Muscle Microstructure And Contraction Flashcards
What are the 3 different muscle types and what control are they under?
- Smooth muscleMuscle is under involuntary control from the autonomic NS.
- Cardiac muscleMuscle is under involuntary control from the autonomic NS.can contract autonomously but is under influence of autonomic NS and circulating chemicals
- Skeletal muscleMuscles under voluntary control, attached to bones and contract to facilitate movement-
Which nervous system innervates skeletal muscle
Somatic nervous system
Explain the macroscopic structure of skeletal muscles
Myofilaments→ Myofibrils →Myofibres (endomysium) → Fascicles (perimysium) → Muscle (epimysium)
Individual muscle → Wrapped in sheath of connective tissue (Epimysium) enabling muscle to contract and move powerfully while maintaining structural integrity.
Epimysium → separates muscle from other tissues.
Muscle fibres are arranged in bundles (fascicles) surrounded by a layer of connective tissue called the perimysium. Fascicular arrangement enables system to trigger specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle.
Each myofibre is encased by a thin layer of collagen, and reticular fibres → the endomysium. Endomysium surrounds extracellular matrix of cells and plays a role in transferring force produced by muscle fibres to tendons
Sarcoplasmic reticulum
Network of fluid-filled tubules, constitutes the main intracellular calcium store in striated muscle, cardinal role in the regulation of excitation-contraction coupling.
What is the cytoplasm of myofibres called and what does it contain?
Sarcoplasm
Myoglobin and mitochondria
Sarcolemma
Plasma membrane of myofibres
Transverse tubules tunnel into centre
Myofibril structure
myofibrils extend along entire length of myofibre
Composed of repeating units of sarcomere
- myofibrils are composed of thin myofilaments made of actin (light band) and thick filaments of myosin (dark band) that don’t extend along length of myofibre
- myofilaments overlap and are arranged in repeating units called sarcomeres from Z disc to Z disc
Sarcomere
Region of a microfibril contained between 2 cytoskeletal structures (Z-lines) and the striated appearance of skeletal muscle fibres due to the arrangement of the thick and thin myofilament within each sarcomere
- What is the structure of myofilaments like?
-
- Thick myosin and thin actin filaments overlap
- Dense protein Z-discs separate sarcomeres
- Which bands are referred to as dark and light?
A band (thick - myosin) → Dark
I band (thin - actin) → Light
Describe what the A-band, M-line, I-band and H-zone contain
- A-bandComposed of thick filaments containing myosin, span the centre of the sarcomere extending towards the Z-discs
- M-lineThick filaments are anchored at the middle of the sarcomere by myomesin
- I-bandLighter regions contain thin actin filaments anchored at the Z-discs by alpha-actinin. Thin filaments extend into the A-band towards the M-line, overlapping with regions of the thick filamentA band is dark due to thicker composition of myosin filaments, in addition to overlapping actin filaments
- H-zone
Middle of A band,thin filaments not present
During muscle contraction what happens to lengths of the the bands
During contraction:
- I-band shortens
- A-band same length
- H-zone narrows/disappears
- Z discs get closer to each other
Diameter of myofibrils
1-2 micrometers
Myosin structure
- 2 globular heads and a single tail formed by 2 alpha helices
- Tails of 100s of molecules form 1 filament
Actin structure
- Actin molecules are twisted into a helix
- Each molecule has a myosin binding site
- Actin filaments also have troponin and tropomyosin associated which move and uncover binding sites when calcium present
Describe the steps of the initiation of muscle contraction at the NMJ
1) Action potential opens voltage gated Ca2+ channels
2) Ca2+ enters presynaptic terminal
3) causes exocytosis of vesicles
4) Acetylcholine diffuses across cleft at neuromuscular junction
5) binds to ACh receptors (on the post-synaptic membrane within the motor-end plate of the sarcolemma) and induces action potentials in muscle
6) Local currents flow from polarised region and adjacent region and AP spreads along surface of muscle fibre membrane
7) ACh broken down by acetylcholine esterase and muscle fibre response to that ACh molecule ceases
Describe the steps of activation of muscle contraction
- Propagation of AP along sarcolemma into T-tubules.
