Skeletal Muscle 骨骼肌 XIA Qiang (夏强), PhD Department of Physiology Zhejiang University School of Medicine Tel: 88206417, 88208252 Email: xiaqiang@zju.edu.cn

Skeletal muscle骨骼肌 Cardiac muscle 心肌 Smooth muscle平滑肌

Plasma membrane invaginations

Skeletal muscles are attached to the skeleton by tendons. Skeletal muscles typically contain many, many muscle fibers.

The sarcomere（肌小节） is composed of: thick filaments called myosin, anchored in place by titin fibers, and thin filaments called actin, anchored to Z-lines .

A cross section through a sarcomere shows that: each myosin can interact with 6 actin filaments, and each actin can interact with 3 myosin filaments.

Sarcomere structures in an electron micrograph.

Filaments

Myosin filament (thick filament)粗肌丝

Actin filament (thin filament)细肌丝 Tropomyosin原肌凝蛋白 Troponin肌钙蛋白

Structure of thin and thick filaments

Titin肌联蛋白

Sarcotubular system (1) Transverse Tubule横管 (2) Longitudinal Tubule纵管 Sarcoplasmic reticulum肌浆网

Contraction (shortening): myosin binds to actin, and slides it, pulling the Z-lines closer together, and reducing the width of the I-bands. Note that filament lengths have not changed.

Contraction: myosin’s cross-bridges（横桥） bind to actin; the crossbridges then flex to slide actin.

The thick filament called myosin is actually a polymer of myosin molecules, each of which has a flexible cross-bridge that binds ATP and actin.

The cross-bridge cycle The myosin-binding site on actin becomes available, so the energized cross-bridge binds. 1. The cross-bridge cycle requires ATP The full hydrolysis and departure of ADP + Pi causes the flexing of the bound cross-bridge. 2. Partial hydrolysis of the bound ATP energizes or “re-cocks” the bridge. 4. Binding of a “new” ATP to the cross-bridge uncouples the bridge. 3.

The myosin-binding site on actin becomes available, so the energized cross-bridge binds. 1.

The full hydrolysis and departure of ADP + Pi causes the flexing of the bound cross-bridge. 2.

Binding of a “new” ATP to the cross-bridge uncouples the bridge. 3.

Partial hydrolysis of the bound ATP energizes or “re-cocks” the bridge. 4.

The cross-bridge cycle The myosin-binding site on actin becomes available, so the energized cross-bridge binds. 1. The cross-bridge cycle requires ATP The full hydrolysis and departure of ADP + Pi causes the flexing of the bound cross-bridge. 2. Partial hydrolysis of the bound ATP energizes or “re-cocks” the bridge. 4. Binding of a “new” ATP to the cross-bridge uncouples the bridge. 3.

In relaxed skeletal muscle, tropomyosin blocks Roles of troponin, tropomyosin, and calcium in contraction In relaxed skeletal muscle, tropomyosin blocks the cross-bridge binding site on actin. Contraction occurs when calcium ions bind to troponin; this complex then pulls tropomyosin away from the cross-bridge binding site.

The role of Ca in triggering the contraction of skeletal and cardiac muscle

Excitation-contraction coupling 兴奋-收缩偶联 Transmission of action potential (AP) along T tubules Calcium release caused by T tubule AP Contraction initiated by calcium ions

The latent period between excitation and development of tension in a skeletal muscle includes the time needed to release Ca++ from sarcoplasmic reticulum, move tropomyosin, and cycle the cross-bridges.

The transverse tubules bring action potentials into the interior of the skeletal muscle fibers, so that the wave of depolarization passes close to the sarcoplasmic reticulum, stimulating the release of calcium ions. The extensive meshwork of sarcoplasmic reticulum assures that when it releases calcium ions they can readily diffuse to all of the troponin sites.

Passage of an action potential along the transverse tubule opens nearby voltage-gated calcium channels, the “ryanodine receptor,” located on the sarcoplasmic reticulum, and calcium ions released into the cytosol bind to troponin. The calcium-troponin complex “pulls” tropomyosin off the myosin-binding site of actin, thus allowing the binding of the cross-bridge, followed by its flexing to slide the actin filament.

