BIOPHYSICS OF MUSCLE CONTRACTION

1. BIOPHYSICS OF MUSCLE CONTRACTION Ivan Poliaček

2. TYPES OF MUSCLES • Skeletal muscle - striated muscle tissue existing under control of the somatic nervous system (voluntary control) • Cardiac muscle - special striated muscle tissue of the heart working automatically and under the influence of autonomicnervous system (involuntarily) • Smooth muscle - non-striated muscle tissue activated by autonomic nervous system, hormones, or simply stretching

3. Surrounds each bundle cell Bundles of collagen fibers Fascia Becomes the muscle sheath which fuses with the tendon Surrounds each cell

4. Deep structures of a sceletal muscle

5. Muscle fiber = muscle cell • The sarcolemma – the cell membrane (plasma membrane)of a muscle cell - conduct stimuli - an outer coat (thin layer of polysaccharides with collagen fibrils)that fuses with a tendon fiber (they collect into bundles to form the muscle tendons) • Sarcoplasm - cytoplasm with organelles • Myofibrils - cylindrical organelles – contractive elementsthe actin and myosin filaments (thelength a few μm) – organized in repeated subunits along the length of the myofibril - sarcomeres • Sarcoplasmic reticulum with T-tubules – internal conductive system

6. Structure of a muscle cell

7. A microscopic photo and a scheme of

8. a maximum contraction of sarcomere is about 30%

9. Skeletal muscle • attached to the bones for movement • several types of fibers(speed, stamina, fatique, force, motor unit size, structure...) • cells - long multi-nucleatedcylinders - the length of muscle cell is a few mm (human sceletal muscle), the diameter is typically 100 - 150 μm • cytoskeleton – supporting the cell shape

10. Summary 1 • What is basic and deep structure of skeleton muscle? • What is the sctructure of muscle cell (fiber)? How does this cell look like? • What is the endomysium among the muscle cells for? • What organel is responsible for contraction – how is it arranged? • What are contractille elements – molecules? • How is contraction performed?

11. What does drive the muscle?What is making the muscle to contract?

12. Motor fibers (axons of motoneurons) within the motor nerves conduct action potentials to theneuro-muscular junction - end-plate. Action potential at the pre-synaptic membrane - activates voltage-gated Cachannels - Ca2+ influx - vesicles with neurotransmitter Acetylcholine(ACh)release acetylcholin into the synaptic cleft

13. acetylcholine Acetylcholin binds with nicotinic receptors on postsynaptic membraneof musclecell(motor end-plate) – activation of Nachannels – depolarization = EPSP = end plate potential • acetylcholine is eliminated by breaking down with the acetyl cholinesterase nicotinic ACh receptor

14. Intracellular recordingfrom a muscle fiber A nerve impulse reaching the axon terminal releases ACh into the neuromuscular junction. ACh produces EPSP = end plate potential (EPP) in the membrane beneath the terminal (the subsynaptic = end-plate membrane) (A in the graph), but not farther away (B). When the EPP reaches the threshold (about −50 mv), an action potential is generated that sweeps along the fiber (A to B in the graph).

16. If a single vesicle occasionally release ACh into the synaptic cleft the miniature EPSP (miniature end-plate potential) is produced – NO ACTION POTENTIAL follows Full EPSPfull end-plate potential - EPP (due to synaptic action : AP – Ca2+ influx - many vesicles release) always reaches the firing level and produces action potential at the muscle cell. Action potential spreads along the sarcolemmaandviaT-tubules of the muscle cell depolarizes theterminal cisternae and the sarcoplasmatic reticulum.

17. Motor unit • Connective tissue separates muscle cells - each one must be stimulated by a motoneuron • Motor unit - all muscle cells innervated by the same motoneuron – they will contract at the same time • Motor units vary in size - mostly mixture of motor units of different sizes large motor units >100 cells (typically slow postural muscles) small motor units about 10 cells (precise control fast acting muscles – those moving the eye)

18. Recruitment of motor units Increasing level of central (moto-neuronal) activation – successive activation of higher threshold motor units

19. Summary 2 • Where is the signal activating skeleton muscle coming from? • Where is it transmitted to the muscle? • What is the function of Ca++? • What is the name for EPSP at the muscle cell? • What receptors and what mediator are involved in synaptic transmission from the motoneuron to the muscle cell? • What is the course of action potential at the muscle fiber? • How far (where) the action potential at the muscle cell travel? • How many muscle cells are supplied with single motoneuron? • Can nearby muscle fibers be activated differently? • When and how much are all muscle fibers activated?

