Lecture 7- Muscles

Question 1

What are the antagonist muscle pairs?

Answer 1

Flexor (Bicep)

Extensor (Tricep)

Question 2

What are the two types of skeletal muscle contraction?

Describe how the contraction arises

Answer 2

Isotonic: length changes, tension remains
Concentric: shortening
Eccentric: lengthening
Isometric: Length changes, tension remains

Question 3

What is the function of skeletal muscle?

Answer 3

Attached to bone and produces movements

Question 4

What is skeletal muscle made of?

Answer 4

Bundes of muscle cells (myofibres)

Question 5

Describe the structure of myofibres

Answer 5

Myofibres made from many cells which fused together

1) Large and cylindrical
2) Multinucleate
3) Packed with myofibrils

Question 6

Describe the structure of myofibrils

Answer 6

1) Light and dark bands= striated
2) Have T-tubules: membrane invaginations that contact the EC fluid
3) Sarcoplasmic reticulum: Ca2+ stores surrounding myofibrils

Question 7

What is a sarcomere?

Answer 7

A functional unit of muscle- it lies between two Z-lines

Question 8

What is the sarcolemma?

Answer 8

The myofibril membrane

Question 9

With the aid of a drawing, explain how you get the excitation- contraction coupling of muscles

Answer 9

1) AP propagates along sarcolemma and T-tubules
2) Depolarisation causes the dihydropyridine receptors (DHPR) to change shape
3) Change in shape is transmitted to ryanodine receptors (RyR) on SR
4) RyR open and Ca2+ is released
5) Increase in intracellular Ca2+

Question 10

What are intercalated discs? What do they contain to help with their function?

Answer 10

• specialised regions connecting individual cardiomyocytes

- contain gap junctions to allow AP to spread rapidly

Question 11

What are the contractile units of cardiomyocytes called?

Answer 11

Sarcomeres

Question 12

How is cardiac muscle excited?

Answer 12

• Depolarisation opens voltage gated Ca2+ channels–> Ca2+ influx
• Ca2+ has three effects:
  1) Binds to RyR on SR and causes Ca2+ induced Ca2+ release
  2) initiates contraction by binding to troponin
  3) further depolarisation

Question 13

Where is smooth muscle found?

Answer 13

In the walls of hollow organs

Blood vessels, GI tract

Question 14

Why are is smooth muscle not striated?

Answer 14

Does not contain a regular arrangement of actin and myosin

Question 15

How is smooth muscle activated?

Answer 15

1) depolarisation activates VGCCs
2) Ca2+ / Calmodulin (CaM) complex activates myosin light chain kinase (MLCK)
3) MLCK phosphorylates myosin light chains (MLC20)
4) Cross bridges with actin filaments–> contraction

Question 16

Draw and annotate the structure of a sarcomere

Answer 16

1) Z: lines- define the lateral boundaries of the sarcomere
2) Actin- polymeric thin filament made of two twisted a-helices- displays polarity
3) Myosin- thick filaments ‘motor proteins’. Contain many globular heads which interact with actin
4) Titin- Large spring like filaments aching myosin to the Z-line
5) Nebulin- large filaments associated with actin
6) Tropomyosin- elongated proteins bound to actin
7) CapZ and Tropomodulin- associated with the +ve ad -ve ends of actin respectively

Question 17

Describe the sliding filament theory

Answer 17

1) Presence of Ca2+ –> movement of troponin from tropomyosin chain.
2) Movement exposes myosin binding site on surface of actin chain
3) Charged myosin heads bind to binding site
4) Binding and discharge of ADP causes myosin head to pivot (power stroke) and actin filament drawn to centre of sarcomere
5) ATP binding–> releases myosin head from actin chain
6) ATP hydrolysis–> provides energy to recharge myosin head. So relaxation needs ATP hydrolysis

Question 18

What happens during isometric contraction when you have no muscle shortening? w.r.t. myosin heads

Answer 18

• Muscle tension= force exerted by the load

- so muscle doesn’t contract and myosin heads reattach to the same point on the actin chain

Question 19

In terms of tension and load what happens during isotonic contraction?

Answer 19

Muscle tension> force exerted by load

Question 20

Where can you find pacemaker cells in the heart?

Answer 20

Sinoatrial node

AV node

Question 21

Where can you find conducting fibres in the heart?

Answer 21

1) Bundle of His: join AV node and Purkinje fibres

2) Purkinje fibres

Question 22

What are the walls of the heart called and what are they made of?

Answer 22

Myocardium
Cardiac muscle
Cardiomyocytes (striated muscle)