Transcript Skeletal Muscle Contraction

Skeletal Muscle Contraction
Dr. Wasif Haq
Introduction
• Muscles make 50% of total body mass.
• 40% by skeletal muscles & 10% combination of smooth &
cardiac muscles.
• Sarcolemma: Cell membrane of muscle fiber. Ends fuse with
tendons’ fiber to form muscle tendons inserting in bone.
• Sarcoplasm: Intracellular matrix of muscle fiber having
suspended myofibrils. Abundant ATP & K+,P+,Mg++.
• Sarcoplasmic reticulum: Endoplasmic reticulum of muscles,
store Ca++ needed for contraction.
• Sarcomere: Myofibrils comprise of smaller contractile units,
smallest contractile unit of muscle fiber.
Structure of Myofibril.
• Sarcomeres comprise of series of alternating dark and light
bands causing striated appearance.
• Dark band=A band. (Anisotropic) comprise of Myosin & ends
of Actin.
• Lighter appearance in midsection called H-zone which is
bisected by dark M line.
• Light band=I band (Isotropic) comprise of Actin only.
• Light band have middle Z line.
Structure of Myofilaments
• Myofibrils contain myofilaments that in turn are composed of
Actin and Myosin.
• Actin and Myosin ratio: 2:1
• Myofibrils comprise of 1500 myosin (in center) attached to
3000 actin on each side.
• Portion of myofibril lying between 2 successive Z disc is
Sarcomere.
Myosin
• Myosin has 6 polypeptide chains, 4 light chains & 2 heavy
chains.
• 2 heavy chains form double helix twisting forming tail.
• Ends of these chain folded to form head, thereby making 2
heads lying side by side at one end of double helix myosin.
Myosin
• Multiple tails unite to form body with many heads handing
outwards on the sides of the body with a side hanging
structure called arm that helps to extend the head away from
the body.
• Cross bridges: Protruding arms & head together.
• Cross bridges are flexible at two points called hinges.
• 1. Origin of arm from body
• 2. Site of attachment of head to arm.
Myosin
• Hinged arms cause head to either extend far outward or come
closer to the body.
• The hinged head in turn participate in actual contraction
process.
• Head has ATPase enzyme cleaving ATP and thereby deriving
energy needed to cause muscle contraction.
Actin
• Comprises of 3 proteins: Actin, Tropomyosin and Troponin
• Actin twisted around each other in 2 chains like double helix
structure made of F actin protein molecule, comprising of 1
active site resulting from union of 2 F actin strands.
• Tropomyosin: Attached with Actin. At resting phase,
Troponin-Tropomyosin complex covers 7 active sites on actin
thereby preventing myosin binding and eventually muscle
contraction.
• Troponin: Attached with Tropomyosin. 3 subtypes.
• Troponin I: Affinity for Actin.
• Troponin T: Tropomyosin.
• Troponin C: Calcium ions, for contraction Ca++ uncover active
sites.
Sliding Mechanism
• At resting phase, actin molecules are distant and apart from
myosin and from each other.
• When contraction is needed, actin molecules are pulled closer
to myosin and to each other, overlapping each other.
• Occurs because cross bridges of myosin become attached to
active sites on actin, pulling the actin towards sarcomere.
• Changes in bands:
• Shortening of I & H band
• Z line brought closer to each other.
• Ca++ needed to facilitate binding between actin and myosin and
ATP required to liberate energy.
Walk Along Theory
• In presence of Ca++ ions, head from cross bridges of myosin
become attached to active sites on actin causing contraction.
• Walk along theory=Ratchet theory of contraction.
• Constant attachment and detachment of the head from active
site on actin takes place, hence myosin head moves along the
next active sites on actin in a sequence.
• Power stroke: The tilt of myosin head towards the arm causing
dragging of the actin filament along.
• Followed by detachment of the head, returning to normal
position and again binding with newer active site on actin.
• Fenn effect: Greater the amount of work to be performed by
muscle, greater will be the amount of ATP to be cleaved.
Mechanism for muscle contraction
• 1. Action potential travels through motor nerve to
endings on muscle fiber.
• 2. At the nerve ending, Acetylcholine; neurotransmitter is
released.
• 3. Acetylcholine acts on Acetylcholine channels.
• 4. Inflow of Na+ ions occur causing depolarization.
• 5. Action potential travels through muscle fiber
membrane.
• 6. Sarcoplasmic reticulum releases Ca++
• 7. Ca++ inititate binding between actin and myosin.
• 8. Ca++ pumped back into Sarcoplasmic reticulum hence
stopping contraction.
Extras
• Motor unit: All the muscle fibers innervated by single motor nerve.
• Muscle fatigue: Anaerobic glycolysis, lactic acid accumulation
causing tissue injury.
• Hypertrophy: Increase in size of the muscle fiber.
• Hyperplasia: Increase in number of normal muscle fiber in normal
arrangement.
• Rigor Mortis: Loss of ATP after death fails to separate cross bridges
from actin causing muscles to remain contracted and remain rigid.
Takes 15-25 hours to resolve.
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