Muscle contraction mechanisms Flashcards
structure of muscles, biochemistry of muscle contraction, muscle fibress
What are the three types of muscles?
- Smooth muscle
- Cardiac muscle
- Skeletal muscle
What is the length range of smooth muscle?
30-200 µm
What is the length range of cardiac muscle?
50-100 µm
What is the length of skeletal muscle?
Up to 0.3 m (can be longest in the body)
How many nuclei are found in smooth muscle?
Mono-nucleated (one nucleus), central
How many nuclei are found in cardiac muscle?
Mono-nucleated mainly (<5)
How many nuclei are found in skeletal muscle?
Multi-nucleated (can have hundreds of nuclei)
What is the appearance of smooth muscle?
Spindle shaped, tapered ends, non-striated
What is the appearance of cardiac muscle?
Branched, striated
What is the appearance of skeletal muscle?
Elongated, striated
Where is smooth muscle located?
Lining of the respiratory, cardiovascular, digestive and reproductive tract
Where is cardiac muscle located?
Lining of the heart
Where is skeletal muscle located?
Muscles attached to skeleton
What type of control is smooth muscle under?
Involuntary → autonomic NS
What type of control is cardiac muscle under?
Self-contractible, involuntary → autonomic NS
What type of control is skeletal muscle under?
Voluntary → somatic NS
What is the hierarchical structure of skeletal muscle? - skeletal muscle anatomy
Muscle (organ) → fascicles (bundle of fibres) → muscle fibres (cells) → myofibrils (organelle)
skeletal muscle histology
what is a muscle fibre and what do they contain
single cell that contains 100-1000s of myofybrils
what are myofibrils surrunded by
surrounded by sarcoplasmic reticulum
what does the SR do
store and release calcium
What are the alternations of dark and light bands in skeletal muscle called and what are they?
Striations formed from sarcomeres
What is the smallest contractile unit in a muscle fibre?
Sarcomere
What are the two types of filaments in a sarcomere and what are they bound by?
- Thick filaments
- Thin filaments
(alternate) - bound by Z disks/lines
where do we find sarcomeres
each myofibril contains as many as 20,000 sarcomeres in repeated subunits along the length
sarcomeres: where do the thin and thick filaments project from
- thin from both directions of z band
- thick from centre
What is the composition of thin filaments?
F actin arranged as a helix plus tropomyosin and troponin
what makes up each thick filament?
About 250 myosin molecules
what is special about myosin molecules
globular heads that move
What connects the ends of thick filaments and Z-disks?
Connectins which are fine, thin elastic filaments
What is the role of connectins?
Give muscle its springlike property
Resting state of sarcomere
partial/ intermediate overlap between thick and thin filaments
What occurs during a muscle contraction?
The Z lines of the sarcomere are pulled closer together (shorter myofibril, shorter muscle) and greater overlap between thick and thin filaments
how are the sarcomeres shortened
- thick and thin filaments produce cross bridges between each other to slide filaments over each other
What contractile proteins are involved in the sliding filament mechanism of contraction?
- Myosin (thick)
- Actin (thin)
what is the contractile force in a muscle produced by
cross bridges between thick and thin filaments
What happens in step 1.
Rest
- troponin-tropmyosin complexes on thin filaments block the binding sites on the actin
- myosin heads are ADP bound in ‘cocked’ position
- no AP firing, no or low Ca
- no cross-bridges between thin and thick filaments
5 steps of sliding filament mechanism of contraction
- rest
- activation
- sliding filaments
- myosin detachment
- reactivation of myosin
What happens during the activation step of muscle contraction?
- muscle fibre activated, AP down t-tubules into SR
- Ca2+ is released from the cisternae of the SR
- Ca2+ binds to troponin
what does calcium binding to troponin do
- conformational change in thin filament then exposes actin binding sites
What is formed when myosin heads attach to actin?
Cross-bridge
what happens in the third step or sliding of filaments
- power stroke
- longitudinal force pulls filaments into greater overlap
- shortens muscle fibre
- myosin heads shed bound ADP and resume relaxed state while still attached
What is the power stroke in muscle contraction?
Mechanical energy from ATP dephosphorylation released through myosin heads
What occurs during the myosin detachment step 4?
