Skeletal Muscles (Part 1)

Question 1

what proportion of human body weight is made up by muscle

Answer 1

40%

Question 2

state what it is meant by the key term - excitability

Answer 2

the ability of muscle tissue to be electrically stimulated

Question 3

state what it is meant by the key term - extensibility

Answer 3

the ability of muscle to be stretched it’s resting length (passive)

Question 4

state what it is meant by the key term - elasticity

Answer 4

the ability of a muscle to return to it’s resting length (passive)

Question 5

state what it is meant by the key term - contractility

Answer 5

the ability of a muscle to actively generate tension

Question 6

state 3 facts about cardiac muscle

Answer 6

1. actin and myosin form cross bridges - sliding filament mechanism
2. node cells (pacemaker cells) produce spontaneous AP’s - autorhythmicity
3. electrical coupling between cells

Question 7

what is the refractory period time in cardiac cells? and what is the reason for this?

Answer 7

refractory period of 250 ms to prevent tetanic contractions

Question 8

state 4 facts about smooth muscle

Answer 8

1. actin and myosin form cross bridges - sliding filament mechanism
2. surrounds cell structures
3. changes in lengths changes the shape of cells
4. controlled by the amount of Ca2+ in the cells
5. Ca2+ release controlled by the autonomic NS
6. spontaneous AP’s or drifting of polarity in some smooth muscle cells

Question 9

state 3 facts about skeletal muscle

Answer 9

1. changes in skeletal muscle moves skeleton
2. actin and myosin form cross bridges - sliding filament theory
3. controlled by motor neurones (voluntary and reflex)

Question 10

how does skeletal muscle create and control movement? (2 things)

Answer 10

1. skeletal muscle generates force to act on the skeleton

2. skeletal muscle changes length whilst generating force to move and control the skeleton

Question 11

state 3 characteristics of skeletal muscle fibres

Answer 11

1. multinucleate
2. contains many mitochondria
3. has transverse tubules (t tubules)
4. myofibrils and sarcomeres
5. sarcolemma, sarcoplasm, sarcoplasmic reticulum (SR)

Question 12

state what it is meant by the key term - myofibrils

Answer 12

myofibrils are the structures that give skeletal and cardiac muscle their characterised striated appearance

Question 13

state the two types of myofibrils

Answer 13

1. actin (thin filament)

2. myosin (thick filament)

Question 14

state what it is meant by the key term - sliding filament theory

Answer 14

muscle force and length change is generated buy the overlapping and interaction of actin and myosin filaments

Question 15

which two people came up with the sliding filament theory?

Answer 15

1. Andrew Huxley

2. Hugh Huxley

Question 16

state 3 facts about actin

Answer 16

1. contractile protein
2. structure is like a string of pearls on a string and then the strand of pearls are twisted together
3. each actin has a binding site for myosin

Question 17

state 2 facts about tropomyosin

Answer 17

1. regulatory protein

2. overlaps binding sites for myosin and inhibits interaction when the muscle is in it’s relaxed sate

Question 18

state 3 facts about troponin

Answer 18

1. regulatory protein
2. troponin binds to Ca2+ reversibly and once bound change the conformation to pull tropomyosin away from the myosin binding site
3. Ca2+ binding to troponin regulates skeletal muscle contraction because it moves the tropomyosin away and allow myosin to interact with the actin

Question 19

state 2 facts about the sarcoplasmic reticulum (SR)

Answer 19

1. homologous to ER
2. stores and releases Ca2+
3. T tubules connected to SR via junctions
4. T tubule protein is a modified voltage gated Ca2+ channel known as dihydropyridine

Question 20

explain the 6 steps of the excitation-contraction coupling reaction

Answer 20

1. muscle AP propagated into T tubules
2. Ca2+ released from lateral sac of SR
3. Ca2+ binding to troponin removes blocking action of tropomyosin
4. actin-myosin cross bridges form
5. AP ends, Ca2+ re-uptake by SR
6. Ca2+ removal from troponin restores tropomyosin binding action

Question 21

state 3 things that occur when actin-myosin cross bridges form (expanding upon step 4 in the excitation-contraction coupling reaction)

