Physiology of skeletal muscle contraction

Question 1

What binds to troponin C in skeletal muscles?

Answer 1

4 Ca2+

Question 2

What binds to troponin C in the heart muscle?

Answer 2

3 Ca2+

Question 3

What happens to troponin after calcium binds?

Answer 3

Changes conformation

Question 4

What does a conformational change in troponin (TnC) lead to?

Answer 4

Shuts off TnI
tropomyosin-troponin leaves F-actin groove
Unmasks the myosin binding site on actin

Question 5

What happens to the next myosin that binds?

Answer 5

Next myosin head makes cross bridges (cycling) to actin
Myosin breaks down ATP
Myosin pulls thin filaments

Question 6

What is total TnI used as a marker of?

Answer 6

Total muscle breakdown

Question 7

What is cardiac TnI used as a marker of?

Answer 7

Myocardial infarction

Question 8

What is cross bridge cycling?

Answer 8

Molecular cycle of actin-myosin interaction

Mechanism of contraction at a molecular level

Question 9

What does contraction through cross bridge cycling depend on?

Answer 9

Binding of myosin heads to thin filaments (actin) at specific binding sites

Question 10

Describe the resting state of a sarcomere

Answer 10

Myosin heads are blocked from binding to actin by tropomyosin, which occupies the specific binding sites ( F actin double helical groove)

Question 11

What can increase the force generated?

Answer 11

Increased overlap of thick and thin filaments

Question 12

When does the greatest force generation occur?

Answer 12

When sarcomere is at optimal length

Question 13

Describe the process of cross bridge cycling in terms of te 4 reactions

Answer 13

Myosin releases actin
Myosin head cleaves ATP
Myosin binds actin
Power stroke

Question 14

Where is creatine found?

Answer 14

Muscle fibres

Question 15

What is creatine phosphorylated to?

Answer 15

Creatine phosphate

Question 16

How is energy stored in muscle?

Answer 16

Stored as creatine phosphate in muscle fibres

Question 17

Describe what happens to ATP during cross bridge cycling

Answer 17

ATP is hydrolysed to ADP + Pi , creatine phosphate donates high energy phosphate to ADP restoring it to ATP

Question 18

How are ATP levels in muscle kept stable?

Answer 18

Buffering and regeneration

Question 19

Which enzyme catalyses the reaction of ATP during cross bridge cycling?

Answer 19

Creatine kinase or Creatine phosphokinase (CK, CPK) in both directions

Question 20

What is creatine?

Answer 20

Small molecule that accepts high energy phosphate bonds from ATP

Question 21

What is plasma creatine phosphokinase a marker of?

Answer 21

Muscle destruction

Question 22

What is creatinine a diagnostic marker of?

Answer 22

Kidney function

Question 23

What is creatinine?

Answer 23

Breakdown product of creatine

Question 24

What are the two calcium gradients?

Answer 24

Extracellular vs cytosolic free calcium

SR vs cytosolic free calcium

Question 25

What provides most of the calcium?

Answer 25

Efflux of calcium from the sarcoplasmic reticulum to cytoplasm

Question 26

What does calcium entering the cell from the outside provide?

Answer 26

Only a small fraction of the calcium needed for muscle contraction

Question 27

What does calcium trigger?

Answer 27

Contraction

Question 28

Which neurotransmitter leads to depolarisation?

Answer 28

ACh

Question 29

What does depolarisation cause?

Answer 29

Increase in calcium

Question 30

Name the receptor where there is a net inward current

Answer 30

Active nicotinic AChR

Question 31

How does depolarisation spread?

Answer 31

Through T tubules

Question 32

What do local action potentials trigger?

Answer 32

Ca2+ efflux from terminal cisternae across sarcoplasmic reticulum into the fibre cytoplasm

Question 33

Where is the ryanodine receptor found?

Answer 33

In the sarcoplasmic reticulum

Question 34

What does the ryanodine receptor do?

Answer 34

Releases Ca2+ from SR

Question 35

What is the ryanodine receptor triggered by?

Answer 35

Voltage sensor on Ca2+ channel

Question 36

Where is SERCA found?

Answer 36

In sarcoplasmic reticulum

Question 37

What does SERCA stand for?

Answer 37

Smooth endoplasmic reticulum calcium ATPase

Question 38

What is the function of SERCA?

Answer 38

Pumps Ca2+ back into SR

Question 39

What does SERCA require?

Answer 39

ATP

Question 40

What causes a twitch?

Answer 40

Single action potential leading to calcium release from sarcoplasmic reticulum

Question 41

What causes a twitch to end?

Answer 41

Ca2+ ions are rapidly pumped back into the SR

Question 42

Describe the molecular basis of tetany

Answer 42

Frequent APs Insufficient Ca2+ resequestration Summation of contraction

Question 43

Name the two types of muscle fibres

Answer 43

Slow twitch | Fast twitch

Question 44

Describe the structure of slow twitch fibres

Answer 44

High myoglobin | Many mitochondria

Question 45

Give the size, colour and property of slow twitch fibres

Answer 45

Red Small diameter Oxidative

Question 46

Give the size, colour and property of fast twitch fibres

Answer 46

White Large diameter Non-oxidative

Question 47

How do the two muscle fibre types differ?

Answer 47

Aerobic (slow fibres) vs anaerobic respiration Faster calcium reuptake in fast fibres Maximum tension produced in fast fibres Fatigue resistance in slow fibres

Question 48

Name the fibre types in order of slow to fast fibres

Answer 48

Type 1 Type IIA Type IIX Type IIB

Question 49

What do muscles contain?

