Flashcards in Contraction Of Skeletal Muscles Deck (21):
What is the sliding filament mechanism?
The sliding of actin and myosin filament past each other
If the sliding filament mechanism is correct then what must be observable?
There will be more overlap of actin and myosin in a contracted muscle than in a relaxed one
When a muscle contracts, what changes occur to the sarcomere?
The I-bands and H-bands become narrower and the Z-lines move closer together, or in other words, the sarcomere shortens
Describe the structure of myosin
Myosin is made up of two types of protein - a fibrous protein arranged into a filament and a globular protein formed into two bulbous structures at one end
Describe the structure of actin
Actin is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand
Describe the structure of tropomyosin
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 by attaching themselves to the binding sites on the actin filaments, and then flexing in unison, pulling the actin filaments along the myosin filaments
How do the myosin heads and the actin filament become detached?
Using energy from ATP
What occurs during muscle stimulation in the sliding filament mechanism?
An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion channels to open and calcium ions to move into the synaptic knob and cause the synaptic vesicles to fuse with the postsynaptic membrane and release their acetylcholine into the synaptic cleft and bind with receptors on the postsynaptic membrane
How does the action potential travel deep into the fibre?
It moves through a system of tubules that branch throughout the cytoplasm of the muscle
What are the tubules in contact with?
The endoplasmic reticulum of the muscle which has actively absorbed calcium ions from the cytoplasm of the muscle
What does the action potential do to the calcium ion channels on the endoplasmic reticulum? And what does this cause to happen?
It causes them to open and the calcium ions flood into the muscle cytoplasm down a diffusion gradient
What do the calcium ions do to the tropomyosin?
They cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away
What allows the myosin heads to bind to the actin filament?
There is an ADP molecule attached to the myosin head that allows a cross bridge to form
What do the myosin heads do once they attach to the actin filament that releases the ADP molecule?
They change their angle, pulling the actin filament along as they do so and releasing the ADP molecule
What attaches to each myosin head to, causing it to become detached from the actin filament?
An ATP molecule
What activates the the enzyme ATPase, which hydrolyses the ATP to ADP and what does this provide the energy for?
The calcium ions and the hydrolysis of ATP to ADP provides the energy for the myosin head to return to its original position
In order for the cycle to repeat, what must continue?
Nervous stimulation of the muscle
When muscle stimulation ceases, what happens to the calcium ions?
They are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP
What does the reabsorption of the calcium ions allow the tropomyosin to do?
Block the actin filament again