sliding filament theory

what in the biggest unit that makes up a muscle?

fasicle

what makes up the fasicles?

muscle fibres aka muscle cells aka myocytes

what makes up the muscle fibres/cells/myocytes?

myofibrils

what makes up the myofibrils?

myofilaments

what are the myofilaments?

actin and myosin

which out of actin and myosin is thinner or thicker?

actin - thin

myosin - thick

what is the sarcolemma?

plasma membrane of a muscle fibre/cell/myocyte

what is the myofibril organised into?

sarcomeres - short segments of myofibril

where does the sarcomere span to and from?

Z line to Z line

what is the Z line made of?

actin

what is the centre of the sarcomere called?

M line

what makes up the M line?

myosin

what is the A band?

length of the myosin

what is the I band?

actin only area

what is the H band?

myosin only

what happens to the filaments when the muscle contracts?

they slide over each other

what happens to the A band during contraction?

stays the same (the myosin filament itself does not get any shorter)

what happens to the I band during contraction?

shortens (as the filaments are sliding over each other there less 'actin only' space)

what happens to the H band during contraction?

shortens (as the filaments are sliding over each other there is less 'myosin only' space)

what is excitation-contraction coupling?

mechanism of how an action potential causes muscle fibre contraction

what initiates cardiac muscle contraction?

pacemaker cells in SA and AV nodes initiate an action potential

what happens to the action potential initiated by SA and AV nodes?

travels down the T-tubules between sacromeres which depolarises the whole sarcolemma

what does membrane depolarisation cause?

opens voltage-operated calcium channels (VOCCs) in the T-tubule system and calcium is released

what are the 2 ways that Ca2+ is release from the sarcoplasmic reticulum?

- via IP3

- via ryanodine receptors (RyR)

how is calcium released from the sarcoplamic reticulum via IP3?

- Gq activates Phospholipase C enzyme

- Phospholipase C breaks down PIP2 into IP3 + DAG

- IP3 then binds to the IP3 receptor on the SR

- DAG acts on various proteins (e.g. PKC)

- Calcium channels on the SR open and calcium is released

how is the action of IP3 terminated?

IP3 is cleaved to IP2 by IP3 phosphotase

how is calcium released from the sarcoplasmic reticulum via ryanodine receptors (RyR)?

- calcium binds to RyR on the sarcoplasmic reticulum

- the induces conformational changes in a calcium channel which is closely associated with RyR

- RyR is activated and opens so calcium is released from SR

- this is also known as the 'calcium spark'

what is meant by calcium-induced calcium release?

calcium being released into the intracellular fluid causes more calcium to be released from sarcoplasmic reticulum stores

what initiates skeletal muscle contraction?

nerve signals (action potentials) reaching the neuromuscular junction

what neurotransmitter is released at the neuromuscular junction?

ACh

what does ACh bind to?

nicotinic ACh receptors on sarcolemma (muscle plasma membrane)

what does the binding of ACh cause?

influx of sodium ions and hence depolarisation of membrane potential

where do the action potentials spread?

deep into the muscle down the T-tubule system to depolarise entire sarcolemma

what does depolarisation of the sarcolemma cause?

opening of voltage-gated Ca2+ channels

what happens after the Ca2+ channels open?

influx of Ca2+

what does the Ca2+ influx also cause?

sarcoplasmic reticulum in the muscle cells to release even more Ca2+

what does the Ca2+ do?

binds to troponin C on tropomyosin which is wrapped around the actin filaments

what happens when the Ca2+ binds to troponin C?

tropomyosin undergoes a conformational change and reveals the myosin head binding sites on actin

what needs to happen before myosin head can bind to the actin?

ATP binds to myosin head

what happens to the ATP on the myosin head?

breaks down into ADP and Pi but remains on the myosin head

what does the breakdown of ATP cause?

the myosin head to move backwards and bind to the actin

what happens once the myosin head is bound?

ADP and Pi are released and the myosin head moves forward into its original position, but still bound to the actin filament = power stroke

what does the power stroke cause the actin filament to do?

slide forwards

what happens after the power stroke?

ATP binds and releases the myosin head again

what binds to troponin C?

calcium - c for calcium

what does troponin I bind to?

actin - i and a are vowels

what does troponin T bind to?

tropomyosin - t for tropomyosin