LECTURE 27

Question 1

Which muscle fibres fatigue more easily?

Answer 1

Fast, larger

Question 2

How many MyHC species in mammals?

Answer 2

10

Question 3

What is the slowest most oxidative muscle, and the fastest most glycolytic muscle?

Answer 3

Soleus and masseter

Question 4

How do fibre types exist?

Answer 4

Pure (one MyHC type) or hybrid

Question 5

What are myosin isoform properties influenced by?

Answer 5

hormone levels, exercise, injury and ageing

Question 6

What is the function of myosin light chains?

Answer 6

Modulate actin-myosin interactions, stabilise myosin head during force development, influence velocity of shortening

Question 7

What is the effect of treating muscle with clenbuterol?

Answer 7

After 4 weeks no exercise there is a greater proportion of fast twitch fibres

Question 8

What determines the contractile properties of the muscle fibre?

Answer 8

Nerve supplying muscle and electrical signals

Question 9

What determines the fatiguablity of fibres?

Answer 9

The total number of impulses

Question 10

What is SAID?

Answer 10

specific adaptation to imposed demand - should be able to predict changes to muscle if we know inputs

Question 11

What happens when a muscle is denervated?

Answer 11

Atrophy

Question 12

What happens to a muscle after 6 weeks in a cast?

Answer 12

• 40% loss of volume
• atrophy of individual fibres
• slow type I fibres more effected

Question 13

What happens if immobilisation is in shortened position?

Answer 13

Accentuated muscle loss

Question 14

What happens if immobilisation is in a lengthened position?

Answer 14

Muscle mass and function better preserved

Question 15

When fibre type is changing what is the order of changes?

Answer 15

Metabolic then contractile properties changed

Question 16

What is muscle plasticity?

Answer 16

Ability of muscle to change types (slow, fast)

Question 17

What happens when muscle is changed from fast to slow?

Answer 17

• increased capillary density, constant action so need more nutrients (week 1-2)
• proliferation of mitochondria, oxidative (week 1-2)
• decrease SR and calcium ATPase, slower contractions (week 2-3)
• express different myosin and troponin isoforms (week 5)
• muscle atrophy

Question 18

What happens with chronic electrical stimulation to normal human muscles?

Answer 18

Changes in fatiguability but rarely alterations is myosin isoform expression - some evidence of type II - type I fibre in spinal cord injury patients

Question 19

What is the timeline of muscle plasticity?

Answer 19

• Within first few hours of stimulation SR begins to swell (demanded to perform continuous contraction and lots of calcium left over because reuptake is not as efficient, SR overall ends up decreasing in density)
• Within 2-12 days have metabolic changes: mitochondrial number, oxidative enzyme activity, capillarity, blood flow, oxygen consumption (all reflect the change in requirements of the muscle – increased metabolic activity of muscle)
• Sequence of adaptive events: calcium handling – enzyme activities – MyHC
• After 14 days the Z-band begins to increase in width and calcium ATPase is observed
• After a prolonged dose of increased activity, muscle fibre CSA, maximum tetanic tension, and muscle mass have decreased significantly

Question 20

Are metabolic enzymes etc. more easy to chance than contractile proteins?

Answer 20

Yes

Question 21

What is the best way to make a muscle weaker?

Answer 21

Chronic low frequency stimulation