Lecture 28

Question 1

What is the key difference between faster twitch muscle fibers and slower twitch muscle fibers?

Answer 1

Larger, faster muscle fibers fatigue more easily.

Smaller, slower muscle fibers are fatigue resistant

Question 2

What are some different factors that influence the variability of muscle fibers?

Answer 2

Fast and slow twitch fiber designations are very oversimplified names. 10 MyHC species have been identified in mammalian muscle fibers but there could be more.

Muscle fibers may contain various proportions of myosin isoforms.

Fibers can be one fiber type or hybrid.

Muscles that are known as a specific twitch muscle still have a small portion of their fibers being other fiber types.

A slow fiber in the mouse is not necessarily the same as a slow fiber in a cat or a human.

Question 3

What are the fiber types found in human limb muscles?

Answer 3

Limb muscles typically contain types 1, 2a, 2x, and 2b.

Question 4

What factors have a potential effect on MyHC composition?

Answer 4

MyHC isoforms differ in their functional properties and different isoforms are expressed by different hormone levels, injuries, and aging.

Question 5

What are the functions of MyLC?

Answer 5

Myosin light chains modulate actin-myosin interactions.

They stabilize myosin head during force development.

They also influence velocity of shortening.

Question 6

What is the result of the variation of isoforms of muscle proteins?

Answer 6

Many different combinations of heavy and light chains are possible due to the variation.

Question 7

What determines the proportion of muscle fibers that are present in every person?

Answer 7

Very strong influence of genetics on fiber proportion. It is very difficult for fiber types to change permanently. The transitions are often temporary and happen very rarely.

*Fiber type transition is often seen in muscle injuries 7 days after regeneration but this effect is reversed after another 7 days.

Question 8

What is the effect of clenbuterol on fiber type composition?

Answer 8

Clenbuterol increases the fast twitch fiber amounts.

Question 9

How can muscles be changed from one fiber type to another?

Answer 9

Skeletal muscles can change from one type to another via innervation changes. This means that fast contracting muscles can be made from slow twitch fibers by changing their innervation.

Question 10

How does innervation of the muscle change fiber type?

Answer 10

The change in fiber type from innervation occurs due to the frequency of innervation of the muscle fibers.

Question 11

What is the result of low frequency stimulation of a muscle fiber?

Answer 11

slow-twitch characteristics

Question 12

What is the result of high frequency stimulation of a muscle fiber?

Answer 12

Fast-twitch characteristics

Question 13

What influences the fatiguablility of muscle fibers?

Answer 13

The total number of impulses

Question 14

What is the idea of muscle plasticity?

Answer 14

Muscles adapt specifically to imposed demand (SAID: Specific Adaptation to Imposed Demand). Every structural element in muscles can be changed if given proper stimulus.

Question 15

What are some stimuli for disuse muscle atrophy?

Answer 15

Immobilization of limbs, plaster casting, prolonged bed rest, spaceflight, aging, denervation, etc..

Question 16

Which fiber type atrophies when immobilized?

Answer 16

If a limb is immobilized type 1 fibers atrophy. This is different in other animals.

Question 17

What effect does the contraction status of an immobilized limb affect atrophy?

Answer 17

If the fiber is immobilized while contracting this makes muscle loss faster.

If the muscle is in a lengthened position the muscle is better preserved.

Question 18

What happens to a muscle fiber when it is stimulated to change from fast twitch to slow twitch?

Answer 18

Fast muscle changes its metabolic properties and then its contractile properties. I.e the muscle fibers themselves change from one type to another.

Question 19

What are the major changes that occur to a fast twitch muscle fiber when it is converted into a slow twitch fiber?

Answer 19

Increased capillary density

Proliferation of the mitochondria

Decrease in SR and Ca2+ ATPase

Expression of different myosin and troponin isoforms takes place

These changes all occur with different time courses

Question 20

How do muscle type changes occur over time?

Answer 20

Different changes occur at different times. Changes in myosin and the sarcoplasmic reticulum density take a long time.

Question 21

What is the sequence of events taking place during a muscle type change?

Answer 21

Calcium handling -> Enzyme Acitivities -> myosin heavy chain

Question 22

How do muscles’ external appearance change with change in fiber type?

Answer 22

Type 1 fibers are usually smaller in size and this makes them more efficient for their function. The opposite occurs in fast twitch fibers.