Central pattern generators

"Central pattern generators (CPGs) can be defined as neural networks that can endogenously (i.e. without rhythmic sensory or central input) produce rhythmic patterned outputs" or as "neural circuits that generate periodic motor commands for rhythmic movements such as locomotion." 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: 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.

Anatomy
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. 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-centre oscillator consists of two neurons that individually have no rhythmogenic ability, but which 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 in which 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. For a more detailed description of the neural circuitry underlying the leech heartbeat rhythm generator and the pyloric network of decapod crustacea see Hooper's Review of Central Pattern Generators.

Neuromodulation
Organisms must adapt their behavior to meet the needs of their internal and external environments. Central pattern generators, as part of the neural circuitry of an organism, can be modulated to adapt to the organism's needs and surroundings. Three roles of modulation have been found for CPG circuits:
 * 1) Modulation in CPG as Part of Normal Activity
 * 2) Modultion Changes the Functional Configuration of CPGs to Produce Different Motor Outputs
 * 3) Modulation Alters CPG Neuron Complement by Switching Neurons Between Networks and Fusing Formerly Separate Networks into Larger Entities

For example, the Tritonia diomedea swimming CPG can produce reflexive withdrawal in response to weak sensory input, escape swimming in response to strong sensory input and crawling after escape swimming has ceased. The dorsal swim interneurons (DSIs) of the swim CPGs not only cause the rhythmic escape swimming, but also connect to cilia-activating efferent neurons. Experimental evidence confirms that both behaviors are mediated by the DSIs. "Given the extreme differences between these behaviors—-rhythmic versus tonic, muscular versus ciliary, and brief versus prolonged—these findings reveal a striking versatility for a small multifunctional network." "Part of this flexibility is caused by the release of serotonin from the DSIs, which causes cerebral cell 2 (C2) to release more transmitter and strengthen its network synapses. Application of serotonergic antagonists prevents the network from producing the swimming pattern, and hence this intranetwork modulation appears essential for network oscillation."
 * Modulation in CPG as Part of Normal Activity

Data from experiments by Harris-Warrick in 1991 and Hooper and Marder in 1987 suggest that the functional target of modulation is the entire CPG network. This idea was first observed through experiments with the neuromodulator in the lobster. The effect of proctolin could not be understood by looking only at the neurons it directly affected. "Instead, neurons that are not directly affected both alter the response of the directly affected neurons and help to transmit the changes in the activity of these neurons throughout the network," allowing the entire network to change in consistent and synchronized way. Harris-Warrick and colleagues have conducted many studies over the years of the effects of neuromodulators on CPG neural networks. For example, a 1998 study showed the distributed nature of neuromodulation and that neuromodulators can reconfigure a motor network to allow a family of related movements. Specifically, dopamine was shown to affect both individual neurons, and synapses between neurons. Dopamine strengthens some synapses and weakens others by acting by pre- and post-synaptically throughout the crustacean stomatogastric ganglion. These responses, as well as other effects of dopamine, can be opposite in sign in different locations, showing that the sum of the effects is the overall network effect and can cause the CPG to produce related families of different motor outputs.
 * Modulation Changes the Functional Configuration of CPGs to Produce Different Motor Outputs

A single neuronal network, such as a central pattern generator, can be modulated moment-to-moment to produce several different physical actions depending on the needs of the animal. These were first coined "polymorphic networks" by Getting and Dekin in 1985. An example of one such polymorphic central pattern generator is a multifunctional network of the mollusk Tritonia diomedea. As described by Hooper, weak sensory input to the swimming CPG produces reflexive withdrawal, while strong input produces swimming. The dorsal swim interneurons (DSIs) of the circuit release serotonin to convert to "swim mode," while application of serotonergic antagonists prevents the swim pattern. Additionally, the same single interneuronal network has been found to produce not only "rhythmic, muscle-based escape swimming," but also "nonrhythmic, cilia-mediated crawling." Evidence also suggests that although the CPG controls related but separate functions, neuromodulation of one function can occur without affecting the other. For example, the swim mode can be sensitized by serotonin without affect the crawl mode. Thus, the CPG circuit can control many separate functions with the appropriate neuromodulation.
 * Modulation Alters CPG Neuron Complement by Switching Neurons Between Networks and Fusing Formerly Separate Networks into Larger Entities

