Muscle physiology week 1 Flashcards
Structure of Skeletal Muscle (Micro anatomy)
Sarcomere Actin Myosin Troponin Tropomyosin Titin
Muscle Tissue Types
makeup 30 - 40 % of body weight
Smooth Muscle
Cardiac Muscle
Skeletal muscle
Skeletal muscle
- striated muscle
- Single muscle cell = muscle fiber
- multiple nuclei per fiber
- voluntary muscle
- attach to bones via tendons and enable movement
- Can produce powerful fast movements or small precise movements
- Can stretch and retract
Fixator muscles
- skeletal muscle
- stabilizes joints
[ rotator cuff muscles ]
Involuntary muscle
- smooth muscle
- Found in walls of hollow organs
[ stomach, esophagus, bronchi, and blood vessels ] - contract and relax automatically
Cardiac muscle
- only found in walls of the heart
- similar to skeletal muscle - has a striped appearance
- similar top smooth muscle - involuntary
- Highly specialized
> resistant to fatigue
> due to presence of large number of mitochondria,
myoglobin and good blood supply
Skeletal muscle:
Myofibril
= specialized contractile elements
- Contain highly organized microfilaments
> Thick and thin filaments
The functional unit
sarcomere
A myofibril displays alternating bands
dark bands (the A bands) and light bands (the I bands)
A band
stacked set of thick filaments along with the portions of the thin filaments
H zone
Middle of A band
M line
supporting proteins
I band
Thin filaments that don’t project into the A band
sarcomere
the distance between the Z-lines
Z line
any of the dark thin bands across a striated muscle fiber that mark the junction of actin filaments in adjacent sarcomeres.
Growth
increase in length by adding new sarcomeres
Titin
[ Highly elastic protein ]
- Scaffolding
- Elastic spring
- Signal transduction
Myosin
- can bend at hinge points in two locations
- Heads form the cross bridges
- Actin binding site
- Myosin ATPase site
Thin filaments contain 3 proteins
Actin
Tropomyosin
Troponin
Actin
- Contains binding sites for attachment with myosin cross bridge
Tropomyosin
covers the actin sites that bind with the cross bridges
Troponin
stabilizes tropomyosin in its blocking position over actin’s cross-bridge binding sites
in a relaxed muscle fiber
- No excitation
- No cross-bridge binding because cross-bridge binding site on actin is physically covered by troponin-tropomyosin complex
- muscle fiber is relaxed
in an excited muscle fiber
- muscle fiber is excited and Ca2+ is released
- released Ca2+ binds with troponin, pulling troponin-tropomyosin complex aside to expose cross-bridge binding site
- cross-bridge binding occurs
- binding of actin and myosin cross-bridge triggers power stroke that pulls thin filament inward during contraction
in an excited muscle fiber
- muscle fiber is excited and Ca2+ is released
- released Ca2+ binds with troponin, pulling troponin-tropomyosin complex aside to expose cross-bridge binding site
- cross-bridge binding occurs
- binding of actin and myosin cross-bridge triggers power stroke that pulls thin filament inward during contraction
Sliding Filament Theory
Thin filaments on each side of a sarcomere slide inward over the thick stationary filaments toward the A band’s center during contraction.
Thin filaments pull the Z lines closer together,
so in effect, the sarcomere shortens.
All sarcomeres throughout the length of the Muscle fiber shorten simultaneously – the entire muscle fiber shortens
Power Stroke
- Myosin cross bridges “walk” along an actin filament to pull it inward relative to the thick stationary filament.
- When actin and myosin bind, myosin’s head tilts inward
- Bending at this neck hinge point creates a “stroking” motion
- Thin filament is pulled toward the center of the sarcomere
- stroke of a cross bridge
In contracting muscle all cross-bridge strokes directed towards:
The center of thick filament
How does cross-bridge interaction between actin and myosin bring about muscle contraction?
Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling. Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction.
How does a muscle action potential trigger this contractile process?
A Muscle Contraction Is Triggered When an Action Potential Travels Along the Nerves to the Muscles. Muscle contraction begins when the nervous system generates a signal. The signal, an impulse called an action potential, travels through a type of nerve cell called a motor neuron.
The neuromuscular junction is the name of the place where the motor neuron reaches a muscle cell. Skeletal muscle tissue is composed of cells called muscle fibers. When the nervous system signal reaches the neuromuscular junction a chemical message is released by the motor neuron. The chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fiber. That starts a chemical reaction within the muscle.
What is the source of the Calcium that permits cross-bridge binding?
The sarcoplasmic reticulum stores calcium ions, which it releases when a muscle cell is stimulated; the calcium ions then enable the cross-bridge muscle contraction cycle.
