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Generation of rhythmical behaviour by the nervous system: Evidence for Central Pattern Generators (CPG) Refresh crayfish Simple reflex evolving not much neural machinery but also sufficiently flexible to be adapted to stimulus and flexibility in internal state. Aim More complex behaviour and their neural machinery Cockroach Live in leaf litter and are model organisms for walking behaviour. This is a rhythmical behaviour; the legs are co-ordinated in a tripod gate, so at any time opposing tripods are on or off the ground. Most six legged animals adopt this gate, but if you could remove the middle two legs of the animal it will adopt a tetrapod gate. So again, the neural machinery is adaptable. Each individual limb shows alternate movements of extension and flexion. This behaviour is episodic - not occurring continuously (animal isn't constantly moving) 1950s Research Big question was whether this neural circuitry was built-in, or whether the cyclical behaviour responds to cyclical sensory feedback. In other words, could say every time leg steps onto ground, receives sensory input from weight bearing which triggers a reflex causes the legs to bend again, which stimulates a receptor in the knee joint which triggers a simple reflex to extend again. This is the chain reflex hypothesis, where each action in the cycle triggers sensory input to feed back and trigger the next phase. In the 50s, researchers wanted to find out whether the chain reflex is true or whether the circuitry is innate, arising from a central rhythm generator. It was found out that there are hardwired neural circuits that can trigger these behaviours in the absence of any sensory input. Appears to be centrally generated. Does not mean they don't take account of sensory input, but means that they don't require it to keep going. It was also shown the nervous system could generate fictive motor patterns - if you take the nervous system out of the animal, you can record the output for a specific motor pattern like walking. Isolate nervous system generates cyclical output for actions such as limb extension and flexion, even in absence of sensory input. Termed fictive as it wasn't a real motor pattern as the rest of the animal wasn't intact. Wilson - Locusts 1950s Extract from Wilson's studies in the 1950s of the motor patterns for locust flight. The way locusts fly is that they have elevator and depressor muscles at wing bases that cause alternating contraction of the two muscles. Wilson recorded electrical impulses in the nerves of the muscles causing contraction.
Had a locust on its back on a platform and use electrodes to record nerves that innervate muscles. Even when wings held still, can still record rhythmical output in the nerves. When the nerves are cut from the muscles, you can still record cyclical output to contract muscle contraction even though no longer intact. Subsequently shown that could isolate thoracic ganglion and the nerves altogether and could still record rhythmic pattern. Example of fictive behaviour. Wilson also showed that if wing movement rate was manually controlled, you could control the frequency of the output of the nervous system. So sensory input, e.g. sensory receptors in the wing that affect the position in the wing, could influence the speed of output. Crayfish (Weirsma 1960s) Looked at other large diameter neurons in nerve cord by stimulation with an electric current. He found the he could trigger certain behaviours. For example, stimulating one of the fibres could trigger postural reflex here animal lifts up claws to make itself more imposing. Could also trigger rhythmical movements of the external gills, causing water to flow over them. When females lay their eggs, they are held at the gills as they have to be aerated, and the gills move to aerate them. If you stimulated a neuron with an unpatterned, continuous electrical activity it would stimulate patterned movement in the swimmerets. Obviously some activity to move in a rhythmical way, even if sensory input not rhythmical.
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