Muscle contraction Flashcards
What is the tissue that surrounds the muscle?
Epimysium - Dense irregular connective tissue surrounding entire muscle.
Perimysium - connective tissue sheath grouping muscle fibres into bundles.
Endomysium - connective tissue surrounding each myocyte.
How do myocytes look under microscopes?
Myocytes are multinucleated - during development, there is fusion of cytoplasms.
Striated, and parallel.
Striated = Dark and Light bands
Dark band is A band = contains the myosin and thin filaments.
= Anisotropic = non-homogenous.
Light band is I band = just the Z disc and some thin filaments.
= Isotropic = Homogenous.
You can see the sarcolemma/sarcoplasmic membrane = PM of myocytes.
What is the sarcomere and What can be found within it?
The repeated motif and is between Z-discs.
Z dics found in the centre of I band (Light, isotropic band), containing thin Filaments.
THin filament = Actin, troponins and tropomyosin.
A band = is anisotropic, heterogenous containing thick myosin filaments as well as thin actin filaments.
M line is the centre of sarcomere, within the A band and H zone - containing no thin filaments, just myosin filaments.
What is observed in sarcomeres during contraction?
What is contraction?
Contraction is the shortening of sarcomeres by sliding of thin/thick filaments via actin-myosin interactions.
Z discs get closer.
I and H bands get smaller
Excluding the sarcomere, what can be found inside a myocyte?
Myocytes contain bundles of myofibrils
Myofibrils are surrounded by Sarcoplasmic reticulum (SR).
Sarcoplasmic reticulum forms invaginations (T-tubules), which transverse into the myocyte.
Establishing Triads
SR-T-tubule-SR
Where T-tubules come as close to SR as possible.
Muscle vs myocytes vs myofibrils?
Each muscle contains multiple muscle fibres/myocytes.
Each myocyte contains many bundles of myofibrils.
What are thin and thick filaments?
Thin filaments extend from Z disc, towards the centre of the A band (Except in the M-line/H zone).
Thin filaments are about 7nm diameter:
Containing Actin, tropomyosin and troponins.
Thick filaments = 15nm diameter. Containing Myosin II.
What is the structure of each Thin filament?
7nm diameter.
Actin, Tropomyosin and Troponins.
Monomeric G-actin is polymerised in a process using ATP, to form F actin.
F actin forms linear, helical chains from 2 chains of F-actin
= Forming double helix.
Tropomyosin is a dimeric, elongated protein.
Two monomers wrap around to form a superhelix.
Tropomyosin super helix binds to F-actin in its grooves - spanning 7 G-actin monomers.
At both ends of tropomyosin superhelix, Troponin binds.
Troponin is a tripeptide:
Troponin-T = binds to Tropomyosin.
Troponin-I - inhibits Actomyosin ATPase.
(prevents contraction without Ca2+)
Tropinin-C - binds up to 4 Ca2+ ions, to relieve inhibition of actin-myosin interaction.
What is the structure of Troponin?
At both ends of tropomyosin superhelix, Troponin binds.
Troponin is a tripeptide:
Troponin-T = binds to Tropomyosin.
Troponin-I - inhibits Actomyosin ATPase.
(prevents contraction without Ca2+)
Tropinin-C - binds up to 4 Ca2+ ions, to relieve inhibition of actin-myosin interaction.
What is the structure of Tropomyosin?
Tropomyosin is a dimeric, elongated protein.
Two monomers wrap around to form a superhelix.
Tropomyosin super helix binds to F-actin in its grooves - spanning 7 G-actin monomers.
At both ends of tropomyosin superhelix, Troponin binds.
What is the structure of thick filaments?
15nm diameter.
Myosin II is formed from 2 heavy chains and 2 pairs of light chains.
2 heavy chains form a dimer = containing 2 globular heads, connected with swining neck domain, to the alpha-helical tails.
The elongated heavy chain tails wrap around to form a super helix, with the two heads side by side.
Each head has an actin binding site, and ATP hydrolysis site.
The small conf. change by ATP hydrolysis etc is amplified by the swinging neck domain into large mechanical motion, crucial to Power Stroke mechanism.
How is Myosin II arranged in thick filaments?
Several hundred Myosin II molecules assemble in thick filaments in parallel.
Elongated heavy chain tails in superhelix arrange in parallel, allowing globular heads to pertrude.
= Available to bind to actin.
The myosin II molecules are arranged in 2 groups head to tail - where the central H-zone has no globular heads!
What is the excitation part of excitation-contraction coupling?
ACH released by motor neurone triggers an EPP by nicotinic AChRs.
EPP triggers AP, propagating along sarcolemma and down into myocytes via T-Tubules.
At the level of the SR-T-tubule-SR (TRIADS):
DHP receptors in PM are L-type VG Ca2+ channels.
Depolarisation triggers opening of DHP receptors, which are coupled with RyR receptors - Ca2+ channels in SR membrane.
= Results in Ca2+ release from SR into sarcoplasm.
= Followed by Ca2+ dependent contraction.
How does Ca2+ trigger muscle contraction?
Ca2+ release via DHP-RyR coupling in SR causes increase in cytoplasmic Ca2+.
Ca2+ binds to Troponin C.
Troponin complex undergoes conformational change.
Troponin T moves, displacing Tropomyosin (to which it is attached) , which unmasks myosin binding sites in F-actin.
This enables the globular heads of myosin II to bind to actin and contraction via POWER-STROKE
What is the Power stroke mechanism?
When Ca2+ permits the unmasking of actin binding sites for myosin II:
Myosin Heads in high energy conformation (Containing ADP and Pi) form Cross-bridges via the release of Pi.
Myosin heads bind to actin.
Power stroke:
Myosin head releases ADP causing conf. change that moves Myosin head towards centre of sarcomere via swinging neck (Whilst crossbridge intact)
= Myosin moves thin filament towards M-line.
Myosin now in low-energy configuration, binds ATP - causing dissociation of Cross Bridges.
Myosin head undergoes ATP hydrolysis, cocking myosin head away from centre of sarcomere.
= Back into high-energy configuraton to enable reformation of Cross-bridges.
