Despite many years of research, the precise organization of spinal circuitry involved in locomotion is poorly understood. Motoneurons are thought to be the sole mediators of motor output from the spinal cord. However, new evidence suggests that they may play an active role in the production of locomotor activity. We found that stimulation of motor axons can trigger locomotor activity in the isolated neonatal mouse cord. We also discovered that a second fast excitatory neurotransmitter (glutamate or aspartate), in addition to acetylcholine, is released from recurrent collaterals of neonatal motoneurons (Mentis et al, 2005). To explore this issue in more detail, I am concentrating on two aspects of spinal network organization: (i) the structural and functional interactions amongst motoneurons and (ii), between motoneurons and spinal interneurons.

Current experiments are focused on the possibility that motoneurons are not only connected electrically but also synaptically. Preliminary results demonstrate that some motoneurons receive cholinergic synaptic boutons from other motoneuron axon collaterals. To test if these apparent connections are functional, I am recording intracellularly from motoneurons and examining their ventral root-evoked synaptic responses in the presence of cholinergic and glutamatergic receptor antagonists. To shed further light on the spatial organization of motoneuron recurrent collaterals, intracellular dyes are used in combination with immunohistochemical markers of various neurotransmitters.

To identify putative interneurons that are involved in locomotor activity we combine optical imaging techniques with new methods to load calcium sensitive dyes into interneurons. I can then monitor changes in the intracellular calcium concentration during episodes of locomotor-like activity.