Physiology Revision: Muscle Structure and Function

Question 1

What are the layers of muscle fibers?

Answer 1

Muscle tissue - Fascicles (bundles of muscle fibers) - Muscle fibers - Myofibrils - Sacromeres

Question 2

What are the two muscle fiber architectures?

Answer 2

Parallel - Parallel fibers can quickly change their length but are weaker because they have less fibers per cross-sectional area

Pennate - Pennate fibers are diagonal and stronger because they have a greater PCSA (physiological cross sectional area) but slower

Question 3

What are the components of the neuromuscular junction?

Answer 3

1) Axon terminal - ending of the axon

2) Neuromuscular endplate - interface btw motor neurons and muscle fibers

3) Synaptic cleft- where neurotransmitters (ACH) diffuse

Question 4

Describe the AP propagating sequence.

Answer 4

1) AP reaches the axon terminal and depolarizes the membrane

2) Ca2+ channels open and calcium diffuses into the axon terminal and binds to proteins

3) Synaptic vesicles release Ach into neuromuscular junction and then bind to nicotinic receptors

4) Binding of Ach opens Na+ and K+ ion channels and more Na+ comes into muscle cell than K+ leaves, causing local depolarization of the membrane

5) Muscle fiber AP is generated at the depolarization travels down the T-tubules and then activates a DHR receptor which in turn activates a ryanodine receptor that opens Ca2+ channels

6) Ca2+ is then released from the SR and binds to troponin to move tropomyosin away from the actin binding sites causes sliding filament theory to take place

Question 5

Can a single muscle fiber be innervated by two or more neurons?

Answer 5

No, there is only one neuromuscular junction per muscle fiber

Question 6

What are the 4 stages of X-bridge cycling?

Answer 6

1. Energized myosin on thick filaments binds to actin
2. X-bridge binding triggers release of ATP hydrolysis from myosin, producing angular movements
3. Another ATP binds to myosin and hydrolysis occurs to break bond btw actin and myosin head
4. cycle restarts and ATP binds to myosin and energizes it into a cocked position

• ATPase is the enzyme that determines speed of ATP hydrolysis

Question 7

What is the latent period?

Answer 7

After the AP, the period before the muscle fiber produces tension

Question 8

What are the three types of muscle fibers? Give brief description of each

Answer 8

1) Fast glycolytic fibers

• High ATPase activity

2) Fast oxidative fibers

• High ATPase activity, highly oxidative/moderately glycolytic

3) Slow Oxidative fibers

• Low ATPase activity, highly oxidative

Question 9

Difference btw Hypertrophy and Hyperplasia?

Answer 9

Hypertrophy

• increase in muscle fiber size
• protein synthesis greater than protein breakdown

Hyperplasia

• increase in the # of muscle fibers (not possible in humans)

Question 10

Anabolism vs. Catabolism?

Answer 10

Anabolism is muscle GAIN

Catabolism is muscle LOSS