Lecture 9-Skeletal muscle- cross bridge and muscle recruitment Flashcards
Describe the basic structure of the Muscle cell, including myofilaments, myofibrils, thin filament, thick filament, sarcomere,
Muscle cell ( muscle fibre)
inside that, we have multiple myofibrils. they are like cylinders.
Then Inside those, we have bundles of myofilaments.
Then we have the sarcomere, which is a single contraction unit of the myofilament.
The sarcomere gets its striped appearance from ACTIN( thin filament) and Myson( thick filament).
Describe the attached state
The muscle has just been contacted and is getting ready for the next contraction.
Myofilament has just finished a Powerstroke. Actin is being pulled by Myosin, and CROSS BRIDGES between actin and myosin ARE STILL PRESENT.
Describe the detached state
A molecule of ATP binds to the myosin head, in preparation for releasing some energy(ATP releases energy) to prime the myosin head.
The BINDING of ATP causes myosin to release ACTIN, NO MORE CROSS BRIDGES.
Describe the Cocked state
The Myosin head burns ATP to make energy( and some ADP and Phosphate as waste).
It stores the energy by CHANGING THE SHAPE OF THE MYOSIN, getting it ready to pull on actin again.
Describe the Cross Bridge state
IF CALCIUM is PRESENT and bound to myofilament, then it’s time for the next contraction. Remember that the calcium would be there if EC coupling had occurred. This is how EC coupling and cross bridge cycle are linked.
The energised myosin head ( changed shape) can now be bound to actin again forming a CROSS BRIDGE.
Describe the Powerstroke state
We now have a CROSS BRIDGE with an energised myosin head attached to actin.
The myosin uses its stored energy to pull, causing the actin filaments to SLIDE ( overlapped now).
Thus the sarcomere shortens, and contraction has occurred.
The sarcomere must shorten for contraction to occur but the thick and thin filaments will SLIDE not shorten.
What 2 things does muscle force/tension depend on?
- The number of muscle fibres recruited
- The rate at which the muscle is stimulated
Describe and explain Recruitment
The different nerves are communicated with many different fibres. if we needed to pick something heavier, we would engage more nerves, so thus we have more muscle fibres activated.
SLIDE:
A small number of nuerons would produce less force from the muscle, the a mount of force would increase as more nuerons are activated. THE PROCESS OF ACTIVATING MORE MUSCLE FIBRES IS CALLED RECRUITMENT.
Describe and explain the frequency of stimulation, including definitions of terms like twitch and tetanus.
A single action potential will result in a pulse of ca release into the cytoplasm, A short period of tension is called a TWITCH.
Twitch summation:
Many action potentials fired in a rapid sequence result in a sustained release of ca from the SR( ca keeps being released), this means SUSTAINED ACTIN-MYOSIN INTERACTION, and thus a sustained period of contraction.
Eventually, we reach maximal signalling and contraction capability of the muscle, at which point the force will be plateu. This is called TETANUS.
Explanation: signals coming through are nerves are called action potentials.
TWITCH SUMMATION
If the brain wants to pick something heavier, we can increase the frequency of the action potentials. Before we even get to relaxation( when SERCA pumps CA back to SR), we get another single straight away, causing a twitch summation(STRONGER CONTACTION). Release of ca keeps going
MORE calcium, more cross bridges, more pulling power
TETANUS:
increased action potentials, ca keeps going out, stronger and stronger contraction as more cross bridges form until we hit PLATEU, NO MORE CA this is called tetanus. Tetanus is the point where we have reached the maximum contraction point.
Describe and explain the length and tension relationship.
At the resting sarcomere, the muscles are at optimal length, we can do a powerful contraction.
But if we change the length of muscles, we change the length of Myofilaments( SACROMERE).
Each muscle has an optimal length where it will be the strongest, and when either longer or shorter it will weaken.
If there’s too much overlap between actin and myosin due to the shortening of muscle length, we can make lots of cross bridges BUT there’s no space left to contract.
If a muscle has stretched, it means the sarcomere has stretched. This means actin gets pulled away from myosin, with no cross-bridges but lots of space to contract.
Fast fibres vs slow fibres
Fast Fibers, really big diameters, their job is to contact powerful and fast. eg lifting a bar over head.
-Few capillary supply
-few mitochondria so white colour (the colour comes from Mitochondria.
fatigue resistance: low, will get tired fast
Time to peak tension( max force): fast
Slow fibres: eg standing
Slow fibres have white spaces, this is where the blood vessels go, Slow fibres want lots of blood comin to them, they don’t get tired easily. They are constantly making lots of energy.
-small diameter
-many capillary supply
-many mitochondria, so RED.
-fatigue resistance-high( can stand for long)
-Time to peak tension, slow
