How the mesencephalic locomotor region recruits hindbrain neurons

This chapter summarizes experiments which were designed to reveal how repetitive electrical stimulation of the mesencephalic locomotor region (MLR) recruits nearby hindbrain neurons into activity, such that locomotion can ensue in the tiger salamander, A. tigrinum. The MLR stimulus strength was subthreshold or near-threshold for locomotor movements to ensue. Such relatively weak stimulation of the MLR produced locomotor movements after a relatively long delay, which featured neuronal interactions in the hindbrain. MLR-evoked spike responses of single hindbrain neurons were recorded before locomotor movements began. This allowed consideration of the build-up of the hindbrain neuronal activity, which was subsequently impressed upon the spinal cord such as to evoke locomotor movements. Each train of MLR stimulus pulses evoked monosynaptic responses in but a small proportion of the hindbrain's neurons. Rather, oligosynaptic responses were routinely evoked, even in the "input" neurons that were activated monosynaptically. Consecutive stimulus volleys recruited a given neuron after a variable number of synaptic translations. It is argued that the hindbrain's input neurons excited a much larger number of other hindbrain neurons. By this means, an MLR-evoked, short-lived propagating wave of excitation (i.e., approximately 2-4 successive synaptic activations) can be spread throughout the hindbrain.