Transcript Part B Muscles - St. Thomas More school Science Student Site

Muscle Fiber Contraction
Chapter 9
Section 9.4 -2
Muscle Contraction
• When muscles contract they are physically
shortening.
• They contract because of nerve impulses sent
from our brain.
Remember…
Skeletal Muscle Organization
• Different types of Myofilaments
1. Thick Filaments
 Made of bundles of protein called
MYOSIN
2. Thin Filaments
 Made of bundles of protein called
ACTIN
• Actin and Myosin bind to each other,
causing muscle contraction
Actin Myofilament
-THIN
• Made up of 2 strands of protein (ACTIN)
molecules
▫ Like 2 strings of beads loosely wound together
Myosin Myofilament
- THICK
• Made up of 2 strands of protein (MYOSIN)
molecules
• 10 x larger than thin myofilament
• 2 ends
▫ Long rod
▫ Double-headed
globular region
How Myofilaments Contract…
1. Myosin head attaches to actin.
How Myofilaments Contract…
2. Myosin head moves.
▫
▫
Heads bend backwards, like flexing hand or wrist
Because head is chemically bonded to actin, it
makes actin get pulled along with the myosin head
as it flexes
How Myofilaments Contract…
• Result: Actin myofilament SLIDES past myosin
myofilament in the direction of flex
▫ One myosin head flexes, then another and then
another, and essentially “walks” along actin
How Myofilaments Contract…
• Takes energy from ATP to release myosin head from
actin
▫ This repositions myosin head for each new flex
▫ To “lift foot from actin” regarding walking analogy
Check it out…
• http://www.mcgrawhill.ca/school/applets/abbi
o/quiz/ch10/myofilament_contraction.swf
Sliding Filament Model
• Z line
▫ Muscle tissue that is anchored at
one end of EACH actin myofilament
▫ Because its anchored, when the
actin moves, it drags the z line along
with it
▫ Area between Z lines = sarcomere
When Muscle is Relaxed…
• Myosin heads are raised and ready to be attached.
• But they are unable to bind to actin.
• They are unable to bind because the myosin head’s
attachment site on actin is PHYSICALLY BLOCKED
by a protein called tropomyosin.
So Logically…
• Tropomyosin needs to get out of the way for
muscle to contract.
• To do this, we require a protein called
TROPONIN.
Regulating Muscle Contraction
• Muscle contraction is regulated by calcium ions
(Ca2+) in the sacroplasm.
• Sarcoplasm - muscle fibre cytoplasm.
• The calcium ion concentration regulates the
troponin - tropomyosin complex.
Regulating Muscle Contraction
• LOW concentration causes tropomyosin
to inhibit myosin binding.
• HIGH concentration causes
tropomyosin to get out of the way so binding
can occur.
Where does this Ca2+ come from?
• Sarcoplasmic Reticulum
▫ When muscle is stimulated to
contract, SR releases Ca2+
into myofilament.
▫ When muscle is stimulated to relax, and stop
contracting, Ca2+ returns to SR via ACTIVE
TRANSPORT.
Sliding Filament Theory Video
• http://www.youtube.com/watch?v=0kFmbrRJq
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