Feedback Mechanism
Although the theory of central pattern generation calls for basic rhythmicity and patterning to be centrally generated, CPGs can respond to sensory feedback to alter the patterning in behaviorally appropriate ways. Alteration of the pattern is difficult because feedback received during only one phase may require changed movement in the other parts of the patterned cycle to preserve certain coordination relationships. For example, walking with a pebble in the right shoe will alter the entire gait, even though the stimulus is only present while standing on the right foot. Even during the time when the left foot is down and the sensory feedback is inactive, action is taken to prolong the right leg swing and extend the time on the left foot, leading to limping. This effect could be due to widespread and long-lasting effects of the sensory feedback on the CPG or due to short-term effects on a few neurons that in turn modulate nearby neurons and spread the feedback through the entire CPG in that way. Some degree of modulation is required to allow one CPG to assume multiple states in response to feedback.

Additionally, the effect of the sensory input will vary depending on the phase of the pattern in which it occurs. For example, during walking, resistance to the top of the swinging foot (i.e. by a horizontal stick) causes the foot to be lifted higher to move over the stick. However, the same input to the standing foot cannot cause the foot to lift or the person will collapse. Thus, depending on the phase, the same sensory input can cause the foot to be lifted higher or held more firmly to the ground. "This change in motor response as a function of motor pattern phase is called reflex reversal, and has been observed in invertebrates (DiCaprio and Clarac, 1981) and vertebrates (Forssberg et al., 1977). How this process occurs is poorly understood, but again two possibilities exist. One is that sensory input is appropriately routed to different CPG neurons as a function of motor pattern phase. The other is that the input reaches the same neurons at all phases, but that, as a consequence of the way in which the network transforms the input, network response varies appropriately as a function of motor pattern phase."

A recent study by Gottschall and Nichols studied the hindlimb of a decerebrate cat during walking (a CPG controlled function) in response to changes in head pitch. This study describes the differences in gait and body position of cats walking uphill, downhill and on level surfaces. Proprioceptive (Golgi tendon organs and muscle spindles) and exteroreceptive (optic, vestibular and cutaneous) receptors work alone or in combination to adjust the CPG to sensory feedback. The study explored the effects of neck proprioceptors (giving information about the relative location of the head and body) and vestibular receptors (giving information about the orientation of the head relative to gravity). Decerebrate cats were made to walk on a level surface with their heads level, tilted up or tilted down. Comparing the decerebrate cats to normal cats showed similar EMG patterns during level walking and EMG patterns that reflected downhill walking with the head titled up and uphill walking with the head tilted down. This study proved that neck proprioceptors and vestibular receptors contribute sensory feedback that alters the gait of the animal. This information may be useful for treatment of gait disorders.

Functions of Central Pattern Generators
Central pattern generators can serve many functions in vertebrate animals. CPGs can play roles in movement, breathing, rhythm generation and other oscillatory functions. The sections below will focus on specific examples of locomotion and rhythm generation, two key functions of CPGs.

Locomotion
The first modern evidence of the central pattern generator was produced by isolating the locust nervous system and showing that it could produce a rhythmic output in isolation resembling that of the locust in flight. This was discovered by Wilson in 1961. Since that time, evidence has arisen for the presence of central pattern generators in vertebrate animals. This section will address the role of the central pattern generator in locomotion for the lamprey and humans.

