1. Cross Bridge Cycle

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

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

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

5. The action potential triggers contraction
How does the AP trigger contraction?
This question has the beginning (AP) and the end (contraction) but it misses lots of things in the middle!
We should ask:
how does the AP cause release of Ca from the SR, so leading to an increase in [Ca]i?
how does an increase in [Ca]i cause contraction?

6. Structures involved in EC coupling
A band
(myosin)
I band
(actin)
Z disc
Contractile收缩 proteins in striated muscle are organised into sarcomeres
T-tubules and sarcoplasmic reticulum are organised so that Ca release is directed toward the regulatory (Ca binding) proteins
The association of a t-tubule with SR on either side is often called a ‘triad’ （三联管）(tri meaning three)
Z disc
M line
Z disc
sarcoplasmic
reticulum
t-tubules
Triad
junctional feet

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

8. Ca2+ Controls Contraction
Ca2+ Channels and Pumps
Release of Ca2+ from the SR triggers contraction
Reuptake of Ca2+ into SR relaxes muscle
So how is calcium released in response to nerve impulses?
Answer has come from studies of antagonist对抗 molecules that block Ca2+ channel activity 19

9. Lumen 管腔

10. Transverse tubules connect plasma membrane of muscle cell to SR

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

12. 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)

13. 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

14. Summary: Excitation-Contraction Coupling

15. Factors that Affect the Efficiency of Muscle Contraction

16. Tension张力 and Load负荷
The force exerted on an object by a contracting muscle is known as tension张力.
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.

17. 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.

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

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

20. Effects of Repeated Stimulations
Tetanus单收缩 treppe 阶梯
Figure 10.15

21. 1/sec
5/sec
10/sec
50/sec

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

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

24. Isotonic and Isometric Contractions

25. Resistance and Speed of Contraction

26. Rectangular hyperbola直角双曲线

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