Organization of rhythm and pattern generation networks

Transcription

1 Organization of rhythm and pattern generation networks Modeling the mammalian locomotor CPG: insights from mistakes and sensory control David A. McCrea & Ilya A. Rybak Dept of Physiology Winnipeg, MB Canada Drexel University College of Medicine Philadelphia, PA USA SPINAL CORD RESEARCH CENTRE

2 Basic Half-centre CPG Lundberg-Brown simple organization with mutually inhibitory interactions between half centres - ensures - alternation CPG Antagonist Inhibition peturbations CPG excitability changes affect both rhythm and Mn activity Perturbations affecting the CPG shift the phase of the step cycle and change the timing of the following steps. Motoneurons How can this basic scheme explain cycle period maintenance during failed motoneuron recruitment? Grillner & Zangger 1979 chronic spinal cat

3 A possible solution: two-level CPG with reciprocal Inhibition at both levels Hypotheses: 2 level CPG provides an explanation for: 1. different modes of sensory control 2. non-reseting and resetting deletions resetting Generation resetting Proprioceptive feedback maintaining Perturbations producing deletions maintaining Earlier discussions of independent rhythm and Mn excitation networks Koshland Smith Perret Cabelguen Kriellaars et al (Jordan) A computational approach using a large (medium) scale network will be used to test these hypothesis Burke et al Brownstone

4 Building the two-level CPG architecture each population consists of 2 Hodgkin-Huxley style neurons: interneurons are one-compartment models motoneurons have a soma and one dendritic compartment (Booth et al 1997) Ionic channels: Na+; NaP+; K+DR; Ca2+L; Ca2+N; K+(Ca2+); Leak Classical half-centre organization. Inpf-E Ia-E Ia-F Inpf-F Excitatory interneurons connected to motoneurons. Additional inhibitory interneurons for flexor-extensor alternation Outside the CPG Basic reciprocity module - antagonist motoneuron pair. Reciprocal inhibitory interneurons and Renshaw cells R-E R-F apologies: unlikely the only source of Mn Inhibition during locomotion

5 Building the two-level CPG architecture Generator Inrg-E Inrg-F The RG (rhythm generator) network has strong reciprocal inhibitory and weak reciprocal excitatory interconnections. Inpf-E Ia-E Ia-F Inpf-F Controls the PF (output) layer which can oscillate but doesn t because of rhythm generator R-E R-F

6 Building the two-level CPG architecture Generator Drive (MLR) Inrg-E Inrg-F Inpf-E Inpf-F Descending command signal (drive)tonically activates the networks by increasing excitability at both the RG and PF levels. ic flexor and extensor alternation ensues. persistent sodium currents produce sustained depolarization of excitatory CPG neurons. Ia-E Ia-F excitation within the population synchronizes activity Outside the CPG half-centre inhibition and NaP inactivation generate oscillations R-E R-F

7 Locomotor simulations Hz n mv raster single h NaP simulations performed using NSM-3. developed at Drexel University by Sergey Markin, Natalia Shevtsova & Ilya Rybak persistent sodium currents (NaP) depolarize excitatory CPG neurons. half-centre inhibition plus NaP inactivation (via h NaP) generate oscillations generation Ia IN Renshaw Ia-F Ia-E R-F R-E motoneurons 4 4

8 Locomotor modes: single -8 h NaP Normal pattern produced by MLR stimulation increased interneuron excitability (DOPA, 5- HT, K+,etc.) Synchronous styrchnine rhythm SmAB TA EDL MG single h NaP Inrg-E 1 sec Drive (MLR) Inrg-F 2 1 sec Inrg-E 2 Inrg-F 1 sec normalized step in decerbrate cat x x Inrg-E x x Inrg-F

9 Activities of motoneuron pools Model Experimental recordings Mn -F TA Mn -F raster Sart Mn -F single Sart (Mn) Mn -E raster MG SmAB Mn -E single 4s 6s SmAB (Mn) 1s simulations can reproduce firing rates of individual Mns and activity of Mn populations (ENG)

10 Changing phase durations and cycle period with RG drive 2 2 Increased Drive to Drive (MLR) Generator 2 2 Increased Drive to 2 2 Ia-E Ia-F R-E R-F 4 sec extensor-phase dominated rhythm sec flexor-phase dominated rhythm Intensity of Mn recruitment is independent of cycle period and phase CPG Motoneurons

11 Normal RG operation (MLR stimulation) ( range of cat locomotion) 3. cycle MLR Drive cycle period 2. period (sec) (sec) MLR Drive to RG Inrg-E Generator Inrg-F ( standard simulation value) RG g 2 NaP, ms/cm Inpf-E Inpf-F Reciprocal inhibition Motoneurons τmaxnap inact. (sec)

