1. Skeletal Muscle 骨骼肌 XIA Qiang (夏强), PhD Department of Physiology
Zhejiang University School of Medicine
Tel: ,

2. Skeletal muscle骨骼肌
Cardiac muscle 心肌
Smooth muscle平滑肌

3. Plasma membrane invaginations

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

5. 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 .

6. 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.

7. Sarcomere structures in an electron micrograph.

8. Filaments

9. Myosin filament (thick filament)粗肌丝

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

11. Structure of thin and thick filaments

12. Titin肌联蛋白

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

14. 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.

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

16. 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.

17. 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.

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

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

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

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

22. 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.

23. 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.

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

25. 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

26. 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.

27. 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.

28. 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.

29. EC coupling in skeletal muscle

30. Mechanisms of Ca removal from the cytoplasm

32. 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

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

34. The vertebrate neuromuscular junction or motor end plate

35. 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.

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

37. Structure of the nicotinic AChR

38. 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: )

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

40. 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.

41. 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 New York, McGraw-Hill, 1994.)

43. 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.

44. 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)

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

46. 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.

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

48. Load-velocity relation 长度-速度关系

49. 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.

50. 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.

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

52. 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

53. 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.

54. 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.

55. 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)

56. 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

57. 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.

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

59. 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

61. Thank you!