Initiation and generation of Movements: 1. Central Pattern Generators

The complexity of motor control is, to a great extent, overcome by hierarchical organization of the controlling system. Lower levels of this system contain a set of central pattern generators (CPGs). the neuronal networks capable of producing the basic spatio-temporal pattern underlying different “automatic” movements (rhythmic movements like locomotion, respiration, as well as non-rhythmic ones like swallowing and defense reactions) in the absence of peripheral sensory feedback. Instead of controlling individual muscles involved in generation of a definite motor pattern, higher centers (through command system) activate the corresponding CPG that generates this pattern. The most detailed analysis of CPGs has been performed for rhythmical movements. In these experiments, sensory feedback was abolished using in vitro (see Figures 1D; 2D; 3D), immobilized (see Figure 4B-D), or deafferented preparations.

To figure out how a CPG operates one has to address the following questions: First, what is the source of rhythmicity in the network? Second, what mechanisms determine the temporal pattern of the motor output, that is, its frequency and the relative duration of the cycle phases? Third, what mechanisms shape the motor output, that is determine the number of phases in the cycle and the transition from one phase to another?

In the majority of CPGs, two parts can usually be distinguished: a rhythm generator and an output stage. The rhythm generator is the neuronal network in which the rhythm originates; it also determines a relative duration of the cycle phases. This network is usually formed by interneurons and does not include motoneurons. The output stage is formed by interneurons and motoneurons; they receive inputs from the rhythm generator but do not affect the rhythm. The output stage produces a final shaping of the motor output.