Question

Describe how central pattern generators accomplish rhythmic signaling. Include both pacemaker cells and neuronal networks in your answer.

Answer 1

• Central pattern generators (CPGs) are biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. The ability to function without input from higher brain areas still requires modulatory inputs, and their outputs are not fixed. Flexibility in response to sensory input is a fundamental quality of CPG-driven behavior.

Localization:

• 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).

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.
• 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).

Neuromechanical control:

• ​​​​​​​If step cycle durations and muscle activations were fixed, it wouldn't be possible to change body velocity and adapt to varying terrain. It has been suggested that the mammalian locomotor CPG comprises a “timer” (possibly in the form of coupled oscillators) which generates step cycles of varying durations, and a “pattern formation layer,” which selects and grades the activation of motor pools. Increasing the neural drive from the midbrain locomotor region (MLR) to the spinal CPG increases the step cycle frequency (the cadence). Swing and stance phase durations co-vary in a fairly fixed relationship, with stance phases changing more than swing phases.
• 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 context.

Oscillating network with a pace-maker cell:

• One way that a collection of neurons can oscillate is to have one of the neurons oscillate on its own, and have the other cells oscillate because they are told to do so (through their synaptic interactions) by the neuron that intrinsically oscillates. In this situation, we call the intrinsically oscillating neuron the "pace-maker" since it sets the pace for the entire network. But this isn't the only way to have a network that oscillates. Just as a single oscillating neuron can contain a collection of currents that don't oscillate on their own, but when they get together they start to oscillate, a network of neurons can contain only neurons that don't oscillate on their own, but when they get together they do. This is an example of an emergent property. Sometimes emergent properties are seen as spooky, imprecise things (until you've looked at them for a long time and gotten tired of feeling spooked out), and without mathematics, they are imprecise things. With mathematics, they can be made precise, but they're still a little spooky.

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