- Dihydropyridine (DHP) receptor in T tubule membrane sense change in voltage & changes shape of a protein linked to a ryanodine receptor
- Ryanodine receptor Ca2+ channel in sarcoplasmic reticulum opens, allowing Ca2+ efflux from SR into sarcoplasm (specifically the space around filaments)
- Ca2+ binds to troponin and tropomyosin moves out of way
- reveals myosin binding site on actin
-
Ca2+ actively transported into the SR continuously while AP continue. ATP-driven pump (where uptake rate < or = release rate)
- Muscle contraction stops when there is a signal termination from the motor neurone, re-polarisation of sarcolemma and T-tubules, ryanodine receptor close, and tropomyosin occupy the myosin binding sites on the actin
Describe the steps of excitation contraction coupling
1) Ca2+ binds to troponin and tropomyosin moves out of way
2) This exposes myosin binding site on surface of actin chain
3) ‘Charged’ myosin heads bind to exposed site on actin filament forming a cross-bridge. (Cross-bridge formation occurs when myosin-head attaches to actin, ADP and Pi bound to myosin.)
4) This binding and discharge of ADP causes myosin head to pivot (the power stroke) which pulls actin filament towards centre of sarcomere (M line)
5) ATP binds and releases myosin head from actin chain
6) ATP hydrolysis provides energy to recharge the myosin head
This is process happens repeatedly across many sarcomeres and myofibres and the muscle shortens
Describe neuronal control/pathway of muscle contraction
- Upper motor neurones are in brain (primary motor cortex)
- These synapse onto lower motor neurones in brainstem or spinal cord
- We have voluntary neural control from upper and lower motor neurones
What is a motor unit
Single motor neurone along with all muscle fibres that it innervates
Stimulation of one causes contractions of all
In muscles with fine control there are fewer fibres per neurone so we can finely adjust force in muscles
Innervating ratio
Defines the number of muscle fibres innervated by a single motor neurone
What will the innervation ratio be for fine control/ delicate movements broadly speaking?
Innervating ratio will tend to be small,allowing nuances of movement
Slow type I
Smallest diameter
Smallest dendritic trees
Thinnest axons
Slowest conduction velocity,low force,fatigue resistance
High myoglobin,high aerobic,low anaerobic capacity
Red
Fast fatigue resistant type IIa
Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity,moderate force and fatigue resistance
High myoglobin,moderate aerobic,high anaerobic capacity
Pink
Fast fatiguable type IIb
Larger diameter
Larger dendritic trees
Thicker axons
Faster conduction velocity,high force,high fatigue
Low myoglobin,low aerobic,high anaerobic capacity
White
List the properties of type IIB, type IIA and type I motor units
Type IIB - Fast twitch, high tension, high fatigue
Type IIA - Fast twitch, moderate tension, fatigue resistant
Type I - Slow twitch, low tension, fatigue resistant
By what two ways does brain regulate muscle force
Recruitment- motor neurones are recruited in order of size,smallest first. As more force needed more units are recruited
Rate coding-A motor neurone fired at a range if frequencies ,slow units fire at lower frequencies
Summation occurs when units fire at a frequency too fast
What is the effect of these factors on muscle fibres in motor units (in terms of crossing over)?
- Motor unit and fibre characteristics are dependent on the nerve innervating them
- If a fast and slow twitch muscle are cross innervated, the slow one becomes fast and vice versa
- The motor neurone has some effect on the properties of the muscle fibres it innervates
3 types of muscle contraction
- Isometricmuscle produces force but doesn’t change in lengthTension in the muscle = constantSarcomere shortening and increasing muscle tension, though the force produced can’t overcome resistance provided by loade.g. holding a shopping bag
- ConcentricMuscles shorten to move a load
- Eccentric
Muscle produces force but is getting longer eg holding something heavier than can be managed
What type of contraction is most likely to cause damage
Eccentric
Muscle plasticity
Ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it
In what cases does muscle fibres type I get converted to type II typically?
Severe de-conditioning or spinal cord injury or microgravity
- Microgravity in spaceflight results in shift from slow to fast muscle fibre types as not as much postural control needed
Common conversion of muscles following training
Conversion from IIb to IIa
What is ageing associated with
Associated with loss of type I and II fibres, preferential loss for type II fibres
Results in a larger proportion of type I fibres in aged muscle (Evidence from slower contraction times)
What are muscle fascicles and muscle fibres surrounded by
Fascicles surrounded by perimysium
Fibres surrounded by endomysium
Neurotrophic factors
Are a growth factor
Prevent neuronal death
Promote growth of neurons
