Review of the CPGHow 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?

Simple CPG

ComplexityCPG 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

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.

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.

Must meet the following criteria:1.Phasically locked/correlated to the rhythmic behavior2.Ablation of cell decreases/disrupts the CPG rhythm 3.Stimulation of the cell can initiate/modulate/maintain the CPG rhythm 4.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

Identification of CPG cells

Example: IN501Cell normally fires in rhythmic relation to the depressor muscle activityDepolarizing 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

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.

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

Hinge stretch receptor affects muscle activity

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.

Supine

Pronate

Stimulation of Campaniform sensilla produced rhythmic

•IPSPs in depressor MNs•EPSPs in elevator MNs

•Opposite of Hinge stretch receptor

The campaniform sensilla on the wings activate by wing distortion (flexion).

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

Tegula phasically locked to wing beat cycle•Initiates following the initiation of downstroke

Activation of tegular nerve depolarizes IN566

Activation of IN566 depolarizes elevator MN

Tegula nerve activation indirectly mediates elevation MNs

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

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

Central control of Flight CPG

Major roll is in course correction along 3 flight axes

Flight control sensors of the head

Yaw (fast responding)•Sensitive to directional airflow

Roll (fast responding)•Photo sensitive (light vs. dark)•Collectively represent horizontal attitude with respect to the horizon.

Pitch roll and yaw(slow responding)

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

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

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.