Skeletal Muscle Contractile Mechanisms

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  • Created by: LBCW0502
  • Created on: 19-01-19 11:29
What are the two types of striated muscle?
Skeletal (muscles attached to skeleton, uses tendons, contraction for force, movement and maintaining body posture) and cardiac (heart muscle arranged to form 4 chambers, contracts to pump blood into vascular system)
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Where is smooth muscle located?
In wall of hollow organs e.g. blood vessels, airways and gut. Controls vessel or tube diameter (constrict/dilate to regulate movement)
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Describe features of a transverse section of skeletal muscle
Muscle fibres, nuclei (controls muscle cells) randomly distributed, blood vessels, capillaries, endomysium (hold muscle fibres together)
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Describe features of a longitudinal section of skeletal muscle
Nuclei scattered, striations (due to how muscle fibres are arranged, thick/thin filaments) only seen in l.s. Striations cased by highly organised array of intracellular structures (myofibrils) with nuclei bulging out
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Which type of muscle is the largest muscle?
Skeletal muscle
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What are myofibrils?
Units which form muscle fibres, run parallel. Light (actin) fibres and dark (myosin) filaments. Myofibrils are broken down into sarcomeres (between Z discs). Contraction of each sarcomere unit
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What does a build up of muscle mean?
Protein synthesis in muscle (actin/myosin). Striated pattern due arrangement of proteins
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Describe the arrangement of filaments within a sarcomere
Thick filaments (myosin) and thin filaments (actin) overlap within sarcomere (Z-lines/disc anchors the thin/actin filaments)
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What is the I-band?
The part where only the thin/actin filaments are present
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What is the A-band?
The part where thick/myosin filaments are located
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What is the sarcomere?
The segment of a myofibril between adjacent Z-lines
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What gives rise to the striations?
A and I bands bend light differently
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What is the M-line?
Region where the myosin/thick filament is anchored to
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What is the H-zone?
Only see myosin/thick filament in region (no action)
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Describe features of the sarcomere
The contractile unit of both skeletal and cardiac muscle fibres repeated many times along each myofibril. Contains thick/thin myofilaments in their interdigitating arrangement
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What is the sarcomere surrounded by?
Sarcoplasmic reticulum (SR) containing Ca 2+ ions (needed for contraction). Transverse (T) tubules which bring it the signal to start contraction. Organelles and enzymes which produce the ATP needed to 'fuel' contraction
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Contraction of skeletal muscle can mean what?
Muscle shortens actively, causing a movement
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What is isometric contraction?
Muscle produces force but does not shorten e.g. supporting a weight, maintaining posture
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What is concentric contraction?
Muscle may produce force while also shortening e.g. lifting a weight
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What is eccentric contraction?
Muscle may produce active force while it is being stretched by the action of other muscles e.g. a 'braking' action, walking down stairs/hill
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Describe the mechanism of contraction
Head portion of myosin molecules making up thick filaments make contact with thin filaments and pull them towards the centre of the sarcomere. Myosin heads can be seen using EM and are called cross-bridges (bridge gap between thick/thick filaments)
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What are the two important binding sites on the myosin head for binding sites?
Actin binding site and ATP binding site (ATP used to break actin-myosin cross bridge/relax muscle, ATP hydrolysis)
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Describe the contractile mechanism in skeletal muscle fibres
Ca 2+ released from SR binds to troponin C which changes shape. This moves tropomyosin out of the way of myosin heads. Myosin heads can bind to actin molecule and pull/power stroke (similar mechanism in cardiac muscle but not all Ca 2+ comes from SR)
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Describe the cross-bridge cycle
Myosin splits ATP, retains products and binds to new actin. Myosin head attaches to actin. Myosin head release Pi and rotates (heads pull on actin). Myosin releases ADP, binds ATP and detaches - cycle repeats
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What is force-generation?
Strain energy in myosin head is released by pulling on thin/actin filament
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What happens in the contractile mechanism at a molecular level?
Cross-bridges pull on thin filaments (in a series of steps - cross-bridge cycle, through thick myosin in thick filaments interacts with actin in thin filaments). Hydrolysis (splitting) of ATP during the cross-bridge cycle
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Describe the shortening and the sliding filament mechanism
During contraction, cross-bridges pull the thin filament towards the centre of each sarcomere. So sarcomere shortens, I-band shortens and H-zone shortens/disappears but A-band doesn't change. Overlap of thick/thin filaments within A band increase
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Force production (action tension) is affected by what?
Muscle length (experiments/experience show that the length of the muscle/stretching affects amount of force it can produce). Summarised in graph (length-tension curve) which shows tension (force) relating to muscle and sarcomere length
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How is the length-tension curve determined?
Electrical signal across muscle for contraction, see how force varies
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Describe features of the length-tension curve (1)
Too short/too stretched sarcomere produces low force. There is an optimal length at which muscles contract at
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Describe features of the length-tension curve (2)
Long sarcomere (no overlap, fewer points of contact). Short sarcomere (lose force/cancel out, complete overlap, not pulling muscle, actin squashed)
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Other cards in this set

Card 2

Front

Where is smooth muscle located?

Back

In wall of hollow organs e.g. blood vessels, airways and gut. Controls vessel or tube diameter (constrict/dilate to regulate movement)

Card 3

Front

Describe features of a transverse section of skeletal muscle

Back

Preview of the front of card 3

Card 4

Front

Describe features of a longitudinal section of skeletal muscle

Back

Preview of the front of card 4

Card 5

Front

Which type of muscle is the largest muscle?

Back

Preview of the front of card 5
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