Lecture 10 - Structure And Regulation Of Skeletal Muscle Flashcards
Describe the organisation of skeletal muscle (define the two new words)
Myonuclei > Myofibrils > muscle fibre > perimysium > muscle > epimysium
Perimysium is connective tissue that wraps bundles of muscle fibres called fasicles
Epimysium is connective tissue that wraps the whole muscle
Define: A band I band Z band M band
A band - entire thick myosin filaments and overlap of thin actin filaments
I band - thin actin filaments not superimposed by myosin filaments
Z band - disk between I bands
M band - the middle of the sarcomere
What do A and I bands look like under an electron microscope and why?
A band has thick lines and so is dark, while I band has thin lines and is light. This is because A band doesn’t let polarised light in while I band does
What happens to the width of the I band during contraction and relaxation?
Contraction makes the I band thinner in width, while relaxation makes the I band thicker in width
Describe the structure of myosin
Thick filaments are formed from * numbers of ** molecules
Myosin has a alpha helix tail region and a hinge region at the bottom, and alkali light chain and regulatory light chain heads at the top
Large; myosin II
Myofilaments are **
Thin filaments are formed from * in complex with * and *
* binds * complex which causes exposure by pulling *
When * binding site exposed, a * is formed
* head and * filament bind and slide, the generated * causing *
Protein complexes;
Actin; troponin; tropomyosin
Myosin; crossbridge
Myosin; actin; force; contraction
What is Titin?
Where does it extend to/from?
What is it bound to?
What happens upon muscle activation?
What does titin provide when muscle is stretched, and what does it resist?
The 3rd filament that stabilises myosin filament position
From Z disk to M line
Bound to thick filament
Calcium binds titin and alters it’s stiffness
Provides more force but resists overstretching
What receptors are at the neuromuscular junction?
What happens upon stimulation?
Nicotinic Ach receptors
Stimulation leads to depolarisation, opening of voltage operated sodium channels and an action potential in the muscle fibre
What do the transverse tubules on the sarcolemma and sarcoplasmic reticulum do?
Control intracellular calcium and carry action potential to muscle fibre interior
Describe the structure of t tubules and the receptors surrounding them?
They are * coupled
T tubules in the middle
On the left is the dihydropyridine channels which are voltage operated and sense potential changes during action potentials
On the right is ryanodine receptors which release calcium
Mechanically
What are the five steps of the crossbridge cycle?
ATP binds to myosin head ATP hydrolysed and myosin head resting New crossbridge forms Phosphate released, causing a shape change and power stroke ADP released
What are the four steps of excitation contraction coupling?
1) ATP end plate potential arrives as action potential
2) The AP travels down sarcolemma and into the tubules
3) The T tubules depolarisation is sensed by dyhydropyridine receptors, which trigger ryanodine receptors to open, releasing calcium into the cytoplasm and causing crossbridge cycling and contraction
4) Calcium goes back into the sarcoplasmic reticulum by SERCA which stops the crossbridge cycle
What is the motor unit made up of?
Fibres and alpha motor neuron
Type 1 fibres are * twitch, make up * of fibres and are useful in * (aerobic)
Type 2 fibres are * twitch, and have two types, * which is * resistant and makes up *, and * which is * fatigue and makes up *
Slow; 50%; endurance
Fast; IIa; fatigue; 25%
IIx; fast; 25%
What are three differences between type 1 and 2 fibres in terms of:
Speed of myosin ATPase?
Sarcoplasmic reticulum?
Motor units?
Speed varies - faster means bigger contraction
Type 2 has a more developed SR which increases calcium release
Type 1 has less motor units (<300) which Type 2 has more
What does type 1 need to make ATP?
What about type 2? What is IIa more suited to, and what is IIx more suited to?
Oxygen, to make ATP from fat and carbs
Type 2 is anaerobic and so doesn’t need oxygen.
IIa provides more force e.g in 1600m runs, and IIx is for bursts
What are three determinants of ratio/development of type one and type two fibres?
Genetics (which alpha motor neurons innervated what)
Training
Age (less type 2)
Is the minimum or maximum number of muscle fibres recruited?
What is the order related to? And what is the order?
What is force of contraction dependent on?
Minimum number of fibres
Related to size of alpha motor neuron; order is type 1 (smallest), then type IIa then IIx (largest)
Dependent on AP firing rate
Define incomplete tetanus
What does incomplete tetanus look like? Give Y axis title
When muscle fibers are stimulated at a rate where they don’t completely relax before the next stimulus
Looks like a staircase, with force on the Y axis
What is the calcium sensor on actin?
Troponin
Calcium is released from **
** release is coupled to * at *
Amount of force generated is influence by **, * of ** and * of **
Sarcoplasmic reticulum
Sarcoplasmic reticulum; depolarisation; sarcolemma
AP frequency; recruitment of motor units; number of crossbridges cycling
Type 1 is slow *, type IIa is fast / and type IIx is fast *
Oxidative; oxidative/glycolytic; glycolytic
