Scott L Hooper, Ohio University, Athens, Ohio, USA
Central pattern generators (CPGs) are neural networks that can produce rhythmic patterned outputs without rhythmic sensory or central input. CPGs underlie the production of most rhythmic motor patterns and have been extensively studied as models of neural network function.
A central goal of neuroscience is to understand how nervous systems produce movement. The simplest movements are reflexes (knee jerk, pupil dilation), which are involuntary, stereotyped and graded responses to sensory input, and have no threshold except that the stimulus must be great enough to activate the relevant sensory input pathway. Fixed action patterns (sneezing, orgasm) are involuntary and stereotyped, but typically have a stimulus threshold that must be reached before they are triggered, and are less graded and more complex than reflexes. Rhythmic motor patterns (walking, scratching, breathing) are stereotyped and complex, but are subject to continuous voluntary control. Directed movements (reaching) are voluntary and complex, but are generally neither stereotyped nor repetitive. Rhythmic motor patterns comprise a large part of behaviour. They are also complex (unlike reflexes) yet stereotyped (unlike directed movements) and, by definition, repetitive (unlike fixed action patterns). As a consequence of this combination of behavioural importance and experimental advantage, rhythmic motor pattern generation has been studied extensively. This work has shown that the basic rhythmicity and patterning of rhythmic motor patterns are produced by neural networks termed central pattern generators.
What is a Central Pattern Generator?
Central pattern generators (CPGs) are neural networks that can endogenously (i.e. without rhythmic sensory or central input) produce rhythmic patterned outputs; these networks underlie the production of most rhythmic motor patterns (Marder and Calabrese, 1996; Stein et al., 1997). The first modern evidence that rhythmic motor patterns are centrally generated was the demonstration that the locust nervous system, when isolated from the animal, could produce rhythmic output resembling that observed during flight (Wilson, 1961 cited in Marder and Calabrese, 1996). Subsequent work showed that, in a wide variety of animals, nervous systems isolated from sensory feedback could produce rhythmic outputs resembling those observed during rhythmic motor pattern production. This work has further shown that rhythmic pattern generation does not depend on the nervous system acting as a whole, but that CPGs are instead relatively small and autonomous neural networks. Concept of Fictive Motor Patterns A fictive motor pattern is a pattern of motor neuron firing that would, if the motor neurons were still attached to their muscles, result in the motor pattern in question being produced. This concept arose because the best evidence for CPGs comes from work on isolated nervous systems. However, these preparations raise a question: is the neural activity observed in vitro the same as that which generates the motor pattern observed in vivo? An unambiguous demonstration has been achieved in only a few invertebrate preparations in which the CPG and the muscles it innervates can be simultaneously maintained in vitro, and it can be shown that a given neural output pattern induces a specific motor output. Only slightly less ambiguous are preparations (again primarily invertebrate) in which the motor nerves contain sufficiently few axons that the activity of each motor neuron can be identified in vivo from chronic extracellular nerve recordings; in vivo and in vitro motor neuron activity can thus be compared directly. In preparations with nerves that have large numbers of axons, it is more difficult to show that the same motor neurons are active in vivo and in vitro. In these cases, demonstrating that correct nerve activity continues in vitro (e.g. flexor and extensor motor neuron activities maintain the correct phase relationship), without showing that identical motor neurons are firing, is often considered sufficient evidence of a correct fictive motor pattern.