PBL 9 Flashcards
(31 cards)
What are the 3 layers of connective tissue that skeletal muscle is arranged within?
Epimysium - collagen fibres surrounding the entire muscle
Perimysium - divides the skeletal muscle into fasicles - bundles of fibre
Endomysium - surrounds the individual skeletal muscle fibres
How are skeletal muscle fibres specialised for their function?
- Large
- Multinucleated
- T tubules - allow action potential to reach whole cell at once so whole region can contract at the same time
What is a muscle cell triad?
Terminal cisternae plus T tubercle
Terminal cisternae - is an expanded area at the end of each sarcoplasmic reticulum in muscle cells. It contains calcium stores which are released at the beginning of muscle contraction
What does the sarcomere A band contain?
M line - where thick myosin filaments connect
H band - contains only thick myosin filaments
Zone of overlap - a dark region where actin filaments are loacted between the thick myosin filaments
What does the sarcomere I band (light band) contain?
Actin - extending from the A band of one sarcomere to the A band of the next sarcomere
Z line - marks the boundary between adjacent sarcomeres, constipation of proteins which interconnect the thin filaments
Titin - elastic protein that extends from the thick filaments in the A band to the Z line
Which proteins comprise a thin filament?
F actin - twisted strand composed of two rows of G actin
Nebulin - exerts along F actin between G actin strands to hold them together
Tropomyosin - covers active sites on G actin (7 active sites)
Troponin - bound to tropomyosin
What are the 3 subunits of troponin and what do they bind to?
Troponin T - binds to tropomyosin
Troponin I - binds to G-actin
Troponin C - has receptors that bind to calcium ions
What is a motor unit?
A motor unit is a single lower motor neurons and all of the muscle fibres that it innervates
What are the two types of lower motor neurons?
Alpha neruons - innervate extrafusal muscles (involved in strength and power)
Gamma neurons - innervate intrafusal muscles (involved in tone and tension)
Explain ‘calcium induced calcium release’ within muscle cells
Action potentials arriving at motor end plate pass into T tubules which cause voltage gated calcium channels to open in T tubules, so calcium flows into sarcoplasm
Influx of calcium activates ryanodine receptor channels on the sarcoplasmic reticulum membrane, which then release further stored calcium into the sarcoplasm
Explain the contraction cycle?
Arriving calcium at zone of overlap binds to troponin causing it to change shape and pull tropomyosin away from actin
Myosin with charged ADP+P binds to actin to form cross bridge
Power stroke occcurs where myosin head swings towards M line, ADP + P are lost
Binding of another ATP to myosin causes the cross bridge to detach
Myosin is reactivated by ATP hydrolysis ready restart the cycle
Cycle is inhibited when calcium is no longer present
What causes the contraction cycle to end?
Ach is broken down by acetylcholinesterase
Action potential is no longer fired
Calcium is taken back up by the sarcoplasmic reticulum
Troponin can reattach to actin bringing tropomyosin with it to recover the actin binding sites
What is the size principle of motor unit activation?
Small, slow motor fibre units are recruited first followed by fast large motor fibres:
Slow > fast resistant > fast fatigue
What is fused tetanus
Where all the motor units are activates so all the muscle fibres are contracting at the same time. This is peak tension.
What happens to motor units during sustained contraction
They are activating on a rotating basis, so some are resting and recovering while others are actively contracting
What is muscle tone?
This is the contraction of some motor units even when the muscle is at rest. The contractions do not produce enough tension to produce movement, but they tense and firm the muscle
What is the point in muscle tone?
Helps stabilises positions of bones and points
Helps maintain body posture when producing tension in muscles involved with balance and posture
Prevents sudden uncontrolled changes in positions of bones and joints
What is the difference between an isotonic and isometric contraction?
Isotonic - where tension rises and skeletal muscle length changes
Isometric - where tension rises but skeletal muscle length doesn’t change
What are the two types of isotonic contraction?
Concentric - shortening of the muscle
Eccentric - lengthening of the muscle
What are type I muscle fibres?
- Slow oxidative fibres
- Red in colour
- Good supply of nutrients
- get ATP from oxidative phosphorylation (oxygen)
- Resistant to rapid depletion of energy reserves
- Found in marathon runners
What are type IIA muscle fibres?
- Fast twitch oxidative fibres
- Fast resistant
- have faster contraction speed
- still relatively resistant to fatigue
- get ATP from oxidative phosphorylation
- need more energy to operate than type I
What are type IIB muscle fibres
- Fast twitch glycolytic
- White in colour
- become short of energy very quickly “fast fatigue”
- larger cells which have limited blood supply and nutrient supply
- generate ATP via substrate level phosphorylation (glucose>lactic acid) (aerobic)
- also can use creative kinase as immediate source of ATP (requires neither glucose or oxygen)
- good for sprinting
How does mitochondrial biogenesis occur?
- Exercise increases demand for ATP
- Activation of AMPK (adenosine monophosphate activated protein kinase)
- AMPK inhibits anaerobic energy consumption and stimulating energy producing, catabolic pathways e.g, oxidative phosphorylation
What signalling molecules can lead to muscle cell atrophy?
Akt, mTOR - decrease protein synthesis rate
Glucocorticoids, myostatin NF kappaB, ROS - increase protein degradation and decrease protein synthesis
