muscle contraction Flashcards
how are myocytes specialised?
- multi- nucleated due to fusion of myoblasts during development
what does the fusion of myoblasts form?
- myotubes or muscle fibres
what are muscle fibres filled with?
- around 1000 parallel rods of contractile material
how are rods packed in muscle fibres?
- cytosol
- myofibrils
how are filaments arranged? what is the purpose?
- striated pattern
- deliberate arrangement in order to maximise muscle contraction
what is the main functional unit of the muscle?
- sarcomere
- between Z lines
what does sarcomere consist of?
- thick and thin filaments overlapping
what does the thick filament consist of?
- myosin
- highly abundant
what does the thin filament consist of?
- actin, tropomyosin and troponin
what do tropomyosin and troponin act as?
- regulators
what does I band consist of?
- thin filament only
what does H zone consist of?
- thick band only
what does A band consist of?
- overlap of thick and thin filament
what is the m line made up of?
- myomesin and M protein
what does the Z line contain?
- alpha actinin
what are ATPases?
- proteins that catalyse the hydrolysis of ATP to liberate free energy for cellular wok
what does the sliding filament model suggest?
- lengths of thick and thin filaments doesn’t change when the sarcomere contracts instead they overlap
- contraction caused by active sliding of thick and thin filaments past each other
what is the specialised protein in contraction and what is it effectively?
- myosin
- acts as an enzyme as it breaks ATP down so acts like ATPase
describe the structure of myosin
- very large (520kda)
- abundant in the muscle
- consists of two large heavy chains and small light chains
describe what the light and heavy meromyosin chains do
- light meromyosin chain forms filaments spontaneously (self- assembly)
- heavy meromyosin forms cross bridges and S1 sub fragment hydrolyses ATP and binds actin
describe the S1 fragment
- fragment on myosin has an ATP binding site which combines actin
- allow the conversion of ATP into movement via hydrolysis
how many gene products are labelled as ATPase?
- 430 gene products
- most processes unlock energy via enzymes
what forms does actin exist in?
- G- actin (globular)
- F- actin (fibrous)
describe what F actin monomers do and what they form
- monomers intertwine and form the trunk of thin filaments to which tropomyosin and troponin attach
what does F actin increase?
- ATPase activity of myosin by increasing the rate at which ADP and Pi are released from the active site
what happens at rest to myosin and actin?
- they are close together but there is no binding of myosin and actin
at rest does ATP hydrolysis happen?
- it does happen but it is hydrolysed by myosin slowly since actin is not involved to help release ADP+ Pi
what happens when the muscle is excited?
- myosin head binds to actin and ADP and Pi is released from the active site
- causes a conformational change in S1 to create a lever arm and power stroke
what is the power stroke?
- main generator of force that stimulates the muscle
- causes ATPase to break down ATP which allows attachment of myosin to actin
what bit of myosin uses energy that’s liberated and why?
- little cross bridge utilises energy in order to switch as conformational change
what happens when the active site is empty of ADP and Pi?
- ATP returns causing detachment of actin and myosin
what is the cycle dependent on?
- availability of ATP
what does myosin convert free energy to?
- converts free energy of ATP hydrolysis to kinetic energy
what are myosin isoforms?
- genes encode proteins but variants in proteins can arise from one gene via alternative splicing or RNA editing
- adult humans have myosin heavy chain (MHC), 1, 11A and 11X
describe type 1 fibres
- slow twitch fibres
- low myosin ATPase activity
- low shortening velocity
- high economy
- high resistance to fatigue
describe type IIa fibres
- fast twitch fibre
- intermediate myosin ATPase activity, shortening velocity, economy and resitsance to fatigue
describe type IIx fibres
- fast twitch fibre
- high myosin ATPase activity
- high shortening velocity
- low economy
- low resistance to fatigue
describe the experimental assessment of myotube contraction
- create muscles in a dish ‘ in culture’ and contract the myotubes without a nervous system
- contraction is enabled via electrical stimulation as it releases calcium ions
what is calcium?
