Biomedical Engineering Reference
In-Depth Information
Sensory-Motor Synaptic Connections
movement, the polysynaptic assistance pathway
is concomitantly recruited to activate agonistic
motoneurons while the monosynaptic resistance
pathway is inhibited (see below), thereby silencing
the antagonistic motoneurons. In contrast, during
stance the monosynaptic connections prevail, and
the resistance reflex is now expressed.
ARIN is a nonspiking interneuron that gen-
erates graded depolarizations in response to in-
creasing CBCO stretches, which produce graded
long-lasting EPSPs in Dep MNs. Considering that
the ARIN only receives inputs from velocity-
coding afferents, the activation of the assistance
pathway thus depends on dynamic movement
parameters. However, because ARIN gradedly
and long-lastingly activates motoneurons, the
result will not consist of a phasic motoneuronal
activation, but rather of additional excitation to
the motoneurons carrying out the movement. In
addition, the ARIN receives phasic disynaptic
inhibitory inputs from stretch-sensitive afferents
via another interneuron, likely to prevent the
over-excitation of the motoneurons involved in
the ongoing movement. Thus, the CPG not only
recruits different sets of motoneurons, but also
controls the sensory-motor pathways in such a
way as to finely adapt their inputs to the prevail-
ing motor behavior.
Resistance reflexes evoked by CBCO stimula-
tion are mediated by parallel, direct and indirect,
excitatory and inhibitory connections between
primary sensory afferents and motoneurons
(El Manira et al. , 1991; Le Bon-Jego & Cattaert,
2002). For instance, releasing the CBCO activates
release-sensitive sensory neurons that elicit both
mono- and polysynaptic excitatory postsynaptic
potentials (EPSPs) in the motoneurons that would
oppose the imposed movement (Dep MNs in this
case). In parallel, the same sensory neurons inhibit
the antagonistic motoneurons (here, Lev MNs)
through a polysynaptic GABAergic pathway
(Figure 3B). Extensive studies of the monosyn-
aptic sensory-motor connections (El Manira et
al ., 1991; Le Ray et al. , 1997b) have shown that
each Dep or Lev MN receives information from
four to eight CBCO afferents.
This implicates that (i), the motoneurons them-
selves possess strong integrative capabilities, and
(ii), the proprioceptive information is distributed
to the two motoneuronal populations that control
the joint, whatever the phase in locomotor cycle.
This therefore requires that specific mechanisms
regulate cyclically the transmission of sensory
inputs toward their postsynaptic motoneurons
(see below).
Motoneuronal Reflex Responses
Mechanisms of Reflex Reversal
Because a major part of sensory-motor processing
is achieved through direct afferent-to-motoneuron
connections (El Manira et al. , 1991), motoneu-
rons integrate the various parameters of CBCO
information. Interestingly, among the pool of
12 Dep MNs (Le Ray & Cattaert, 1997), three
motoneurons show no monosynaptic response,
whereas eight others display a characteristic re-
sistance reflex response ( i.e. , they are activated by
CBCO release). Surprisingly, one Dep MN always
exhibits a direct assistance reflex response ( i.e. ,
activation by proprioceptor stretch). The popula-
tion of resistance Dep MNs can be further divided
When the central pattern generator (CPG) is active
(during actual or fictive locomotion), the sign of
the reflex is cyclically reversed, which consists
of a phase-dependent switch from a resistance
(negative feedback) to an assistance (positive
feedback) reflex. In crayfish, a key interneuron
is specifically involved in this switch for the Dep
MN population (Le Ray & Cattaert, 1997). This
'assistance reflex interneuron' (ARIN; Figure 3B)
is accessed directly by up to eight velocity-coding,
stretch-sensitive CBCO neurons and is likely to
project directly onto Dep MNs. During active leg
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