The lamprey has been used as a model for vertebrate CPGs because, while its nervous system has a vertebrate organization, it shares many positive characteristics with invertebrates. When removed from the lamprey, the intact spinal cord can survive for days in vitro. It also has very few neurons and can be easily stimulated to produce a fictive swimming motion indicative of a central pattern generator. As early as 1983, Ayers, Carpenter, Currie and Kinch proposed that there was a basal CPG responsible for most undulating movements in the lamprey including swimming forward and backward, burrowing in the mud and crawling on a solid surface. The different movements have been found to be altered by neuromodulators, including serotonin in a study by Harris-Warrick and Cohen in 1985 and tachykinin in a study by Perez, CT et. al. in 2007. The lamprey model of CPG for locomotion has been very important to the study of CPGs and is now being used in the creation of artificial CPGs. For example, Ijspeert and Kodjabachian used Ekeberg's model for the lamprey to create artificial CPGs and simulate swimming movements in a lamprey-like substrate using controllers based on a SGOCE encoding. Essentially, these are the first steps toward the use of CPGs to code for locomotion in robots.

Central pattern generators also contribute to locomotion in higher animals and humans. In 1994, Calancie, et. al. claimed to have witnessed the "first well-defined example of a central rhythm generator for stepping in the adult human." The subject was a 37-year-old male who suffered an injury to the cervical spinal cord 17 years prior. After initial total paralysis below the neck, the subject eventually regained some movement of the arms and fingers and limited movement in the lower limbs. He had not recovered sufficiently to support his own weight. After 17 years, the subject found that when lying supine and extending his hips, his lower extremities underwent step-like movements for as long as he remained lying down. "The movements (i) involved alternating flexion and extension of his hips, knees, and ankles; (ii) were smooth and rhythmic; (iii) were forceful enough that the subject soon became uncomfortable due to excessive muscle 'tightness' and an elevated body temperature; and (iv) could not be stopped by voluntary effort." After extensive study of the subject, the experimenters concluded that "these data represent the clearest evidence to date that such a [CPG] network does exist in man." As described in Neuromodulation, the human locomotive CPG is very adaptable and can respond to sensory input. It receives input from the brainstem as well as from the environment to keep the network regulated. Newer studies have not only confirmed the presence of the CPG for human locomotion, but also confirmed its robustness and adaptability. For example, Choi and Bastian showed that the networks responsible for human walking are adaptable on short and long timescales. They showed adaptation to different gait patterns and different walking contexts. Also, they showed that different motor patterns can adapt independently. Adults could even walk on treadmills going in a different direction for each leg. This study showed that independent networks control forward and backward walking and that networks controlling each leg can adapt independently and be trained to walk independently. Thus, humans also possess a central pattern generator for locomotion that is capable not only of rhythmic pattern generation but also remarkable adaptation and usefulness in a wide variety of situations.

Rhythm Generators
Central pattern generators can also play a role in rhythm generation for other functions in vertebrate animals. For example, the rat vibrissa system uses an unconventional CPG for whisking movements. "Like other CPGs, the whisking generator can operate without cortical input or sensory feedback. However, unlike other CPGs, vibrissa motoneurons actively participate in rhythmogenesis by converting tonic serotonergic inputs into the patterned motor output responsible for movement of the vibrissae." Breathing is another non-locomotive function of central pattern generators. For example, larval amphibians accomplish gas exchange largely through rhythmic ventilation of the gills. A study by Broch, et. al. showed that lung ventilation in the tadpole brainstem may be driven by a pacemaker-like mechanism, whereas the respiratory CPG adapts in the adult bullfrog as it matures. Thus, CPGs hold a broad range of functions in the vertebrate animal and are widely adaptable and variable with age, environment and behavior.

Functions in Invertebrates
As described earlier, CPGs can also function in a variety of ways in invertebrate animals. In the mollusc Tritonia, a CPG modulates reflexive withdrawal, escape swimming and crawling. CPGs are also used in flight in locusts and for respiration systems in other insects. Central pattern generators play a broad role in all animals and show amazing variability and adaptability in almost all cases.