achieved through multiple effects that
act cooperatively at different levels in the sen-
sory-motor synaptic pathway (Figures 6B2-4):
(1) increased motoneuron input resistance, (2)
increased efficacy of synaptic transmission from
sensory afferents to excitatory interneurons, and
(3) increased excitability of these interneurons
contributing to an improvement of the amplitude
of motoneuronal polysynaptic EPSPs.

CONCLUSION

The temporal dynamics of sensory feedback
processing are critical factors in the generation
of adapted behaviors. While difficult to tackle in
complex models such as mammals and especially
the human nervous system, the main principles
of neuronal integration have been identified and
described in detail in invertebrates. Consequently,
artificial systems will probably continue to gain a
lot from knowledge in invertebrates and, especial-
ly, crustacean neurobiology. For example, twenty
years ago, population vector coding was proposed
to support movement commands (Georgopoulos
et al. , 1986). In crayfish, like human skilled hand
tasks (Roll et al. , 2000), the sensory information
associated with limb movement is also coded in
terms of a vector sum, suggesting that this process
may be a general feature of sensory coding. On
this basis, a future challenge will be to understand
how the CNS dynamically integrates multiple

Serotonergic Neuromodulation

Serotonin is an endogenous biogenic amine that
plays critical roles in many different behaviors,
such as aggressiveness or locomotion, and also
in the establishment and maintenance of social