Review of the CPG How do we know the circuit pattern: Who connects to who?

1. Review of the CPG • How do we know the circuit pattern: • Who connects to who? • Excitatory vs inhibitory connections? • How do we know a neuron is part of a CPG vs driven by a CPG? • What is the role of proprioreception in pattern generation • Are proprioreceptors actually part of the CPG?

2. Simple CPG

4. Complexity • CPG circuit diagram for one depressor and one elevator neuron • Keep in mind that there are dozens of motor units that must be coordinated; this is just a part of the entire circuit. • Some INs are not part of the CPG per se but work in coordination with the CPG

5. Complexity2 • CPG circuit diagram simplifies the synaptic interactions • Diagrams typically show only one synapse per collateral, in reality there can be dozens of collaterals and thousands of synapses.

6. Mapping the synaptic interactions between cells in a circuit. • To identify a relationship you must show a correlation between APs in one cell and: • EPSPs/APs • IPSPs • …in the other • Methods: • Passive dual recordings • Stimulation experiments • Hyperpolarizing experiments • All should show a predictable pattern of: • Influence from the upstream cell to the down stream cell • Downstream EPSP/AP/IPSP lags consistently • Upstream consistently elicits/produces the same effect • If hyperpolarizing upstream causes: • Depolarization: Downstream cell should become less active. • Hyperpolarization: Downstream cell should be released from inhibition and become more active • Depolarization/AP generation experiments on upstream cells should produce the reverse of hyperpolarizing exps.

7. Identification of CPG cells • Must meet the following criteria: • Phasically locked/correlated to the rhythmic behavior • Ablation of cell decreases/disrupts the CPG rhythm • Stimulation of the cell can initiate/modulate/maintain the CPG rhythm • They must be able to reset the CPG • The reset experiment: • Indicates whether a neuron participates in a CPG circuit • Disrupting the activity of a cell should alter the pattern of • That cell • The CPG rhythm

8. Example: IN501 • Cell normally fires in rhythmic relation to the depressor muscle activity • Depolarizing it for a period shifts: • Its relationship to the projected rhythm • The actual muscle activity • Note that the rate of the rhythm is unchanged after depolarizing current is released Wing is temporarily stuck in the down position

9. Sensory receptors that provide proprioreceptive feedback during flight. • Wing Hinge stretch receptors: • Activate at the peak of the elevation phase of the stroke cycle • Magnitude of response is dependent on the amplitude of the elevation phase • The higher the amplitude (of the wing stroke) the more activity the hinge receptor produces • Likely responsible for constraining wing movement to a safe limit.

10. Hinge stretch receptor affects muscle activity AP in hinge stretch receptor EPSP in depressor motor neuron • Stimulation of hinge stretch receptor initiates rhythmic MN responses: • EPSPs in Depressor MNs • IPSPs in elevator MNs

11. Pronate Supine • The campaniform sensilla on the wings activate by wing distortion (flexion). • Spikes are elicited when wing is supine: • Start of each down-stroke and ending at the start of each upstroke. • Signal lift and thrust forces during the depression phase of each wing movement.

12. The campaniform sensilla on the wings activate by wing distortion (flexion). • Stimulation of Campaniform sensilla produced rhythmic • IPSPs in depressor MNs • EPSPs in elevator MNs • Opposite of Hinge stretch receptor

13. Tegula receptors Activation of the sensory output from the tegula sensory nerve begin just after the start of the downstroke.

14. Activation of tegular nerve depolarizes IN566 Activation of IN566 depolarizes elevator MN Tegula nerve activation indirectly mediates elevation MNs • Tegula phasically locked to wing beat cycle • Initiates following the initiation of downstroke

15. Tegula passes reset test: • Prolonged stimulation delays of tegular nerve lags pattern

16. Can we consider the proprioreceptive system as part of the CPG? • They meet the criterion • Are not necessary for rhythmic activity • They are not “central” • Since they contribute to the pattern generation perhaps the concept of CPG needs to be revised

17. Central control of Flight CPG Major roll is in course correction along 3 flight axes

18. Flight control sensors of the head • Yaw (fast responding) • Sensitive to directional airflow Pitch roll and yaw (slow responding) • Roll (fast responding) • Photo sensitive (light vs. dark) • Collectively represent horizontal attitude with respect to the horizon.

19. Deviation detection neurons (DDNs) • Three basic types receive input from ocelli (and L/R WS-hairs) • DNI (descending fiber ipsilateral to ocellus) • DNM (innervates medial ocellus) • DNC (descending fiber contralateral to ocellus) • Each type encodes a different aspect of flight deviation • Example: DNC • Responds specifically to roll to contralateral side

20. Simulation of light change effects of a roll inhibits the DNC as well Simulation of air flow also causes DNC response. Interaction of air and light

21. Putting it all together • Key point here is that the CPG acts as a gate keeper • Only allows correction of flight path at the appropriate phase of the cycle.