How does the lamprey swim? All you ever wanted to know about the CPG for lamprey locomotion

1. How does the lamprey swim?All you ever wanted to know about the CPG for lamprey locomotion The role of coupling, mechanics and sensory feedback in shaping motor patterns – and our models Zurich-02

2. Locomotion: Biological Control In all animals locomotion is a function of several types of signals: • Initiation from the organisms’ brain or by sensory input • Feedforward signals from a central pattern generator (CPG) • Sensory inputs signaling the position of the body or limbs in the world • Guidance and regulation from the brain • Proper integration of control signals with mechanics of the organism ALL ARE NECESSARY FOR ADAPTIVE LOCOMOTION Zurich-02

3. Overview • Normal lamprey swimming – a simple behavior • CPG – structure and function in isolation • Adding sensory feedback – some surprising effects • Putting the body back on • Coupling and how to measure it • Coupling in body vs spinal cord • Conclusions we can/cannot make from the coupling data • New ways of looking at it – not there yet, but closer • Onward to whole animal swimming Zurich-02

4. Lamprey swimming • No paired fins sticking out • Frequency is proportional to speed • Left and right alternate activity • Activity travels uniformly down the body • One wavelength/body of ~100 segments Zurich-02

5. Fictive swimming • Recording from a 50 segment isolated spinal cord piece • Alternation between left and right • Traveling wave of activity along the body • Traveling wave almost uniform Zurich-02

6. Structure of CPG: a distributed system • Segmental oscillators: 1-3 segments • Intersegmental coupling: • long and short • Ascending and descending • Distributed among the fiber tracts Zurich-02

7. Sensory feedback: sensing bending • Edge cells: mechanoreceptors on the edges of the spinal cord • Sense stretch > bending motion • Feedback upon motoneurons and interneurons • Can entrain the CPG when the spinal cord is periodically bent • Another action: slowly decaying excitation – outlasts the bending Zurich-02

8. Entrainment by bending Zurich-02

9. Entrainment by bending Zurich-02

10. Slowly decaying excitation • Bending faster: • Captures frequency • Bending stops > • Frequency slowly recovers Zurich-02

11. Slowly decaying excitation • Bending slower: • Captures • Overshoots • Recovers slowly • Breaks free and doesn’t capture • Slowly recovers • Slowly decaying excitation is powerful influence Zurich-02

12. Putting the body back on • The “trunk preparation” • Remove head and tail • Skin and muscle over the spinal cord • Stabilize with small pin through end • Activate to swim with D-glutamate • Record activity from muscles during movement Zurich-02

13. Motion of trunk preparation Zurich-02

14. Comparison between spinal cord and trunk preparation • Frequency is higher • Phase lags are shorter • Coupling is VERY strong in both (more later) Zurich-02

15. Weakening the coupling • Activity in whole swimming animal • Trunk preparation with hemisection – still coordinated • Remove muscle and its movement – no longer coordinated Movement increases the coupling Zurich-02

16. Estimating coupling • Use a maximal likelihood method to find the best values for parameters: b = total coupling strength g = relative ascending fraction Zurich-02

17. Intersegmental coordinating system: structure and function experimental conditions of study: long range short range Zurich-02

18. Means: coupling and direction • Total coupling strength: b • Very strong for all but 20 segments inhibited and two hemisections • Ascending fraction: g • Caudal inhibited: descending • Hemisections: ascending • Others indeterminant Zurich-02

19. Conclusions: coupling strength • Isolated spinal cord: Coupling is very strong • Long range coupling is very strong (inhibited segments) • Short range coupling strong, but not as strong as long range (hemisections) • Coupling decreases with loss of either lots of long or lots of short range fibers Zurich-02

20. Conclusions: directions of coupling • Caudal inhibited: g < 0.5 • Long range is predominantly descending • Hemisections: g > 0.5 • Short range is predominantly ascending Zurich-02

21. Individual values Zurich-02

22. Individual values • Directionality – highly variable • Animals differ in their relative proportions • Rough patterns exist, but values vary • Strengths are more consistent • Strong • Long and short Zurich-02

23. Models of intersegmental coupling • Early models: all nearest neighbor • Short • Weak • Reality: long, short and strong • Early models: phase difference coupling • Weak is required for phase difference coupling • Reality: coupling is very strong Phase difference coupling should produce coupling dominance: estimates should be all ascending or all descending NOT SEEN >> another reason these models are insufficient Zurich-02

24. New approaches to modeling • Simulated bursting of connectionist model based on lamprey neurons (EIN, CCIN, LIN) • Test behaviors of different configurations Zurich-02

25. Conclusions so far: • Model needs long range coupling to provide the total coupling strength of the real spinal cord • Model will need to be “tweaked” to get the phase delays correct • This is just the first pass – other more complex models may yield more interesting and more accurate results Zurich-02

26. Bye now! Ontomechanics: Adding vortices and muscle mechanics Zurich-02