Contraction of skeletal muscle 2 Flashcards
What happens when an action potential reaches a neuromuscular junction?
Calcium ion protein channels, open allowing calcium to diffuse into the synaptic knob
What does the influx of calcium ions cause?
It triggers synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
How does acetylcholine affect the muscle fibre?
It diffuses across the synaptic cleft and binds to receptors on the muscle cell membrane
What happens when acetylcholine binds to its receptors?
It depolarises the muscle fibre membrane, initiating muscle contraction
How does the action potential travel through the muscle fibre?
The action potential travels deep into the muscle fibre through T-tubules, which are extensions of the cell surface membrane
What is the role of the T- tubules in muscle contraction?
They branch throughout the sarcoplasm and are in contact with the sarcoplasmic reticulum, helping to transmit the action potential into the muscle
What does the sarcoplasmic reticulum do with calcium ions?
It actively transports calcium ions out of the muscle cytoplasm, maintaining a low calcium ion concentration in the cytoplasm
How does the action potential affect the sarcoplasmic reticulum?
It opens calcium ions channels on the sarcoplasmic reticulum, causing calcium ions to diffuse into the cytoplasm
What role does ADP play in the myosin head during contraction?
ADP molecules attached to the myosin head allow it to bind to the actin filament, forming a cross-bridge
What does the influx of calcium ions do to the tropomyosin?
Calcium ions cause the tropomyosin molecules to move, exposing the binding sites on the actin filament
What happens when the myosin head attaches to the actin filament?
The myosin head changes its angle, pulling the actin filament along and releasing ADP
What happens after the myosin head has pulled the actin filament along?
ATP attaches to the myosin head, causing it to detach from the actin filament
What does ATP hydrolysis do during muscle contraction?
ATPase activates and hydrolyses ATP into ADP, providing the energy for the myosin head to return to its original position
What happens after the myosin head returns to its original position?
The myosin head, now with ADP attached, reaches further along the actin filament, and the cycle repeats as long as calcium ions concentration remains high
How do myosin heads move during muscle contraction?
As myosin molecules are aligned in two oppositely facing sets, the movement of one set of myosin heads is in the opposite direction to the other set
What is the result of the actin filaments moving in opposite directions?
The actin filaments are pulled towards each other, shortening the distance between the Z-lines, resulting in muscle contraction
What is the overall effect of repeated muscle contractions?
Simultaneous contraction of myofibrils shortens the muscle, leading to movement of body parts
What happens when nervous stimulation ceases?
Calcium ions are actively transported back into the sarcoplasmic reticulum using ATP
What does the reabsorption of calcium ions allow?
It allows tropomyosin to block the actin bind sites again
What happens after tropomyosin blocks the actin binding sites?
The myosin heads can no longer bind to actin, and contraction ceases, leading to muscle contraction
What can antagonistic muscles do when the muscle relaxes?
They can pull the actin filaments out from between the myosin filaments (to a certain extent)
What is the main energy source during muscle contraction?
The hydrolysis of ATP to ADP and inorganic phosphate provides energy for muscle contraction
What are the key processes that require energy from ATP during muscle contraction?
- The movement of myosin heads
- The reabsorption of calcium ions into the sarcoplasmic reticulum by active transport
How is ATP primarily regenerated in muscle cells?
ATP is mainly regenerated by respiration of pyruvate in the mitochondria, which are abundant in muscle cells
What happens when oxygen supply cannot meet the demand for ATP in very active muscles?
Muscles use anaerobic pathways to generate ATP, including the use of phosphocreatine and glycolysis
What role does phosphocreatine play in energy supply during muscle contraction?
Phosphocreatine regenerates ATP by donating its phosphate group to ADP, providing a rapid source of energy
Where is phosphocreatine stored, and how is it replenished?
Phosphocreatine is stored in the muscle and is replenished by phosphate from ATP when the muscle is relaxed
