MSS12 Skeletal Muscle Contraction Flashcards
Muscle fibre structure
- ***Myofibril
- thin filament
- thick filament - ***Sarcoplasmic reticulum
- ***T tubules (invagination of sarcolemma)
- Sarcolemma
- Sarcoplasm
- Nuclei
- Mitochondria
Myofibril: Thin filament + Thick filament
Thin filament (attach at Z line (formed by Actinin)):
- Troponin (C + I + T) (豆)
- Tropomyosin (strand)
- -> F-actin strand (Troponin + Tropomyosin) - G-actin molecules (with Active site) (珠)
- ***Nebulin (中軸)
Thick filament (attach at Z line by ***Titin (彈弓)):
- Myosin head
- Hinge
- Myosin tail
- M line
Sarcomere: ***Z line to Z line
Titin and Nebulin
Both: Giant Accessory Protein
Titin:
- ***from Z disk to M line
- provides elasticity
- stabilizes myosin
- aids in ***restoring resting sarcomere length after contraction (i.e. during relaxation)
Nebulin:
- lying along thin filament
- attach to Z disk (formed by Actinin)
- ***do not extend to M line
- ***align actin (作為中軸)
Sliding filament theory
During contraction:
- ***Z lines move closer
- A band stays same width (whole length of thick filament)
- I band + H band smaller (unoverlapped area of **thin and **thick filament respectively)
—> Sarcomere (Z line to Z line) shorten simultaneously
—> ends of myofibrils pulled towards its centre (M line)
—> thick filament 將 thin filament 拉向自己中心
Optimal resting length
normal range of sarcomere length: 75-130% of the optimal length
Length of overlap
Maximum tension produced when:
Zone of overlap is large but thin filament do not extend across sarcomere’s centre (M line) (i.e. thin filament唔好撞到M line)
Short resting lengths:
thin filament extend across centre of sarcomere (thin filament撞到M line)
–> **interfere with the normal orientation of thick and thin filament
–> ↓ tension production
–> further shorten
–> thick filament contact Z line (thick filament撞埋Z line)
–> sarcomere cannot shorten
–> **myosin heads cannot pivot
–> tension cannot be produced
Long resting lengths:
sarcomere stretched too far
–> zone of overlap ↓
–> **cross-bridge interaction ↓
–> further ↓ in zone of overlap
–> thick and thin filament cannot interact
–> cannot produce active tension (cannot contract)
–> normally prevented by **titin filaments (connect myosin to Z disk) + connective tissue
Skeletal muscle contraction
3 stages:
- Neural control:
- NMJ
- Graded / Action potential
- Neurotransmitter release - Excitation-contraction coupling
- Propagation of action potential
- ***Ca release
- Actin-myosin interaction - Tension production
- Threshold / Maximal stimulation
- Twitch / Summation / Tetanus
***Exicitation-contraction coupling
- Motor neurone release ACh at NMJ
- Net entry of Na through ACh receptor-channel initiates muscle action potential along ***sarcolemma
- Action potential reaches **T-tubule alters **Dihydropyridine receptor (DHP) conformation
- DHP receptor open ***Ryanodine (RyR) receptor Ca release channel on SR
- Ca release into cytoplasm
- Ca bind to ***Troponin C (thin filament)
- Troponin-Ca complex pulls Tropomyosin away from Actin’s Myosin-binding site (exposing binding site on Actin)
- Actin-Myosin binding
- Myosin heads power stroke
- Actin filament slide towards centre of sarcomere (M line)
Myosin head uncoupled from Actin **after binding to ATP
—> **Hydrolysis of ATP advances Myosin head by short distances (recock head)
—> Cross-bridge formation: myosin head tightly bound to actin by **releasing Pi
—> Force generation: power stroke after **releasing ADP, Myosin head move towards M-line
—> Reattachment: Myosin bind tightly to new actin
Sources of ATP at different muscular activities
Resting muscle:
Breakdown **Fatty acids —> ATP —> used to build **energy reserves:
- Creatine phosphate (Creatine + phosphate from ATP)
- Glycogen
Moderate activity:
Breakdown Glucose (from glycogen) / **Fatty acids —> ATP
—> supply to myofibrils (*Oxidative phosphorylation, require adequate supply of oxygen)
—> used for Power contraction
Peak activity:
Breakdown **Glucose (from glycogen) / **Creatine phosphate —> ATP
—> **Glycolysis (pyruvate —> lactate)
—> **Lactate as by-product
—> Mitochondrial activity (Oxidative phosphorylation) produce about 1/3 of ATP consumed
CP produce **less energy and **shorter duration than Glycogen
Delayed-onset muscle soreness (DOMS)
- several hours after exercise
- may last 3-4 days
- highest when activity involves ***eccentric contractions
- **CK + **myoglobin blood levels elevated –> indicating muscle plasma membrane damage
3 Mechanisms:
- small tears in muscle tissue
- -> loss of enzymes, myoglobin, other chemicals that stimulate pain receptors
- -> however amount of pain is not directly correlated to amount of biochemical changes - muscle spasms
- tears in CT framework / tendons
Summation of contractions
- Single twitch:
muscle relaxes completely between stimuli - Summation:
stimuli close together do not allow muscle to relax fully - Unfused tetanus:
summation but stimuli are still far enough apart to allow muscle to relax slightly between stimuli - Complete tetanus:
close stimuli summation leading to ***steady tension / maximum tension
–> after a period tension ↓ despite continuing stimuli due to fatigue
Development of tension in a twitch
Fast muscle (short time to reach maximum tension after stimulus): Eye muscle
Intermediate: Gastrocnemius muscle
Slow muscle: Soleus muscle
Presence of latent period:
from stimulus開始計: time needed for **conduction of action potential (very quick) and **subsequent release of Ca by SR (less quick but still quicker than development of twitch force)
Isotonic vs Isometric contractions
Isotonic contraction:
- muscle contracts
- ***shortens
- creates enough force to move the load
- ***peak tension = amount of load (< peak tension capabilities)
2 types: 1. Concentric: muscle length shorten on stimulation 2. Eccentric: muscle elongates as it generates tension --> when ends, unopposed load stretches muscle until either: - muscle tears - tendon breaks - elastic recoil of skeletal muscle sufficient oppose the load
Isometric contraction:
- muscle contracts
- but ***NOT shorten
- force CANNOT move the load
Muscle performance vs Endurance
Muscle performance:
***maximum amount of tension / force produced by a particular muscle group
Endurance:
amount of ***time during which the individual can perform a particular activity
Factors determining performance capabilities:
- ***Type of muscle fibres
- ***Distribution of muscle fibres
- ***Size of muscle fibres
- Physical ***conditioning / training
Fast vs Slow muscle fibres
Slow (Red muscle):
- **Slow oxidative, slow-twitch oxidative / Type I S
- ***smaller diameter
- darker colour due to ***myoglobin
- ***fatigue resistant
- surrounded by more ***extensive capillary network
- many mitochondria (for ***oxidative phosphorylation)
- ***low glycolytic enzyme conc in sarcoplasm
- main substrate for ATP generation: Lipids, Carbs, a.a. (***aerobic)
Intermediate:
***Fast resistant, fast-twitch oxidative / Type II-A
Fast (White muscle):
- **Fast fatigue, fast-twitch glycolytic / Type II-B
- ***larger diameter
- paler colour
- ***easily fatigue
- -> reach peak twitch tension in 0.01s after stimulation
- ***few mitochondria
- ***high glycolytic enzyme conc in sarcoplasm
- main substrate for ATP generation: Carbs (***anaerobic)
