Module 632 Lecture 9 JCS Control of Muscle Contraction
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
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
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
Twitch and tetanus – depend upon pattern of nerve impulses Force Tetanus Unfused tetanus Twitch Time – sec.
SR T-tubule Ryanodine receptor Dihydropyridine receptor Cytosol The dihydropyridine-ryanodine receptor complex
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.
7-actin repeat structure (14 - F-actin helix is double) in thin filament Structure repeats (half-turn) every 36.5nm
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
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)
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)
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
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.
Insect flight and Molluscan catch + adductor muscles: Pecten maximus Catch muscle Adductor muscle Insect flight muscles
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).
MOLLUSCAN Adductor Catch state? Catch muscle
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
Drosophila indirect flight muscles (dorso-longitudinal muscles)
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
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.
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
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.