Skeletal muscles Flashcards
what is another term for skeletal muscle
- striped/striated muscle
describe the structure of muscle fibres
- cylindrical in shape and enclosed by a sarcolemma
- possess many nuclei (multinucleate)
- possess many protein strands/myofibrils
- arranged parallel to each other
describe the fine/ultrastructure of the muscle fibre
- z-line connects actin filament
- sarcomere is length between each z-line
- A-band (dark band) is the length of myosin filaments
- h-zone is centre of A-band and only contains myosin
- I-band (light band) is outer region of A-band and only contains actin
- m-line connects myosin filaments in A-band
describe how the muscle fibre structure changes during contraction?
- H-zone narrows
- I-band narrows
- outer region of A-band becomes wider
- A-band remains same size
- length of each sarcomere decreases (z-lines closer)
describe the mechanism of muscle contraction
- tropomyosin covers the binding site on the actin filaments
- calcium ions are released from the sarcoplasmic reticulum when the muscle fibre is stimulated
- calcium ions binds to tropomyosin causing it to change shape and move from binding site
- this allows myosin heads to bind to the actin filaments forming actinomyosin crossbridges
- calcium ions stimulates ATP hydrolase which hydrolyses ATP for the release of energy to enable the actin filament to be pulled and to detach the myosin head breaking the actinomyosin bridges
what happens to the calcium ions after muscle contraction
- they are actively transported back into the sarcoplasmic reticulum which requires energy from the hydrolysis of ATP
describe the transmission of nerve impulse across a neuromuscular junction
- arrival of action potential at synaptic knob causes depolarisation of presynaptic membrane which stimulates calcium channels to open and calcium ions to diffuse into the synaptic knob
- calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) which diffuses across the synaptic cleft
-acetylcholine attaches to specific protein receptor sites on the sarcolemma stimulating the entry of sodium ions causing depolarisation - this results in an action potential along the sarcolemma and into the muscle fibre stimulating the release of calcium ions which initiates muscle contraction
- acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming acetyl and choline which are both taken up to the synaptic knob by active transport and acetylcholine is re-synthesised
- active transport and re-synthesis require ATP yielded from mitochondria
describe the differences between slow and fast muscle fibres
- slow contract slowly, fast contracts quickly
- slow has many mitochondria, fast have few mitochondria
- slow is red due to presence of myoglobin (stores oxygen), fast is whitish due to minimal amount of myoglobin
- slow has low conc of phosphocreatine, fast has high conc of phosphocreatine
- slow has low conc of glycogen as lots of energy released per glucose, fast has high conc of glycogen as less energy released per glucose
what is ATP required for in muscles
to provide energy for:
- the swivel movement of myosin head to pull the actin filament
- the detachment of myosin head breaking the actinomyosin bridges
- the reabsorption of calcium ions into the sarcoplasmic reticulum via active transport
what are the three sources of ATP
- ATP-Phosphocreatine system
- Anaerobic Respiration
- Aerobic Respiration
describe the ATP-Phosphocreatine system
- ADP reacts with phosphate donated by phosphocreatine to form ATP and creatine
- produces ATP very quickly but phosphocreatine is used up very quickly
- phosphocreatine is reformed with a using phosphate from ATP when the muscle is relaxed
what type of muscle fibre is the ATP-Phosphocreatine system particularly important and why
- fast muscle fibres in order to help produce ATP quickly in anaerobic conditions
