skeletal muscle physiology - finish on ipad

Question 1

myofibril structure =

Answer 1

a long cylindrical organelle within a muscle fibre that is highly specialised for contraction

Question 2

sarcomere =

Answer 2

space between two Z-lines

Question 3

A-band =

Answer 3

part of the myofibril where thick myosin filaments are present

Question 4

I-band =

Answer 4

part of the myofibril where only thin actin filaments are present

Question 5

H-band =

Answer 5

part of the myofibril where only thick myosin filaments are present

Question 6

thin filaments =

Answer 6

are formed from actin in complex with troponin and tropomyosin

Question 7

thick filaments =

Answer 7

are formed from large numbers of myosin II molecules

Question 8

regulation of contraction:

Answer 8

Ca2+ dependent

at high calcium conc. the myosin binding site-cross bridge cycling can occur

Question 9

titin:

Answer 9

titin forms fine filaments that stabilise the myosin filament position

titin extends from Z-disk to M-line

with muscle activation calcium binds to titin and alters its stiffness = provides increased force when muscle is stretched and resists over stretching

Question 10

mechanism of muscle contraction:

Answer 10

1. action potential arrives at neuromuscular junction, causing Ca2+ to enter the motor neurone, depolarising it = acetylcholine is released
2. acetylcholine binds to sarcolemma causing Na+ diffusion, depolarising it - action potential sped up by transverse tubules
3. action potential triggers Ca2+ release from sarcoplasmic reticulum, it binds to troponin on the actin filaments
4. troponin changes conformation exposing binding sites
5. myosin heads attach to binding sites and tilt 45 degrees (powerstroke) - ADP and Pi are released
6. actin filaments are pulled passed myosin filaments (sarcomere contracts)
7. ATP attached to myosin heads are hydrolysed to ADP and Pi - this energy is used to detach myosin heads from actin filaments

Question 11

control of intracellular calcium in muscle is done by…

Answer 11

transverse tubules on the sarcolemma and sarcoplasmic reticulum

Question 12

dihydropyridine receptors =

Answer 12

located on transverse tubules - are voltage-gated channels that sense change in membrane potential during action potentials

are coupled to ryanodine receptors

Question 13

ryanodine receptors =

Answer 13

release calcium from sarcoplasmic reticulum into cytoplasm

coupled to dihydropyridine receptors

Question 14

excitation-contraction coupling:

Answer 14

1. End plate potential triggers action potential in muscle fibres
2. Action potential propagates along sarcolemma and down T-tubules
3. Depolarisation of T-tubules is sensed by dihydropyridine receptors that are couples to ryanosine receptors on sarcoplasmic reticulum causing them to open
4. Ca2+ is released into cytoplasm - initiates cross bridge cycling and contraction
5. Ca2+ is pumped back into sarcoplasmic reticulum (by sarcoplasmic + endoplasmic reticulum calcium ATPase) and this terminates cross bridge cycling

Question 15

the cross-bridge cycle:

Answer 15

1. Ca2+ binds to troponin on actin filaments - troponin changes conformation exposing binding sites
2. myosin heads attach to binding sites and tilt 45 degrees (powerstroke) - ADP and Pi are released
3. actin filaments are pulled passed myosin filaments (sarcomere contracts)
4. ATP attached to myosin heads are hydrolysed to ADP and Pi - this energy is used to detach myosin heads from actin filaments

Question 16

Muscle fibre type 1:

Answer 16

(slow)

‣ 50% of fibres in an average muscle

‣ Peak tension in 110ms (slow twitch)

Question 17

Muscle fibre type 2:

Answer 17

‣ Peak tension in 50ms (fast twitch)

‣ Type IIa = fatigue resistant (25% of fibres in an average muscle)

‣ Type IIx = fast fatigue (25% of fibres in an average muscle)

Question 18

Differences of type I and II fibres:

Answer 18

• Speed of myosin ATPase varies:
‣ Fast myosin ATPase = fast contraction cycling
‣ Slower myosin ATPase = slower contraction cycle

• sarcoplasmic reticulum:
‣ Type II fibres have more highly developed SR
‣ Faster Ca2+ release

• motor units:
‣ Type I motor units = smaller neuron
‣ Type II motor unit = larger neuron

Question 19

Role of type 1 fibre during exercise:

Answer 19

slow contracting - high aerobic endurance:

‣ Can maintain exercise for prolonged periods

‣ Require oxygen for ATP production

‣ Low intensity aero I exercise, daily activities

Question 20

Role of type 2 fibre during exercise:

Answer 20

poor aerobic endurance, fatigue quickly, produce ATP anaerobically

• Type IIa (FR = faster contracting, fatigue resistant)
‣ More force, faster fatigue than type I
‣ Short, high intensity endurance events

• Type IIx (FF = fast contracting, fast fatigue)
‣ Short, explosive sprints

Question 21

Fibre type determinants:

Answer 21

• genetic factors
◦ Determine which alpha-motor neurones innervate fibres
◦ Fibres differentiate based on alpha-motor neurone

• training factors
◦ Endurance versus strength training, de-training
◦ Can induce small changes in fibre type

• Ageing - muscles lose type II motor units

Question 22

Orderly recruitment and the size principle:

Answer 22

• recruit minimum number of motor units needed
• Recruited in same order each time ( type I - type IIa - type IIx)
• Size principle: order of recruitment of motor units directly related to side of alpha-motor neurone

Question 23

Fibre type and athletic status:

Answer 23

• endurance athletes = type I predominates
• Sprinters = type II predominates

Fibre type not sole predictor of success...
• cardiovascular function
• Motivation 
• Training habits 
• Muscle size