Answer 49

A mixture of fibre types

Question 50

What does muscle composition depend on?

Answer 50

Muscle action

Question 51

Give the composition of soleus

Answer 51

80% Type 1 (slow) | 20% Type IIA

Question 52

Give the composition of vastus lateralis

Answer 52

Mixture of type 1, IIA, IIX

Question 53

Name the different groups of physical fitness

Answer 53

Inactive Moderately active Endurance athlete Anaerobic athlete

Question 54

What colour do type 1 fibres stain?

Answer 54

Dark

Question 55

What colour do type II fibres stain?

Answer 55

Light

Question 56

List the 3 types of coordination of contraction

Answer 56

Motor units Tetany Fusion of myocytes into long myofibres

Question 57

Define motor unit

Answer 57

A single alpha motor neuron and all muscle fibres that it innervates

Question 58

What do motor units function as?

Answer 58

A single contractile unit of skeletal muscle

Question 59

What type are all muscle fibres in a single motor unit?

Answer 59

The same

Question 60

Describe the synapses in large muscles responsible for powerful gross contractions

Answer 60

Single motor neuron may synapse on 1000 fibres

Question 61

Describe the synapses in small muscles mediating precision movements

Answer 61

Single motor neuron may synapse with as few as 2 - 3 fibres

Question 62

What determines the muscle fibre?

Answer 62

Type and function of lower motor neuron

Question 63

Define isometric contraction

Answer 63

Generates a variable force while length of the muscle remains unchanged

Question 64

Define isotonic contraction

Answer 64

Generates a constant force while the length of the muscle changes

Question 65

Describe the types of force generation when picking up a glass

Answer 65

stage 1: isometric – force increases, joint does not move Muscle Force < force of gravity –> force increases biceps and brachioradialis generate force by isometric contraction as muscles have not yet shortened stage 2: isotonic – force remains the same, arm moves Glass moves upward in response to force an isotonic contraction starts as the force generated by the muscles overcomes gravitational and inertial forces keeping glass on the table glass starts to rise as the muscles shorten and the elbow bends and force generated by the muscle is constant as the glass is moving

Question 66

What does muscle contraction not necessarily mean?

Answer 66

Muscle shortening

Question 67

What is meant by concentric?

Answer 67

Force during contraction

Question 68

Give an example of concentric force

Answer 68

Tossing a ball into the air

Question 69

What is meant by eccentric?

Answer 69

Force during muscle elongation

Question 70

Give an example of eccentric force

Answer 70

When braking | When the weight of the object is overwhelming - catching a ball

Question 71

When can both types of force occur?

Answer 71

In one behaviour

Question 72

What does proprioception control?

Answer 72

Force generation

Question 73

What is proprioception based on?

Answer 73

Length and strength

Question 74

Describe the size principle

Answer 74

As the initial isometric contraction occurs more and more motor units are recruited starting with smaller ones and progressively adding larger ones Allows fine gradation of force for small movements

Question 75

What does lower motor neuron disease cause?

Answer 75

Weakness | Muscle atrophy

Question 76

What does upper motor neuron disease cause?

Answer 76

Spasticity, hypertonia

Question 77

Describe the stretch reflex

Answer 77

Controls muscle length | Increases muscle force

Question 78

What is westphal's sign?

Answer 78

Lack of patellar reflex

Question 79

What is the function of the patellar reflex

Answer 79

Posture and balance

Question 80

Name the fibres for sensation in the stretch reflex

Answer 80

Intrafusal fibres

Question 81

Name the fibres for contraction in the stretch reflex

Answer 81

Extrafusal fibres

Question 82

What does the muscle spindle fibre do?

Answer 82

Detects stretch

Question 83

Give the location of the muscle spindle

Answer 83

Parallel to muscle fibres

Question 84

What do the muscle spindles contain?

Answer 84

3-12 intrafusal fibres

Question 85

What do gamma motor neurons do?

Answer 85

Increase sensitivity | Drive contraction of edge of intrafusal fibres

Question 86

What are sensors from muscle spindles called?

Answer 86

Type 1a and Type 2

Question 87

Where are sensors from muscle spindles found?

Answer 87

Wrap around the intrafusal fibres

Question 88

What do sensors from muscle spindles do?

Answer 88

Detect stretch of central non-contacting region using stretch receptors

Question 89

What is the function of the muscle spindle?

Answer 89

Like a thermostat that regulates the relationship between the muscle length and muscle contractility ie. the relationship between neural drive and force generation

Question 90

What can absence of the muscle spindle reflex suggest?

Answer 90

Receptor damage Femoral nerve damage Peripheral nerve disease eg. peripheral neuropathy

Question 91

What can happen to the muscle spindle reflex in upper motor neuron disease

Answer 91

Can lead to hypertonia and spasticity UMN inhibits normal descending inhibitory input to spinal interneurons Spindle reflex can become over sensitive - can attempt to contract the muscle all the time

Question 92

What is the function of the tendon reflex?

Answer 92

Protects from overloading | Decreases muscle force- dropping the load - sensor firing - decreased contraction

Question 93

Describe the path of the tendon reflex

Answer 93

Sensor to spinal cord Interneuron to motor neuron Motor neuron inhibited Motor neuron to muscle

Question 94

What is the sensor of the tendon reflex?

Answer 94

Golgi tendon organ - detects tension in series with muscle in tendon - near border of muscle