Muscle Contraction Flashcards
What are 5 groups we can break skeletal muscle down into?
Skeletal muscle Muscle fascicles Muscle fibre Myofibrils Sarcomeres
Skeletal muscle layer
Surrounded by epimysium
Separates the muscle from other muscles, bones and soft tissues
It contains fascicles of muscle fibres
Muscle fascicles
Contains muscle fibres
Surrounded by perimysium
Perimysium has blood vessels and nerves to supply muscle fibres
Muscle fibre
Surrounded by endomysium
Contains capillaries and nerve fibres for muscle cells. Also contains satellite cells in this layer to repair damaged tissue.
Contains myofibrils composed of actin and myosin
Myofibril
Surrounded by sarcoplasmic reticulum, T tubules and their terminal cisternae (for calcium release)
Triad = 2 terminal cisternae and a T tubule
Consists of sarcomeres
Sarcomeres
Thousands within a myofibril
Features: Z lines at each end M line in middle I bands of actin A bands of actin and myosin H zone around M line (only myosin)
Also contains proteins
Titin within the I band
Z disc connects thin filaments to next sarcomere
3 categories of proteins in muscle
(Can’t resist science acronym)
CONTRACTILE PROTEINS = myosin and actin. Myosin binds to actin and pulls them along in contraction
REGULATORY PROTEINS= Switch on and off contractions. Eg, troponin and tropomyosin
In relaxed muscle, these two proteins block binding site on actin so myosin cannot bind and there is no contraction
STRUCTURAL PROTEINS- Provide alignment, elasticity and extensibility eg, titin, myomesin, nebulin and dystrophin
Myosin structure
Makes up thick filaments
Has 2 globular heads, a hinge and a tail
Myosin heads rotate and bind to actin binding sites to make cross bridges. Each myosin head has binding site for actin and binding site for ATP. Heads reach the nearest thin filaments.
Myosin tail extends to the M line and binds to other myosin tails
Hinge enables them to rotate and move
Actin structure
Thin filaments made of = Actin + troponin + tropomyosin
Relaxed muscle = actin-myosin binding site covered by troponin and tropomyosin
In contraction , calcium ions bind troponin so troponin and tropomyosin move away from binding site to expose them
Thin filaments held in place by Z lines
TITIN STRUCTURE
Titin protein anchors thick filament to M line and Z disc
End of myosin filaments to the Z discs can stretch up to 4 times resting length and spring back unharmed.
Plays role in eccentric contractions as it recovers muscle from being stretched.
Describe the other structural proteins
Myomesin makes the M line of sarcomeres. It connects to titin and adjacent thick filaments.
Nebulin is inelastic protein, connects to actin filaments and aligns actin filaments
Dystrophin links thin filaments to sarcolemma to transmit tension generated to the tendon
Sliding filament theory
H zone gets smaller in contraction
I band also gets smaller in contraction
A band stays same length in relax or contraction
Sarcomeres shorten in contraction
Myosin cross bridges pull on actin filaments and thin filaments slide inwards towards M line
Z discs move closer together as sarcomeres shorten
Sarcomere shortens, muscle fibre shortens and muscle therefore shortens,
HOWEVER myosin and actin remain same length (this is why A band same length)
Skeletal muscle pre-contraction
Nerve impulse reaches axon terminal at neuromuscular junction
Acetylcholine is released by synaptic vesicles and released into synaptic cleft between axon terminal and muscle cell.
Ach diffuses to bind to receptors on sarcolemma + sodium ion channels open.
Sarcolemma depolarised
Sodium ions rush into muscle cell
An actin potential spreads over sarcolemma and down T tubules to reach a triad (2 terminal cisternae and T tubule)
Terminal cisternae release calcium ions into sarcoplasm
Calcium binds to troponin so the troponin-tropomyosin complex moves away to reveal myosin binding site on actin.
