Central pattern generator

Central pattern generators (CPGs) are biological neural networks that produce rhythmic patterned outputs without sensory feedback. A CPG could be crudely analogized to the pendulum of a clock, generating a repeating signal at a constant frequency in order to coordinate rhythmic motions. CPGs have been shown to produce rhythmic outputs resembling normal "rhythmic motor pattern production" even in isolation from motor and sensory feedback from limbs and other muscle targets. To be classified as a rhythmic generator, a CPG requires:

CPGs have been found in practically all vertebrate species investigated, including human.

Various molecular, genetic and imaging studies have been conducted as for the localization of the CPGs. The results have shown that the networks responsible for locomotion are distributed throughout the lower thoracic and lumbar regions of the spinal cord. Rhythmic movements of the tongue, that participate in swallowing, mastication and respiration, are driven by hypoglossal nuclei, which receive inputs from the dorsal medullary reticular column (DMRC) and the nucleus of the tractus solitarius (NTS). The hypoglossal nucleus receives rhythmic excitatory inputs also from brainstem respiratory neurons within the pre-Boetzinger complex, which appears to play an important role in the origin of respiration rhythmogenesis.

Although anatomical details of CPGs are specifically known in only a few cases, they have been shown to originate from the spinal cords of various vertebrates and to depend on relatively small and autonomous neural networks (rather than the entire nervous system) to generate rhythmic patterns. Many studies have been done to determine the neural substrate of locomotor CPGs in mammals. Neural rhythmicity can arise in two ways: "through interactions among neurons (network-based rhythmicity) or through interactions among currents in individual neurons (endogenous oscillator neurons)". A key to understanding rhythm generation is the concept of a half-center oscillator (HCO). A half-center oscillator consists of two neurons that have no rhythmogenic ability individually, but produce rhythmic outputs when reciprocally coupled. Half-center oscillators can function in a variety of ways. First, the two neurons may not necessarily fire in antiphase and can fire in any relative phasing, even synchrony, depending on the synaptic release. Second, half-centers can also function in an "escape" mode or a "release" mode. Escape and release refer to the way the off-neuron turns on: by escape or release from inhibition. Half-center oscillators can also be altered by intrinsic and network properties and can have dramatically different functionality based on variations in synaptic properties.

...
Wikipedia