What are the two types of muscle contraction and how do they differ?
Isotonic – tension stays the same and length changes
Isometric – tension changes and length remains the same
What are the two subtypes of one of the types of muscle contraction?
Isotonic = Concentric (shortening) + Eccentric(lengthening)
What do myofibres have and how do they look?
- T-tubules: membrane invaginations that contact the extracellular fluid
- Sacrcoplasmic Reticulum: extensive network of ca2+- stores surrounding each myofibril
Look:
- large and cylindrical
- multinucleate
- packed with myofibrils
Explain the excitation-contraction coupling in skeletal muscles.
- Action potential (AP) propagates along myofibre membrane (sarcolemma) and T-tubules
- Depolarisation activates dihydropyridine receptors (DHPR) causing conformational change in DHPR
- This change is transmitted to ryanodine receptors (RyR) on SR opening RyR channel; Ca2+ release from intracellular stores
- Thus depolarisation—-> Increase in intracellular Ca2+
What is the Z-line made up of?
Alpha-actinin CapZ
What are the two types of receptors involved in excitation-contraction coupling of skeletal muscle?
Dihydropyridine receptor Ryanodine receptor
What are the different components of a sarcomere and draw it?
Z-line Actin filaments CapZ and Tropomodulin Nebulin Titin Myosin Tropomyosin Nebulin
Check with the tissues lecture powerpoint, of the drawing and the labels.
What is the structure of actin?
Two twisted alpha helices- displays polarity
What does Titin do?
It holds the myosin in place to the z-line It is very large. It is spring like
Where are CapZ and Tropomodulin found?
CapZ is at at the end near the Z line and tropomodulin is near the end close to the myosin.
What is nebulin associated with and what is tropomyosin?
Large filaments associated with actin.
Tropomyosin - Elongated protein bound to actin
What does myosin have?
Thick filaments- ‘motor proteins’. contains numerous ‘globular heads’ that interact with actin
Explain the sliding filament theory
- In the presence of Ca2+ movement of troponin from tropomyosin chain
- Movement exposes myosin binding site on surface of actin chain
- ‘Charged’ myosin heads bind to exposed site on actin filament
- This binding & discharge of ADP causes myosin head to pivot (the ‘power stroke’)pulling actin filament towards centre of sarcomere
- ATP binding releases myosin head from actin chain
- ATP hydrolysis provides energy to ‘recharge’ ( charge= ADP bounded) the myosin head
What is the relationship between tension and load in isotonic and isometric contraction?
Isotonic – Tension > Force
Muscle contracts —> fibres shortens
Isometric – Tension = Force
Muscle DOES NOT contract —-> myosin heads reattach to the same point on actin chain
Describe 5 of the structure present on a myofibril seen under a light microscope?
A-band: Dark bands, intersected by a darker region.
H-Zone: Dark region intersecting the A-Band
I-band: Light bands, intersected by a dark line
Z-Line: made up of a-actinin and CapZ
Sarcomere: the functional unit of muscle that lies between 2 z-lines.
What type of muscle are cardiomyocytes?
They are striated muscle
What specialised structure links adjacent cardiomyocytes?
Intercalated Discs
What junctions are present in these structures?
Desmosomes (holds membrane structures together) Gap Junctions (allows electrical communication between cells)
Which receptors are involved in excitation-contraction coupling of cardiac muscle?
Voltage Gated Calcium Channels
Ryanodine Receptors
Describe the process of excitation-contraction coupling of cardiac muscle.
-Action potential in the heart muscle is generated by pacemaker cells in the nodes.
The AP moves down T tubules and comes into contact with VGCCs. This causes the VGCC to open allowing ca2+ influx.
The ca2+ then binds to RyR and causes calcium induced calcium release (CICR).
Same as skeletal.
What are the effects ca2+ can have
- Ca2+ induced Ca2+ release (CICR) by binding to RyR on SR
- Initiate contraction binding to troponin
- Further depolarisation
Describe the process of excitation-contraction coupling of smooth muscle
Action potential reaches the VGCC and makes it open leading to Ca2+ influx Ca2+ binds to Calmodulin forming a Ca2+-CaM complex .This complex activates myosin light chain kinase .MLCK phosphorylates myosin light chains and leads to
cross-bridges with actin filaments leading to vasoconstriction.
Where are smooth muscles found?
Present within walls of all hollow organs e.g. blood vessels, gastrointestinal tract. It does not contain the striated pattern of actin and myosin (irregular arrangement of actin and myosin filaments)
What is the difference between exictation of cardiac muscles compared to Skeletal muscles?
In cardiac muscles there is no contact between VGCC and RyR.
There is also a different kind of troponin in cardiac excitation.