This part is excitation- contraction coupling
Contraction cycle begins
Skeletal muscle contraction cycle
Terminal cisternae of the SR release Ca++ ions that bind to troponin
Troponin and tropomyosin move away to expose binding sites on actin
Cross bridge formation: Myosin head rotates and binds to actin-myosin binding site to form actin-myosin cross bridge
Myosin head bends to pull actin along to the M line called POWER STROKE. ATP gets hydrolysed to ADP + Pi releasing energy and sarcomeres will shorten.
Cross bridge detachment: myosin head is bound to another ATP to release energy to detach myosin head from actin-myosin binding site
It will re attach to another site and happens in many sarcomeres at same item
Process occurs in plenty of ATP and Ca++
Define contraction cycle
The repeating sequence of events that causes thin filaments to slide between thick filaments
Describe muscle relaxation steps
Acetylcholinesterase breaks down acetylcholine in synaptic cleft
Ach can not bind to receptors on sarcolemma as it has been broken down = no action potentials in sarcolemma
Calcium ion release channels close
Calcium ions pumped back into terminal cisternae via active transport
Ca++ decreases in muscle cell
Calsequestrin (calcium binding protein) maintains calcium in terminal cisternae
Troponin and tropomyosin return to blocking binding sites
Muscle passively lengthens to rest
Muscle fibre lengths
Optimal overlap of thick and thin filaments = max. Cross bridges can be formed and greatest tension produced
Muscle is stretched = fewer cross bridges and less force produced
Muscles shortened too much = fewer cross bridges, less forced and z lines crush thick filaments
Resting muscle length = 70-130% optimum muscle length
Length tension curve
The curve would show that mid range of muscle work produces most tension in muscle
Too stretched or shortened too much= fewer cross bridges and less tension, so less force produced
What is ATP used for?
To release energy for muscle contraction
To detach myosin heads from binding sites
For active transport of calcium ions back into terminal cisternae
Muscles use ATP quick and sarcoplasmic ATP only gives a few secs of energy
What are 3 sources of atp production in muscle?
CREATINE PHOSPHATE
ANAEROBIC CELLULAR RESPIRATION
AEROBIC CELLULAR RESPIRATION
What happens in creatine phosphate system?
Phosphate transferred from creatine phosphate to ADP to regenerate ATP
PHOSPHATE + ADP = ATP + CREATINE
3-6 more phosphocreatine in muscles than ATP
Regeneates ATP rapidly
Maximal contraction lasts for 15 secs , eg explosive sports
Another system used after 15 secs
Relaxing muscle = ATP breaks down into ADP and CREATINE phosphate and stores until needed
Contraction= phosphate transferred from CREATINE phosphate to ADP to form more ATP + (CREATINE)
How do we obtain creatine?
Eating meat
Internal production in kidneys and liver
Creatin supplements
Disadvantages to creatine supplements?
Body may stop producing own creatine
Waste product creatinine may increase and damage kidneys , creatinine levels in urine X90 more than normal
Side effects may be nausea, bad stomach and muscle cramps
Increased creatine not useful for events lasting longer than 15 seconds
Anaerobic cellular respiration
Aka lactic acid system
Doesn’t need any oxygen
glycogen stored in muscle
Glucose phosphorylated to glucose phosphate and releases an ADP
ATP used to add another phosphate to form hexose bisphosphate
This slits into 2 TP
Triose phosphates oxidised to 2 pyruvates
Net gain of 2 ATP
Pyruvic acid converted to lactic acid and diffuses into blood.
Continues for 30-40 seconds like 200m run
Aerobic cellular respiration;
Cellular respiration can use: Pyruvic acid from glycolysis amino acids Fatty acid from adipose tissue Oxygen from blood or myoglobin
Produced ATP for activities over 30 secs. Eg, long distance run
Provides 90% ATP energy if activity lasts more than 10 mins
Troponin
Made of 3 polypeptides and binds to :
Actin
Calcium
Tropomyosin
