L24 - Skeletal Muscle

Question 1

What are the three major types of muscle?

Answer 1

• skeletal
• cardiac
• smooth

Question 2

What is skeletal muscle?

Answer 2

Muscle attached to the bone
- voluntary

Question 3

What is cardiac muscle?

Answer 3

Muscle not attached to the bone
- specialised form of skeletal muscle
- involuntary

Question 4

What is smooth muscle?

Answer 4

Muscle not attached to the bone
- myocytes with a fusiform shape
- involuntary

Question 5

What are key characteristics of skeletal muscle?

Answer 5

• attached to bone
• composed of fibres
• posture and movement
• nervous control (voluntary)

Question 6

What are the different skeletal muscle structures?

Answer 6

• muscle
• tendons
• myofibrils

Question 7

What are muscles?

Answer 7

Skeletal muscle made from bundles of multinucleated muscle cells

Question 8

What are tendons?

Answer 8

Bundles of collagen fibres that attach muscle to bone

Question 9

What are myofibrils?

Answer 9

Muscle cell formed from bundles of actin and myosin filaments organised into myofibrils

Question 10

How many actin filaments surround a single myosin filament?

Answer 10

6 actin filaments

Question 11

What is the sarcoplasmic reticulum?

Answer 11

Homologous to the ER found in other cells
- wrapped around the myofibril

Question 12

What does the SR act as in skeletal muscle?

Answer 12

Ca2+ store

Question 13

What type of pattern is visible on skeletal muscle? What is it caused by?

Answer 13

Striated pattern
- Accumulation of protein at different points of the fibre

Question 14

What are the functinal units of striated muscle?

Answer 14

• Z-disc (a-actinin)
• thin filament (actin, tropomyosin, troponin)
• M-band (myomesin)
• thick filament (myosin, titin)

Question 15

What is the A band?

Answer 15

Thick filaments (myosin)
- dark band at the centre of the sarcomere

Question 16

What is the I band?

Answer 16

Contains thin filaments (actin)
- don’t overlap with thick filaments
- either side of A band with the Z disc in the centre

Question 17

What is the H zone?

Answer 17

Light area found between the ends of thin filaments
- the centre of the sarcomere

Question 18

What is the M line?

Answer 18

Protein that link central regions of thick filaments

Question 19

What is the Z disc?

Answer 19

Network of proteins that anchor the thin filaments

Question 20

What is one sarcomere?

Answer 20

Z line to Z line

Question 21

What are thick filaments like?

Answer 21

• 15nm in diameter
• hundres of motor proton myosin
• heavy and light chains

Question 22

What is the tail of a myosin molecule formed of?

Answer 22

• 2 intertwined chains
• double globular head projecting from it at an angle (half left, half right)
  = middle filament region (M-region)

Question 23

What are thin filaments like?

Answer 23

• F-actins stranfs, coposed of string of globular G actin subunits
• has active site that binds to head of myosin molecule
• has tropomyosin that blocks active sites
• bound by Ca-binding troponin
• attached to Z disc

Question 24

What does a-actinin do?

Answer 24

Bind and cross-link the ends of F-actin filaments from adjacent sarcomeres at the Z line

Question 25

What is a-actinin-2?

Answer 25

Ubiquitous in all types of muscle fibres

Question 26

What are the different parts of troponins?

Answer 26

• Troponin I (inhibitory)
• troponin C (Ca binding)
• troponin T (tropomyosin binding)

Question 27

What is the sliding filament mechanism?

Answer 27

Contraction - development of force rather than shortening of muscle fibres
- myofilaments remain same length but overlap to a greater extent

Question 28

What are the sequence of steps in the cross bridge cycling?

Answer 28

• resting muscle
• activation of contraction
• terminating cross bridge

Question 29

What happens while the muscle is resting (in the cross bridge cycling)?

Answer 29

• myosin molecules bound to ADP and Pi
• tropomyosin covers ‘myosin binding sites’ on actin filaments

Question 30

What happen in the activation of contraction (in the cross bridge cycling)?

Answer 30

• muscle stimulated, Ca levels inc
• Ca2+ bind to troponin complex
  = moves tropomyosin, unmasks myosin binding sites
• +ADP binds to actin
• cross bridge formation releases ADP and Pi = movement of crossbridge

Question 31

What happens in the termination of the cross bridge (in the cross bridge cycling)?

Answer 31

• ATP binds myosin, breaks actin-myosin crossbridge
• ATP converted to ADP, myosin returns to energised position

Question 32

What do skeletal muscle contractions require?

Answer 32

Synaptic input from motor neurons

Question 33

What can a single motor neuron do?

Answer 33

Innervate multiple muscle fibres
- causin simultaneous contraction

Question 34

What is the structure of the neuromuscular junction (NMJ)?

Answer 34

• myelin sheath ends near surface of muscle fibre
• axon terminal has vesicles containing Ach
• ## motor end plate direclty under terminal portion of axon

Question 35

What happens at NMJs?

Answer 35

• AP depolarises the axon terminal, open VGCC
• releases Ach containing vesicles
• Ach diffuses to motor end plate, activates nAch receptors (ionotropic receptors)
• depolarises motor end plate = end plate potential

Question 36

What are the different fibre types of skeletal muscle?

Answer 36

• fast twitch (glycolytic, explosive power)
• slow twitch (aerobic, fatigue resistant)

Question 37

What is the speed of contraction determined by?

Answer 37

Myosin heavy chain isoform expressed by the fibre

Question 38

What is the gene for speed?

Answer 38

Alpha actinin 3
- expressed in a subset of fast twitch muscles
- presence of absence of variant correlates with athletic performance