Muscle Structure and Function - II

Question 1

What is isotonic?

Answer 1

Muscle tension remains unchanged where muscle length decreases

Question 2

What is isokinetic?

Answer 2

Muscle length decreases with constant velocity

Question 3

What are the three key
biochemical characteristics important to muscle fibre function?

Answer 3

1) Oxidative capacity (quantity of mitochondria, capillaries and myoglobin)
2) Type of myosin isoform expressed (3 types that differ in activity (rate of ATP breakdown)
3) Amount of contractile protein within fibre (amount of actin and myosin)
- (Type of motor neuron innervation)

Question 4

On average, what % of type I fibres in arm and leg muscles?

Answer 4

45-55%

-Trend in distribution consistent across muscle groups (within individuals).

Question 5

EXAMPLE: What % of type I and II fibres may a distance runner have?

Answer 5

Type I - 70 to 80%
Type II - 20-30%

Question 6

What is speed of contraction regulated by?

Answer 6

myosin ATPase activity

Question 7

What is the Maximal power output equation?

Answer 7

Maximal power output = force × shortening velocity.
* High force, fast fibres produce high power output.

Question 8

What does muscle contraction speed depend on?

Answer 8

Rate of cross bridge cycling (depends on the myosin ATPase isoform)

Question 9

What is Immunohistochemical staining?

Answer 9

• Selective antibody binds to unique myosin proteins
• Fibre types differentiated by colour difference

Question 10

What is gel electrophoresis?

Answer 10

• Identify myosin isoforms specific to different fibre types

Question 11

Why is the speed of shortening greater in fast fibres?

Answer 11

• SR releases Ca2+ at a faster rate.
• Higher ATPase activity.

Question 12

What are the 4 main factors in force regulation of muscle?

Answer 12

• Number and types of motor units recruited (more + fast MU = greater force)
• Muscle length (‘ideal’ length for force generation, increased C-B formation)
• Firing rate of motor neurons (frequency of stimulation)
• Contractile history of muscle (rested vs muscle exposed to fatigue, warmup exercise results in postexercise potentiation)

Question 13

Can the muscle fibres belonging to a single motor unit vary?

Answer 13

No - all muscle fibres that belong to
a single motor unit are of the
same fibre type

Question 14

Muscle force is increased by recruiting more motor units. What factor influences muscle function?

Answer 14

Number of fibres per Motor Unit will determine how that muscle functions.

E.g., Small eye muscles. Large
amount of units @ ~ 10 fibres / motor unit.

Large weight bearing medial
gastrocnemius ~ 600 motor units
@ ~2000 fibres / motor unit

Question 15

Motor neuron structure relates to function. Motor neurons supplying larger faster motor units have..?

Answer 15

• Larger cell bodies
• Larger diameter axons
• Greater number of axonal branches
• Sparse afferent innervation (less spindle excitatory input)
• More complex and extensive motor end plate of neuromuscular junction

Question 16

Describe the Henneman Size Principle.

Answer 16

There is a consistent recruitment pattern.
- Smaller slow units produce less force and are more fatigue resistant.
- Larger fast units produce more force and are less fatigue resistant

Question 17

Why are fast motor units harder to excite than slow motor units?

Answer 17

Because fast motor units have higher threshold motor neurones.

Question 18

Once enough summation occurs that tetanus is achieved, can more force be produced past this point?

Answer 18

No - tetanus is the maximal force the muscle is capable of producing

Question 19

What is the relationship between speed of movement and muscle fibre-type?

Answer 19

At any absolute force exerted by the muscle, the speed of movement is greater in muscles with a higher percentage of fast-twitch fibers.

Question 20

When is maximum velocity of shortening greatest?

Answer 20

At the lowest force

Question 21

The peak power increases with velocity
up to movement speed of..?

Answer 21

200 to 300 degrees /second.

Question 22

Why does power decrease at higher velocities?

Answer 22

Because force decreases with increasing movement speed.