Muscles Flashcards
Describe Skeletal Muscle?
- Attached to the bone
- Comes in antagonistic pairs: Flexor (Bicep) + Extensor (Tricep
- Consists of a bundle of muscle cells known as myofibres.
- Myofibres are: Large + Cylindrical, Multinucleate, Packed with myofibrils
Difference between Isotonic and Isometric contraction?
Isotonic - muscle length changes but Tension stays the same.
Concentric = Shortening
Eccentric = Lengthing
Isometric Contraction = Muscle length stays same but the tension changes
What are Myofibrils?
Sarcomere is the functional unit of muscle - lies between 2 Z-lines
Z-lines: made up of alpha-actinin and Capz
A-band: Dark bands intersected by a darker region -> H-zone
I-band: Light bands, intersected by a dark line –> Z-line
Describe Excitation-Coupling in the Skeletal Muscle?
- AP propagates alon the myofibirl membrane and T-tubules.
- Depolarisation activates dihyrdopiridine receptors (DHPR) causing a conformation change in DHPR.
- The change in shape allows DHPR to make physical contact with Ryanodine Receptors on the SR.
- This leads to a conformational change in RyR which opens RyR channel and causes Ca2+ release from the SR.
Describe the Sarcomere?
Z-line: Defines lateral boundaries of Sarcomere.
Actin - Polymeric thin filament, 2 twisted alpha helices, Polarity
Myosin - Thick filaments, motor proteins, have numerous globular heads that interacts with actin
Titin - large spring-like filaments, anchoring myosin to the Z-line.
Nebulin - Large filaments associated with actin
Tropomyosin - elongated protein bound to actin
CapZ and Tropomodulin - associated with the +ve, -ve ends of actin.
What is the Sliding Filament Theory?
- in the presence of Ca2+ –> movement of troponin from tropomyosin chain
- movement exposes myosin binding site on actin
- charged myosin heads bind to actin
- binding and discharge of ADP causes myosin head to pivot –> pulling actin filament towards centre of sarcomere
- ATP binding –> releases myosin head from actin chain
- ATP hydrolysis –> provides energy to ‘recharge’ the myosin head
Summarise the Excitation-Contraction Coupling
Excitation:
- AP propagates along the muscle membrane via T-tubules of myofibres
- DHPR on T-tubules activated by AP –> opening of RyRs
- RyRs are on the Sarcoplasmic reticulum –> opening allows Ca2+ efflux from SR into the myofibre.
Contraction:
- Rise in Ca2+ in myofibre –> contraction initiation
- Ca2+ binds to troponin on actin fibres
- Movement of tropomyosin –> myosin heads binds to actin
- ADP phosphorylation –> myosin head pivots pulling actin towards sarcomere centre
- ATP hydrolysis - recharges myosin heads
- Sliding of actin along myosin = SLIDING FILAMENT THEORY
What is the Tension-Load Relationship?
Isotonic Contraction =
- Tension>Load
- Muscle contracts and the fibres shorten
Isometric Contraction =
- Tension = Load
- Actin is pulled in and out
- Use ATP as it needed to return myosin heads to the charged state.
What is Cardiac Muscle?
Cardiomyocytes: - striated muscle - Intercalated discs: specialised discs connecting individual cardiomyocytes. Desmosomes + Gap Junctions
Describe Cardiac Excitation-Contraction Coupling?
- AP in the heart muscle is generated by pacemaker cells in the nodes
- AP moves down T-tubules and comes into contact with VGCCs
- No contact between VGCC and RyR
- The Ca2+ binds to the RyR and causes Calcium Induced Calcium Release
- Rest of contraction phase is same as SKM
Describe Smooth Muscle?
Smooth - no striated pattern of actin and myosin (still present)
Contraction different in smooth muscle
- depolarisation activates and opens the VGCC –> Ca2+ influx.
- calcium enters the cell and binds to CaM forming a complex
- The complex activates Myosin Light Chain Kinase
- MLCK phosphorylates myosin light chains (MLC20)
- This changes the appearance of smooth muscle cells from elongated to contracted –> vasoconstriction.
