Muscle Contraction Flashcards
Describe the cellular and molecular mechanisms involved in muscle contraction.
Structure of skeletal muscle:
- contain myofibrils (proteins) that bring about contraction - actin/myosin
- Actin and myosin are arranged in a sarcomere (structural unit of muscle)
- Z line - where protein filaments (titin/nebulin) are anchored
- A band - thick filaments in centre of sarcomere
Myosin-
- thick filament
- polypeptide chains twisted around each other
- 2 globular heads and a long tail - anchor to actin molecules
- heads are site of myosin ATPase enzyme
Actin-
- thin filament
- intertwined chains
– troponin - small globular protein bound to actin and tropomyosin
– tropomyosin - rod shaped, located end to end along the thin filament
- Thick and thin filaments slide across one another to shorten the sarcomere and produce a contraction
Events during muscle contraction:
- resting state - troponin controls the position of tropomyosin on the thin filament, blocking the myosin binding site on the actin molecules
2. excitation-contraction coupling - calcium ions bind to troponin which changes its shape which moves tropomyosin on the thin filament away from the myosin binding site
3. binding - myosin heads bind to actin on the thin filament causing the detachment of ADP and P molecules
4. power stroke - myosin heads move, performing a ‘power stroke’ which drags the thin filament towards the centre of the sarcomere
5. detachment - ATP binds to myosin, changing its shape, causing it to lose affinity for actin and to detach from its binding site - ATP is hydrolysed by ATPase which re-energises the myosin heads to return to their original position
6. no calcium = resting state / calcium present = returns to stage 3
Describe why skeletal muscle contraction requires energy and discuss potential sources of this energy.
Functions of ATP in skeletal contraction:
1. energy released from ATP hydrolysis re-energises the myosin head, providing energy for the cross-bridge movement and force generation
2. binding of ATP to myosin causes the release of the myosin head from actin, allowing for repeated contractions
3. Ca-ATPase hydrolyses ATP in the sarcoplasmic reticulum to take up Ca2+ ions back into the SR to end contraction
Energy for contraction:
1. phosphorylation of ADP by creatine phosphate-
- very rapid
- onsets muscle contraction
- short duration
- limited ATP production
2. oxidative phosphorylation of ADP in mitochondria-
- aerobic respiration
- slow
- moderate levels of activity
- uses blood-borne fuels (glucose, oxygen, fatty acids) and muscle glycogen
3. phosphorylation of ADP in the cytosol-
- anaerobic respiration
- lactic acid byproduct
- requires muscle glycogen / blood-borne glucose
- if intense exercise - rate of ATP use becomes very important
Compare the properties of the main types of skeletal muscle fibre.
Slow oxidative-
- type I
- low myosin ATPase activity
- high oxidative capacity (aerobic)
- small diameter ; weak ; fatigue resistant
Fast oxidative-glycolytic-
- type IIa
- high myosin ATPase activity
- high oxidative + glycolytic capacity
- high glycolytic capacity
Fast glycolytic-
- type IIb
- high myosin ATPase activity
- high glycolytic capacity (anaerobic)
- large diameter ; strong ; fatigue easily
- Type I muscle fibres are typically used in postural activities and low intensity movements / keeping joints in place
- Type II muscle fibres are used in short bursts but cannot be sustained over long periods of time
- Muscle has a distribution of all 3 muscle fibre types within (though can be influenced by age and lifestyle)
Name and describe the types of muscle contraction and give examples of when each might be used in a standing or moving animal.
-Isometric - muscle contracts but does not shorten, so no movement generated (eg: supporting a load)
- Concentric - muscle actively creates the movement of a joint (eg: elbow flexion)
- Eccentric - resist/control movement at joints (eg: lowering dumbbell in hand - bicep is a natural flexor but controls lowering the weight through extension)
