15.8 Contraction of skeletal muscle Flashcards
Describe movement of skeletal muscle
Antagonistic pairs
- Muscle can’t push it can only pull
- So to move a limb, antagonistic pairs of muscle are needed so one contracts to move it in one direction whilst the other relaxes.
Vice versa in the opposite direction
So antagonistic pairs pull in opposite directions and when one is contracted the other is relaxed
What is the sliding filament mechanism
This shows how arrangement of various proteins in muscle brings about a contraction in muscle fibre
Process involves actin and myosin filaments sliding past one another
How does appearance of sarcomere change in a contracted muscle as opposed to a relaxed one
- When contracting, more actin and myosin overlap so you will see darker myofibrils here.
- The I band becomes narrower ( this is where it is just actin)
- The Z lines move closer together
( these are at the centre of each I band) - The H zone becomes narrower
How does A band change between a contracted and relaxed muscle
It stays the same width, as its width is determined by the length of myosin filaments
These filaments don’t become shorter themselves
What two types of protein is myosin made of
- The tail is made of a fibrous protein arranged into a filament made of 100s of molecules
- A globular protein is formed into two bulbous structures at one end (the head)
What is actin made of
Where is tropomyosin
Actin is a globular protein whose molecules are arranged into long chains that are twisted around each other to form helical strand
Tropomyosin forms long thin threads that weave around actin filaments
How does sliding filament muscle contraction work
Vague overview
- Bulbous heads of myosin form cross bridges with actin filaments.
They attach to binding site of actin filament and then all flex in unison to pull the actin filament along myosin filaments
They then detach and use ATP as a source of energy to return to original angle and they can reattach further along actin filaments and repeat.
How are muscles stimulated
- An action potential reaches many neuromuscular junctions simultaneously
- This causes calcium protein channels to open so calcium ions diffuse into synaptic knob
- These calcium ions cause synaptic vesicles to fuse with presynaptic membrane and release acetylcholine into synaptic cleft
- Acetylcholine diffuses across synaptic cleft and binds with receptors on muscle cell membrane which causes it to depolarise
Once a muscle has been stimulated, how does it contract
- Action potential travels deep into muscle fibre through a system of T tubules that are an extension of cell membrane
- These branch throughout the sarcoplasm (cytoplasm of muscle)
- The tubules contact ER of muscle called sarcoplasmic reticulum. This has been actively transporting calcium ions from sarcoplasm leading to a very low concentration of Ca2+ ions in sarcoplasm.
- This action potential opens calcium protein channels so calcium ions diffuse back into sarcoplasm down conc gradient.
- The calcium ions cause tropomyosin molecules that were blocking binding sites on actin filaments to pull away
- There are already ADP molecules attached to myosin heads so they are prepared to bind to actin filament and form a cross bridge
- Once attached to actin filament, the myosin heads change their angle and pull the actin filaments along as they do so, and release a molecule of ADP
- An ATP molecule then attaches to each myosin head and causes it to detach from actin filament
- Calcium ions then activate enzyme ATPase which hydrolyses ATP to ADP which provides energy to make myosin head return to its original position
- Then process can repeat further along the actin filament, as myosin head with ADP can attach further along the chain.
How does this contracting decrease length of sarcomere
Myosin molecules are joined tail to tail so are moving actin filaments in opposite directions from the other set.
This means the actin filaments moving in opposite directions pull them closer together which shortens the distance between two adjacent Z lines.
How does a muscle relax
- When nervous stimulation stops, calcium ions are actively transported back into endoplasmic reticulum using energy from hydrolysis of ATP
- This causes tropomyosin to move back and block binding sites of actin filaments
- Myosin heads are now unable to bind to actin filaments and contraction ceases
- So actin fibrils are pulled out from between myosin by antagonistic muscles
What is energy from ATP to ADP required for
Loads of mitochondria in sarcoplasm of muscles!
- Returning myosin heads to original angle
- Reabsorbing calcium ions into the endoplasmic reticulum by active transport
Rapidly generating ATP anaerobically is required in a very active muscle.
How does this occur
Phosphocreatine can’t supply energy directly to the muscle so instead it can regenerate ATP
It is stored in muscles and acts as a reserve supply of phosphate to combine with ADP
This phosphocreatine store is replenished when muscle is relaxed
