Institute of Physiology Medical School of SDU Tel 88381175 (lab) LIU Chuan Yong 刘传勇 Institute of Physiology Medical School of SDU Tel 88381175 (lab) 88382098 (office) Email: liucy@sdu.edu.cn Website: www.physiology.sdu.edu.cn

Section 3 Muscle Contraction

Classification of the Muscle According to the structure: Striated Muscle, Smooth Muscle According to the nerve innervation: Voluntary Muscle, Involuntary Muscle According to the Function: Skeletal Muscle, Cardiac Contraction, Smooth Muscle

Skeletal Muscle Cardiac Muscle Smooth Muscle

I Signal Transmission Through the Neuromuscular Junction

Skeletal Muscle Innervation

Illustration of the Neuromuscular Junction (NMJ)

New Ion Channel Players Voltage-gated Ca2+ channel in presynaptic nerve terminal mediates neurotransmitter release Nicotinic Acetylcholine Receptor Channel in muscle neuromuscular junction (postsynaptic membrane, or end plate) mediates electrical transmission from nerve to muscle

Nerve Terminal Ca2+ channels Structurally similar to Na+ channels Functionally similar to Na+ channels except activation occurs at more positive potentials activation and inactivation much slower than Na+ channels

Neuromuscular Transmission Myelin Axon Axon Terminal Skeletal Muscle

Neuromuscular Transmission: Step by Step - - Nerve action Depolarization of terminal opens Ca channels Nerve action potential invades axon terminal - + + - + + Look here

postsynaptic membrane ACh is released and diffuses across Binding of ACh opens channel pore that is permeable to Na+ and K+. ACh binds to its receptor on the postsynaptic membrane ACh is released and diffuses across synaptic cleft. Ca2+ induces fusion of vesicles with nerve terminal membrane. ACh ACh ACh Nerve terminal Ca2+ Ca2+ Na+ Na+ Na+ Na+ K+ Na+ K+ Na+ K+ K+ ACh Na+ Na+ Na+ K+ Outside Muscle membrane Na+ Na+ Inside K+ K+ Na+ K+ K+ K+ K+ K+ Na+ K+ Na+

End Plate Potential (EPP) The movement of Na+ and K+ depolarizes muscle membrane potential (EPP) VNa EPP Muscle Membrane Voltage (mV) Threshold Presynaptic terminal -90 mV VK Time (msec) Presynaptic AP Outside Muscle membrane Inside Voltage-gated Na Channels ACh Receptor Channels Inward Rectifier K Channels

Meanwhile ... ACh ACh ACh Choline resynthesized ACh is hydrolyzed by into ACh and repackaged into vesicle Choline is taken up into nerve terminal ACh is hydrolyzed by AChE into Choline and acetate ACh unbinds from its receptor so the channel closes Nerve terminal Choline ACh Acetate ACh Outside Muscle membrane Inside

Structural Reality

Neuromuscular Transmission Properties of neuromuscular junction 1:1 transmission An unidirectional process Time delay. 20nm/0.5-1ms Easily affect by drugs and some factors The NMJ is a site of considerable clinical importance

Clinical Chemistry Related compounds are Ach is the natural useful in the neuroscience research Suberyldicholine is a synthetic neuromuscular agonist. Ach is the natural agonist at the neuromuscular junction. Carbachol and related compounds are used clinically for GI disorders, glaucoma, salivary gland malfunction, etc. Tubocurarine and other, related compounds are used to paralyze muscles during surgery. Tubocurarine competes with ACh for binding to receptor- but does not open the pore. So tubocurarine is a neuromuscular blocking agent. Carbachol is a synthetic agonist not hydrolyzed by acetylcholinesterase. Tubocurarine is the primary paralytic ingredient in curare.

Anticholinesterase Agents Anticholinesterase (anti-ChE) agents inhibit acetylcholinesterase （乙酰胆碱酯酶） prolong excitation at the NMJ

Anticholinesterase Agents 1. Normal: ACh Choline + Acetate AChE 2. With anti - AchE: ACh Choline + Acetate anti - AChE

Uses of anti-ChE agents Clinical applications (Neostigmine, 新斯的明, Physostigmine毒扁豆碱) Insecticides (organophosphate 有机磷酸酯) Nerve gas (e.g. Sarin 沙林，甲氟膦酸异丙酯。一种用作神经性毒气的化学剂))

Sarin comes in both liquid and gas forms, a highly toxic and volatile nerve agent developed by Nazi scientists in Germany in the 1930s. 500 times more toxic than cyanide （氢化物） gas.

