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Amazing self-organization: How does CPG maintain flexibility in different actions?
Central pattern generators (CPGs) are self-organizing biological neural circuits that can generate rhythmic outputs in the absence of rhythmic inputs. These tightly coupled patterns of neural activity drive the rhythmic and typical motor behaviors we experience such as walking, swimming, breathing, or chewing. Although they can function normally in the absence of input from higher brain areas, they still require regulatory input, and their output is not fixed. Flexible responsiveness to sensory input is an essential trait of CPG-driven behavior.
Physiology"To be classified as a rhythmic generator, a CPG requires two or more processes to interact such that each process increases and decreases in sequence, with the result that the interaction repeatedly returns the system to its starting state."< /p>
CPG Neurons
CPG neurons can have different intrinsic membrane properties. Some neurons can fire action potentials in bursts, whereas others are bistable and can be triggered by depolarizing current pulses and terminated by hyperpolarizing current pulses. Many CPG neurons fire after release of inhibition (termed post-inhibitory rebinding), while another common property is a reduction in firing rate during periods of constant depolarization (termed spike rate adaptation).
Rhythm Generation
Rhythm generation in CPG networks depends on the intrinsic properties of CPG neurons and their synaptic connections. There are two basic rhythm-generating mechanisms at work here: metronome/follower and mutual inhibition. In a metronome-driven network, one or more neurons act as a core oscillator (the metronome), driving other non-bursting neurons (the followers) into a rhythmic pattern. In a network driven by mutual inhibition, two (groups of) neurons inhibit each other. These networks are called semi-central oscillators. When isolated, these neurons do not have rhythmic activity, but when coupled through inhibitory connections, they can produce alternating patterns of activity.
"Gap junctions also contribute to rhythmic oscillations and neuronal synchronization in CPGs."
Short-term synaptic dynamics
CPG networks have extensive repetitive synaptic connections, including mutual excitation and mutual inhibition. Synapses in CPG networks undergo short-term activity-dependent modifications. Short-term synaptic depression and facilitation can play a role in both bursts of activity and the termination of activity.
CPG circuit
The CPG circuitry thought to be involved in motor control consists of motor neurons and intraspinal neurons located in the lower thoracic and lumbar regions of the spinal cord, as well as in each nerve segment in the ventral nerve cord of invertebrates. The CPG neurons involved in swallowing are located in the brainstem, specifically in the lingual nucleus in the medulla oblongata. Although the general location of CPG neurons can often be inferred, the specific location and identity of the participating neurons are still under exploration.
Neuromodulation
Organisms must adapt their behavior to meet the demands of their internal and external environments. As part of an organism's neural circuitry, central pattern generators can be tuned to adapt to the organism's needs and surroundings. Regulation has three roles in a CPG circuit:
- Regulation is an intrinsic property of the CPG network or is necessary to trigger its activity.
- Tuning changes the functional configuration of the CPG to produce different outputs.
- Regulation changes the composition of CPG neurons by switching neurons between networks and fusing previously independent networks into larger entities.
In rodents, blocking neuromodulatory connections significantly reduces rhythmic activity and may completely abolish drug-induced simulated locomotion. This phenomenon suggests that neural regulation is crucial for the flexibility of CPG.
Sensory feedback
While central pattern generation theories require that basic rhythm and pattern generation be generated centrally, CPGs can respond to sensory feedback to alter patterns in behaviorally appropriate ways. Even feedback received during a specific phase of a movement pattern may require changes in other parts of the pattern cycle to maintain certain coordinated relationships. For example, a pebble inside the right shoe will change the entire gait, even though the stimulus is only present when standing on the right foot. This adjustment may be due to widespread and long-lasting effects of sensory feedback on the CPG, or due to short-term effects on a few neurons that in turn modulate nearby neurons, thereby extending the feedback to the entire CPG.
Function
CPGs can serve a variety of functions, including movement, breathing, rhythm generation and other oscillatory functions. The diversity of these functions demonstrates their key roles in a variety of activities.
When considering the flexibility and responsiveness of CPGs, we can't help but wonder how the self-organization mechanisms of these neural circuits can inspire our exploration of new technologies?
