Central Pattern Generators (CPGs)

Central Pattern Generators (CPGs)

1. How do CPG circuits generate rhythmic patterns? How can this be applied to behavior?

2. 1. SPINAL CORD CAN GENERATE MOTOR PATTERNS INDEPENDENT OF THE BRAIN: it is the simplest vertebrate motor system

   • c. Two alternative hypotheses for generating rhythmic and alternating pattern
     • i. REFLEX CHAIN MODEL
       • Sensor neurons innervating a muscle fire and excite interneurons that activate motor neurons to the antagonist muscle
     • ii. CENTRAL PATTERN GENERATOR
       • A central circuit generates rhythmic patterns of activity in the motor neurons to antagonist muscles
   • b. Evidence for CPGs
     • Fictive motor patterns generated by an isolated neural circuit resemble those recorded in vivo
       • First observed in lobsters
       • Placing neurons in vitro (petri dish) showed three phase activity between motor and interneurons, when microelectrodes were attached. This occured without sensory input
   • a. Background
     • LOCOMOTION CYCLES ARE ORGANIZAED BY CPGs
       • Independent of the brain
     • Movement has two stages:
       • 1 THE STEP CYCLE: Leg flexion (f) and extension (e) are related to the swing and stance stages of locomotion
       • 2. TRANSECTION OF THE SPINAL CORD
       • We know this from recordings of electrical activity of extensor and flexor muscles
     • Electrical activity of this can be recorded using electro-myographic recordings. The procedure included:
       • 1. The hind limbs are still able to walk on the treadmill
       • 2. Reciprocal burst of electrical activity can be recorded from flexors during the swing phase and from extensor during the stance phase of walking (alternating activity of muscles parallel to swing and stance movements
       • 3. Therefore, there must be a central pattern generator present in the spinal cord

3. CPGs: A network of neurons capable of generating patterned (rhythmic) activity in the absence of sensory input to drive the timing of output Examples include breathing, walking and chewing in mammals, swimming in fish and flying in birds. Can be overridden by voluntary control.

   • MUST KNOW THIS DEFINITION!!!!

4. 2. CELLULAR MECHANISMS UNDERLYING PATTERN GENERATION

   • a. Neurons have intrinsic properties including:
     • i. Endogenous bursting
     • ii. Plateau potenitals
     • iii. Postinhibitory rebound
     • iv. Spike frequency adaptation
   • b. Rhythms can be generated by endogenous bursters or can be an emergent property of synaptic coupling between non-bursting neurons 'NETWORK OSCILLATOR'
     • In pacemaker networks, a PACEMAKER NEURON can synaptically drive and antagonist to fire in alteration
       • I.e. Spike activity of one neuron inhibits the other
         • Resulting in postinhibitory rebound after spike excitation decreases
           • This repeats due to the pacemaker property of the pacemaker neuron
     • Simplest exaple of a network oscillator: one formed between two neurons that fire non-rhythmically in isolation
       • BUT fire in bursts as a result of reciprocal inhibition
       • Tonic firing is observed in uncoupled receptors
       • Although it shows similar results to pacemaker neurons during coupling, they are not generated by endogenous bursting
       • Therefore, a COMPLETELY DIFFERENT SYSTEM

5. 4. EXAMPLES OF CPGs

   • b. Tritonia swim system
     • Highly dependent on sensory input as it is an escape mechanism
     • Using in tact animals, it is possible to record the central motor programme and behaviour at the same time
       • STAGES:
         • i. Sensory input activates transient trigger-type command neuron Tr1 and Dorsal Ramp Interneuron (DRI), a command gating neuron
         • ii. DRI excites Dorsal Swim Interneuron (DSI)
         • iii. DSI excites Cerebral Neuron 2 (C2)
         • iv. C2 feeds back and excites DRI, further exciting DSI via a positive feedback loop
         • v. C2 also excites Ventral Swim Interneuron (VSI), which inhibits DSI and C2, thereby momentarily interrupting the positive feedback loop
       • It generates a two-phase rhythm
         • When DSI starts bursting as a result of sensory and command neuron
           • The C2 and VSI neurons are silent
             • The DSIs excite themselves and C2, but because C2 has a high threshold it takes 1-4 seconds to start firing (i.e. there is a delay)
               • C2 starts to fire and re-excites DSI which keeps firing
                 • It begins to depolarize VSI but this is delayed by the activation of an A current
                   • When VSI starts to fire, it terminates the DSI burst and C2 is also eventually inhibited
                     • When VSI stops, the cycle repeats
                       • None of the CPG neurons have bursting properties
   • c. Clione limacine swim system
     • A mix of mostly inhibitory synapses and intrinsic burst-generating mechanisms
       • The predominant form of connections are RECURRENT inhibition
         • However there are a couple of excitatory connections
           • All of which use endogneous bursting
   • d. Lymnaea feeding
     • How multiple connections between SO, a command-like fedding interneuron
       • It generates a three-phase rhythm generated by three types of CPG neuron:
         • Protraction
         • Rasp
         • Swallow
       • The interaction of intrinsic propreties, synaptic properties and connectivity underlies the generation of three phases of CPG activity
   • a. Simple oscillating circuit: the half-centre model
     • The half-centre model is two neurons (A+B) interconnected by inhibitory synaptic connections either through MUTUAL /RECIPROCAL inhibition
       • Could use different intrinsic properties:
         • Endogenous bursting
         • Plateau potentials
         • Post-inhibitory rebound
       • Shos there is not set principle/design for CPG networks
     • Underlies locomotory circuits in vertebrates

6. 3. CENTRAL PATTERN GENERATORS - PUTTING IT TOGETHER

   • a. Intrinsic/endogenous properties of neurons
     • Firing theshold
     • Frequency-current relationship
     • Spike frequency adaptation
     • Post inhibitory rebound
     • Plateau potentials
     • Endogneous or conditional bursting
   • b. Synaptic properties
     • Sign (inhibitory/excitatory)
     • Time course
     • Mode of transmission - chemical/electronic etc.
     • Faciliation/depression
   • c. Patterns of connection
     • Reciprocal inhibition
     • Recurrent inhibition
     • Patallel excitation and inhibition
     • Mutual excitation
   • By taking all different components, networks can be generated by wiring them up

7. 5. SUMMARY

   • a. Develop a good general understanding of whre CPG circuits are, what building blocks they have and how they function
   • b. To be able to use specific examples of the best known molluscan CPG circuits to illustrate the different combinations of intrinsic properties and synaptic connectivity that can underlie CPG-driven rhythmic circuit activity
     • Swimming in Tritonia
     • Swimming in Clione
     • Feeding in Lymnaea

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