When networks walk a fine line: balance of excitation and inhibition in spinal motor circuits

Investigations on spinal motor circuits have primarily been related to direct connections to motoneurons and supraspinal input, while the motor pattern generation circuit itself has remained elusive. In the classical half-center model (HCM), motor patterns are generated by feedforward excitation with reciprocal inhibition. However, experiments over the last decade have indicated that inhibition, besides providing reciprocal coordination, may serve additional roles similar to that seen in the brain. Such organization relies on recurrent inhibition to give stability of the spiking activity manifested by simultaneous increases in excitation and inhibition within the network, that is, a 'balanced network'. Here we discuss the theoretical concepts and experimental data for and against this architecture in motor circuits, and suggest how it can be integrated in the conventional HCM.