12 Creating fictive and real locomotor periods and phase durations 1.5 phase 1 duration (sec).5 model increased drive to orange RG shortens the green phase and cycle period Drive (MLR) Generator cycle duration (s) more - RG drive - less 1 burst duration T b (s) MLR fictive locomotion flexor dominated extensor dominated flex burst duration T b (s) ext treadmill locomotion phase duration T b (s) stance.5 ext flex.5 swing cycle duration (s) cycle duration (s) cycle duration (s) Yakovenko et al 25 Halbertsma 1983

13 Sensory (reflex) control of CPG reflex networks are integrated into CPG networks promote flexion flexor group I EDL PLong R. femoris Iliopsoas Sartorius EDL sural (cutaneous) F E promote extension ankle PBST TA Sartorius Plantaris MG LG Soleus SmAB Quadriceps flexor group II extensor group I specialized stumbling correction SP (dorsal foot, cutaneous ) Tibial (plantar foot, cutaneous) Selective afferent control of both sides of the CPG with and without rhythm alteration argues for the Clock projecting to both the extensor and flexor sides (CPG symmetry) MLR INTRACELLULAR ST ENG all data from MLR evoked fictive locomotion

14 Stimulation of a single nerve affects motoneuron activity throughout the limb: CPG- mediated actions increased Mn recruitment graded extension enhancement, excitation outlasts stimulus 1.16T LGS stim. 1.28T SmAB ENG (hip extensor) 1.75T 1.4T Guertin Angel Perreault McCrea 1 ms Activation of extensor Ia (spindle) and Ib (tendon organ) afferents control of cycle timing transition from extension to flexion delayed but cycle period unaffected! Pl 1.6T AB extensors flexor Q MG Sart 1 sec hip knee ankle hip during real locomotion, sensory-evoked component accounts for 3-7% of extensor activity (e.g. Hiebert & Pearson 1999)

15 Ib Ia{ CPG model for afferent stimulation during locomotion Drive (MLR) Adding extensor group I afferent input: at the rhythm generator level Irg-E Generator at the pattern formation Ipf-E disynaptic excitation of extensors Iab-E mono. Mn excitation reciprocal inhibition Ib Ia group I

16 group I afferent input to PF & RG Weak RG input (rhythm maintained) Strong RG input (rhythm altered) Stim s 6s 8s Pl stim. 1.6T Ipf-E Irg-E s 6s 8s Sm stim. 2T Sart AB 1s period unaffected (phase prolonged more Mns recruited) Experiment TA MG 1s period lengthened

17 group I afferent input to PF & RG Weak RG input Strong RG input (rhythm maintained) (rhythm altered) Stim s 6s 8s Pl stim. 1.6T Ipf-E Irg-E s 6s 8s Sm stim. 2T Sart AB period unaffected (phase prolonged more Mns recruited) 1s Experiment TA MG 1s period lengthened

18 During flexion: PF level resetting (extensor group I afferents) Drive (MLR) 2 Irg-E Generator stimulation Ipf-E 2 In-E Ia-E Ia-F sec Reciprocal inhibition afferent input to pattern formation level generates a quick step without changing the clock (no RG effect) Ib Ia { group I R-E R-F LGS 1.6 T TA AB locomotor periods 1s Guertin et al J Physiol 1995

19 Problem: spontaneous reflex reversals with flexor nerve stimulation Different effects of flexor muscle nerve stimulation in different preparations Spontaneous reflex reversal: variable effects in the same preparation EDL 5T 2nd prolongs flexion EDL 5T Stecina et al J Physiol 25 group II hypothesis E F E F Generation group I 1st resets to extension Hypothesis: competing CPG actions of flexor afferents: 1) spindle primaries excite flexor side of CPG - secondaries excite extensor side. 2) preparation-dependent regulation of group II synaptic transmission to CPG motoneurones II flexor spindle afferents I

20 STIM CPG control by afferents in flexor muscles synaptic effectiveness of gr. II flexor afferents least most group II Generator s small increase in flexor activity marked increase in flexor activity phase switch to extension EDL 5T II I Sart GS 1s Sart

21 What happens when Mn activation falters? spontaneous deletions of either flexor or extensor activity is another argument for the Clock projecting to both the extensor and flexor sides (CPG symmetry) hip extensor SMAB ankle LGS hip flexor ankle Sart TA For the whole story see: Lafreniere-Roula & McCrea JNeurophysiol 94: (25). Duysens, McCrea & Lafreniere-Roula J Neurophysiol 95: (26). 5 1 sec 15 2 Deletions: despite continuous MLR stimulation, spontaneous omissions of activity often occur that briefly interrupt alternating flexor and extensor activity

22 Reciprocity of motoneuron excitation is maintained during deletions both Mn excitation and cycle timing affected (one layer CPG?) This is a resetting deletion: note the phase shift Sart hip flexor intracell population (ENG) SmAB hip extensor intracell sec population (ENG)

23 Resetting deletions experiment Drive (MLR) excitation Sart TA SmAB Generator LGS Sart (Mn) Ia-E Ia-F SmAB (Mn) drive 5 1 s phase shift 4 4 R-E R-F Generator disturbances shift the rhythm. single single -4 mv -8-4 mv S simulation phase shift

