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A central pattern generator is network of neurons found in the nervous system which is responsible for the generation of rhythmic behaviors such as walking, swimming, and breathing. This “motor symphony” of movement is not orchestrated by one unit, or conductor, but is the result of interaction between many semi-autonomous parts. Therefore, with CPG, an organism is able to exhibit rhythmic activity in the absence of sensory inputs. In class we discussed the development of CPGs as being genetic and/or the result of experience. CPGs are formed during early embryonic development where neuronal circuits are organized to enable spinal cord and brain stem circuits to generate efficient motor patterns, such as breathing. An illustration of the genetic origin of CPGs comes from the experiment discussed in class where new born birds where placed in straight jackets. After the period where a bird would normally ‘learn’ to fly had passed, the jackets where removed and the young birds were still able to fly.  This study indicates that the motor patterns necessary for flight are not learned. In order to further prove the genetic origin theory of CPGs it would be necessary to locate and change the specific gene and observe if the pattern changes (the motor patterns in flight, for example).  

What intrigues me is the idea that CPGs may be a result of experience and that these systems are modulated by sensory feedback. If CPGs induce natural rhythmic behaviors can the frequency and phases of a CPG be selectively modified to increase the accuracy and efficiency of a particular behavior? In searching for the answer to this question I came across an article by Paul S. Katz, David J. Fickbohm and Christina P. Lynn-Bullock. The article physiologically and morphologically identified specific neurons in opisthobranch mollusks shown to be part of the central pattern generator underlying dorsal swimming. They refer to these neurons as the dorsal swim interneurons, or DSIs.

The motor pattern for swimming is generated as a result of the properties of CPG neurons and their synaptic interconnections. Sensory neurons, which are located in the fused cerebropleural ganglion, synapse directly and indirectly on the dorsal ramp interneuron which monosynaptically excites the CPG dorsal swim interneurons. The DSIs synapse onto two other CPG interneuron types: cerebral neuron 2 (C2) and the ventral swim interneurons (VSIs).

The DISs are immunoreactive for the neurotransmitter serotonin. “Physiological evidence strongly suggests that the DSIs release serotonin and that it is used both as a classical neurotransmitter, evoking fast and slow synaptic actions, and as a neuromodulator, altering the cellular and synaptic properties of other neurons”. The effects of this modulation include the enhancement of synaptic strength and increased excitability of CPG cerebral interneuron 2.

When the DISs are active they evoke neuromodulatory actions on other CPG neurons modulating their cellular and synaptic properties. It would then be logical to assume that by naturally or artificially modifying DISs activity the resulting behavioral pattern of swimming can be changed, or enhanced.

What are some natural neuromodulators that affect central pattern generated behaviors in humans and can these be artificially modified in order to enhance behaviors? I’ll keep looking.

http://icb.oxfordjournals.org/cgi/content/full/41/4/962