Skeletal Muscle

Question 1

What does motor unit provide

Answer 1

Interaction between spinal cord and muscle via motor neurone, sending action potentials to multiple muscle fibres. Connected with muscles via neuromuscular junctions.

Question 2

For large muscle …

Answer 2

More muscle fibres must be innervated to generate large enough force for contraction
.

Question 3

Sequence Of neuromuscular junction

Answer 3

1.motor neurones action potential arrives at axon terminal, this depolarises plasma membrane opening Ca2+ channels
2. The Ca2+ ions diffuse in the axon terminal and bind the protein.
3. The synaptic vesicles release acetylcholine and this diffuses through axon terminal, to motor end plate, binding to nicotinic receptors , opening ion channels.
4. Na and K pass through channel via electrochemical gradient. Causing depolarisation of motor end plate, and muscle fibre action potential is initated.
5. This produces a end plate propagation.

Question 4

Skeletal muscle characteristics

Answer 4

Vertical fibres where the sarcomere is stored to make contraction
Voluntary muscles
Need a lot of energy

Question 5

Afferent sensors of the muscle

Answer 5

Golgi tendon organs transmit information about muscle tension and send to spinal cord to activate agonist
Muscle spindles detect lengthening of the muscle and contracts the Agonist to prevent injury

Question 6

What is Excitation contraction coupling

Answer 6

Sequence of events which an action potential in plasma membrane activates the force generating mechanism

Question 7

Excitation contraction coupling sequence

Answer 7

1. Muscle fibre receives action potential
2. Impulse propagates via tubles and sarcoplasmic recticulum, releasing Ca2+ to help contracts muscle.
3. when the muscle is rel.axed myosin surrounding the actin filament blocks the bind that sits for mysosin to create a cross bridge.
4. When Ca2+ is released it binds to tropohin, uncovering binding sites of actin filaments , allowing a bind of a cross bridge from myosin to generate force.
5. When muscle is relaxed Ca2+ is removed from troponin and restored back into sarcoplasmic rectinulum , this requires ATP.

Question 8

Calcium level in relaxed muscles

Answer 8

In relaxed muscle: Low Ca2+ so the cross bridge cannot bind to actin because tropomyosin is covering the binding site (troponin holds tropomyosin over binding site)

Question 9

Calcium levels in active muscle

Answer 9

High Ca2+ so binds to troponin, the tropomyosin moves away from the cross bridge binding site, and so the actin binds to cross bridge.

Question 10

2 other proteins important in linking membrane action potential with Ca2+ release

Answer 10

Dihydropyridine (DHP) receptor
Ryanodine receptor

Question 11

Explain the sliding filament mechanism

Answer 11

Myosin doesn’t move, and the actin filaments slides over myosin.
Muscle shortening is a result of certain parts of actin and myosin interacting with each other, when they move one end is fixed and other moves towards Or away from the fixed point.

Question 12

Explain the cross bridge cycle

Answer 12

1. Energised myosin cross bridges on thick filaments bind to actin
2. Cross bridge binding triggers release of ATP hydrolysis products from myosin, producing myosin movement.
3. ATP bound to myosin, breaking link between actin and myosin, dissociation of cross bridge
4. ATP bound to myosin, and is split energising the myosin cross bridge
   All due to enzyme AT-Pase which determines the speed of ATP hydrolysis and results in sarcomer shortening velocity

Question 13

What is muscular tension

Answer 13

When a forces exerts on a joint when contracting this is called tension of the muscle.
If the tension exceeds load of muscle, muscle fibres can shorten to move.
If load exceeds muscle tension, then muscle fibres stay the same length

Question 14

What is load

Answer 14

Load: the force that is exerted on a muscle by an object is called the load of the muscle

Question 15

What’s a Shortening contraction

Answer 15

Shortening contraction (concentric contraction) : constant load, muscle shortens, tension > load

Question 16

What’s an isometric contraction

Answer 16

Isometric contraction : constant muscle length, free object ( load = tension) eg. a bag, fixed object ( load to tension) eg. A wall.

Question 17

Whats a Lengthening contraction

Answer 17

Lengthening contraction (eccentric contraction): muscle length increases , load > tension

Question 18

What’s a twitch

Answer 18

Mechanical response of muscle fibres to single action potential

Question 19

What’s a contraction time

Answer 19

After action potential there’s a latent period before tension of muscle increases. The time interval from the begin of tension development to peak tension

Question 20

In a shortening contraction increase load causes

Answer 20

Latent period to increase
Velocity of shortening to slow down
Total duration of the twitch to become shorter
The distance shortens to become less.

