11

Cross Bridge Cycle

22

Regulation of ContractionRegulation of Contraction

Tropomyosin bloTropomyosin blocks myosin bindicks myosin binding in absence of ng in absence of CaCa2+2+

Low intracellular Low intracellular CaCa2+2+

when musclwhen muscle is relaxede is relaxed

33

CaCa+2+2 binds to troponin during contbinds to troponin during cont

ractionraction Troponin-CaTroponin-Ca+2+2 pul pul

ls tropomyosin, uls tropomyosin, unblocking myosinblocking myosin-binding sitesn-binding sites

Myosin-actin crosMyosin-actin cross-bridge cycle can s-bridge cycle can now occurnow occur

44

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 cyto

plasm

The action potential triggers The action potential triggers contractioncontraction

The action potential triggers The action potential triggers contractioncontraction

How does the AP trigger How does the AP trigger contraction?contraction?

This question has the beginning This question has the beginning (AP) and the end (contraction) (AP) and the end (contraction) but it misses lots of things in the but it misses lots of things in the middle!middle!

We should ask:We should ask:how does the AP cause release of how does the AP cause release of

Ca from the SR, so leading to an Ca from the SR, so leading to an increase in [Ca]increase in [Ca]ii??

how does an increase in [Ca]how does an increase in [Ca]ii cause contraction?cause contraction?

Z disc

A band(myosin)

I band(actin)

Z disc M line Z disc

sarcoplasmicreticulum

t-tubules

junctional feetTriad

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)

Structures involved in EC coupling

Structures involved in EC coupling

Structures involved in EC couplingStructures involved in EC coupling- - Skeletal Muscle -Skeletal Muscle -

Structures involved in EC couplingStructures involved in EC coupling- - Skeletal Muscle -Skeletal Muscle -

outin

voltage sensor? junction foot

sarcoplasmic reticulum

sarcolemmaT-tubule

88

CaCa2+2+ Controls Contraction Controls Contraction

CaCa2+2+ Channels and Pumps Channels and Pumps Release of CaRelease of Ca2+2+ from the SR triggers cont from the SR triggers cont

raction raction Reuptake of CaReuptake of Ca2+2+ into SR relaxes muscle into SR relaxes muscle So how is calcium released in response tSo how is calcium released in response t

o nerve impulses? o nerve impulses? Answer has come from studies of antagoAnswer has come from studies of antago

nistnist 对抗 对抗 molecules that block Camolecules that block Ca2+2+ chan channel activitynel activity

99

1010

Transverse tubules connect plasma membrane of muscle cell to SR

1111

Ca2+ release during Excitation-Contraction coupling

Ryanodyne RCa-release ch.

Action potential on motor endplate travels down T tubules

1212

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)

1313

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

1414

Summary: Excitation-Contraction CouplingSummary: Excitation-Contraction Coupling

Factors that Affect the Efficiency of Muscle Contracti

on

1616

Tension 张力 and Load 负荷 The force exerted on an object by a contrThe force exerted on an object by a contr

acting muscle is known as tensionacting muscle is known as tension 张力张力 .. The force exerted on the muscle by an obThe force exerted on the muscle by an ob

ject (usually its weight) is termed load.ject (usually its weight) is termed load. According to the time of effect exerted bAccording to the time of effect exerted b

y the loads on the muscle contraction the y the loads on the muscle contraction the load was divided into two forms, preload load was divided into two forms, preload and afterload.and afterload.

1717

Preload PreloadPreload is a load on the muscle before is a load on the muscle before

muscle contraction. muscle contraction. Determines the initial length of the muscle Determines the initial length of the muscle

before contraction. before contraction. Initial length is the length of the muscle Initial length is the length of the muscle

fiber before its contraction. fiber before its contraction. It is positively proportional to the preload.It is positively proportional to the preload.

1818

The Effect of Sarcomere Length on Tension

The Length – Tension Curve

Concept of optimal length

1919

Types of Contractions I TwitchTwitch 单收缩单收缩 : a brief mechanical con: a brief mechanical con

traction of a single fiber produced by a straction of a single fiber produced by a single action potential at low frequency single action potential at low frequency stimulation is known as single twitch.timulation is known as single twitch.

TetanusTetanus 强直收缩强直收缩 : It means a summati: It means a summation of twitches that occurs at high frequeon of twitches that occurs at high frequency stimulationncy stimulation

2020

Effects of Repeated Stimulations

Figure 10.15

2121

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

2222

AfterloadAfterload AfterloadAfterload is a load on the muscle after the b is a load on the muscle after the b

eginning of muscle contraction. eginning of muscle contraction. The reverse force that oppose the contractile foThe reverse force that oppose the contractile fo

rce caused by muscle contraction. rce caused by muscle contraction. The afterload does not change the initial lenThe afterload does not change the initial len

gth of the muscle, gth of the muscle, But it can prevent muscle from shortening becaBut it can prevent muscle from shortening beca

use a part of force developed by contraction is use a part of force developed by contraction is used to overcome the afterload.used to overcome the afterload.

2323

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

Types of Contractions (II)

2424

Isotonic and Isometric Contractions

2525

Resistance and Speed of Contraction

2626

2727

Muscle PowerMuscle PowerMaximal power occurs where the product of

force (F) and velocity (V) is greatest (P=FV)

X Max Power= 4.5units