EC coupling in skeletal muscle

Mechanisms of Ca removal from the cytoplasm

General process of excitation and contraction in skeletal muscle 骨骼肌兴奋与收缩的基本过程 Neuromuscular transmission Excitation-contraction coupling Muscle contraction

A single motor unit（运动单位） consists of a motor neuron and all of the muscle fibers it controls.

The vertebrate neuromuscular junction or motor end plate

1. The exocytosis of acetylcholine from the axon terminal occurs when the acetylcholine vesicles merge into the membrane covering the terminal. 2. On the membrane of the muscle fiber, the receptors for acetylcholine respond to its binding by increasing Na+ entry into the fiber, causing a graded depolarization. 3. The graded depolarization typically exceeds threshold for the nearby voltage-gate Na+ and K+ channels, so an action potential occurs on the muscle fiber.

Nicotinic acetylcholine receptor 烟碱型乙酰胆碱受体 Acetylcholinesterase 乙酰胆碱酯酶

Structure of the nicotinic AChR

The effect of a local anesthetic on the AChR. A, Single-channel recording of nicotinic AChR expressed in a Xenopus oocyte. The patch was in the outside-out configuration, and the holding potential was −150 mV. The continuous presence of 1 μM ACh caused brief channel openings. B, This experiment is similar to that in A except that in addition to the ACh, the lidocaine analogue QX-222 (20 μM) was present at the extracellular surface of the receptor channel. Note that the channel opening is accompanied by rapid flickering caused by many brief channel closures. The time scale of the lower panel is expanded 10-fold. (Data from Leonard RJ, Labarca CG, Charnet P, et al: Evidence that the M2 membrane-spanning region lines the ion channel pore of the nicotinic receptor. Science 1988; 242:1578-1581.)

End plate potential (EPP) 终板电位 End-plate potentials elicited at the frog neuromuscular junction by stimulation of the motor neuron

Miniature end plate potential 微终板电位 Small fluctuations (typically 0.5 mV) in the resting potential of postsynaptic cells. They are the same shape as, but much smaller than, the end plate potentials caused by stimulation of the presynaptic cell. Miniature end plate potentials are considered as evidence for the quantal release of neurotransmitters at chemical synapses, a single miniature end plate potential resulting from the release of the contents of a single synaptic vesicle.

Evoked and spontaneous MEPPs. A, The investigators recorded V in frog skeletal muscle fibers that were exposed to extracellular solutions having a [Ca ] of 0.5 mM and a [Mg ] of 5 mM. These values minimize transmitter release, and therefore it was possible to resolve the smallest possible MEPP, which corresponds to the release of a single synaptic vesicle (i.e., 1 quantum). The investigators stimulated the motor neuron seven consecutive times and recorded the evoked MEPPs. In one trial, the stimulus evoked no response (0 quanta). In two trials, the peak MEPP was about 0.4 mV (1 quantum). In three others, the peak response was about 0.8 mV (2 quanta). Finally, in one, the peak was about 1.2 mV (3 quanta). In one case, a MEPP of the smallest magnitude appeared spontaneously. B, The histogram summarizes data from 198 trials on a cat neuromuscular junction in the presence of 12.5 mM extracellular Mg . The data are in bins with a width of 0.1 mV. The distribution has eight peaks. The first represents stimuli that evoked no responses. The other seven represent stimuli that evoked MEPPs that were roughly integral multiples of the smallest MEPP. The curve overlying each cluster of bins is a gaussian or “normal” function and facilitates calculation of the average MEPP for each cluster of bins. The peak values of these gaussians follow a Poisson distribution. (Data from Magleby KL: Neuromuscular transmission. In Engel AG, Franzini-Armstrong C [eds]: Myology, Basic and Clinical, 2nd ed, pp 442-463. New York, McGraw-Hill, 1994.)

Pharmacology of the vertebrate neuromuscular junction Pharmacology of the vertebrate neuromuscular junction. Many of the proteins that are involved in synaptic transmission at the mammalian neuromuscular junction are the targets of naturally occurring or synthetic drugs. The antagonists are shown as minus signs highlighted in red. The agonists are shown as plus signs highlighted in green.

