skeletal muscles and muscle contraction

Question 1

what is the gross strutcure of a muscle?

Answer 1

1. muscle
2. bundle of muscle fibres (surrounded by a connective tissue sheath)
3. muscle fibre
4. myofibril

Question 2

what is myofbril made of?

Answer 2

actin and myosin protein filaments

Question 3

describe the structure of actin

Answer 3

1. composed of thin filaments
2. 2 long protein molecules forming a helix structure
3. the chain has myosin binding sites
4. tropomyosin wound around the actin fibres, containing Ca2+ binding sites

Question 4

describe the structure and function of tropomyosin

Answer 4

1. fibrous protein wound around actin
2. has Ca2+ binding sites
3. acts as a ‘switch’ to control contractions

Question 5

what is the structure of myosin?

Answer 5

1. composed of thick filament

2. long rod shaped fibrous protiens with globular/ bulbous head shaped regions at one end, projecting to the side

Question 6

what is found on the myosin head?

Answer 6

1. ATPase

2. actin binding site

Question 7

why is ATPase present on the myosin head?

Answer 7

1. hydrolyses ATP into ADP and Pi

2. breaks the cross bridge between the myosin and actin filaments so the muscle can relax

Question 8

what are the 3 reactions that are sources of energy for muscle contraction?

Answer 8

1. aerobic respiration
2. anaerobic respiration
3. phosphocreatine system

Question 9

explain when aerobic respiration is used to release energy for muscle contraction

Answer 9

1. low intensity exercise
2. slower less powerful contractions
3. for a long period of time
   aerobic respiration produces the maximum amount of ATP
   requires O2 and glucose

Question 10

when is anaerobic respiration used to release energy for muscle contractions?

Answer 10

1. rapid/ hard muscle contractions
   2.for short periods of time (90s)
2. lactate produced causes muscle fatigue
   produces reduced amount of ATP
   glycolysis of stored glycogen

Question 11

how is the phosphocreatine system used to release energy for muscle contractions?

Answer 11

1. ATP is produced from the donation of phosphate from phosphocreatine
   used for very short term, vigorous exercise (10s)
   no lactate is formed
   PCr is re-synthesised from ATP while the muscle is relaxed
   this system maintains the muscles supply of ATP during vigorous exercise

Question 12

what types of muscle fibres do skeletal muscles contain?

Answer 12

1. fast twitch fibres

2. slow twitch fibres

Question 13

describe how a muscle contraction is stimulated

Answer 13

1. an action potential (AP) arrives at the neuromuscular junction
2. causes an influx of Ca2+ ions into the neurone and the release of Ach
3. Ach binds to receptors sites causing an influx of Na+ and so an AP in the sarcolemma
4. Impulse is carried out throughout the muscle fibre through the T-tubules
5. sarcoplasmic reticulum releases Ca2+ into the fibre

Question 14

what are features of fast twitch fibres?

Answer 14

during short burst of activity, contractions are quick and powerful
anaerobic
store a large amount of PCr in cytoplasm, provides a quick source of ATP during sudden exercise
lactate produced causes muscle fatigue quickly

Question 15

features of a slow twitch fibres

Answer 15

during endurance exercise, contractions are slow and work for a longer time
aerobic
large no. mitochondria
high concentration of myoglobin
excellent blood supply: helps maintain aerobic respiration in tissue, so fibres are slow to fatigue
ATP generation is slower so contractions are weaker

Question 16

list the differences between a neuromuscular junction and a neurone synapse

Answer 16

1. NM junctions always use Ach while NS use many different neurotransmitters
2. the post sn membrane in nmjs is folded into many cleft sthat store the achesterase, while in ns the achesterase is found in the synaptic cleft
3. nmjs has more receptors, ns had less
4. in nmj the impulse is slways excitatory, the ns impulse can be either excitatory or inhibitory

Question 17

how do muslces provide movement

Answer 17

1. force occurs during contration
2. skeletal muscles pull bones
3. movement requires a minium of 2 muslces working in opposite directions; antagonisitc pairs

Question 18

what is the structure and function of the t-tubules in muscle contraction?

Answer 18

1. from the sarcolemma, they pass down into the muslce cell and g around the myofibrils.
2. conduct impulses from the sarcolemma into the sarcoplasmic reticulum
3. allowing the release of ca2+

Question 19

explain the sliding filament theory

Answer 19

The sliding filament theory describes the process by which muscles contract.

1. ) A nerve impulse arrives at the neuromuscular junction, releasing acetylcholine. Depolarisation continues down the t-tubules, causing Ca2+ release.
2. ) Ca2+ binds to tropomyosin, changing the shape, causing to move off actin binding sites. Myosin heads bind to the exposed actin-myosin binding sites, forming a cross-bridge.
3. ) Hydrolysis of ATP releases energy, allowing myosin to ‘cock’ its head,pulling actin to create an overlap. This is muscular contraction (shortening).
4. ) ATP binds to myosin heads, causing the cross-bridge to be broken.Once the ATP is hydrolysed it can bind to another actin binding site, further down actin. Allowing contraction to continue.
5. ) When ATP and Ca2+ are depleted in the muscle contraction terminates. Actin binding sites are covered again with tropomyosin.

Question 20

how do muscles relax?

Answer 20

1. muscles enter a refractory period following contratioc
2. Achesterase hydrolyses the Ach
3. Ca2+ is actively transported back into the sarcoplasmic reticulum

Question 21

how is posture maintained?

Answer 21

1. muscles work in antagonistic pairs
2. when one muscle contrats is shortens
3. this pulls out the other muscle
4. the second muscle will reverse the movement caused by the first
5. both muscles are needed to maintain posture