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1.  Synaptic integration of rhythmogenic neurons in the locomotor circuitry: the case of Hb9 interneurons 
Innovative molecular and genetic techniques have recently led to the identification of genetically defined populations of ipsilaterally projecting excitatory interneurons with probable functions in the rhythm-generating kernel of the central pattern generators (CPGs). The role of interneuronal populations in specific motor function is determined by their synaptic inputs, intrinsic properties, and target neurons. In this review we examine whether Hb9-expressing interneurons (Hb9 INs) fulfill a set of criteria that are the hallmarks of rhythm generators in the locomotor circuitry. Induced locomotor-like activity in this distinct population of ventral interneurons is in phase with bursts of motor activity, raising the possibility that they are part of the locomotor generator. To increase our understanding of the integrative function of Hb9 INs in the locomotor CPG, we investigated the cellular mechanisms underlying their rhythmic activity and examined the properties of synaptic inputs from low-threshold afferents and possible synaptic contacts with segmental motoneurons. Our findings suggest that the rhythmogenic Hb9 INs are integral components of the sensorimotor circuitry that regulate locomotor-like activity in the spinal cord.
doi:10.1111/j.1749-6632.2010.05533.x
PMCID: PMC3057624  PMID: 20536922
locomotor-like rhythms; rhythmogenic interneurons; Hb9 interneurons; rhythm-generating kernel; locomotor central pattern generator; Hb9:eGFP transgenic mouse
2.  Mechanisms of excitation of spinal networks by stimulation of the ventral roots 
It has recently been demonstrated that motoneurons in neonatal rodents release an excitatory amino acid, in addition to acetylcholine, from their central terminals onto Renshaw cells. Although the function of this amino acid release is not understood, it may mediate the excitatory actions of motor axon stimulation on spinal motor networks. Stimulation of motor axons in the ventral roots or muscle nerves can activate the locomotor central pattern generator or entrain bursting in the disinhibited cord. Both of these effects persist in the presence of cholinergic antagonists and are abolished or diminished by ionotropic and metabotropic glutamate antagonists.
Calcium imaging in the disinhibited cord shows that a ventral root stimulus evokes ventrolateral activity initially which subsequently propagates to the rest of the cord. This finding suggests that excitatory interneurons excited by motoneuron recurrent collaterals are located in this region. However, motoneurons do not exhibit short latency excitatory potentials in response to ventral root stimulation indicating that the excitatory effects are mediated polysynaptically. The significance of these findings is discussed.
doi:10.1111/j.1749-6632.2010.05535.x
PMCID: PMC3033581  PMID: 20536921
calcium imaging; motoneuron; recurrent excitation; spinal cord
3.  Mechanisms regulating the specificity and strength of muscle afferent inputs in the spinal cord 
We investigated factors controlling the development of connections between muscle spindle afferents, spinal motor neurons and inhibitory Renshaw cells. Several mutants were examined to establish the role of muscle spindles, muscle spindle-derived NT3 and excess NT3 in determining the specificity and strength of these connections. The findings suggest that although spindle-derived factors are not necessary for the initial formation and specificity of the synapses, spindle-derived NT3 seems necessary for strengthening homonymous connections between Ia afferents and motor neurons during the second postnatal week. We also found evidence for functional monosynaptic connections between sensory afferents and neonatal Renshaw cells although the density of these synapses decreases at P15. We conclude that muscle spindle synapses are weakened on Renshaw cells while they are strengthened on motor neurons. Interestingly, the loss of sensory synapses on Renshaw cells was reversed in mice over-expresssing NT3 in the periphery, suggesting that different levels of NT3 are required for functional maintenance and strengthening of spindle afferent inputs on motor neurons and Renshaw cells.
doi:10.1111/j.1749-6632.2010.05538.x
PMCID: PMC3027487  PMID: 20536937
Proprioceptor; muscle spindle; motor neuron; Renshaw; stretch reflex

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