skeletal muscles are stimulated to contract by nerves and act as effectors

Question 1

what does the phrase “antagonistic pair of muscles” mean

Answer 1

• muscles can only pull so they work in pairs to move bones around joints
• pairs pull in opposite directions: agonist contracts while antagonist is relaxed

Question 2

define ligaments

Answer 2

attach bones to bones

Question 3

define tendons

Answer 3

attach bones to muscles

Question 4

what do agonist muscles do

Answer 4

contract (pull)

Question 5

what do antagonist muscles do

Answer 5

relax (push)

Question 6

name the 3 types of muscle in the body and where they are located

Answer 6

1. cardiac - heart (contract with no conscious control)
2. smooth - walls of blood vessels and intestines (contract with no conscious control)
3. skeletal - attached to incompressible skeleton by tendons

Question 7

describe the gross structure of skeletal muscles

Answer 7

• muscle cells are fused together to form bundles of parallel muscle fibres called myofibrils
• muscle fibres are multinucleate
• arrangement ensures there is no point of weakness between cells
• each myofibril is surrounded by an endomycium (loose connective tissue with many capillaries)

Question 8

describe the microscopic structure of a skeletal muscle

Answer 8

• myofibrils: site of contraction
• sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria (for respiration to provide ATP) and endoplasmic reticulum
• sarcolemma: folds inwards towards sarcoplasm to form transverse (t) tubules

Question 9

describe the ultrastructure of a myofibril

Answer 9

1. Z-line: boundary between sarcomes
2. I-band: only actin (light colour on electron micrograph)
3. A-band: overlap of actin and myosin (dark colour on electron micrograph)
4. H-zone: only myosin

Question 10

how do you differentiate between A-band and I-band on an electron micrograph

Answer 10

• I-band: thin and light
• A-band: thick and dark

Question 11

what is the function of tropomyosin

Answer 11

blocking/inhibiting the actin-myosin binding site

Question 12

describe a relaxed muscle

Answer 12

• the actin-myosin binding site is blocked by tropomyosin
• this prevents an actinomyosin bridge

Question 13

describe a contracted sarcomere

Answer 13

• myosin and actin filaments slide over one another to make the sarcomeres contract
• sarcomere becomes shorter
• I-band becomes shorter
• H-zone becomes shorter
• a-band stays the same

Question 14

describe the role of calcium ions and ATP in myofibril contraction (sliding filament theory)

Answer 14

• calcium ions cause the tropomyosin to move out of the actin-myosin binding site
• allowing the actinomyosin bridge to form
• calcium ions activate ATPase (catalyses hydrolysis of ATP)
• atp is used to change the shape of the myosin head (power stroke)
• continues as long as the binding site is open (calcium ions = action potential)
• atp is used to detach the myosin head from the binding site
• atp is used to return the myosin head to resting (original) position
• atp is needed to reabsorbed calcium ions via active transport against concentration gradient

Question 15

explain how muscle contraction is stimulated

Answer 15

• in neuromuscular junction action potential occurs opening voltage-gated calcium channels
• vesicles move towards and fuse with presynaptic membrane
• exocytosis of acetylcholine which diffuses across synaptic cleft
• acetylcholine binds to receptors on Na+ channel protein on skeletal muscle cell membrane
• influx of Na+ causes depolarisation

Question 16

explain the role of Ca2+ in muscle contraction

Answer 16

• action potential moves through t-tubules in the sarcoplasm, ca2+ channels in sarcoplasmic reticulum open
• ca2+ binds to troponin, triggering conformational change in tropomyosin
• exposes binding sites on actin filaments so actinomyosin bridges can form

Question 17

outline the role of ATP and phosphocreatine in muscle contraction

Answer 17

1. phosphocreatine
   
   • phosphate group is from phosphocreatine is removed and added to adp to synthesise atp
   • cells store phosphocreatine, it is a short and simple reaction so fastest way to create atp
   • phosophocreatine is used up quickly so is used for high intensity and short duration
   • anaerobic process
   • alactic process (no lactic acid produced)
2. anaerobic respiration (glycolysis)
   
   • 2x ATP made by glycosis
   • pyruvate is converted into lactate (causes muscle fatigue)
   • short duration, high intensity
3. aerobic respiration
   
   • lots of atp mostly by oxidative phosphorylation
   • slow, requires many reactions but no harmful waste products

Question 18

where are slow and fast-twitch muscle fibres found in the body

Answer 18

• slow twitch: sites of sustained contraction e.g calf muscle
• fast twitch: sites of short-term, rapid powerful contraction e.g biceps

Question 19

explain the role of slow and fast-twitch muscle fibres

Answer 19

• slow-twitch: long-duration contraction, well-adapted to aerobic respiration to prevent lactate buildup
• fast-twitch: powerful short-term contraction, well-adapted to anaerobic respiration

Question 20

explain the structure and properties of slow-twitch muscle fibres

Answer 20

• glycogen store: many terminal ends can be hydrolysed to release glucose for respiration
• contain myoglobin: higher affinity for oxygen than haemoglobin at lower partial pressures
• many mitochondria: aerobic respiration produces more atp
• surrounded by many blood vessels: high supply of oxygen and glucose
• darker in colour