Léna Jami

Reduction of Presynaptic Action Potentials by PAD: Model and Experimental Study

A compartmental model of myelinated nerve fiber was used to show that primary afferent depolarization (PAD), as elicited by axo-axonic synapses, reduces the amplitude of propagating action potentials primarily by interfering with ionic current responsible for the spike regeneration. This reduction adds to the effect of the synaptic shunt, increases with the PAD amplitude, and occurs at significant distances from the synaptic zone. PAD transiently enhances the sodium current activation, which partly accounts for the PAD-induced fiber hyperexcitability, and enhances sodium inactivation on a slower time course, thus reducing the amplitude of action potentials. In vivo, intra-axonal recordings from the intraspinal portion of group I afferent fibers were carried out to verify that depolarizations reduced the amplitude of propagating action potentials as predicted by the model. This article suggests PAD might play a major role in presynaptic inhibition.

Flexible processing of sensory information induced by axo-axonic synapses on afferent fibers.

Recent experiments indicate that afferent information is processed in the intraspinal arborisation of mammalian group I fibres. During muscle contraction, Ib inputs arising from tendon organs are filtered out by presynaptic inhibition after their entry in the spinal cord. This paper reviews the mechanisms by which GABAergic axo-axonic synapses, i.e., the morphological substrate of presynaptic inhibition, exert this filtering effect. Using confocal microscopy, axo-axonic synapses were demonstrated on segmental Ib collaterals. Most synapses were located on short preterminal and terminal branches. Using a simple compartmental model of myelinated axon, the primary afferent depolarisation (PAD), generated by such synapses, was predicted to reduce the amplitude of incoming action potentials by inactivating the sodium current, and this prediction was experimentally verified. A further theoretical work, relying on cable theory, suggests that the electrotonic structure of collaterals and the distribution of axo-axonic synapses allow large PADs (about 10 mV) to develop on some distal branches, which is likely to result in a substantial presynaptic inhibition. In addition, the electrotonic structure of group I collaterals is likely to prevent PAD from spreading to the whole arborisation. Such a non-uniform diffusion of the PAD accounts for differential presynaptic inhibition in intraspinal branches of the same fibre. Altogether, our experimental and theoretical works suggest that axo-axonic synapses can control the selective funnelling of sensory information toward relevant targets specified according to the motor task.

Short-latency synaptic patterns among cat peroneal motoneurons

Motoneurons innervating peroneal muscles in the cat leg (PB, PT and PL, respectively, for peroneus brevis, tertius and longus) were examined for their connections with afferents from these and other leg muscles and with cutaneous afferents. The aim was to investigate (1) whether inputs from nearby muscles and cutaneous areas are likely to assist or oppose the excitation elicited in peroneal motoneurons by PB contractions, and (2) whether reflex connectivity might allow distinction of alpha (i.e. motoneurons innervating skeletal muscle fibres) and beta (i.e. motoneurons innervating both skeletal and intrafusal muscle fibres) subgroups among PB and PT motoneurons. In the three peroneal pools, every motoneuron had excitatory monosynaptic connections with Ia afferents from each of the three peroneal muscles, and nearly every motoneuron received di- or trisynaptic excitation from low-threshold cutaneous afferents in sural or superficial peroneal nerves. Inputs from these sources might facilitate the contraction-induced positive feedback. In contrast, the patterns of short-latency synaptic connections with group I afferents from pretibial flexor and post-tibial extensor muscles were heterogeneous among peroneal motoneurons but did not point to any specific beta pattern.

A strategy of faster movements used by elderly humans to lift objects of increasing weight in ecological context

It is not known whether, during the course of aging, changes occur in the motor strategies used by the CNS for lifting objects of different weights. Here, we analyzed the kinematics of object-lifting in two different healthy groups (young and elderly people) plus one well-known deafferented patient (GL). The task was to reach and lift onto a shelf an opaque cylindrical object with changing weight. The movements of the hand and object were recorded with electromagnetic sensors. In an ecological context (i.e. no instruction was given about movement speed), we found that younger participants, elderly people and GL did not all move at the same speed and that, surprisingly, elder people are faster. We also observed that the lifting trajectories were constant for both the elderly and the deafferented patient while younger participants raised their hand higher when the object weighed more. It appears that, depending on age and on available proprioceptive information, the CNS uses different strategies of lifting. We suggest that elder people tend to optimize their feedforward control in order to compensate for less functional afferent feedback, perhaps to optimize movement time and energy expenditure at the expense of high precision. In the case of complete loss of proprioceptive input, however, compensation follows a different strategy as suggested by GLs behavior who moved more slowly compared to both our younger and older participants.

Corrigenda for Glycogen-depletion method of intrafusal distribution of γ-axons that increase sensitivity of spindle secondary endings

Page 16: Lena Jami, Dominique Lan-Couton, and Julien Petit. The title of the article should read: “A study with the glycogen-depletion method of intrafusal distribution of ggr-axons that increase s...