20. the rest The shape and some parameters of muscle fiber action potential intracellular extracellular

21. Extracellular recording of parasternal muscle EMG – 2 different motor units are present 200 μV 10 ms

22. The mechanism.What happens on the way from action potential to the contraction of the muscle?

23. Action potential on T-tubules - depolarizes sarcoplasmic reticulum - activates voltage-gated Ca channels - Ca2+released into intracellular space of muscle fiber Ca2+initiates muscle contraction The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the Ca2+ ions. A transverse T - tubules are the pathway for the action potential to signal the sarcoplasmic reticulum.

25. Muscle contraction is provided by interaction of ACTIN (thin) and MYOSIN (thick) fibers (filaments). Tropomyosin covers binding spots of actin at a rest state.

27. Muscle contracts when myosin heads make a cross-bridges (bonds) with actin and the heads bend pulling the actin filament deeperamong the myosin filaments. the cycle : bind – bend – release 1 stroke – 10-12 nm As far as Ca2+ and ATP is present (around the Actin and Myosin) the cycle : binding, bending, releasing and straightening of myosin heads continue (contraction). Ca2+ is constantly pumped (by a Ca pump) back into the sarcoplasmic reticulum (& extracellular space, tubules, cisternae). In order to keep high Ca2+ concentration - further action potentials has to release it again and again. Without Ca2+ tropomyosin returns back (conformation) over actin filament blocking its binding loci – stop contraction.

28. Crossbridge binding Schemeof chicken muscle myosin head. The movement is likely much moredistributed including the movementat the actin-myosin interface. (Rayment et al. Science 261: 50-58, 1993) Eachmyosin molecule contains two peptide heavy chains which are cleaved enzymatically to produce a myosin tail and two globular heads or crossbridges. The crossbridge heavy chain has a tail region (shown in blue) around which two smaller peptides wrap. These peptides are the light chains (yellow and red) which stabilize,and in some cases, regulate the myosin.

29. Myosin filament with myosin heads each 14.5 nm. 294 myosin moleculeson the thick filament (588 ATP-ase binding spots).

30. Ca2+binds with troponin, changes shape (conformation) of tropomyosin that exposes the actin binding spots on actin filament. The muscle contraction requires the ATP. As the ATP is required to release the actin – myosin binding – without ATP : RIGOR MORTIS.

31. For the unbinding the myosin head from the actinand its “straightening”, the ATP is required. Myosin hydrolyze ATP (lyse it into ADP and phosphate) gathering the energy

32. Summary 3 • What is sarcoplasmic reticulum for? • What happens when sarcoplasmic reticulum membrane is depolarized? • What is the function of Ca++? • What happens when the Ca++ is pumped back to the reticulum and other areas out of muscle cell intracellular space? • How tropomyosin affect the muscle cell contraction? • What is the role of ATP in muscle contraction? • What happens when there is no ATP in the muscle cell? • Why the sequence actin myosin connection – bending myosin heads (pulling the actin molecules) – release and stretch of myosin heads occur? • For what exactly is the ATP energy used?

33. Single muscle twitch • Increased intracellular Ca2+ concentrations (Ca2+ released from sarcoplasmic reticulum and T – tubules) lasts longer than action potentials • even longer lasts single muscle twitch - no action potential from other than related motoneuron - SUMMATION is TEMPORAL at the level of Ca2+ release and twitches overlapping

34. Single muscle twitch – unfused & fused tetanus • A tetanic contraction - motor unit stimulated by a high frequency of action potentials from its motoneuron • The time between stimuli is shorter than single twitch durations – no time to relax - superposition of muscle twitches and (temporal) summation of contraction • The strength of contractions increases - it is added to the previous twitch Incomplete (unfused) tetanus Complete (fused) tetanus

35. The opposite acting muscle to the agonist is the antagonist(e.g. flexing the elbow biceps brachii - agonist, the triceps brachii - antagonist) An antagonist isnever totally relaxed - control and damping of movement by maintaining tone against the agonist (eccentric movement)

36. Summary 4 • What is the duration of the action potential, increased Ca++ intracellular concentration, and single muscle twitch? • What type of summation works for muscle contraction? • How many motoneurons can stimulate the particular muscle cell? • Compare the frequencies of activation of a muscle cell during unfused and fused tetanus? • How is maximum contraction of the muscle cell achieved? • How are working oppositely connected muscles?