24 Non-resetting deletions - cycle timing maintained deletions often involve omission of an integer number of cycles: rhythm is not reset Sart EDL SmAB MG TA AB LGS contra TA 1 sec spontaneous disturbance CPG? Motoneurons How can this simple CPG network remember cycle timing? contra MG 1sec contralateral rhythm is not required for maintaining ipsilateral cycle timing during non-resettting deletions

25 Timing maintenance during full and partial deletions TA LG 1 sec LGS mn #1 1 mv LGS mn #2 SmAB TA PerL LGS 5 ms Note the maintained timing with complete failure of depolarization

26 Non resetting deletions: type 1 deletion of antagonists with tonic excitation of agonists excitation drive Generator Drive (MLR) Model 5 5 Ia-E 1 4 drive 1 1 Ia-F 1 Ia-E 4 R-F 4 R-E 4 disturbances at the level cause deletions without shifting the phase of post-deletion rhythm 4 4s 6s 8s 1s

27 Non resetting deletions: type 1 deletion of antagonists with tonic excitation of agonists excitation drive Generator Drive (MLR) Sart Experiment EDL MG SmAB 1s drive 5 4 single 4-4 mv -8 disturbances at the level cause deletions without phase shifting the post-deletion rhythm. single -4 mv seconds Model 1

28 Non resetting deletions: type 1 deletion of antagonists with tonic excitation of agonists excitation drive Generator Drive (MLR) Sart Experiment EDL MG SmAB 1s drive 5 4 disturbances at the level cause deletions without phase shifting the post-deletion rhythm increased excitation causes deletions with tonic agonist activity single single 4-4 mv -8-4 mv seconds Model 1

29 Non resetting deletions: type 2 deletion of agonist activity with maintained antagonist rhythm Drive (MLR) A inhibition Sart TA SmAB MG EDL (Mn) B drive 1 1s 4 single single 4-4 mv -8-4 mv sec decreased causes deletions with maintained rhythmic activity in antagonists

30 Complex PBSt activity from a two-phase CPG = original 2 phase CPG specialized PF network complex Mn activity Drive (MLR) + Drive (MLR) Drive (MLR) PF-PBSt PF-PBSt R-PBSt PBSt PBSt specialized circuitry creates complex Mn activity (e.g. PBSt) during locomotion Perret & Cabelguen

31 Complex PBSt activity from a two-phase CPG Generator Drive (MLR) Model performance 1s 2 1 PF-PBSt PF-PBSt Mn-PBSt Mn-PBST (single) mv PBSt -7

32 Extending the model to more than two muscles Drive (MLR) Distributed Generator Generator (Clock) Network (Units) Ia-E Ia-F Unit R-E R-F ankle knee hip Unit Burst Generator (Grillner 1981)

33 Model predictions for interneuron identification 1. excitatory 2. monosynaptic excitation from MLR 3. rhythmically active in extension 4. project to extensor Mns and flexor IaINs 5. activity fails during non-resettting deletions 6. strong extensor group I input Drive (MLR) 1. excitatory 2. monosynaptic excitation from MLR 3. rhythmically active in extension 4. do not project to motoneurons or Ia interneurons 5. rhythmic activity continues during non-resetting deletions 6. weaker extensor group I input Inpf 1 inhibitory Ipf-E 2. no MLR excitation 3 rhythmically active in extension Inpf 4 do not project to motoneurons or Ia interneurons Ipf-E Ia-E Ia-F 5 tonically active or suppressed during type 1 and 2 deletions, respectively extensor group I afferent R-E R-F

34 Observations on deletions and sensory control of the CPG in the adult cat has lead to the proposal for a two level CPG architecture computational modelling shows that this simple network: reproduces both resetting and non-resetting deletions reproduces different modes of afferent feedback control 1. Mn recruitment via the PF level 2. locomotor phase transitions and step cycle period timing via the RG generates bifunctional Mn activity with simple additional circuitry creates criteria for functional identification of CPG interneurons creates hypotheses for sensory circuitry organization Generator (Clock) Resetting deletions targets of sensory & descending control Non-resetting deletions Network (Units)

35 Ilya & David would like to thank SPINAL CORD RESEARCH CENTRE 1 Katinka Stecina 2 Myriam Lafreniere-Roula 3 Samit Chakrarbatry Michael Angel Pierre Guertin Marie-Claude Perreault 4 Technical assistance: Sharon McCartney, Maria Setterbom 2 sorry, no pics! Sergi Markin Natalia Shevtsova Ilya Rybak s modeling facility With teletype interface and Fortran language, we have made computer easy to use NIH Bioengineering Research Partnership grant R1NS previous support from: NIH R1NS4662 and R1HL72415 (Rybak) NIH (R1NS4846) and the Canadian Institutes of Health Research (McCrea)