Muscle Contraction Flashcards
Learning Outcomes
- Explain the sliding-filament hypothesis of contraction and outline the main events in the cross-bridge cycle
- Relate the sliding-filament hypothesis to the length-tension relationship in muscle
- Describe the events responsible for excitation-contraction coupling in striated muscle
- Explain the functional classification of skeletal muscle fibres
- Outline the similarities and differences between skeletal, cardiac and smooth muscles
What is contraction? (broad definition)
Contraction: the ability of a muscle to actively shorten and develop tension
Difference between Relaxed and Contracted muscle tissue in terms of actin?
Relaxed state–ends of actin filaments from Z discs barely overlap
Contracted state –actin filaments pulled inward, overlap to maximum extent
What is the difference between a myosin molecule and an actin molecule?
-Myosin molecule made up of a twisted tail of 2 heavy chains, a hinge between it and 2 myosin heads ( contain ATP and actin binding sites), with light chains on each head: filament= 200/300 molecules
- Actin molecule- double-stranded F-actin molecules comprising polymerised G-actin molecules
- 2 strands of tropomyosin molecules
- troponin complex on each tropomyosin for contraction (binds Ca2+)
Each myofibril contains 1500 myosin filaments and 3000 actin filaments- see lecture 42 for diagrams
Myosin is thick filament, actin thin
What are the first steps of the sliding filament theory of contraction?
- in low Ca2+ conditions, tropomyosin blocks access to the myosin binding site of actin
- in high Ca2+ conditions, Ca2+binds to troponin
- the positions of troponin and tropomyosin are altered on actin -myosin can now access its binding site on actin.
Sliding filament theory part 2: after the binding sites are uncovered what occurs?
- ATP on myosin, is hydrolysed to ADP and Pi. This reorientates and energizes the myosin head
- Myosin head attaches to myosin-binding site on actin, releases phosphate.
- Cross-bridges form, power stroke occurs, myosin head pivots from 90o to 45o angle.
- ADP released. Cross-bridge generates force, sliding actin past myosin.
- Crossbridge remains attached to actin until another ATP is bound. When ATP binds to myosin, the myosin head detaches from actin.
Sliding filament theory part 3-
Neither thick nor thin filaments change length during active muscle contraction
-Myosin heads walk along the actin filament
-Walk-along” mechanism (or ratchet theory) of muscle contraction
Many cross-bridges formed/broken asynchronouslyalong filaments
-SMOOTH CONTRACTION -in spite of individual cross-bridges continually attaching/reattaching
What is the length-tension relationship?
Length-tension relationship depends on sarcomere length within a muscle beforecontraction begins
- Resting length (2.2μm), optimal overlap in sarcomere, maximum tension possible
- Tendon attachment of skeletal muscle to bone enables optimal resting muscle fibre length
In muscle where do action potentials arrive?
Action potential in a motor nerve arrives at the neuromuscular junction or motor end plate.
How is excitation initiated in the NMJ?
- Action potential travels along motor nerve to neuromuscular junction
- Nerve secretes acetylcholine
- Acetylcholine diffuses across synapse
- Ach binds to nicotinic receptors on muscle membrane –opens receptor-operated channels, inward current produces local depolarization (a graded potential)
- Local currents depolarise the muscle membrane by influx of Na+ions into muscle cell –initiating an action potential
- AP travels along muscle fibre membrane via voltage-dependent Na+channels
How does this excitation spread in the T- tubule sarcoplasmic reticulum system? ( after initiation, steps 1-6)
- Muscle AP flows rapidly through centre of fibre via the T-tubules
- Dihydropyridine receptors (L-type channels) on T-tubule membrane sense the AP voltage. They trigger Ca2+-release channels on sarcoplasmic reticulum, called ryanodine receptors, to open and release Ca2+ into sarcoplasm
- Rise in intracellular Ca2+enables Ca2+-binding to troponin, tropomyosin moves, and cross-bridge cycling occurs
- Ca2+actively pumped back into SR store by a pump causing relaxation
What are the different types of muscle contraction?
-Isotonic (Constant tension, change in length)
Concentric- Muscle shortens
-Isotonic( Constant tension, change in length)
Eccentric- Muscle lengthens
-Isometric (Constant length, produces tension)
Muscle length stays constant
What are the characteristics of Slow Twitch Type 1?
Slow twitch, type I •postural muscles of back •slow, sustained contractions required •oxidative metabolism •many mitochondria •red -myoglobin •resist fatigue
What are the characteristics of Fast Twitch muscle fibres? (Types IIA and B)
Fast twitch, type IIA •soleus in calf •fast contractions •oxidative metabolism •many mitochondria •red-pink myoglobin •Intermediate fatigue resistance
2. Fast twitch, type IIB •extraocular muscle •rapid contractions •anaerobic glycolysis •few mitochondria•white –little myoglobin •high glycogen content
Characteristics of smooth muscle excitation and spontaneous activity
Smooth muscle can be excited by nerves, or, generate spontaneousaction potentials using internal ‘pacemakers’
Smooth muscle action potentials show variability, depending on location
