Lecture 30: Skeletal Muscle Part I Flashcards
What are the features of skeletal muscle?
- Under voluntary control
- Striated
- Single long cylindrical cells
- Multiple peripheral nuclei
What are the features of cardiac muscle?
- Striated
- Branched cells with 1-3 central nuclei
- Connected via intercalated discs
- Invountary control
What are the features of smooth muscle?
- Involuntary
- Found in the walls of internal organs
- Spindle shaped, uninucleated cells
- Not striated
What are the contractile units that make up muscle fibres called?
Myofibrils
How are skeletal muscles attached within the body?
They are attached to bones via tendons
Where are thick filaments found on a sarcomere?
Running the length of the A band
Where are thin filaments found on a sarcomere?
Running the length of I band and partway into the A band
What is the Z disc?
Sheet of proteins that anchor thin filaments and connects myofibrils to one another
What is the H zone?
The part of the A band in which only thick filaments exist
What is the M line?
Line of protein myomesin that holds thick filaments together - located within the H zone
What are T-tubules?
Deep infoldings of sarcolemma that circle each sarcomere at the junctions of A and I bands.
What is the function of T-tubules?
Carry action potentials deep into muscle cell
What is the SR?
Sarcoplasmic reticulum is the storage site for calcium - they surround T-tubules forming a triad
What are thick filaments?
Parts of myofibrils made up of myosin -> each myosin has a globular head and a tail
Two myosin molecules come together as the two tails intertwine to form a helix
The head allows myosin molecules to bind to actin -> enzyme that is able to breakdown ATP
How are thick filaments arranged within myofibrils?
Myosin heads project away from M line
What are thin filaments?
Parts of myofibrils that are made up of globular actin proteins
Filaments are composed of double stranded helical actin chain
Thin filaments have regulatory proteins called troponin and tropomyosin
What is troponin?
Regulatory protein found on thin filaments -> when bound by calcium ions they cause tropomyosin to move away from the actin binding site allowing myosin heads to form cross bridges with the actin chain
What is tropomysoin?
Regulatory protein found on thin filaments -> at rest, they cover the actin binding site preventing muscle contraction from occurring
How does the sarcomere change as muscle contraction occurs?
Z discs pulled towards M line
I band and H zone become narrower
What are the 4 steps of the cross bridge cycle?
- Cross-bridge formation
- Power stroke
- Detachment
- Energisation of myosin head
What occurs in the cross-bridge formation stage of the cross bridge cycle?
Calcium ions bind to troponin causing tropomyosin to move positions exposing actin binding site
ATP binds to myosin head and it broken down to ADP and Pi allowing myosin head to be in the cocked position
Release of inorganic phosphate ion allows myosin heads to bind
Myosin head binds to actin binding site on thin filament
What happens in the power stroke stage of the cross bridge cycle?
ADP is released providing energy for the myosin head to rotate to its lower energy state pulling the actin filament along
-> sarcomere is therefore shortened
What occurs in the detachment stage?
A new ATP molecule binds to the myosin head -> this weakens the actin-myosin bond causing myosin to detach from the thin filament
What happens in the energisation of the myosin head stage?
Myosin head breaks down the ATP to ADP and Pi
Hence, energy is released allowing myosin to return to its higher energy state (position) making it ready for muscle contraction again
What is the role of calcium in muscle contraction?
It is the on switch for muscle contraction to begin
-> the cross-bridge cycle will continue if calcium levels are above a critical threshold (0.001-0.01mM)
How is the release of calcium to allow muscle contraction regulated?
Calcium ions are released from the sarcoplasmic reticulum by the opening of calcium channels
Active transport pumps (Ca2+ ATPase) constantly move Ca2+ ions from the muscle fibres back to the sarcoplasmic reticulum
What is isotonic contraction?
Muscle contraction in which there is a shortening of the muscle.
- > tension is constant
- > velocity variable
What is isometric contraction?
Muscle contraction in which there is no shortening of the muscle
- > length of muscle constant
- > tension is variable
