Neural Mechanisms of Leg Motor Control in Crayfish - Advancing Artificial Intelligence Through Biological Process Applications - page 29

into two groups according to the shape of their
response to 'ramp and plateau' CBCO stimula-
tion (Figure 3C; Le Ray et al. , 1997b): one pool
generates phasic bursts of EPSPs during release
(upward) ramps (Figure 3C1) while the second
develops an increasing depolarization throughout
the CBCO release, on top of which phasic EPSP
barrages are generated during ramps (Figure 3C2).
Although compelling, it has not been possible
to correlate the response shape with the type of
sensory input (phasic, phaso-tonic or tonic) that
the motoneuron receives, nor with its intrinsic
electrical properties. In contrast, simulation
experimentation suggested that the localization
of sensory inputs along the non-homogeneous
postsynaptic dendritic tree of a motoneuron de-
termines the shape and time constant of EPSPs.
Consequently, this may (or not) allow the develop-
ment of a tonic depolarizing response: proximal
synapses generate fast and large EPSPs involved
in phasic reflex responses, while distal synapses
produce slow and smaller EPSPs responsible for
tonic depolarization. Nevertheless, although the
motoneuron electrical properties appear to play a
minor role in the monosynaptic response to passive
movements, locomotor circuit activation confers
active properties on these motoneurons, thereby
modifying their integrative abilities (Cattaert et
al. , 1994a,b).

phase between returnstroke (where it preserves
muscular tonicity and initiates downward leg
movement at the end of the swing phase) and
powerstroke (where resistance Dep MNs coun-
teract gravity during stance and propel the body
forward). Moreover, the interconnection of Dep
MNs via electrical tight junctions (Chrachri &
Clarac, 1989) also permits a role for the assistance
motoneuron in the automatic recruitment of the
other Dep MNs at the transition between swing
and stance phases, thereby initiating the power-
stroke. In addition to this role, electrotonic con-
nections between motoneurons may substantially
organize the output motor response by allowing
all coupled motoneurons of a given pool to share
their sensory input information.
How does the CNS coordinate the responses
of the various motoneurons that receive distinct
inputs? Bernstein (1967) proposed that the CNS
elaborates motor synergies to prevent the overuse
of central resources that would require the process-
ing of all components independently. Evidently,
the control of leg movement in crayfish relies on
the 'simple' organization of two parallel sensory-
motor pathways that are activated according to the
phase of the locomotor cycle: one resists imposed
displacements, while the other participates in the
ongoing movement. Indeed, distinct mono- and
polysynaptic connections convey sensory infor-
mation toward motoneuron pools involved in either
the resistance to, or assistance of movement. In the
active phase, the CPG selects specific polysynaptic
pathways that activate a large proportion of, if
not all, motoneurons within a pool. Moreover, it
unmasks active properties in motoneurons that,
through gap junctions, may propagate to neighbor-
ing motoneurons and participate in the synergistic
activation of the pool involved in a given motor
task. In contrast, in the reflex mode, the response
has to be directly proportional to the perturbation,
and sensory inputs constitutes the only source of
motoneuronal activation, individual motoneuro-
nal integration shaping the unitary motor response
according to this specific input. In addition, the

Sensory-Motor Integration

In a mirror image of sensory coding, motoneu-
ronal reflex responsiveness is mainly dynamic,
allowing fast reactions to passive joint movements.
However, a static component is also present in
some motoneurons in order to prevent shifting or
saccadic responses that would, finally, produce
dis-equilibrium. Curiously, a purely phasic assis-
tance Dep MN exists whose function within the
network remains unclear. Because motoneurons
within a pool are recruited progressively during
walking, one can imagine that this particular as-
sistance Dep MN is recruited close to the transition