Contraction Of Skeletal Muscle Flashcards
Do muscles pull or push
They can only pull
What do antagonistic Pairs do
These pairs pull in opposite directions and when one is contracted the other is relaxed
What happens to the sarcomere when a muscle contracts
The I band becomes narrower
The Z lines move closer together
The H zone becomes narrower
The A band remains the same width
Why does the A band remain the same width
As the width of this band is determined by the length of myosin filaments, it follows that the myosin filaments have not become shorter.
This discounts the theory that muscle contraction is due to filaments themselves shortening
What is a myosin
Made up of two types of proteins; a fibrous protein arranged into filament made up of several hundred molecules ( tail ). A globular protein formed init two bulbous structures at one end (the head)
What is actin.
A globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand
What is tropomyosin
Forms long thin threads that are wound around actin filaments
By what mechanism do the filaments slide past one another
The bulbous heads of the myosin filaments form cross bridges with the actin filaments.
They do this by attaching themselves to binding sites on the action filaments, and then flexing in unison pulling the actin filaments along the myosin filaments
They then become detached and, using ATP as a source of energy, return to their original angle and reattach themselves further along the actin filaments
Describe muscle stimulation
An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob
The calcium ions cause the synaptic vesicles fo fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft
Acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell surface membrane, causing it to depolarise
Describe the muscle contraction
- Sarcolemma depolarised
- Sarcoplasmic reticulum releases calcium ions, which move through T tubules
- Calcium ions bind to troponin I’m actin filaments. This causes troponin to move out of the way to reveal binding sites for the myosin head to form cross bridges
- Myosin head binds to actin to form cross links
- Myosin head changes position pulling actin over the myosin (adp is released )
- ATP is hydrolysed to reposition myosin head to allow the crossbridge cycle to begin again
Describe muscle relaxation
When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP
this reabsorption of the calcium ions allows tropomyosin to block the actin filament again
Myosin heads are now unable to bind to actin filaments and contraction ceases, that is, the muscle relaxes
In this state force from antagonistic muscles can pull actin filaments out from between myosin
What is the energy release from muscle contraction needed for
The movement of myosin heads
The reabsorption of calcium ions into the endoplasmic reticulum by active transport
Where is most atp regenerated
From adp during the respiration of pyruvate in the mitochondria, which are particularly plentiful in the muscle.
How is rapidly generating ATP anaerobically?
Particularly achieved using a chemical called phosphocreatine and partly by more glycolysis
What does phosphocreatine do
It can’t supply energy directly to the muscle, so instead it refer ages ATP, which can.
Where is phosphocreatine store and what does it act as
In the muscle
And acts as a reserve supply of phosphate, which is available immediately to combine ADP and so reform ATP.
How is the phosphocreatine store replenished
Using phosphate from atp when the muscle is relaxed
Explain how the shape of the myosin molecule is adapted to its role in muscle contraction
Myosin is made of two proteins. The fibrous protein is long and thin in shape, which enables it to combine with others to form a long thick filament along which the actin filament can move. The globular protein forms two bulbous structures at the end of a filament. This shape allows it to exactly fit recesses in the actin molecule, to which it can become attached. It’s shape also means it can be moved at an angle. This allows it to change its angle when attached to actin and so move it along, causing the muscle to contact
Trained sprinters have high levels of phosphocreatine in the muscles. Explain the advantage of this.
Phosphocreatine store the phosphate that is used to generate atp from adp in anaerobic conditions. A sprinters muscles often work so strenuously that the oxygen supply cannot meet the demand. The supply of atp from mitochondria during aerobic respiration therefore ceases. Sprinters with the most phosphocreatine have an advantage because atp can be supplied to their muscles for longer and so they perform better
Dead cells can no longer produce ATP. Soon after death, muscles contract, making the body stiff - a state known as rigor mortis. From your knowledge of muscle contraction, explain the reasons why rigor mortis occurs after death
One role of atp in muscle contraction is to attach to myosin heads, thereby causing them to detach from the actin filament and making the muscle relax. As no atp is produced after death, there is none to attach to the myosin, which therefore remains attached to actin, leaving the muscle in a contracted state
