Lecture 30 Muscle Flashcards
Features of skeletal muscle x4:
- Under voluntary control
- Striated
- Single long cylindrical cells
- Multiple peripheral nuclei
Features of cardiac muscle x5
- Located only in the heart,
- Striated
- Branched cells with 1-3 central nuclei
- Connected via intercalated discs
- Involuntary control
What is excitotoxicity
neurons are damaged and killed by the overactivations of receptors for the excitatory neurotransmitter glutamate, such as the NMDA receptor and AMPA receptor.
Features of smooth muscle
- Involuntary
- Found in the wall of internal organs (gut, blood vessels)
- Spindle shaped, uninucleated cells
- Not striated
Skeletal muscle cells are composed of __
fibrils containing highly organised contractile filaments
Thick filaments:
run the entire length of an A band
Thin filaments:
run the length of the I band and partway into the A band
Z disc:
coin shaped sheet of proteins that anchor the thin filaments and connect myofibrils to one another
H zone:
lighter mid region where filaments do not overlap
M line:
Line of protein myomesin that holds adjacent thick filaments together
T-tubules
deep invagination continuous with the sarcolemme (cell membrane) and circle each sarcoemer at each of the junctions of the A and I bands. Allows action potentials to be carried deep within the muscle cell.
Sarcoplasmic reticulum:
The calcium storage site. The terminal cisternae of the SR lies close to the T-tubules
The thick filaments are:
composed of myosin.
Myosin structure:
Each myosin has 2 subunits each with a globular head and a tail, the two tails intertwine to form a helix. The heads have a binding site for actin.
Myosin head:
an enzyme that hydrolyzes ATP (an ATPase).
Arranged in a polarised fashion i.e. with the myosin heads projection away from the M-line.
Titin anchors the thick filament to the Z-line
Thin filaments are:
composed primarily of globular actin proteins.
The filaments are composed of double stranded helical actin chain )polymers)
-Troponin and tropomyosin are regulatory proteins associated with actin in skeletal and cardiac muscle
Sliding filament theory of muscle contraction:
- The sarcomere shortens as the thin filaments are pulled over the thick filaments.
- The Z-line is pulled over the M line
- The I band and H zone become narrower
Troponin is regulated by:
Ca2+
Tropomyosin interacts with:
the myosin binding sites
4 steps of the cross bridge cycle:
- Cross bridge formation
- Power stroke
- Detachment
- Energization of myosin head
- Cross bridge formation:
- Myosin binds to the actin binding site to form a cross-bridge
- Note: cross bridges can only occur in the presence of calcium when the myosin binding site on actin is exposed.
- The power stroke:
-ADP is released, myosin head rotates, pulling it with the thin filament sarcomere is shortened.
- Detachment:
A new ATP molecule binds to the myosin, actin-myosin bind is weakened and the myosin detaches.
-Note: No ATP = no detachment
- Energization of the myosin head
Myosin head hydrolyzes the ATP to ADP + Pi. The myosin head moves back to its high energy (cocked formation) about 90 degrees to the actin
Importance of calcium
Calcium ions provide the on switch for the cross bridge cycle to begin.
When the calcium binds with troponin the tropomyosin moves to expose the myosin binding sites on actin.
The cross-bridge cycle as long as ____
calcium levels remain above the critical threshold.
;Isotonic
- Shortening
- Tension constant
- Velocity variable
Isometric:
- No shortening
- Length constant
- Tension variable
Length-tension relationship
During isometric contraction, at the level of the sarcomere the maximum active force (tension developed) is dependent on the degree of actin and myosin overlap.
At lengths <2.0 um:
-Filaments collide and interfere with each other reducing force developed.
At lengths > 2.2 um:
Active forces decline as the extent of overlap between filaments reduces, reducing the number of cross-bridges
Maximal force between:
2.0-2.2
Total tensions =
active + passive force
