Module 632 Lecture 9 JCS

1. Module 632 Lecture 9 JCS Control of Muscle Contraction

2. MODULE - 632Lecture 8Muscle Contraction • Lecture outcomes: • At the end of this lecture a student will be aware how : •   1)most muscles are activated (sometimes inhibited) by neural control. • 2) single impulses produce a twitch; multiple impulses a tetanus • 3)impulses reach the motor end plate, a modified synapse • 5)this causes the muscle action potential to spread along the sarcolemma, • 6)propagation through the sarcolemma, into the T-tubules and the SR causes release of calcium into the sarcoplasm • 7)in skeletal muscle this binds to the troponin complex and activates muscles • 8) smooth muscle is regulated. • 9)insect flight muscle are activated by calcium and stretch • 10)molluscan muscles are regulated by Ca2+ binding to myosin

3. Muscle regulation (1) Vertebrate striated muscle Vetebrate skeletal muscle (skeletal and cardiac) is activated (regulated) by neuronal signals. In addition there may be modulation by hormones etc. A single axon activates many fibres

4. Innervation types Not all muscle fibres are “all or none” – but most are! a) single motor unit with one (or sometimes two) endplate: all-or-nothing electrical response. Vert. fast twitch fibres. b) multi-terminal innervation: usually graded electrical responses like synaptic potentials, variable in magnitude. Vert. ‘slow’ fibres (most amphibian fibres); many invert. muscles. c) polyneuronal innervation. Several nerves to one muscle: different response from the different nerves, - fast, slow and inhibitory. note: excit. and inhib. control; fish, many invert. (jump). Number of fibres/axon varies: ‘Fast’ muscles - 1000-2000 fibres/axon ‘Slow’ muscles 180-200 fibres/axon

5. Twitch and tetanus – depend upon pattern of nerve impulses Force Tetanus Unfused tetanus Twitch Time – sec.

6. 3-D section of a skeletal muscle cell

7. SR T-tubule Ryanodine receptor Dihydropyridine receptor Cytosol The dihydropyridine-ryanodine receptor complex

8. Regulation: Control by nerves (striated muscle) • Summary: • Neuromuscular junction • Muscle plasma membrane depolarises • Propagates down ‘T’ tubules • Trhough di-hyropyridine receptor  ryanodine receptor • into sarcoplasmic reticulum (SR) at centre of fibre • Calcium release from SR - induces calcium release • Ca++ binds to Troponin C – which moves Troponin I – pushes tropomyosin – reveals actin binding sites – myosin can bind and cycle, producing contraction.

9. 7-actin repeat structure (14 - F-actin helix is double) in thin filament Structure repeats (half-turn) every 36.5nm

10. Relaxed state (muscle not contracting): • TnI is bound to actin; holds TM over actomysin binding sites • TnC has no Ca2+ bound to its regulatory sites. • Tropomyosin lies across myosin binding site on F-actin. • Activation: • Calcium binds to TnC, causing a conformational change in TnC, • Changes binding relationship of TnC and TnI relieving the binding of TnI to actin. • Tn-TM complex now free to move across the actin surface • Movement, co-ordinately through TM ‘opens’ up a large number (>7) of actins to the binding of myosin.complex • Muscle contracts

11. In the absence of Ca2+ the C-terminus of TnI binds to actin, holding the Tm-Tn complex over the myosin binding site on actin. - Ca2+ When Ca2+ binds to TnC, the C-terminus of TnI binds to TnC, Tm-Tn complex moves across the actin surface, and the myosin can bind producing contraction. + Ca2+ Figure from Berchtold et al., (2000)

12. Three state model of thin filament regulation - Geeves

13. Smooth muscle regulation (1): • Occurs through phosphorylation: • Calcium released into smooth muscle cells binds to calmodulin (homologous structure to TnC); • This binds to and activates myosin light chain kinase (MLCK) • This phosphorylates the RLC leading to: • Activation of myosin ATPase (changes kinetics of the product – ADP + Pi – release steps) • Increases assembly of thick filaments (see next slide)

