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Myosin Contracts Skeletal Muscle

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Myosin Contracts Skeletal Muscle . Jonathan P. Davis, Ph.D. Assistant Professor Office/Lab Phone 247-2559 Email: [email protected] Department of Physiology and Cell Biology, The Ohio State University, 400 Hamilton Hall. Muscle.

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slide1

Myosin Contracts Skeletal Muscle

Jonathan P. Davis, Ph.D. Assistant Professor

Office/Lab Phone 247-2559

Email: [email protected]

Department of Physiology and Cell Biology, The Ohio State University, 400 Hamilton Hall

slide2

Muscle

Skeletal

Cardiac

Smooth

slide3

Thick Filament or Myosin Regulation

In these cases: Myosin needs to be “activated” before it can interact with actin or move cargo

slide4

Actin Binding Proteins Like Tropomyosin Regulate Myosin Thin Filament Regulation

“Blocked State”

“Open or M State”

“Closed State”

Strong Hydrophobic Myosin Binding Sites

Weak Electrostatic Myosin Binding Sites

Tropomyosin “Rocks and Rolls”

slide5

Structure of Skeletal (striated) Muscle

Comprised of fibers (cells)

Each fiber contains many myofibrils in parallel

Each myofibril contains many sarcomeres in

series

Striations due to characteristic banding pattern

of sarcomere

Electron micrograph of Skeletal Muscle Fiber

slide8

The Thin Filament Is Composed Primarily of Actin but Also Contains Tropomyosin and the TroponinComplex

Complex

The Troponin Complex Contains Three Proteins

  • Troponin C – Binds Calcium
  • Troponin I – Inhibits Cross-Bridge Binding
  • Troponin T – Binds Tropomyosin
slide9

T-tubules and the Sarcoplasmic Reticulum

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.

slide10

ACTION POTENTIAL CAUSES RELEASE OF CALCIUM FROM SR

Cell Membrane

Ryanodine Receptor

Sarcoplasmic Reticulum

T -Tubule

SR Ca2+ ATPase

Dihydropyridine Receptor

Calsequestrin

Calcium

slide11

Mechanism of Skeletal Muscle Activation by Ca2+

**Tm can occupy 2 positions:

“off” and “on” state

1) “off” state- absence of Ca2+

Tm blocks myosin binding

2) “on” state-

Ca2+ binds to TnC

Tm moves toward center of actin

Myosin binding sites exposed

Muscle contracts

slide12

Regulation of Striated Muscle Contraction

1) Action Potential

2) Calcium Transient

Plasma Membrane

Plasma Membrane

Sarcoplasmic Reticulum

[Ca2+]

T-Tubule

Sarcoplasmic Reticulum

**SR Ca2+ ATPase**

Time

Calcium

3) Calcium Binds Troponin C

4) Myosin Power Stroke

5) Force Production

Actin

Actin

–Ca2+ Relaxed

Tropomyosin

Troponin Complex

- Ca2+

Actin

Myosin

Myosin

+ Ca2+

Myosin Binding Site

+Ca2+ Contracted

*** ATP Driven Power Stroke & Detachment***

slide13

ISOTONIC AND ISOMETRIC CONTRACTIONS

Greater the force against which shortening occurs, the slower the velocity of shortening

A – 100% Maximal Force

B – 75% Maximal Force

Force

Maximal Velocity (VMAX)

C – 50% Maximal Force

D – 25% Maximal Force

D

C

= L/T

D

B

C

A

Muscle Shortening

B

A

Time

Muscles exhibit > 200-fold variation in maximum velocity of shortening. Why?

Maximum velocity of shortening

Reflects speed of cross-bridge cycling

Is  actomyosin ATPase activity

Is determined by differences in the myosin molecule

slide14

RELATIONSHIP OF ELECTRICAL TO MECHANICAL EVENT IN SKELETAL MUSCLE CONTRACTION

Calcium Transient

(Shortening or Force Generation)

slide16

ACTIVE, PASSIVE AND TOTAL FORCE VERSUS MUSCLE LENGTH

Isometric contraction at each length

In the body

Skeletal muscle operates at plateau of length-force relation

Cardiac muscle operates on the ascending limb of length-force relation

slide18

CLASSIFICATION OF SKELETAL MUSCLE FIBERS

Classification system of muscle fibers is based on:

Rate of ATP utilization and capacity to re-synthesize ATP

Physiological implications of these parameters

Muscles are heterogeneous with different proportions of fiber types depending on function

slide19

RELATIONSHIP OF MOTOR UNITS TO INNERVATED MUSCLE FIBERS AND RECRUITMENT

Fast-

glycolytic

Fast-oxidative

Slow-oxidative

slide21

EFFECTS OF FATIGUE ON SKELETAL MUSCLE FIBERS TYPES

  • What Could be Happening?
  • Conduction Failure
  • Energy Metabolism Biproducts
  • A) Lactic Acid
  • B) Phosphate and ADP
slide22

Geometry of Muscle

a. Direction fibers run in the muscle

b. Lever system

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