Answer 21

1. cross bridges move through power stroke sliding actin past myosin
2. ATP cause release of myosin head and return to original state
3. if Ca2+ still present, myosin head will attach again to new binding site

Question 22

state what it is meant by the key term - motor unit

Answer 22

a motor unit is a motor neurone plus all the muscle fibres it innervates

Question 23

state 2 additional facts about motor units

Answer 23

1. one motor unit can innervate many muscle fibres, but one muscle fibre can only be innervated by one motor unit
2. within a whole muscle there are many motor units

Question 24

state what it is meant by the key term - neuromuscular junction

Answer 24

the neuromuscular junction is where the axon terminals establish synaptic contact with the motor end plate

Question 25

state what it is meant by the key term - motor end plate

Answer 25

the region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon is known as the motor end plate

Question 26

state the 8 steps of a neuromuscular junction

Answer 26

1. AP propagates down motor neurone to axon terminals 2. causes voltage-gated Ca2+ ion channels to open 3. inc^ fluidity of ACh containing vesicles + causes them to fuse with membrane + release ACh into synapse 4. ACh diffuses across synapse + binds to muscarinic receptors receptors which open Na+ channels 5. Na+ entry cross motor end plate - depolarisation 6. current spreads down sarcolemma 7. muscle fibre AP propagated 8. AP propagates along sarcolemma

Question 27

state 3 facts about neuromuscular junctions

Answer 27

1. are all excitatory 2. 'all or nothing' principle 3. contain acetylcholinesterase

Question 28

state what it is meant by the key term - twitch contraction

Answer 28

the mechanical response to a single action potential

Question 29

state the 3 phases of a twitch contraction (in order)

Answer 29

1. latent period 2. contraction time 3. relaxation phase

Question 30

state what it is meant by the key term - latent period (of a twitch contraction)

Answer 30

the period of time from the action potential to the onset of contraction. the time delay is due to the excitation-contraction coupling reaction

Question 31

state what it is meant by the key term - contraction phase (of a twitch contraction)

Answer 31

this is the time when tension is developing due to cross-bridge cycling

Question 32

state what it is meant by the key term - relaxation phase (of a twitch contraction)

Answer 32

this is the time where tension is decreasing (i.e., relaxing) and is longer than the contraction phase. this is due to the time it takes to get all the Ca2+ sequestered

Question 33

state 3 facts about tetanic contractions

Answer 33

1. multiple AP's result in summation of tension/force 2. if frequency is high, tetanic contraction occur when force remains constant(ish) for the period of activation 3. typically a muscle force in functional activities

Question 34

state what it is meant by the key term - tetanic contraction

Answer 34

is a sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate

Question 35

skeletal muscle fibres do not have the same mechanical and metabolic characteristics. state the two characteristics muscle fibres are determined by

Answer 35

1. the maximum shortening velocities (fast or slow) | 2. the major pathway they use to re-synthesise ATP

Question 36

state what it is meant by the key term - slow oxidative (SO) fibres

Answer 36

SO fibres combine low myosin ATPase activity with high oxidative activity

Question 37

state what it is meant by the key term - fast oxidative glycolytic (FOG) fibres

Answer 37

FOG fibres combine high myosin ATPase activity with high oxidative capacity and intermediate glycolytic capacity

Question 38

state what it is meant by the key term - fast glycolytic (FG) fibres

Answer 38

FG fibres combine high myosin ATPase activity with a high glycolytic capacity

Question 39

state what it is meant by the key term - myosin isoforms

Answer 39

Some isoforms have specialised functions in certain cell types (such as muscle), while other isoforms are ubiquitous

Question 40

state the 3 types of myosin isoforms

Answer 40

1. myosin heavy chain 1 2. myosin heavy chain 2a 3. myosin heavy chain 2x

Question 41

what is the function of the myosin isoforms ?

Answer 41

the type of myosin isoform determines the maximal rate of cross-bridge cycling, and thus, the maximal shortening velocities

Question 42

state what it is meant by the key term - electromyography

Answer 42

the recording of the electrical activity of muscle tissue, or its representation as a visual display or audible signal, using electrodes attached to the skin or inserted into the muscle

Question 43

what do oxidative fibres depend on ?

Answer 43

blood flow - the delivery of oxygen and nutrients