- non- energy nutrient that controls muscle contraction by permitting the binding of myosin to F actin
- via troponin and tropomyosin
what does the tropomyosin do at rest?
- blocks the binding site of actin
what does TnI bind to?
- actin
what does TnC bind to?
- Ca2+
what does TnT bind to?
- tropomyosin
what happens when the nerve impulse arrives at the muscle?
- calcium is released from the sarcoplasmic reticulum
what does calcium bind to once released? what does this allow?
- binds to troponin C to cause a conformational change
- causes detachment of troponin L from actin so tropomyosin moves
what happens to tropomyosin after calcium binds to tnC?
- tropomyosin moves to expose the binding site and hence permits binding
what happens after the power stroke has occurred?
- sequestration of calcium in the sarcoplasmic reticulum so calcium withdraws
- tropomyosin blocks active site again
how is neural stimulation linked to calcium?
- via signals from neurons to muscles that are carried out chemically in motor units
what are motor units?
- motor neurons and the muscle cells that they innervate
what happens as the motor neuron approaches the muscle?
- it splits into hundreds of branches ending at the neuromuscular junction
what is the biochemical process and when does it occur?
- occurs when nerve impulse reaches the junction that allows downstream effect of calcium release
what does each junction contain? what NT is discharged?
- contains many synapses where acetylcholine is discharged when action potentials arrive at pre- synaptic membrane
how is the signal carried in a muscle?
- via acetylcholine receptor
what happens to the neurotransmitter and what allows transmission after?
- NT released into the synaptic cleft
- ligand receptor allows transmission of impulse to result in calcium release and hence contraction
what does acetylcholine cause?
- a conformational change that causes many sodium ions to flow into cytosol with fewer leaving
- leads to depolarisation (postsynaptic potential)
what is the process at the neuromuscular junction known as?
- feedforward mechanism as postsynaptic potential is aided by voltage gated Na+ channels in plasma membrane
- facilitates Na+ entry after depolarisation
why do voltage gated K+ channels open?
- to let K+ out of the cytosol and resting membrane potential is resumed
- maximises the signal for calcium release
describe the ligand receptor job
- ligand gated receptor changes it conformation only when interacting with its ligand (Ach)
- which is the opposite to Na+ and K+ voltage gated channels
what happens to the free acetylcholine after excitation has passed?
- acetylcholine is hydrolysed in synaptic cleft by acetylcholinesterase
- receptor returns to its original conformation
what links neurons to muscle contraction?
- excitation contraction coupling
what does acetylcholine mediated depolarisation stimulate? what is the definition?
- transverse tubules
- extension of the plasma membrane closely appose to Ca2+ containing sacs called sarcoplasmic reticulum
how is reservoir of calcium maintained?
- by Ca2+ ATPase pump which creates steep gradient across the membrane
what does transmission of action potential across transverse tubules cause?
- opening of Ca2+ channel called ryanodine receptor
how is the opening of the ryanodine receptor stimulated?
- via conformation change of the dihydropyridine receptor
how many folds does Ca2+ rise?
- approx. 100 fold
what are the three disruptions in calcium handling?
- ryanodine is a poison that binds with high affinity to ryanodine receptor
- blocks calcium release from SR, which causes paralysis
- lack of activity of Ca2+ ATPase means resting gradient cannot be stored causes rigor mortis
describe rigor mortis
- no ATP pumping calcium back so no relaxation and hence muscle in cross bridge state
what does passing of Ca2+ to TnC activate? what happens when action potential passes?
- muscle contraction
- when AP passes, ryanodine receptor closes to prevent Ca2+ efflux from SR, Ca2+ ATPase returns resting gradient
what are the major energy consumers in muscles?
- myosin ATPase and Ca2+ ATPase
describe how large ATP is
- 1 ATP used for one power stroke; 100’s myosin head in sarcomere; 100s of thousands of sarcomeres in each muscle fibre; 250,000 fibres in muscle
assuming all fibres are engaged, how many molecules are required for a single muscle twitch?
- 7.5 billion molecules
how high can demand for ATP hydrolysis during strenuous exercise be?
- 12 hexillion molecules of ATP per minute
