7. Muscle

Question 1

Describe skeletal muscle

Answer 1

Attached to bone
Produces body movement
Found in antagonistic muscle pairs: flexors and extensors

Question 2

What are the 2 types of muscle contraction and how do they differ?

Answer 2

Isotonic: tension stays the same and length changes
Isometric: tension changes and length remains the same

Question 3

What are the 2 subtypes of isotonic contraction?

Answer 3

Concentric: shortening
Eccentric: lengthening

Question 4

Skeletal muscle consists of

Answer 4

Bundle of myofibres

Question 5

Describe myofibres

Answer 5

Large and Cylindrical
Multinucleate
Packed with myofibrils

Question 6

Appearance of myofibrils

Answer 6

striated due to light and dark bands

Question 7

What is a sarcomere?

Answer 7

Functional unit of muscle

Lies between 2 Z-lines

Question 8

Describe the process of excitation-contraction coupling of skeletal muscle

Answer 8

An AP propagates along sarcolemma and T tubules
Reaches the Dihydropyridine receptors
Depolarisation causes a conformational change in the DHPRs
Change transmitted to Ryanodine Receptors on sarcoplasmic reticulum
RyR opens: causing Ca2+ release from intracellular stores
Causes depolarisation due to increase in intracellular Ca2+

Question 9

T-Tubules

Answer 9

Membrane invaginations that contact the extracellular fluid

Found in myofibres

Question 10

Sarcoplasmic reticulum (SR)

Answer 10

extensive network of Ca2+ stores surrounding each myofibril

Question 11

What are the different components of a sarcomere?

Answer 11

Z-line 
Actin filaments 
Myosin 
Titin 
Nebulin 
Tropomyosin
CapZ and Tropomodulin

Question 12

What is a Z line?

Answer 12

Defines lateral boundaries of sarcomere

Question 13

What is actin?

Answer 13

Polymeric thin filament composed of 2 twisted alpha-helices

Displays polarity

Question 14

What is myosin?

Answer 14

Thick filaments
‘motor proteins’
Contain numerous ‘globular heads’ that interact with actin

Question 15

What is titan?

Answer 15

Very large ‘spring-like’ filaments

Anchor myosin to the Z-line

Question 16

What is nebula?

Answer 16

Large filaments associated with actin

Question 17

What is tropomyosin?

Answer 17

Elongated protein bound to actin

Question 18

What are CapZ and Tropomodulin?

Answer 18

Associated with +ve and –ve ends of actin, respectively

Question 19

Describe the sliding filament theory

Answer 19

Calcium binds to Troponin, causing tropomyosin chain to move
Movement exposes myosin binding site on surface of actin chain
‘Charged’ myosin heads bind to exposed binding sites
Binding and discharge of ADP caused myosin head to pivot (Power stroke), pulling actin filament towards centre of sarcomere
ATP binds to myosin head, releasing it from the chain
ATP hydrolysis provides energy to ‘recharge’ the myosin head

Question 20

What is the relationship between muscle tension and load in isotonic and isometric contraction?

Answer 20

Isotonic: Tension > Force exerted by load
Isometric: Tension = Force exerted by load

Question 21

What does the tension-load relationship in isotonic contraction cause?

Answer 21

Muscle to contract
Fibres shorten
Energy expenditure (ATP): ‘recharging’ of myosin heads

Question 22

What does the tension-load relationship in isometric contraction cause?

Answer 22

Muscle DOES NOT contract: myosin heads reattach to the same point on actin chain
Energy expenditure (ATP): ‘recharging’ of myosin heads

Question 23

What are cardiomyocytes?

Answer 23

Cells in heart that contract causing movement of blood

Question 24

What are the pacemaker cells in the heart?

Answer 24

Sinoatrial node

Atrioventricular node

Question 25

Describe sinoatrial nodal cells

Answer 25

Small, ‘empty’, spindle shaped cells

spontaneously active

Question 26

Describe atrioventricular node

Answer 26

spindle-shaped network of cells located at base of right atrium

Question 27

Conducting fibres in the heart

Answer 27

Bundle of His

Purkinje fibres

Question 28

What is the bundle of his?

Answer 28

fast conducting cells adjoining the AV node and Purkinje fibres

Question 29

What are purkinje fibres?

Answer 29

large cells that rapidly conduct electrical impulses

Question 30

Describe the appearance of Cardiomyocytes

Answer 30

striated muscle cells that are distinct but are linked by intercalated discs

Question 31

What are intercalated discs?

Answer 31

Specialised regions connecting individual cardiomyocytes

Contain numerous gap junctions: allow action potentials to spread rapidly from cell to cell.

Question 32

What are sarcomeres (in the heart)?

Answer 32

Contractile units of cardiomyocytes

Mechanism of contraction is same as skeletal muscle

Question 33

Describe the process of excitation-contraction coupling of cardiac muscle.

Answer 33

Action Potential propagates down T-tubules and leads to the opening of VGCCs: leads to influx of Ca2+
Ca2+ then binds to the RyR making them open leading to Ca2+ release from the SR and a further increase in intracellular Ca2+ (CICR)
Ca2+ initiates contraction by binding to troponin
Ca2+ causes further depolarisation

Question 34

What are the 2 types of receptors involved in excitation-contraction coupling of skeletal muscle?

Answer 34

Dihydropyridine receptor

Ryanodine receptor

Question 35

Which receptors are involved in excitation-contraction coupling of cardiac muscle?

Answer 35

Voltage Gated Calcium Channels

Ryanodine Receptors

Question 36

Describe smooth muscle

Answer 36

present within walls of hollow organs e.g. Blood vessels
Do not contain regular actin/myosin arrangement
Acts slowly
No control over smooth muscle

Question 37

Describe the process of excitation-contraction coupling of smooth muscle

Answer 37

Action potential reaches VGCC and makes it open leading to Ca2+ influx
Ca2+ binds to Calmodulin forming a Ca2+-CaM complex
This complex activates myosin light chain kinase (MLCK)
MLCK phosphorylates myosin light chains (MLC20) that form cross bridges with actin filaments
Leads to smooth muscle contraction