Central pattern generators (CPGs) are networks of neurons that endogenously produce rhythmic output, typically used in motor control. Scott Hooper’s accepted definition of CPGs is that they must have rhythms in which: “(1) two or more processes that interact such that each process sequentially increases and decreases, and (2) that, as a result of this interaction, the system repeatedly returns to its starting condition.” Developmentally, indications are that CPG properties are innately established, and that new motor patterns are acquired by the animal but old capabilities are not lost, such that the CPG becomes increasingly multifunctional. There are two major types of CPGs:
1) CPGs driven by an endogenous oscillator neuron, whose oscillations are induced via interactions of its own membrane currents. An example of this is the pyloric network of crustaceans, driven by an endogenous oscillator called the anterior burster neuron. If you isolate the endongenous oscillator neuron in vitro, it will still produce its typical oscillatory output as driven by its own membrane currents.
2) CPGs driven by the activity of a large network. For example, consider the leech heartbeat rhythm generator. In this CPG, motorneurons have two possible states on each side of the heart, one of which beats the heart from back to front, and one of which causes the heart to beat in unison. Around every 20 beats the two sides of the heart switch states. Two of the neurons of the network possess both a hyperpolarization–activated inward current and a low-threshold persistent Na current. Neurons 3 and 4 are reciprocally inhibited by neurons 1 and 2. The result of this inhibition is that neurons 3 and 4 fire in antiphase, to allow only one of each state to be activated at a given time. This second type of CPG is more common.
CPGs are believed to underly many human functions. These include locomotion (with CPGs found in the thoracolumbar segments of the spinal cord), swallowing (with independent CPGs in the brainstem controlling oral, pharyngeal, and esophageal phases, and requring sensory feedback), respiration, ejaculation (neurons called the spinal generator for ejaculation in the spinal cord are capable of self-sustained rhythmic output to relevant motoneurons), and scratching.
Inspired by CalTech’s Question #12 for cognitive scientists: “What is a central pattern generator (CPG)?”
Lang IM. 2009 Brain stem control of the phases of swallowing. Dysphagia DOI: 10.1007/s00455-009-9211-6.
Hooper SL. 1999 Central Pattern Generators. Embryonic ELS. Pdf here.
Guertin PA, et al. 2009 Key central pattern generators of the spinal cord. Journal of Neuroscience Research DOI: 10.1002/jnr.22067
Buono PL, et al. 2002 A mathematical model of motorneuron dynamics in the heartbeat of the leech. Physica D: Nonlinear Phenomena doi:10.1016/j.physd.2003.08.003