muscle physiology Flashcards
how are muscles attached to the skeleton?
via tendons
describe the structure of a myofilament
protein complexes formed from large numbers of myosin II molecules. the myosin heads contain regulatory light chain and an alkali light chain. there is a hinge region and an a-helical tail region of heavy chains
what are the thin filaments (skeletal) formed from?
actin in complex with troponin and tropomyosin
what is the calcium sensor in skeletal muscle?
troponin
outline the cross bridge cycle
1- ATP binds to myosin head
2- ATP is hydrolysed, myosin head returns to resting position
3-cross bridge forms at a new position on actin
4-P is released and a conformational change results in the power stroke
how does the crossbridge cycle in skeletal muscle differ from the crossbridge cycle in smooth muscle?
in skeletal muscle, the myosin head has constant ATPase activity, meaning the crossbridge cycle will occur constantly as long as Ca2+ and ATP are elevated. in smooth muscle, ATPase activity requires a phosphorylation event, meaning that crossbridge cycle is prolonged without constant ATP use
explain how [Ca2+] regulates contraction in skeletal muscle when Ca2+ is low
when [Ca2+] is low, tropomyosin sterically hinders the crossbridge cycle by obscuring the myosin binding site
explain how [Ca2+] regulates contraction in skeletal muscle when Ca2+ is high
the tropomyosin-troponin complex undergoes a conformational change, exposing the myosin binding site
what is a ‘triad’?
found in skeletal muscle; 2 sarcoplasmic reticulum with a T-tubule sandwiched between
how is calcium release in the cytoplasm controlled in skeletal muscles?
dihydropyridine receptors are voltage-gated receptors found on the T-tubule, when depolarised, they cause ryanodine receptors on the sarcoplasmic reticulum to open, triggers release of Ca2+ into cytoplasm
outline excitation-contraction coupling in skeletal muscle
1) end plate potential triggers AP in muscle fibre
2) AP propagates along sarcolemma down the T-tubules
3) depolarisation of T-tubules is sensed by dihydropyridine receptors, which are mechanically coupled to RYR
4) RYR opens, and Ca2+ released into cytoplasm
5) influx of Ca2+ initiates the cross-bridge cycle
6) Ca2+ pumped back into the SR by SERCA
which factors determine the force of contraction?
- frequency of action potentials
- number of motor units activated
- active length of the muscle
why does a full tetanus occur when frequency of APs is high?
the effect of the APs on force exerted is summative
what is meant by ‘passive force’?
when you stretch a muscle without electrically stimulating it, and the force increases with length
what is meant by ‘total force’?
the force production when a muscle is stretched and electrically stimulated at the same time
how is active force of a muscle worked out?
total force - passive force
compare cardiac muscle with skeletal muscle
cardiac muscle: smaller cells, electrically and mechanically coupled using gap junctions and desmosomes
skeletal muscle: individual cells are smaller, and mechanically coupled
explain how the heart is myogenic
the threshold in the SA node is reached spontaneously, no external depolarisation is required
outline the conduction system of the heart
1) the SA node is the primary pacemaker region
2) action potentials spread across the atria
3) atrioventricular node is the secondary pacemaker region
4) conduction propagates through AVN along ventricular conduction system consisting of purkinje fibres
what is the role of cardiac refractory periods?
- allows ventricles to fill with blood
- protects heart from premature depolarisation
on what are dihydropyridine receptors and ryanodine receptors in the cardiac muscle dependent on?
Ca2+
how should Frank Starling’s law of the heart be interpreted?
the amount of blood returning to the heart determines how much the muscle is stretched, and thus how much force is generated
why is it important that there is a strong passive component of tension in cardiac muscle?
as stretching increases, force production decreases due to less crossbridge cycling. the passive component of tension prevents overextension of the muscle
how is positive inotropy induced?
adrenaline and noradrenaline interact with ventricular B1-adrenoceptors