NMJ Diseases Myasthenia Gravis （重症肌无力） Autoimmunity to ACh receptor Fewer functional ACh receptors Low “safety factor” for NM transmission Lambert-Eaton syndrome（兰伯特-伊顿综合征 ，癌性肌无力综合征 ） Autoimmunity directed against Ca2＋ channels Reduced ACh release

II Microstructure of Skeletal Muscle

Skeletal Muscle Human body contains over 400 skeletal muscles 40-50% of total body weight Functions of skeletal muscle Force production for locomotion and breathing Force production for postural support Heat production during cold stress

Fascicles: bundles, CT(connective tissue) covering on each one Muscle fibers: muscle cells

Structure of Skeletal Muscle: Microstructure Sarcolemma （肌管系统） Transverse (T) tubule Longitudinal tubule (Sarcoplasmic reticulum, SR 肌浆网) Myofibrils （肌原纤维） Actin 肌动蛋白 (thin filament) Troponin （肌钙蛋白） Tropomyosin （原肌球蛋白） Myosin 肌球蛋白 (thick filament)

Within the sarcoplasm Triad （三联管） Transverse tubules Sarcoplasmic reticulum -Storage sites for calcium Terminal cisternae - Storage sites for calcium

Microstructure of Skeletal Muscle (myofibril)

Sarcomeres Sarcomere 肌小节: bundle of alternating thick and thin filaments Sarcomeres join end to end to form myofibrils Thousands per fiber, depending on length of muscle Alternating thick and thin filaments create appearance of striations

Myosin 肌球蛋白 Myosin head is hinged Bends and straightens during contraction

Thick filaments (myosin) Bundle of myosin proteins shaped like double-headed golf clubs Myosin heads have two binding sites Actin binding site forms cross bridge Nucleotide binding site binds ATP (Myosin ATPase) Hydrolysis of ATP provides energy to generate power stroke

Thin filaments 原肌球蛋白 肌钙蛋白 肌动蛋白

Thin filaments (actin) Backbone: two strands of polymerized globular actin – fibrous actin Each actin has myosin binding site Troponin Binds Ca2+; regulates muscle contraction Tropomyosin Lies in groove of actin helix Blocks myosin binding sites in absence of Ca2+

Thick filament: Myosin (head and tail) Thin filament: Actin, Tropomyosin, Troponin (calcium binding site)

III Molecular Mechanism of Muscular Contraction The sliding filament model Muscle shortening is due to movement of the actin filament over the myosin filament Reduces the distance between Z-lines

The Sliding Filament Model of Muscle Contraction

Changes in the appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber

Cross-Bridge Formation in Muscle Contraction

Energy for Muscle Contraction ATP is required for muscle contraction Myosin ATPase breaks down ATP as fiber contracts

Nerve Activation of Individual Muscle Cells (cont.)

Excitation/contraction coupling Action potential along T-tubule causes release of calcium from cisternae of TRIAD Cross-bridge cycle

Begin cycle with myosin already bound to actin

1. Myosin heads form cross bridges Myosin head is tightly bound to actin in rigor state Nothing bound to nucleotide binding site

2. ATP binds to myosin Myosin changes conformation, releases actin

3. ATP hydrolysis ATP is broken down into: ADP + Pi (inorganic phosphate) Both ADP and Pi remain bound to myosin

4. Myosin head changes conformation Myosin head rotates and binds to new actin molecule Myosin is in high energy configuration

5. Power stroke Release of Pi from myosin releases head from high energy state Head pushes on actin filament and causes sliding Myosin head splits ATP and bends toward H zone. This is Power stroke.

6. Release of ADP Myosin head is again tightly bound to actin in rigor state Ready to repeat cycle

THE CROSS-BRIDGE CYCLE Relaxed state Crossbridge energised Crossbridge attachment A + M l ADP l Pi Ca2+ present A – M l ATP AlMlADPlPi Crossbridge detachment Tension develops ADP + Pi ATP AlM A, Actin; M, Myosin

Cross Bridge Cycle

Rigor mortis Myosin cannot release actin until a new ATP molecule binds Run out of ATP at death, cross-bridges never release

Need steady supply of ATP! Many contractile cycles occur asynchronously during a single muscle contraction Need steady supply of ATP!

The action potential triggers contraction How does the AP trigger contraction? We should ask: how does the AP cause release of Ca2+ from the SR, so leading to an increase in [Ca2+]i? how does an increase in [Ca2+]i cause contraction?

