L25 - skeletal muscle

Question 1

what is skeletal muscle?

Answer 1

• voluntary, conscious
• attached to bone

Question 2

key characteristics of skeletal muscle

Answer 2

• made of fibres
• involved in posture and movement

Question 3

what are tendons?

Answer 3

bundles of collagen fibres that attach muscle to bone

Question 4

what is skeletal muscle made of?

Answer 4

bundles of multinucleate muscle cells

Question 5

what is a muscle fibre?

Answer 5

single muscle cell

Question 6

what is a myofibril?

Answer 6

each muscle cell formed from bundles of actin and myosin filaments which are organised into myofibrils

Question 7

describe arrangement of actin filaments around a myosin filament

Answer 7

6 actin around 1 myosin

Question 8

what is the sarcoplasmic reticulum?

Answer 8

• like endoplasmic reticulum
• acts as a Ca2+ store

Question 9

structure of skeletal muscle

Answer 9

striated pattern

Question 10

Z-DISC

Answer 10

• borders of sarcomeres
• smallest functional units
• core made of actin
• alpha actions bind and cross like the ends of F-actin from sarcomeres at z line
• alpha actinin-2 in all muscle fibres

Question 11

what is the thin filament made up of?

Answer 11

actin, tropomyosin, troponin

Question 12

M-Band

Answer 12

myomesin

Question 13

what is the thick filament made up of?

Answer 13

myosin and titin

Question 14

A band

Answer 14

• dark band
• myosin
• centre of sarcomere

Question 15

I band

Answer 15

• lighter band
• only contains thin filaments
• actin

Question 16

M line/disc

Answer 16

proteins that link central regions of the thick filaments

Question 17

H zone

Answer 17

light area at end of thin filaments

Question 18

describe thick filaments

Answer 18

• 15nm in diameter
• lots of motor protein myosin
• myosin. made of heavy and light chains

Question 19

describe myosin

Answer 19

• tail made of 2 chains
• globular hear which projects at an angle
• half of the heads orientated to left, other to right
• creates area in middle called M-region

Question 20

describe thin filaments

Answer 20

• made of filamentous F-actin strands
• which are made of a string of globular G-actin subunits
• contain tropomyosin: blocks active sites
• filaments attached at z disc

Question 21

describe tropomyosin

Answer 21

• blocks active sites
• bound by calcium binding protein troponin

Question 22

what is the interaction between actin and myosin filaments regulated by?

Answer 22

troponins

Question 23

troponin I

Answer 23

inhibitory

Question 24

troponin C

Answer 24

calcium binding

Question 25

troponin T

Answer 25

tropomyosin binding

Question 26

sliding filament theory

Answer 26

1. contraction causes force 2. myofilaments stay same length but it causes more overlap

Question 27

what does an increase in calcium initiate?

Answer 27

contraction

Question 28

cross bridge cycling 3 stages

Answer 28

1. resting muscle 2. activation of contraction 3. terminating cross bridge

Question 29

describe stage one of cross bridge cycling: resting muscle

Answer 29

- myosin molecule bound to ADP + Pi - tropomyosin covers myosin binding sites on acting filaments so they can't bind

Question 30

describe stage 2 of cross bridge cycling: activation of contraction

Answer 30

- muscle stimulated - calcium levels in cytosol increase - calcium binds to troponin - so uncovers myosin binding sites on actin - myosin molecule + ADP binds to actin - cross bridge formation causes release of ADP and Pi and movement of the cross bridge

Question 31

describe stage 3 of cross bridge cycling: terminating cross bridge

Answer 31

- ATP binds to myosin - breaks actin-myosin cross bridge - ATP converted to ADP and Pi - so myosin returns to energised position

Question 32

how are skeletal muscle contractions neurogenic?

Answer 32

- contractions need synaptic input from motor neurones - a single motor neurone can innervate muscle fibres so they all contract at same time

Question 33

describe the neuromuscular junction

Answer 33

- axon terminal and the motor endplate = NMJ myelin sheath that surrounds the motor neurone ends near the surface of the muscle fibre

Question 34

what do the axon terminals in the NMJ contain?

Answer 34

vesicles with ACh

Question 35

motor end plate of NMJ

Answer 35

area of muscle fibre directly under the axon terminal

Question 36

describe how NMJ works

Answer 36

1. A.P at motor neurone depolarised axon terminal 2. so voltage dependent calcium channels open 3. causes release of ACh vesicles through synapse 4. ACh diffuses to motor end plate 5. activates NICOTINIC ACh receptors (ionotropic receptors) 6. motor end plates is depol 7. this is end plate potential

Question 37

fast twich fibres in skeletal muscle

Answer 37

- glycolytic - explosive power

Question 38

slow twich fibres in skeletal muscle

Answer 38

- aerobic - fatigue resistant

Question 39

what is the speed of contraction determined by?

Answer 39

the myosin heavy chain isoform expressed by the fibre

Question 40

is there a gene for speed?

Answer 40

- alpha actinin 3 is expressed in a subset of fast twitch muscles - genetic variant encoding premature stop codon results in truncated form of this actinin - absence of variant correlates with athletic performance

Question 41

myopathies (genetic skeletal muscle diseases)

Answer 41

- present at birth - develops at infancy - non progressive - non degenrative - defect in actin

Question 42

dystrophies

Answer 42

- birth or later - progessive - degenerative - defects in memb / proteins