The neuronal networks underlying vertebrate locomotion have been studied in considerable detail using a lower vertebrate model system, the lamprey. The segmental network consists of ipsilateral excitatory glutamatergic and inhibitory crossed glycinergic interneurons. In addition, there is a sensory movement related input to the network from ipsilateral excitatory and crossed inhibitory stretch receptor neurons that help adapt the movements to external events (see Grillner et al. 2001). The network is activated from the brainstem via reticulospinal neurons, which in turn can be driven from mes- and diencephalic glutamatergic pathways. Visual and olfactory stimuli elicit goal-directed behaviour, most likely via the direct projections from the olfactory bulb and the optic tract to the diencephalic locomotor centre.

For the segmental pattern generation the intrinsic properties of the different network neurons play a critical role. One focus will be on the role of different subtypes of Ca²⁺ and Ca²⁺-dependent K⁺ channels for neuronal network function. The modulation of different ion channel subtypes affects neuronal function and causes thereby characteristic changes at the network level. Different modulators like aminergic and peptidergic transmitters often exert neuron- and synapse-specific effects. Modulators like tachykinins, in addition to short-term effects, also have effects on the cellular and network levels that are dependent on protein synthesis and last more than 24 h. In the lamprey network it is possible to bridge from the molecular and cellular to the behavioural level and predict what changes a modulation of a given type of ion channel in a given cell type will have on the network level.

In the analysis of this system, we have investigated the cellular and network properties experimentally, and explored their potential role by detailed mathematical modelling using model neurons with projection similar to their natural counterparts. These model neurons are connected as in the spinal cord. The simulation has greatly enhanced our insights into the mode of operation of the spinal locomotor system (Ekeberg et al. 1995).