Question 21

Explain load velocity relationship

Answer 21

In absence of a load a shortening contraction reaches its max shortening velocity.
When the load increases to the point where the muscle is not able to move it, then the contraction becomes isometric.
When load increases beyond peak tension = lengthening (eccentric)

Question 22

What’s tetanus

Answer 22

When successive stimulations result in sustained contraction

Question 23

What’s summation

Answer 23

Increase of muscle tension from successive action potential

Question 24

Explain the frequency tension relationship

Answer 24

1. Stimuli are separated by more time than contraction time.
2. Stimuli introduced half way through contraction = increases tension.
3. 2 stimuli introduced close together results in high tension (temporal summation)
4. Muscle fibres have time to partially relax before next stimulation (tension oscillaties)
5. Muscle fibre has no time to relax between stimuli , creates fused tetanus

Question 25

What happens to actin and myosin when muscle is shortening

Answer 25

They overlap so sarcomere shortens and so force produced is limited

Question 26

What happens to actin and myosin When muscle is at mid length

Answer 26

When max contraction can be produced

Question 27

What happens to myosin and actin when muscle is lengthening

Answer 27

Too far part and so can’t build tension

Question 28

3 types of muscle fibre

Answer 28

IIx: fast twitch, fast glycolytic fibres IIa: intermediate fibres , fast oxidative glycolytic fibres I: slow-twitch, slow oxidative fibres

Question 29

Contraction speeds of muscle fibres

Answer 29

Slow oxidative fibres ( low AT-Pase activity, highly oxidative) Fast oxidative fibres ( high AT-Pass activity, highly oxidative and moderately glycolytic) Fast glycolytic fibres ( high AT-Pase , and highly glycolytic) Muscle contraction speed depends on rate of cross bridge cycle, which depends on rate of myosin heavy chain isoform. Shortening results in changes in I band but not the A

Question 30

How to determine fibre type

Answer 30

Colour type ( I more red - more capillaries) Fibre speed and oxidative capacity. Genomic nomenclature Muscle biopsy

Question 31

Skeletal muscle energy metabolism

Answer 31

1. Creative phosphate (CP) : provides energy very fast to form ATP from ADP but lasts only 1-2 seconds 2. Glycolysis : energy from Glucose in the absence of oxygen ( anaerobic metabolism) 3. Oxidative phosphorylation: energy form glucose fat in the presence of oxygen (aerobic metabolism) Replenishing muscle stores of glycogen and CP and removing lactic acid requires energy, so muscle uses more oxygen to produce energy needed (O2 debt)

Question 32

Mechanism of muscle fatigue ?

Answer 32

1. Conduction failure - due to potassium accumulation in tubles 2. Lactic acid build up - acidic environment in muscle affects the psychological functioning of proteins and the mechanisms involved in calcium release and re uptake. 3. Inhibition of cross bridge cycling - accumulation of ADP and P in muscle fibres slows down the cross bridge cycling by preventing the release of cross bridges from actin molecules 4. Fuel substrates- muscle glycogen, blood glucose and dehydration 5. Central command fatigue - failure to propagate signals from the brain for motor neurones.

Question 33

What’s a spike

Answer 33

Spike = record of a single action potential on muscle fibre , this continues to happen and add up (discharges) If frequency increases so does tension

Question 34

What’s hyperthrophy

Answer 34

Hypertrophy: increase in muscle fibre size , due to addition of proteins in muscle cells, when protein synthesis > protein breakdown. Depends on initial strength, duration of training and. Technique of training.

Question 35

Resistance training components

Answer 35

Resistance training components: 1. Time under tension 2. Intensity 3. Sets 4. Repetitions 5. Velocity 6. Exercise order 7. Recovery between sets 8. Frequency 9. Exercise type

Question 36

Hypertrophy vs hyperplasia

Answer 36

Hypertrophy vs hyperplasia: Hypertrophy: increase in size of muscle fibres Hyperplasia: increase in number of muscle fibres

Question 37

Anabolism vs catabolism

Answer 37

Anabolism - muscle gain, muscle protein synthesis is larger than breakdown of proteins, due to correct food and exercise Catabolism - muscle loss, muscle protein synthesis smaller than breakdown of protein, due to stress, starvation, injury

Question 38

Mechanism of hyperthrophy

Answer 38

Mechanism of hypertrophy: when train muscle fibres break so satellite cells activate and profilerate, these then agrivate to the injury sites of the muscle , and fuse to them to repair, increasing fibre size.

Question 39

Hyperthrophy differences

Answer 39

Muscle groups: upper body muscles appear to elicit greater hypertrophy with resistance training , due to habitual lowere extremities and fibre type composition. Influence of gender: women have 60-neither fibre and muscle CSA of men , absolute changes in strength and muscle mass due to resistance training greater for men , but relative change in strength/mass is similar between genders.

Question 40

Muscle adaptation depending on different stimuli

Answer 40

Training stimuli: more intense repetitions you do more builds up muscular endurance, whereas do more short but intense reps builds strength. Type of training: Strength training increases the force that a muscle can produce, and endurance maintains the force a muscle can produce. However endurance training improves fatigue threshold , and strength training does not. Motor unit adaptations: Strength training increases the amount of motor units recruited building muscle creating larger forces, and increase action potential speed ( due to increase neural activity) , where as endurance training would decrease action potential speed ( so that can last longer contracting) Whole muscle changes: strength training builds the whole cross section of the muscle , which means more muscle fibres activated to produce a large force.

Question 41

Molecular changes of muscles

Answer 41

Endurance : Increased mitochondrial mass, Increased oxidative enzymes,Decreased glycolytic enzymes, Increased slow contractile and regulatory proteins,Decrease in fast-fibre area Strength : Increases in wet mass, Increase in Fibre cross-sectional area, Increase in Protein content, Increase in RNA content