PHYSIOLOGY OF MUSCULAR CONTRACTION

1. INTRODUCTION

• I} Actomyosin complex: During the contraction of the muscle the thin actin filaments glide over thick myosin filament

• II} Molecular basis of muscular contraction: It is responsible for formation of actomyosin complex that results in muscular contraction

Stages of muscular contraction

• 1. Excitation contraction coupling • 2. Role of troponin and tropomyosin • 3. Sliding Mechanism

1. EXITATION CONTRACTION COUPLING

• Excitation contraction coupling is the process that occurs in between the excitation and contraction of muscle. This process involves series of activities which are responsible for the contraction of muscle

• IMPULSE Through motor neuron

• NEUROMUSCULAR JUNCTION • ACETYLCHOLINE IS REALEASED • IT CAUSES OPENING OF SODIUM CHANNELS • SODIUM IONS ENTER THE NEUROMUSCULAR JUNCTION • DEVELOPS ENDPLATE POTENTIAL • ACTION POTENTIAL (AP) IN MUSCLE FIBRE

• AP SPREADS TO SARCOLEMA AND MUSCLE FIBRE • AP REACHES THE CISTERNAE OF ‘L’ TUBULES • CISTERNAES ARE EXITED • Ca++ IONS REALEASED IN SARCOPLASM • Ca++ MOVES TOWARDS ACTIN FILAMETS TO FORM

CONTRACTION

2. ROLE OF TROPONIN AND TROPOMYOSIN

• Head of myosin has strong affinity towards active site of F actin.

• i)RELAXED CONDITION: F actin is covered with tropomyosin. Hence myosin can not bind itself to F actin molecule

• ii)EXCITED CONDITION: Ca++ ions released from ‘L’ tubules bind with troponin C.

• This causes change in the position of troponin molecule. This pulls the tropomyosin molecule away from F actin. Due to this movement of tropomyosin, F actin is uncovered and immediately the head of myosin gets attached.

http://www.blackwellpublishing.com/matthews/myosin.html

3. SLIDING THEORY

• It explains how actin filaments slide over myosin filaments and form actomyosin complex during muscle contraction.

• Also known as Ratchet theory or Walk along theory

• Cross bridge has three components viz. a hinge, an arm, and a head

Power stroke After binding with F actin, myosin head is tilted

towards the arm so that the actin filament is draged along with it. This tilting of head is called POWER STROKE

• After tilting, the head immediately breaks away from the active site and returns to the original position and combines itself with another active site on actin molecule.

• These tilting movement occurs again and again. • Thus the head of the cross bridge bends back and

forth and pulls the actin filament of both the ends towards the centre of sarcomere.

• So the actin filaments of both the ends overlap and form actomyosin complex.

• Formation of actomyosin comlex leads to contraction of muscle

When the muscle shortens further

The actin filament from opposite ends of the sacromere approach each other.

So the ‘H’ zone becomes narrow. And the two ‘Z’ lines come closer with reduction in length of sarcomere.

However, the length of ‘A’ band is not altered.But the length of ‘I’ band decreases.

Muscular contraction becomes severe• When the muscular contraction becomes

severe, the following changes takes place in sarcomere:

1. ‘Z’ line comes closer2. ‘I’ band decrease3. H zone either decreases or disappears4. ‘A’ band remains same

Structure of Sarcomere

Summary • STIMULATION OF MUSCLE FIBRE • GENERATION OF AP • SPREADING OF AP IN MUSCLE FIBRE • ARRIVAL OF AP AT CISTERNAE OF ‘L’ TUBULE • REALEASE OF Ca++ ions INTO SARCOPLASM • BINDING OF Ca++ WITH TROPONIN C AND CHANGE IN POSITION OF TROPONIN C • EXPOSURE OF ACTIVE SITE OF ‘F actin’ • BINDING OF MYOSIN WITH F actin AND POWER STROKES IN MYOSIN HEAD • SLIDING OF ACTIN FILAMENTS OVER MYOSIN FILAMENTS • MUSCULAR CONTRACTION

THANK YOU!!