Excitation Contraction Coupling

Dr. Atif MahmoodMay 14, 2010

EXCITATION- CONTRACTION COUPLING

(SEQUENCE OF EVENTS)

EXCITATION- CONTRACTION COUPLING

EXCITATION- CONTRACTION COUPLING (cont’d)

EXCITATION- CONTRACTION COUPLING (cont’d)

Muscle Contraction and Relaxation

• Four actions involved in this process– Excitation = nerve action potentials lead to action

potentials in muscle fiber– Excitation-contraction coupling = action potentials

on the sarcolemma activate myofilaments– Contraction = shortening of muscle fiber – Relaxation = return to resting length

• Images will be used to demonstrate the steps of each of these actions

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Nerve Activation of Individual Muscle Cells (cont.)

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Action potential along T-tubule causes release of calcium from cisternae of TRIAD

Cross-bridge cycle

Excitation/contraction coupling

Begin cycle with myosin already bound to actin

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1. Myosin heads form cross bridges

Myosin head is tightly bound to actin in rigor state

Nothing bound to nucleotide binding site

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2. ATP binds to myosin

Myosin changes conformation, releases actin

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3. ATP hydrolysisATP is broken

down into:ADP + Pi

(inorganic phosphate)

Both ADP and Pi remain bound to myosin

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4. Myosin head changes conformation

Myosin head rotates and binds to new actin molecule

Myosin is in high energy configuration

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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.

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6. Release of ADP

Myosin head is again tightly bound to actin in rigor state

Ready to repeat cycle

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THE CROSS-BRIDGE CYCLE

ATPADP + Pi

AM

A – M ATP AMADPPi

A + M ADP Pi

Relaxed state

Crossbridge energised

Crossbridge

attachment

Tension

develops

Crossbridge

detachment

Ca2+ present

A, Actin; M, Myosin

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Cross Bridge Cycle

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Rigor mortis

Myosin cannot release actin until a new ATP molecule binds

Run out of ATP at death, cross-bridges never release

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Many contractile cycles occur asynchronously during a single

muscle contraction

• Need steady supply of ATP!

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Regulation of Contraction

Tropomyosin blocks myosin binding in absence of Ca2+

Low intracellular Ca2+

when muscle is relaxed

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Ca+2 binds to troponin during

contraction Troponin-Ca+2

pulls tropomyosin, unblocking myosin-binding sites

Myosin-actin cross-bridge cycle can now occur

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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

The action potential triggers contractionThe action potential triggers contractionHow 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?

Z disc

A band(myosin)

I band(actin)

Z disc M line Z disc

sarcoplasmicreticulum

t-tubules

junctional feetTriad

Contractile 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’ (tri meaning three)

Structures involved in EC coupling

Structures involved in EC coupling

Structures involved in EC coupling- Skeletal Muscle -

Structures involved in EC coupling- Skeletal Muscle -

outin

voltage sensor? junction foot

sarcoplasmic reticulum

sarcolemmaT-tubule

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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 molecules that block Ca2+ channel activity

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Dihydropyridine 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, DHP receptor is a voltage-gated Ca2+

channel In skeletal muscle, DHP receptor is apparently

a voltage-sensing protein and probably undergoes voltage-dependent conformational changes

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Ryanodine Receptor

The "foot structure" in terminal cisternae of SR Foot structure is a Ca2+ channel of unusual

design Conformation change or Ca2+ -channel activity

of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels

Many details are yet to be elucidated!

outin

voltage sensor(DHP receptor) junctional foot

(ryanodine receptor)

sarcoplasmic reticulum

sarcolemmaT-tubule

Skeletal muscleSkeletal muscle The AP: moves down the t-tubule voltage change detected

by DHP （双氢吡啶） receptors DHP receptor is

essentially a voltage-gated Ca channel

is communicated to the ryanodine receptor which opens to allow Ca out of SR activates contraction

Cardiac muscleCardiac muscle The AP:

moves down the t-tubule

voltage change detected by DHP receptors (Ca channels) which opens to allow small amount of (trigger) Ca into the fibre

Ca binds to ryanodine receptors which open to release a large amount of (activator) Ca (CACR)

Thus, calcium, not voltage, appears to trigger Ca release in Cardiac muscle!

outin

voltage sensor& Ca channel

(DHP receptor)

junctional foot(ryanodine receptor)

sarcoplasmic reticulum

sarcolemmaT-tubule

The Answers!The Answers!

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

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

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Transverse tubules connect plasma membrane of muscle cell to SR

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Ca2+ release during Excitation-Contraction coupling

Ryanodyne RCa-release ch.

Action potential on motor endplate travels down T tubules

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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)

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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

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Summary: Excitation-Contraction Coupling

Sliding Filament Model I:

• Actin myofilaments sliding over myosin to shorten sarcomeres– Actin and myosin do not change length– Shortening sarcomeres responsible for skeletal

muscle contraction

• During relaxation, sarcomeres lengthen

Sliding filament model II:

Sarcomere Shortening

Muscle contraction occurs when actin and myosin, the major proteins of the thin and thick filaments, respectively, slide past each other in an ATP-driven enzymatic reaction.

Sliding Filament Theory for Muscle Contraction

Huxley AF and Niedergerke R, Nature 173, 971-973 (1954)Structural changes in muscle during contraction; interferenceMicroscopy of living muscle fibers.

Huxley HE and Hanson J, Nature 173, 973-976 (1954) Changes in cross-strations of muscle during contraction and stretch and their structural interpretation.

Structure of Actin and Myosin

Myosin structure:

Thick filament structure:

Structure of the M-line:

Structure of thin filament:

Cross-bridge formation:

Cross-bridge hypothesis of muscle contraction:Sliding of thin and thick filaments is caused by the cross-bridges that extend from the myosin filament, attach to actin, pull the thin filaments toward the center of the sarcomere and detach. This cyclic interaction is coupled with the hydrolysis of ATP.

Sliding Filament Theory for Muscle Contraction

ATP (Crossbridge) ADP + Pi + Energy

Coupling of chemical reactions with vectorial motion.

Initiation of Contraction, Ca2+ release:

The micrograph shows myosin bound to actin

EM evidence for sliding filament theory

Contraction of one sarcomere

Mechanism of muscle contraction