Muscle physiology Flashcards
What is a muscle unit made up of?
Single motor neurone + muscles fibres innervated by them
What is sarcoplasm?
Muscular cytoplasm
What is a sarcolemma?
A muscle fibre membrane that is made up of 2 parts
1) outer coat (thin polysaccharide/collagen layer that attaches to tendons)
2) T-tubules
What are T-tubules
Deep invaginations of sarcolemma into the centre of the fibre that allows speedy depolarisation of the interior
Sarcoplasmic reticulum
Is enlarged endoplasmic reticulum associated with T-tubules, releases Ca2+ on stimulation
Motor end plate of muscle
Sarcolemma directly under an axon, with nicotinic receptors and in-foldings to increase the surface area
What are the 5 stages of action potential transmission across a neuromuscular junction?
1) Action potential in an axon reaches the terminal bouton, Na+ influxes and causes depolarisation.
2) Ca2+ floods into the bouton, binds with a vesicle causing them to exocytose.
3) Ach is released into the cleft and binds to nicotinic receptors.
4) Nicotinic receptors allow Na+ to flood into the muscle sarcoplasm.
5) Threshold is reached and an end plate potential is generated.
AP through a muscle fibre - 6 steps?
1 - EPP spreads over sarcolemma (Na+ influx)
2 - AP penetrated transverse tubules (Na+ continues to flood into the sarcoplasm)
3 - Na+ in sarcoplasm stimulates sarcoplasmic reticulum to release Ca2+ which diffuses into myofibrils
4 - Ca2+ binds to troponin
5 - Troponin pulls Tropomyosin out of the way the was so that myosin binding sites are clear
6 - Myosin heads then bind to the Actin forming a cross bridge
What is the structure of muscle?
1 - muscle
2 - Fasicle/bundle (surrounded by connective tissue)
3 - Fibre/cell (surrounded by its sarcolemma)
4 - Myofibril (intracellular fibril made up of multiple protein layers)
5 - Myofilaments (single protein chains)
Myofibril proteins are?
Myosin
Actin
Tropomyosin
Troponin
What is myosin?
Thick protein with multiple heads that bind to actin utilising ATP to re-cock their heads
What is Actin?
Thin filaments in myofibrils
What does tropomyosin do?
Lies over the actin filaments and prevents myosin heads from binding (muscle at rest)
What does troponin do?
Binds to Ca2+, changing its shape so that it pulls tropomyosin out of the way
What is a sarcomere band?
The contractile unit of muscle, extends from one z line to another, reduces in length upon contraction.
A bands in a sarcomere?
Dark and stay a constant length on contraction (= length of myosin), otherwise known as dark bands
What are the I bands?
Light and reduce upon contraction = length of the sarcomere - length of myosin, also known as light bands
What is the cross bridge cycle?
1 - ATP binds to myosin head to allow it to dissociate from actin
2- ATP splits to ADP + Pi to re-cock the myosin head
3 - Head binds to actin and Pi dissociates
4 - Power stroke occurs as ADP dissociates
Phosphocreatine
= quickest source of ATP in skeletal muscle
1 x PC phosphorylates 1 ADP to ATP
Limited stores, made from ATP at rest during respiration and used to phosphorylate ADP on contraction
Anaerobic respiration in skeletal muscle
Glycogen splits into lactic acid releasing 3 x ATP (net)
Occurs especially in white fast twitch (2b) muscle
Aerobic respiration in skeletal muscle
Glucose is split into CO2 and H2O producing 38 ATPs (net)
FFA oxidation in skeletal muscle
= slowest source of ATP
split into CO2 and H2O producing v high ATP yield, depends on size of FFA (100-150 ATPs)
Type I muscle fibre
Slow twitch, oxidative fibre
Red with high myoglobin levels(O2 store), v vascular
V resistant to fatigue
Type IIa muscle fibre
Fast twitch, oxidative fibre
Red, high myoglobin and vascular, also contains glycogen stores and machinery for anaerobic respiration
Resistant to fatigue
Type IIb muscle fibre
Fast twitch, glycolytic fibre
White, low myoglobin and little vascular input, few mitochondria and oxidative enzymes
large glycogen stores
Fatigues rapidly
What is the recruitment order of muscle fibres?
1st = Type 1, smallest motor neurones so depolarise quickest 2nd = Type IIa 3rd = Type IIb as largest motor neurones so depolarise slowest
What does contraction strength depend on?
Number of motor units recruited, more = more tension
Frequency of impulses, more = more tension
What is peripheral fatigue due to?
Contraction failure - APs don’t conduct along muscle fibre
Lactic acid build up - low pH affects muscle proteins
ADP & Pi build up inhibits cross bridge cycling
What is central fatigue caused by?
Cerebral cortex not firing appropriate motor APs due to tiredness, low motivation, etc.
