Muscle Organization And Contraction Flashcards
What are skeletal mucscles composed of
They’re usually anchored to the bones
Made of :
Muscle fibres
Myofibrils
Sarcomeres
What are muscle fibres
A skeletal muscle cell made of hundreds to myofibrils
What are myofibrils
Made of actin and myosin and composed of repeating contractile units called sarcomeres
What are sarcomeres
contractile unit the has a characteristic banding pattern
Make up myofibrils
What are the thick and thin filaments of sarcomeres
Thick: purple myosin
Thin: orange actin
How far does one sarcomere extend
From one z line to the other z line
All other components are inside this boundary
What is the z line of sarcomeres
Contains the proteins that are important for the sarcomeres structure and stability
What is the m line of the sarcomere
Dark line down the centre of the sarcomere
Contains anchoring protiens
What are the i bands of the sarcomere
They light stain
Contain only the thin actin filaments with the + end pointing towards the z lines
Between the a band and the z line
What is the h zone
Contains only thick filaments
Surrounds the m line
Inside the A band
What is the A band
Dark staining
Overlap of thick and thin filaments also includes the H zone
In a sarcomere each myosin thick filament is surrounded by
6 thin actin filaments
When a muscle is contracted which zones in the sarcomere decrease in length
Which don’t change
I band and h zone (this means the thin filaments slide toward the centre of the sarcomere)
The a band
Myosin 2 is non processive, what does this mean
It’s only in contact with actin for a fraction of the time
Meaning different myosin heads are acting at different times (not synchronized)
This causes the smooth sliding of the actin filaments to the centre of the sarcomere for contraction
How far does a single myosin power stroke move an actin filament
10nm (large distance)
What does the neck of the myosin filament act as
Acts as a lever to move the actin filament to the center and enhance the power stroke to move it a great distance
What is step 1 of the actin myosin contraction cycle
Myosin is attached to the actin
ATP binds to the myosin head and the myosin dissociates from the actin
What is step 2 of the actin myosin contraction cycle
The atp is hydrolyzed by the myosin head and ADP and Pi are still bound to the myosin
What is step 3 of the actin myosin contraction cycle
The hydrolysis of the atp energized the myosin so that it can bind to actin again
What is step 4 of the actin myosin contraction cycle
The Pi that was bound to the myosin leaves
This trigger a power stroke which helps the actin move towards the centre of the sarcomere
What is step 5 of the actin myosin contraction cycle
The adp still on the myosin head is released and once atp is introduced the cycle continues again
What do motor neurons do
There are muscle fibres within a motor unit
these motor units get stimulated by a single motor neuron
The motor neuron stimulates many muscle fibres/cells at the same time
What is a neuromuscular junction
The point where the motor neuron and muscle fibre contact each other
The site of transmission of the nerve impulse that stimulates the muscle cell
What gets transferred from a. Motor neuron to a muscle cell to get it stimulated?
Neurotransmitters (such as acetylcholine)
These stimulate an action potential to occur to excite the muscle cell
Once a nerve impulse has arrived at a muscles cell, what happens
Excitation contraction coupling
The nerve impulse travels through to the transverse tubules (t-tubules) that are basically membrane folds that bring the impulse to the inside of the muscle cell
It’s brought to the sarcoplasmic reticulum
What is the sarcoplasmic reticulum
The special smooth ER in muscle cells that stores Ca2+ in its lumen
The ca is pump into the lumen from the cytosol
After the action potential reaches the sarcoplasmic reticulum what happens
The presence of the action potential opens the Ca2+ channels
The ca from the sarcoplasm in the SR move into the cytoplasm out side the SR
The myofibrils are now bathing in calcium
The thin filaments contains actin and what else
Tropomyosin (rod)
Troponin (globular)
What does the presence of calcium do for muscle contraction
It controls the interaction between the myosin heads and the actin
In the absense of calcium what happens
Troponin controls the position of tropomyosin which blocks the myosin head binding sites on actin
Stops the head from binding the actin by controlling tropomyosin
This cause a resting relaxed stat of the miscle
In the presence of calcium what happens
Them the myofibrils are bathing in the newly released calcium
So the ca binds to Troponin which then moves tropomyosin so it’s not blocking the myosin head binding site anymore
The head can now bind to the actin and undergo the power stroke atp cycle to contract the muscle