37. Summary of muscle contraction

38. Short rest • The time for thinking and organizing infos.

39. MUSCLE PROPERTIES • Elasticity or recoil - muscles have elastic elements which cause them to recoil to their original size. • Stretchability or extensibility - muscles can also stretch and extend to a longer-than-resting length • Excitability andresponsiveness - muscle tissue can be stimulated by electrical, physical, or chemical means • Contractility – transformation of chemical energy into the mechanical work - movement - the response of muscle tissue to stimulation is contraction, or shortening muscles can only pull, they cannot push

40. Smooth muscle • Visceral (smooth)muscle - in the body's internal organs and blood vessels - no striations - layers in the mucous membranes of the respiratory and digestive systems, bands in the walls of blood vessels, and sphincter muscles. • Smooth muscle fibers - a fusiform shape - a spindle-like shape (wide in the middle and tapers at both ends) - 20-500 μm in length. • The ratio of actin to myosin is ~ 6 : 1 in skeletal muscle, 4 : 1 in cardiac muscle and ~ 16.5 : 1 in smooth muscle. • Greater elasticity and ability to stretch and still maintain contractility than striated muscle.

41. Intestine smooth muscle

42. Smooth muscle contraction • Contraction - sliding of myosin and actin filaments- hormones, neurotransmitters, drugs, etc. • Single unit smooth muscle tissue is connected into a syncytium - the autonomic nervous system innervates a single cell within a sheet or bundle- the action potential propagates bygap junctions to neighboring cells (all contract together). • Smooth muscle may contract spontaneously - it is myogenic (via ion channel dynamics or pacemaker cells - interstitial cells of Cajal in the gastrointestinal tract).

43. Smooth muscle contraction Multiunit smooth muscle tissues - individual cells are innervated by the autonomic nervous system (fine control and gradual responses, much like in skeletal muscle) - the trachea, the elastic arteries, the iris of the eye). No troponin, but instead calmodulin is regulatory protein in smooth muscle, also tropomyosin doesn't serve the regulatory function as in striated muscle. Contraction is initiated by a Ca-regulated phosphorylation of myosin, rather than a Ca-activated troponin system. Smooth muscle thin filaments – mainly actin, tropomyosin, and caldesmon.

44. Summary 5 • What is elasticity, stretchability, excitability, and contractiliry? • What are main differences between skeletal and smooth muscles? • What does it mean syncytium? • What does it mean myogenic activity? • What is the molecule the Ca++ interact with in the smooth muscle vell instead of troponin of skeleton muscle cell? • How is contraction of multiunit smooth muscle cell initiated?

45. Heart muscle • Cardiac muscle - much shorter cells than skeletal muscle - cardiac muscle cells connect to one another - gap junctions - intercalated discs- electrochemical impulse can pass to all connected cells - syncytium (the cells work as a unit) Faint striations - similar, but not identical, arrangement of myofilaments in cardiac and skeletal muscle Many cardiac muscle cells are myogenic – activation arises in the heart not from the nervous system - heart natural rhythm

46. cytosol A scheme of intercalated discs of cardiac muscle connexin proteines gap junctions create cytoplasmic bridges intercellular space cell membrane • In cardiac myocytes, influx of Na trigger calcium-induced calcium release - Ca2+ influx through voltage-gated calcium channels on the sarcolemma leading to the release of Ca2+ from the sarcoplasmic reticulum

49. and its relation to the ECG waveform Long-lasting action potential - long refractory period - no temporal summation - no tetanic contraction

50. Relation of electrical and mechanical characteristics of heart action