14. Smooth muscle regulation (2): Remember smooth muscle has a rather poor ultrastructure. Smooth muscle myosin monomers change solubility depending on its phosphorylation state. In some smooth muscles this is a method of regulation of contraction: Dephosphorylated sm-myosin monomer has a tendency to fold up (10s) and exist in a soluble long-lived, inactive M.ADP state; On phosphorylation it becomes extended (6s) and assembles into thick filaments

15. Smooth muscle regulation (3): • Activation is achieved by increases in cytosolic calcium: • Usually caused by neural signals but often coupled to: • Hormones binding to the -receptors • Hormones and other external factors binding to  - receptors, increasing cAMP levels • cAMP reduces MLCK activity, reduces muscle activity • Different smooth muscles may react differently to the same stimulus due to presence of different receptors: • e.g. adrenalin – contraction of blood vessels to gut, but dilation of coronary arteries (for flight and fight) • caldesmon – Ca2+-binding, may function like Troponin.

16. Insect flight and Molluscan catch + adductor muscles: Pecten maximus Catch muscle Adductor muscle Insect flight muscles

17. Regulation of molluscan muscle: (adductor) Washed molluscan muscle (e.g. scallop) contains little troponin The myofibrils retain calcium sensitive activation Remove regulatory LC - Ca2+ sensitivity is lost. Myosin contains two Ca-binding sites, both essential for regulation. Neither RLC or de-sensitised myosin have a high affinity or specific Ca-binding site. Where does Ca2+ bind to activate? Recent crystallographic studies shows that it binds between the ELC and RLC; when both present the site is created. Regulation of molluscan muscle: (catch) Muscle completes contraction but maintains force. One explanation is that Pi is released but not ADP. Myosin remains in a force-producing state – but requiring no energy – until slight release of tension, allows ADP to dissociate and the crossbridge to bind ATP and detach).

18. MOLLUSCAN Adductor Catch state? Catch muscle

19. S1 HEAD MHC: (N to C terminal) Green – 27K domain; red – 50K domain (upper and lower); blue 20K domain and lever; ELC – yellow; RLC pink Rayment et al., 1993

20. Drosophila indirect flight muscles (dorso-longitudinal muscles)

21. Insect indirect flight muscles Dorso- longitudinal muscles -DLM Indirect because not attached to the wings; they move the wings by distorting the thorax Dorso-ventral Muscles -DVM Jump-muscle Tergal depressor of trochanter TDT

22. Flight muscle – oscillatory and asynchronous • Two muscle groups - DVM dorso-ventral muscles - DLM dorso-longitudinal muscles • Requires Ca2+ activation – but nerve impulses are not in synchrony with the wingbeat/muscle contractions. • Motor is started by the fly jumping – second leg (mesothoracic) – TDT (tergal depressor of the trochanter) • muscles contract, with a delay, after being stretched (strain-activation) by the thorax being deformed by the opposing muscle set • delay in strain activation (with stiffness of the cuticle and the drag on the wings – viscosity) determine the wing beat frequency.

23. Where does strain activation • come from? • 2 models: • Geometry of the filament lattice (Wray) • Crosslinks between thick and thin filaments. • Model 1: • Insect flight muscle has a very regular structure compred to any other. • The spacing of the myosin heads and the actin repeat are the same 38.5nm (note F-actin repeat is normally 36.5nm). • Thus myosin heads and their binding sites have the same spacing along the two sets of filaments Insect flight muscle

24. Image shows a thick filament rolled out flat (O = positions of myosin heads;  = actin monomers that myosin heads can reach and bind to) In the unstretched muscles the ‘offset’ prevents myosin heads binding actin; applied stretch brings them into register and they can bind. Unstretched Stretched Model 2: That insect flight muscle-specific polypeptide extensions to tropomyosin/troponins allow then to contact the thick filaments and ‘detect’ the relative movement of the two sets of filaments – no evidence at all! Vertebrate heart muscle is both calcium and stretch-activated (the Starling effect) – believed this may be a direct effect of stretch on the myosin in actomyosin crossbridge – strain affects myosin kinetics.