Regulation of Contraction Tropomyosin blocks myosin binding in absence of Ca2+ Low intracellular Ca2+ when muscle is relaxed

Ca2+ binds to troponin during contraction Troponin-Ca+2 pulls tropomyosin, unblocking myosin-binding sites Myosin-actin cross-bridge cycle can now occur

How does Ca2+ get into cell? Action potential releases intracellular Ca2+ from sarcoplasmic reticulum (SR) SR is modified endoplasmic reticulum Membrane contains Ca2+ pumps to actively transport Ca2+ into SR Maintains high Ca2+ in SR, low Ca2+ in cytoplasm

Ca2+ Controls Contraction Ca2+ Channels and Pumps Release of Ca2+ from the SR triggers contraction Reuptake of Ca2+ into SR relaxes muscle 19

Structures involved in EC coupling - Skeletal Muscle - T-tubule sarcolemma out in sarcoplasmic reticulum voltage sensor? junction foot

Dihydropyridine （DHP, 双氢吡啶） Receptor In t-tubules of heart and skeletal muscle Nifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubules In heart, a voltage-gated Ca2+ channel In skeletal muscle voltage-sensing protein undergoes voltage-dependent conformational changes 20

Ryanodine (利阿诺定 ) Receptor The "foot structure" in terminal cisternae of SR a Ca2+ channel of unusual design Conformation change or Ca2+ -channel activity of DHP receptor gates the ryanodine receptor, opening and closing Ca2+ channels Many details are yet to be elucidated! 21

Skeletal muscle The AP: moves down the t-tubule voltage change detected by DHP （双氢吡啶） receptors communicated to the ryanodine receptor which opens to allow Ca out of SR activates contraction out in voltage sensor (DHP receptor) junctional foot (ryanodine receptor) sarcoplasmic reticulum sarcolemma T-tubule

Cardiac muscle The AP: moves down the t-tubule voltage change detected by DHP receptors (Ca2+ channels) opens to allow small amount of (trigger) Ca2+ into the fibre Ca2+ binds to ryanodine receptors which open to release a large amount of (activator) Ca2+ (CACR) Thus, calcium, not voltage, appears to trigger Ca2+ release in Cardiac muscle! out in voltage sensor & Ca channel (DHP receptor) junctional foot (ryanodine receptor) sarcoplasmic reticulum sarcolemma T-tubule

The Answers! Skeletal Cardiac The trigger for SR release appears to be calcium (Calcium Activated Calcium Release - CACR) The t-tubule membrane has a Ca2+ channel (DHP receptor) The ryanodine receptor is the SR Ca release channel The ryanodine receptor is Ca-gated & Ca release is proportional to Ca2+ entry Skeletal The trigger for SR release appears to be voltage (Voltage Activated Calcium Release- VACR) The t-tubule membrane has a voltage sensor (DHP receptor) The ryanodine receptor is the SR Ca release channel Ca2+ release is proportional to membrane voltage

Transverse tubules connect plasma membrane of muscle cell to SR

Ca2+ release during Excitation-Contraction coupling Action potential on motor endplate travels down T tubules Ryanodyne R Ca-release ch.

Voltage -gated Ca2+ channels open, Ca2+ flows out SR into cytoplasm Ca2+ channels close when action potential ends. Active transport pumps continually return Ca2+ to SR Ca ATPase (SERCA)

Excitation-Contraction Coupling Depolarization of motor end plate (excitation) is coupled to muscular contraction Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca2+ from SR Ca2+ binds to troponin and causes position change in tropomyosin, exposing active sites on actin Permits strong binding state between actin and myosin and contraction occurs ATP is hydrolyzed and energy goes to myosin head which releases from actin

Summary: Excitation-Contraction Coupling

IV Factors that Affect the Efficiency of Muscle Contraction

Tension and Load The force exerted on an object by a contracting muscle is known as tension. The force exerted on the muscle by an object (usually its weight) is termed load. According to the time of effect exerted by the loads on the muscle contraction the load was divided into two forms, preload and afterload.

Preload Preload is a load on the muscle before muscle contraction. Determines the initial length of the muscle before contraction. Initial length is the length of the muscle fiber before its contraction. It is positively proportional to the preload.

The Effect of Sarcomere Length on Tension The Length – Tension Curve Concept of optimal length

Types of Contractions I Twitch: a brief mechanical contraction of a single fiber produced by a single action potential at low frequency stimulation is known as single twitch. Tetanus: It means a summation of twitches that occurs at high frequency stimulation

Effects of Repeated Stimulations Figure 10.15

1/sec 5/sec 10/sec 50/sec

Afterload a load on the muscle after the beginning of muscle contraction. The reverse force that oppose the contractile force caused by muscle contraction. does not change the initial length of the muscle prevent muscle from shortening because a part of force developed by contraction is used to overcome the afterload

Types of Contractions (II) Afterload on muscle is resistance Isometric Length of muscle remains constant. Peak tension produced. Does not involve movement Isotonic Length of muscle changes. Tension fairly constant. Involves movement at joints Resistance and speed of contraction inversely related

Isotonic and Isometric Contractions

Resistance and Speed of Contraction

Muscle Power Maximal power occurs where the product of force (P) and velocity (V) is greatest (P = FV) Max Power= 4.5units X