Muscle contraction

what is myasthenia gravis?

causes muscle weakness, particularly in muscles of eyes, eyelids and face

Defects in signalling from nerve to muscle at neuromuscular junctions

Untreated → difficulties in swallowing and breathing

what are the 3 types of muscle?

sksletal

Cardiac

Smooth

what are the stages of muscle organisation?

filaments→sarcomere→myofibril→fibre→fasciculus→muscle

what does a muscle ultrastructure look like when relaxed?

ends of actin from Z discs barely overlap

what does a muscle ultra structure look like when contracted?

actin filemnts pulled inwards, overlap to maximum extent

Myosin molecule vs myosin filament:

myosin filament is made up of 200-300 myosin molecules

describe the structure of an actin filament:

double stranded F-actin (filamentous actin) molecules comprising polymerised G-actin molecules

2 strands of tropomyosin molecules

Troponin complex on each tropomyosin for contraction (binds Ca2+)M

How much myosin and actin is found in a myofibril?

1500 myosin filaments

3000 actin filaments

what are the contractile proteins of muscle?

myosin

Actin

what are the regulatory proteins of muscle?

tropomyosin

Troponin

what are the structural proteins of muscle?

titin

a-actinin

Myomesin

Nebulin

Dystrophin

what are the first steps of sliding filament of muscle contraction which allow contraction to be initiated?

1. In low Ca2+ conditions, tropomyosin blocks access to the myosin binding site of actin

2. In high Ca2+, Ca2+ binds to troponin

3. Positions of troponin and tropomyosin are altered on actin

4. Myosin can now access its binding site on actin

5. Contraction now favoured

How does the sliding filament theory take place once myosin can access its binding site?

ATP on myosin is hydrolysed to ADP and Pi, to reorientate and energise the myosin head

Myosin head attached to myosin-binding site on actin, releases phosphate

Cross-bridges form, power stroke occurs, myosin head pivots from 90→45 degree angle

ADP released. Cross bridge generates force, sliding actin past myosin

Cross bridge remains attached to actin until another ATP is bound. When ATP binds to myosin, myosin head detaches from actin

Repeats

What happens to the muscle fibres in low calcium conditions?

tropomyosin blocks access to the myosin binding site of actin

what happens to the muscle fibre in high calcium conditions?

Ca2+ binds to troponin, troponin and tropomyosin change positions, leaving the myosin binding site accessible

How does the length of the filaments change during contraction?

they do not change length, only change position relative to each other

what causes the actomyosin cross bridge to break down?

ATP binds to myosin

lenth-tension relationship depends on:

sarcomere length within a muscle before contraction begins

what is the resting length?

2.2um

Optional overlap in sarcomere, maximum tension possible

what enables optional resting muscle fibre length in skeletal muscle

tendon attachment of muscle to bon

how much tension is possible in unstretched muscle?

little

how much tension is possible at highest stretch?

no tension - actin pulled to ends of myosin - no overlap

what does excitation-contraction in skeletal muscle lead to?

skeletal muscle contraction

Generated high Ca2+ conditions needed for actin-myosin binding

what does acetylcholine bind to in the neuromuscular junction?

nicotinic receptors

Muscle action potential flows rapidly through the centre of the muscle fibre via..

the t-tubules

what senses the action potential voltage in a muscle fibres?

dihydropyridine receptors (L-type channels) on T-tubule membrane

what is the function of L-type channels on t-tubules?

trigger the Ca2+ release channels on sarcoplasmic reticulum called ryanodine receptors to open and release Ca2+ into sarcoplasm

what is the result of ryanodine receptors releasing Ca2+ into sarcoplasm?

increase in intracellular Ca2+ enable Ca2+ binding to troponin, therefore troponin and tropomyosin move, allowing binding to the myosin head

how does Ca2+ re enter the sarcoplasmic reticulum store?

calcium ATPase pump

What are the functional classifications of skeletal muscle fibres?

Type 1 - slow twitch

Type IIA - fast twitch

Type IIB - fast twitch

What are slow twitch muscle fibres?

postural muscles of back

Slow, sustained contractions

Oxidative metabolism

Many mitochondria

Red myoglobin

Resists fatigue

what are IIA fast twitch muscle fibres?

soleus in calf

Fast contractions

Oxidative metabolism

Many in mitochondria

Red-pink myoglobin

Intermediate fatigue resistance

what are fast twitch IIB muscle fibres?

extraocular muscle

Rapid contractions

Anaerobic glycolysis

Few mitochondria

White-little myoglobin

High glycogen content