skeletal muscle physiology - finish on ipad Flashcards
myofibril structure =
a long cylindrical organelle within a muscle fibre that is highly specialised for contraction
sarcomere =
space between two Z-lines
A-band =
part of the myofibril where thick myosin filaments are present
I-band =
part of the myofibril where only thin actin filaments are present
H-band =
part of the myofibril where only thick myosin filaments are present
thin filaments =
are formed from actin in complex with troponin and tropomyosin
thick filaments =
are formed from large numbers of myosin II molecules
regulation of contraction:
Ca2+ dependent
at high calcium conc. the myosin binding site-cross bridge cycling can occur
titin:
titin forms fine filaments that stabilise the myosin filament position
titin extends from Z-disk to M-line
with muscle activation calcium binds to titin and alters its stiffness = provides increased force when muscle is stretched and resists over stretching
mechanism of muscle contraction:
- action potential arrives at neuromuscular junction, causing Ca2+ to enter the motor neurone, depolarising it = acetylcholine is released
- acetylcholine binds to sarcolemma causing Na+ diffusion, depolarising it - action potential sped up by transverse tubules
- action potential triggers Ca2+ release from sarcoplasmic reticulum, it binds to troponin on the actin filaments
- troponin changes conformation exposing binding sites
- myosin heads attach to binding sites and tilt 45 degrees (powerstroke) - ADP and Pi are released
- actin filaments are pulled passed myosin filaments (sarcomere contracts)
- ATP attached to myosin heads are hydrolysed to ADP and Pi - this energy is used to detach myosin heads from actin filaments
control of intracellular calcium in muscle is done by…
transverse tubules on the sarcolemma and sarcoplasmic reticulum
dihydropyridine receptors =
located on transverse tubules - are voltage-gated channels that sense change in membrane potential during action potentials
are coupled to ryanodine receptors
ryanodine receptors =
release calcium from sarcoplasmic reticulum into cytoplasm
coupled to dihydropyridine receptors
excitation-contraction coupling:
- End plate potential triggers action potential in muscle fibres
- Action potential propagates along sarcolemma and down T-tubules
- Depolarisation of T-tubules is sensed by dihydropyridine receptors that are couples to ryanosine receptors on sarcoplasmic reticulum causing them to open
- Ca2+ is released into cytoplasm - initiates cross bridge cycling and contraction
- Ca2+ is pumped back into sarcoplasmic reticulum (by sarcoplasmic + endoplasmic reticulum calcium ATPase) and this terminates cross bridge cycling
the cross-bridge cycle:
- Ca2+ binds to troponin on actin filaments - troponin changes conformation exposing binding sites
- myosin heads attach to binding sites and tilt 45 degrees (powerstroke) - ADP and Pi are released
- actin filaments are pulled passed myosin filaments (sarcomere contracts)
- ATP attached to myosin heads are hydrolysed to ADP and Pi - this energy is used to detach myosin heads from actin filaments
Muscle fibre type 1:
(slow)
‣ 50% of fibres in an average muscle
‣ Peak tension in 110ms (slow twitch)
Muscle fibre type 2:
‣ Peak tension in 50ms (fast twitch)
‣ Type IIa = fatigue resistant (25% of fibres in an average muscle)
‣ Type IIx = fast fatigue (25% of fibres in an average muscle)
Differences of type I and II fibres:
• Speed of myosin ATPase varies:
‣ Fast myosin ATPase = fast contraction cycling
‣ Slower myosin ATPase = slower contraction cycle
• sarcoplasmic reticulum:
‣ Type II fibres have more highly developed SR
‣ Faster Ca2+ release
• motor units:
‣ Type I motor units = smaller neuron
‣ Type II motor unit = larger neuron
Role of type 1 fibre during exercise:
slow contracting - high aerobic endurance:
‣ Can maintain exercise for prolonged periods
‣ Require oxygen for ATP production
‣ Low intensity aero I exercise, daily activities
Role of type 2 fibre during exercise:
poor aerobic endurance, fatigue quickly, produce ATP anaerobically
• Type IIa (FR = faster contracting, fatigue resistant)
‣ More force, faster fatigue than type I
‣ Short, high intensity endurance events
• Type IIx (FF = fast contracting, fast fatigue)
‣ Short, explosive sprints
Fibre type determinants:
• genetic factors
◦ Determine which alpha-motor neurones innervate fibres
◦ Fibres differentiate based on alpha-motor neurone
• training factors
◦ Endurance versus strength training, de-training
◦ Can induce small changes in fibre type
• Ageing - muscles lose type II motor units
Orderly recruitment and the size principle:
- recruit minimum number of motor units needed
- Recruited in same order each time ( type I - type IIa - type IIx)
- Size principle: order of recruitment of motor units directly related to side of alpha-motor neurone
Fibre type and athletic status:
- endurance athletes = type I predominates
- Sprinters = type II predominates
Fibre type not sole predictor of success... • cardiovascular function • Motivation • Training habits • Muscle size
