RK Goit, Lecturer

Department of Physiology

contraction occurs by a sliding filament mechanism

Z discs have been pulled by the actin filaments up to the ends of the myosin filaments

caused by forces generated by interaction of the cross-bridges with the actin filaments

when an action potential travels along the muscle fiber, this causes SR to release Ca++

Ca++ activate forces between myosin & actin filaments

energy comes from ATP molecule

Myosin Filament

composed of multiple myosin molecules (200)

myosin molecule is composed of six polypeptide

chains—two heavy chains, & four light chains

two heavy chains wrap spirally around each other

to form a double helix, called a myosin tail

one end of each of these chains is folded bilaterally,

called a myosin head

four light chains are also part of the myosin head

tails of the myosin molecules bundled together to

form the body of the filament

part of the body of each myosin molecule hangs to

the side along with the head- arm

protruding arms & heads together are called cross-

bridges

each cross-bridge is flexible at two points called

hinges

where the arm leaves the body of the myosin filament

where the head attaches to the arm

ATPase Activity of the Myosin Head

myosin head functions as an ATPase enzyme

this property allows the head to cleave ATP

Actin Filament

composed of: actin, tropomyosin, & troponin

Actin

actin filament is a double stranded F-actin protein molecule

F-actin helix is composed of polymerized G-actin molecules

attached to each one of the G-actin molecules is one molecule of ADP

these ADP molecules are the active sites

Tropomyosin

molecules are wrapped spirally around the sides of

the F-actin helix

in the resting state, lie on top of the active sites of

the actin strands

Troponin

are actually complexes of three loosely bound

protein subunits

troponin I for actin

troponin T for tropomyosin

troponin C for Ca++

active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex

in the presence of large amounts of Ca++, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited

when Ca++ combine with troponin C, the troponin complex undergoes a conformational change

this “uncovers” the active sites of the actin, thus allowing these to attract the myosin cross-bridge heads & cause contraction to proceed

“Walk-Along” Theory (ratchet theory) of Contraction

when a head attaches to an active site, causes the

head to tilt toward the arm (power stroke) & to drag the

actin filament along with it

↓

head automatically breaks away from the active site

↓

it combines with a new active site

↓

then the head tilts again

↓

pulling the ends of two successive actin filaments

toward the center of the myosin filament

ATP as the source of energy for contraction

the heads of the cross-bridges bind with ATP

ATPase activity of the myosin head immediately

cleaves the ATP

Ca++ binds with troponin-tropomyosin complex, active

sites on the actin filament are uncovered

bond between head of the cross-bridge & the active

site of the actin filament

once the head of the cross-bridge tilts, this allows

release of the ADP & phosphate ion

a new molecule of ATP binds

binding of new ATP causes detachment of the head

from the actin

the new molecule of ATP is cleaved to begin the

next cycle

The process by which depolarization of the muscle

fiber initiates contraction is called excitation-

contraction coupling.

myofibrils surrounded by T tubule–sarcoplasmic

reticulum system

penetrate all the way from one side of the muscle

fiber to the opposite side

they communicate with the extracellular fluid

when an action potential spreads over a muscle

fiber membrane, a potential change also spreads

along the T tubules to the deep interior of the

muscle fiber

sarcoplasmic reticulum composed of 2 major parts:

large chambers called terminal cisternae

long longitudinal tubules

muscle contraction continues as long as the Ca++

remain in high concentration

a continually active calcium pump located in the

walls of the sarcoplasmic reticulum pumps Ca++

away from the myofibrils back into the sarcoplasmic

tubules

References

Ganong Review of Medical Physiology, 23/E

Textbook of Medical Physiology, 12/E Guyton &

Hall

Thank You