ACTIN AND MYOSIN INTERACTION

PRESENTED BY : MUHAMMAD NABEEL

Roll no. 62

MOLECULAR MECHANISM OF MUSCLE CONTRACTION

Sliding filament mechanism

Mechanical forces generated by interaction of myosin cross bridges with actin filaments cause the actin filaments to slide inward among the myosin filaments.

Under resting conditions these forces are inhibited. When an action potential travels over the muscle fiber membrane the sarcoplasmic reticulum releases large quantities of calcium ions. Forces between myosin and actin filaments are activated and contraction begins.

MYOSIN FILAMENTComposed of multiple myosin

molecules.

The tails of myosin molecules bundle together to form the body of the filament.

Cross bridges.

ATPase enzyme

THE ACTIN FILAMENT Each actin filament is made up of

actin , tropomyosin and troponin.

INTERACTION OF ONE MYOSIN FILAMENT WITH TWO ACTIN FILAMENTS AND CALCIUM IONS CAUSES CONTRACTION

INHIBITION OF ACTIVE SITES Active sites on actin filament are inhibited

by the troponin - tropomyosin complex.

ACTIVATION BY Ca++ IONS

Ca++ ions combine with troponin causing the troponin complex to tug on the tropomyosin molecule.

Active sites uncovered allowing contraction to proceed.

WALK ALONG THEORY

Myosin head attaches to an active site,

Head tilts automatically toward the arm that is dragging along the actin filament which is called power stroke.

Immediately after tilting the head automatically breaks away from the active site and returns to its normal perpendicular direction.

In this position, it combines with a new active site farther along the actin filament and pulls the ends of the actin filaments towards the centre of the myosin filament.

CYCLE OF MUSCLE CONTRACTION (WALK ALONG THEORY)

CONTRACTION IN SMOOTH MUSCLE

ACTIVATION OF MYOSIN IN SMOOTH MUSCLE