Muscle Flashcards
Describe the structure and organisation of the sarcomere.
- M-Line: Anchores centre of myosin (thick) filament.
- Z disc: Anchores centre of Actin (thin FIlament)
- A band: Length of Myosin Filaments.
- I Band: Space between myosin filaments (only thin filament)
- H zone: Space between actin filaments (only thick filaments)
- Thick filament: Entirely myosin (stabilised by Titin)
- Thin filament: Actin, Troponin, Tropomyosin (Nebulin helps align actin)
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What proteins can be found in each of the following muscle lines/bands: A, I, H, M & Z?
A: Myosin, Actin, Troponin, Tropomyosin, Nebulin, Titin
I: Acting, Troponin, Tropomyosin, Nebulin, Titin
H: Myosin, Titin
M: Myosin, Titin
Z: Actin, attaches to titin, Nebulin
What is/are the functions of the structural proteins of muscle?
Titin is an elastic molecule which stabilises the portion of the contractile proteins and assists in returing the sarcomere to its original length through its elastic properties.
Nebulin act as a stabiliser for actin G proteins in their actin F formations.
Tropomyosin is often “in the groove”; what does this mean?
Its considered ‘in the groove’ as tropoyosin rests in between the double helices of the actin molecules along the thin filament.
How is a nerve AP converted into muscle AP?
- Arrival of nerve impulse causes release of Ach from synaptic vesicles
- Ach Binds to receptors on mucle motor end plate opening Na+gated ion channels
- Inside of the muscle cell becomes much more positive, triggering a muscle AP that travels over sarcolemma
ULTRASTRUCTURE of muscle allows simultaneous contraction.
Complex system of T-Tubes and system of flattened vesicles (Sarcoplasmic reticulum) provides structural basis for simultaneous depolarisation.
Describe the structural and functional relationships between the sarcoplasmic reticulum and the T- tubule system.
When Impulse arries at MEP, it causes a deolarisation of the sarcolemma which continues down the T-Tubule system. T-Tubule - Terminal cisterna leads to sarcoplasmic reticulum which spans over sarcomere.
Describe in detail what triggers the release of calcium ions from the sarcoplasmic reticulum.
AP spreads down T-Tubule and stimulates voltage-sensitive dihidropyridine (DHP) receptors. These receptors are mechanically linked to lateral sacs of adjacent SR via specialised foot proteins which extend from the SR.
These foot proteins are known as ryanodine receptors and function as Ca2+ release channels for the SR.
Describe the step-by-step process of crossbridge formation in skeletal muscle.
- ATP Hydrolysis (ATP-> ADP + Pi)
- Ca2+ binds to troponin; pulls tropomyosin out of the way
- Attachment of myosin to actin to form crossbridges.
- Power strokes as myosin heads pivot
- ADP + Pi released
- New ATP arrives
- Detachment of Myosin from actin
- Repeat of ATP hydrolysis
- Myosin prepares to reattach to actin.
What is the role of ATP in crossbridge formation?
Hydrolysed ATP has potential energy stored in myosin head that is released after a power stroke. FOr myosin to dettach from actin filament, a new ATP molecule needs to arrive.
What physiological event causes skeletal muscle to relax?
- Crossbridges swivel as ATP is hydrolysed and ADP is released.
- Ca2+ is sequestered from thin filament by SR
Ca2+ is released from Ca2+ complex - Troponin permits tropomyosin to return to its blocking position.
- Myosin - actin crossbridge breaks
- ATP-Myosin complex is reformed in the heads of the thick filaments.
What is the “All or None” principle when applied to skeletal muscle contraction?
A motor neuron with all of the muscle fibres it stimulates = motor uni.
individual fibres of a muscle unit will contract to fullest extent or will not contract at all, providing conditions remain constant.
Contraction can be graded according to the number of muscle units recruited.
Why is maximum force related to the resting length of the muscle myofilaments?
When a muscle is contracting isometrically, force is reduced if the distance between the ends of the muscle is less or greater then that of the resting length.
This is caused by relative positions of actin and myosin.
If muscle initally stretched, there is little to no overlap of A+M filaents therefore minimal crossbridge formation may occur.
If muscle is shorter, force is diminished and decreases to 0 as thick filaments make contact with Z lines.
How does smooth muscle contract and relax given that there is very little troponin in this tissue?
A Ca2+ linked activator is used. (modification to myosin molecule occuring through phosphorylation of the molecule.)
Ca2+ dependent myosin light chain kinase (MLCK) enzyme catalyses this reaction.
On stimulation, Ca2+ levels increase inside cell and bind to calmodulin which then binds to MLCK
ATP phosphorylation of myosin brings about actin myosin interaction.