ATP binds to myosin heads, detaching them from actin binding site
- actin-binding site is released and can form another cross-bridge to sustain muscle contraction
What happens during the reactivation of myosin in step 5 ?
- thick filament: energy released by dephosphorylation of ATP to bound ADP is stored in myosin heads
- Myosin heads are ‘re-cocked’
- thin filament:
- high Ca: system remains activated, muscles contraction persists
- low Ca: return to resting sate (step 1): myosin head are cocked but unable to form cross-bridges
role of Ca in excitation-contraction coupling
- AP initiated in NMJ
- travels along surface of muscle fibre
- depolarisation of transverse tubules within muscle fibre
- Ca released from cisternae of SPR
- Ca diffuses along myofibrils
- Ca binds troponin enabling cross-bridges to form
- ca release rapid for rapid contraction
What influences the force of a muscle contraction?
- Frequency of action potentials
- Overlap between thick and thin filaments
What occurs at low frequency of action potentials?
Twitches occur with limited Ca2+ release
- allows time for Ca reuptake and for muscle to relax
What occurs at high frequency of action potentials?
Tetani occur with more Ca2+ release
- less time for reuptake, so less time for muscle to relax
- summation of contractions
e.g. in heavy lifting
tetanus
muscle disease; due to sustained tetani we get problems
What is the length-tension relationship in muscle contraction?
force depends on overlap between thick and thin filaments prior to stimulation
What characterizes red muscles?
- for postural or anti-gravity e.g. standing, walking
- Mainly slow-twitch muscle fibres (type I fibres)
- can sustain small amounts of tension for long periods (fatigue resistant)
- aerobic metabolism, many mitochondria, capillaries, myoglobin rich
what is the length-tension relationship dictated by
number of actin:myosin cross-bridge connection available
too stretched = less cross-bridges = less force
too contracted = all available cross-bridges occupied = no additional force possible
optimal length = optimal overlap between myosin and actin
What characterizes pale muscles?
- Shorter bursts of activity, less mitochondria, less myoglobin
- mix of fast twitch (type II) and slow-twitch (type I) fibres
Type IIA muscle fibres
Fast fatigue resistant fibres
- enough aerobic activity to resist for a few minutes
Type IIB muscle fibres
fast fatigable fibres
- anaerobic catabolism, use glycogen, forms lactic acid
What is the endurance level of Type IIA muscle fibres?
High
What is the endurance level of Type IIB muscle fibres?
Low
red vs white muscle
increased muscle use = more red
- higher proportion of type 1 so more resistant to fatigue
Muscle fibre typing
- stain with ATPase
- type 1 light; negative stain
- type 2 dark; positive stain
roughly 50:50 in non-athletic
summary for muscle fibre types
What type of ATP synthesis occurs in Type I muscle fibres?
Aerobic
What type of ATP synthesis occurs in Type IIA muscle fibres?
Anaerobic + aerobic
What type of ATP synthesis occurs in Type IIB muscle fibres?
Anaerobic (Glycolysis)
What is the myoglobin content in Type I muscle fibres?
High
What is the myoglobin content in Type IIA muscle fibres?
Low
What is the myoglobin content in Type IIB muscle fibres?
Low
What color are Type I muscle fibres?
Red
What color are Type IIA muscle fibres?
Pink
What color are Type IIB muscle fibres?
White
type 1 fibres
- slow contraction (50-110ms)
- small force
- resistant to fatigue (oxidative metabolism, many mitochondria, good blood supply)
- recruited in first contraction
type IIA muscle fibres
- fast contraction time (25-45ms)
- intermediate force
- some resistance to fatigue (oxidative metabolism)
- recruited after type 1
type IIB fibres
- fast contraction (<10ms)
- high force
- fatigue easily (anaerobic, glycogen store, few mitochondria)
- recruited last during contraction
What are hybrid muscle fibres?
Fibres that express more than one myosin heavy chain (MHC) type
What can influence the existence of hybrid muscle fibres?
Training and exercise enables fibres to shift between hybrid types, as well as fast and slow fibres
What primarily governs muscle fibre type?
Genetic predisposition
muscles and performance in running
marathon runners - higher proportion of slow fibres
sprinters - higher proportion of fast fibres
