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PHYSIOLOGY OF MUSCLE CONTRACTION (Sliding filament theory of skeletal…
PHYSIOLOGY OF MUSCLE CONTRACTION
Microscopic
structure of skeletal muscle
muscle fiber or myofibre
single muscle cell
many nuclei (multi-nucleated)
nuclei located at periphery of the cell
myofibril
contractile element of muscle
contain actin and myosin filaments
myofilaments
Thin filament (Actin)
tropomyosin, actin and troponin
Thick filament (myosin)
myosin contain myosin ATPase
bind to active site on thin filament (forming cross bridge)
Elastic filament
stabilises structure during contraction
Macroscopic
structure of muscle
Whole muscle
surrounded by
Fascia
(3 layers)
Epimysium
Endomysium
Perimysium
Arrangement of fasicles are correlated with
power
and
range of motion
Parallel arrangement
Pennate arrangement
form tendon sheath or
aponeurosis
(flat layer of tendon)
FOUR
characteristics of muscles
Conductivity
- propagate action potential
Contractility
- ability to shorten and thicken
Excitability
- respond to stimuli
Elasticity
- return to original shape
Three
types of muscles
cardiac muscle
smooth muscle
skeletal muscle
Sliding filament theory
of skeletal muscle contraction
Depolarisation spread to T tubules reaching the sacroplasmic reticulum, causing voltage-gated Ca2+ channels to open
[Ca2+] increases and binds to troponin which exposes myosin binding site
Actin & myosin filaments bind together forming actomyosin cross-bridge
ATP which binds to myosin head are split by myosin ATPase into ADP and Pi, head bends, propelling the actin filament toward the centre of sarcomere (power stroke)
New ATP binds causing myosin head return to resting position
The ATP splits provide energy preparing myosin head for next cycle
ELECTRICAL PROPERTIES
I. Excitable Tissues
Respond to chemical, mechanical & electrical stimuli
Generated electrical signal propagates through the respective tissues
II. Action potential in muscles
Skeletal muscle
RMP is -80 to -90mV
Duration of action potential is 1 to 5ms
Cardiac muscle
RMP is -90mV
Action Potential is +20mV
Increasingly after initial spike the membrane remains plateau for 0.2sec
III. Refractory period in cardiac action potential
From phase 0-2, cardiac muscle cannot excited again (absolute refractory period)
In phase 2, slow influx of Ca2+ located in sarcroplasmic reticulum
In phase 3, (at the end of action potential) cardiac muscle can excite at weaker stimuli than normal (supranormal excitability period)
IV. Wave summation
If the muscle re-stimulated after it has completely relaxed, second twitch is same magnitude as first twitch
If the muscle is re-stimulated before it has completely relaxed, second twitch is added onto first twitch (summation)
If muscle is rapidly stimulated and has no opportunity to relax in between stimuli resulting a maximal sustained (tetanus)
Mechanical properties
I. Skeletal muscle mechanics
Type of muscle contractions
Static
- no movement of body part
Isometric action
(maintain static body position)
Dynamic action
- movement of body part, consider when
constant load
is used (
isotonic
)
:star: Concentric- muscle shorten,
:star: Eccentric- muscle lengthen
Energy for muscle contraction
Direct phosphorylation (coupled reaction of creatine phosphate and ADP
Anaerobic mechanism (glycolysis and lactic acid formation)
Aerobic mechanism (aerobic cellular respiration)
II. Skeletal muscle fibre types
Red muscle
(slow)
rich in myoglobin and oxidative enzymes
Type 1 muscle fibres, expressing myosin which splits ATP slowly
also referred as
slow-twitch muscle
Used for posture support and power slow repetitive movement eg.
Marathon running
standard ATPase reaction: Light
White muscle
(fast)
lack of myoglobin and rich in glycolytic enzymes
Type 2 muscle fibres
that express fast myosin
require large amount of energy for short periods of time eg.
Sprint running
standard ATPase reaction: Dark
Classification of muscle fibres
Type 1 muscle fibres:
slow oxidative fibres
have slow shortening period
utilise aerobic metabolism (fatigue resistance)
Type 2 muscle fibres:
fast oxidative glycolytic and glycolytic fibres
have fast shortening speed -
utilise glycolytic metablism ( low fatigue resistance)