Mechanics of single-fiber contraction Muscle tension 肌张力 – the force exerted on an object by a contracting muscle Load 负荷 – the force exerted on the muscle by an object (usually its weight) Isometric contraction 等长收缩 – a muscle develops tension but does not shorten (or lengthen) (constant length) Isotonic contraction 等张收缩 – the muscle shortens while the load on the muscle remains constant (constant tension)

Twitch contraction 单收缩 The mechanical response of a single muscle fiber to a single action potential is know as a TWITCH

iso = same tonic = tension metric = length Tension increases rapidly and dissipates slowly Shortening occurs slowly, only after taking up elastic tension; the relaxing muscle quickly returns to its resting length.

All three are isotonic contractions. Latent period潜伏期 Velocity of shortening Duration of the twitch Distance shortened

Load-velocity relation 长度-速度关系

Frequency-tension relation频率-张力关系 Complete dissipation of elastic tension between subsequent stimuli. S3 occurred prior to the complete dissipation of elastic tension from S2. S3 occurred prior to the dissipation of ANY elastic tension from S2. T e m p o r a l s u m m a t i o n.

Frequency-tension relation Unfused tetanus非融合性强直收缩: partial dissipation of elastic tension between subsequent stimuli. Fused tetanus融合性强直收缩: no time for dissipation of elastic tension between rapidly recurring stimuli.

Mechanism for greater tetanic tension Successive action potentials result in a persistent elevation of cytosolic calcium concentration

Length-tension relation Short sarcomere: actin filaments lack room to slide, so little tension can be developed. Optimal-length sarcomere: lots of actin-myosin overlap and plenty of room to slide. Long sarcomere: actin and myosin do not overlap much, so little tension can be developed. Optimal length

In skeletal muscle, ATP production via substrate phosphorylation is supplemented by the availability of creatine phosphate磷酸肌酸. Skeletal muscle’s capacity to produce ATP via oxidative phosphorylation is further supplemented by the availability of molecular oxygen bound to intracellular myoglobin.

In skeletal muscle, repetitive stimulation leads to fatigue疲劳, evident as reduced tension. Rest overcomes fatigue, but fatigue will reoccur sooner if inadequate recovery time passes.

Types of skeletal muscle fibers On the basis of maximal velocities of shortening Fast fibers快肌纤维 – containing myosin with high ATPase activity (type II fibers) Slow fibers慢肌纤维 -- containing myosin with low ATPase activity (type I fibers) On the basis of major pathway to form ATP Oxidative fibers氧化型肌纤维 – containing numerous mitochondria and having a high capacity for oxidative phosphorylation, also containing large amounts of myoglobin (red muscle fibers) Glycolytic fibers糖酵解型肌纤维 -- containing few mitochondria but possessing a high concentration of glycolytic enzymes and a large store of glycogen, and containing little myoglobin (white muscle fibers)

Types of skeletal muscle fibers Slow-oxidative fibers – combine low myosin-ATPase activity with high oxidative capacity Fast-oxidative fibers -- combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity Fast-glycolytic fibers -- combine high myosin- ATPase activity with high glycolytic capacity

Most skeletal muscles include all three types. Slow-oxidative skeletal muscle responds well to repetitive stimulation without becoming fatigued; muscles of body posture are examples. Fast-oxidative skeletal muscle responds quickly and to repetitive stimulation without becoming fatigued; muscles used in walking are examples. Most skeletal muscles include all three types. Fast-glycolytic skeletal muscle is used for quick bursts of strong activation, such as muscles used to jump or to run a short sprint.

Note: Because fast-glycolytic fibers have significant glycolytic capacity, they are sometimes called “fast oxidative-glycolytic [FOG] fibers.

Whole-muscle contraction All three types of muscle fibers are represented in a typical skeletal muscle, and, under tetanic stimulation, make the predicted contributions to the development of muscle tension. Fast- glycolytic Fast-oxidative Slow-oxidative

Thank you!