3.6.3 Skeletal muscle contractions

Question 1

describe the structure of a neuromuscular junction

Answer 1

there are receptors on muscle fibre sarcolemma instead of postsynaptic membrane
muscle fibres forms clefts to store enzyme acetylcholinesterase to break down neurotransmitter

Question 2

compare transmission across cholinergic synapses and neuromuscular junctions

Answer 2

• both transmission is unidirectional
  -cholinergic : neurone to neurone, neurotransmitters can be excitatory or inhibitory, action potential may be initiated in postsynaptic neurone
  -neuromuscular junction- motor neurone to muscle, always excitatory, action potential propagates along sarcolemma down T Tubules

Question 3

effect of drugs on synapse

Answer 3

stimulate th NS
-similar shape to neurotransmitter
-stimulate the release of more neurotransmitter
-inhibit enzyme that breaks down neurotransmitter, Na+ continues to enter
Inhibit NS
-inhibit release of neurotransmitterby prevent opening of Calcium ion channels so vescicles containing neurotransmitters cannot leave via exocytosis
Block receptors

Question 4

Overview of muscle contraction

Answer 4

Myosin head slide actin along mysin causing sacromere to contract
simultaneous contraction of many sacromeres causes myofibrils and muscke fibres to contract
when sacromeres contract
- H zones get shorter
I bands get shorter
A band stays the same
Z lines get closer

Question 5

pick of sacromere

Answer 5

bands

Question 6

Describe myofibril contraction

Answer 6

1.) Depolarisation spreads down sarcolemma via T tubules causing CA2+ release from sarcoplasmic reticulum which diffuse to myofibrils
2.) Calcium ions bind to tropomyosin, causing it to move which exposes binding sites on actin
3.) Myosin heads with ADP bind to binding sites on actin forming actinomyosin crossbridge
4.) Myosin heads change angle pulling actin along myosin (ADP+. Pi released,)
5.)new ATP binds to myosin head causing it to detach from binding site
6.) hydrolysis of ATP releases energy for myosin heads to return to original position (activated by CA2+)
7.) myosin reattaches to different binding site further along actin process is rpeated as long as calcium ion concentration is high

Question 7

what occurs during muscle relation

Answer 7

Ca2+ actively transported back into endoplasmic reticulum using energy from ATP
Tropomyosin moves back to block myosin binding site on actin again, no actinomyosin cross bridges fromed

Question 8

describe the role of phosphocreatine in muscle contraction

Answer 8

Pi source which can rapidly phosphorylate ATP to regenerate ATP
ATP+ phosphocreatine -> ATP + creatine
runs out after a few seconds so used in shorts bursts of vigorous excersize
anaerobic and alactic

Question 9

Slow twitch fibres, properties, location and structure

Answer 9

• slow, sustained contractions, produces more atp slowly from aerobic respiration, fatigues slowly
  -muscles for posture eg, back and calves, & legs of long distance runners
  -high conc of myoglobin, stores oxygen for aerobic respiration
  many mitochondria due to high rate of aerobic respiration
  many capillaries, supply oxygen and glucose for aerobic respiration
  aerobic respiration important to reduce the build up of lactic acid and prevent muscle fatigue

Question 10

structure, properties and location of fast twitch muscle fibres

Answer 10

• brief, intensive contractions, less ATP rapidly from anaerobic respiration, fatigues quickly due to high lactose
  -muscles used for fast movement, eg. eyelids, bicepts
  -low level of myoglobin, lots of glycogen to hydrolyse into glucose for glycolysis (inefficient so needes a lot)
• high conc of enzymes for anaerobic respiration in in cytoplasm
  store phosphocreatine