skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
what does the phrase “antagonistic pair of muscles” mean
- 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
define ligaments
attach bones to bones
define tendons
attach bones to muscles
what do agonist muscles do
contract (pull)
what do antagonist muscles do
relax (push)
name the 3 types of muscle in the body and where they are located
- cardiac - heart (contract with no conscious control)
- smooth - walls of blood vessels and intestines (contract with no conscious control)
- skeletal - attached to incompressible skeleton by tendons
describe the gross structure of skeletal muscles
- 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)
describe the microscopic structure of a skeletal muscle
- 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
describe the ultrastructure of a myofibril
- Z-line: boundary between sarcomes
- I-band: only actin (light colour on electron micrograph)
- A-band: overlap of actin and myosin (dark colour on electron micrograph)
- H-zone: only myosin
how do you differentiate between A-band and I-band on an electron micrograph
- I-band: thin and light
- A-band: thick and dark
what is the function of tropomyosin
blocking/inhibiting the actin-myosin binding site
describe a relaxed muscle
- the actin-myosin binding site is blocked by tropomyosin
- this prevents an actinomyosin bridge
describe a contracted sarcomere
- 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
describe the role of calcium ions and ATP in myofibril contraction (sliding filament theory)
- 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
explain how muscle contraction is stimulated
- 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
explain the role of Ca2+ in muscle contraction
- 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
outline the role of ATP and phosphocreatine in muscle contraction
- 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)
- anaerobic respiration (glycolysis)
- 2x ATP made by glycosis
- pyruvate is converted into lactate (causes muscle fatigue)
- short duration, high intensity
- aerobic respiration
- lots of atp mostly by oxidative phosphorylation
- slow, requires many reactions but no harmful waste products
where are slow and fast-twitch muscle fibres found in the body
- slow twitch: sites of sustained contraction e.g calf muscle
- fast twitch: sites of short-term, rapid powerful contraction e.g biceps
explain the role of slow and fast-twitch muscle fibres
- 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
explain the structure and properties of slow-twitch muscle fibres
- 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
