Muscle Physiology

Question 1

Compare cardiac, smooth and skeletal muscle

Answer 1

Cardiac muscle - Single nucleus - Involuntary - Striated - Connected via intercalated discs Smooth muscle - Single nucleus, involuntary - Non-striated Skeletal muscle - Multinucleated (long cells)

Question 2

Describe the order of structure in a muscle

Answer 2

Individual muscle cells are called myofibers. Myofibers are wrapped in endomysium and bundled up to form fascicles. Fascicles are wrapped in perimysium to and bundled to form muscle. Muscle is wrapped in epimysium At the end of the muscle, epimysium tapers into tendon.

Question 3

Describe the structure of a sarcomere unit.

Answer 3

Actin filaments interdigitate with myosin filaments.

Actin:

• Made of G-actin
• Attacehd to Z line via alpha-actinin
• Tropomyosin (filament) and troponin complexes (embedded in intervals) run in actin filament.

Myosin:

• Made of myosin proteins
• ATtached to Z disc via titin

Question 4

Describe the contraction of a sarcomere.

Answer 4

I bands shorten

H zones shorten

A bands don’t change in length

Power stroke:

• ATP binds myosin, cocking into high energy state
• Myosin binds to actin
• Energy is used to pull filaments together
• ATP then rebinds myosin to re-cock it

Question 5

How is muscle contraction regulated?

Answer 5

Actin-myosin binding sites are occupied by tropomyosin.

Troponin complexes regulate tropomyosin.

When calcium binds troponin, it pulls tropomyosin out of the way.

Sarcoplasmic reticulum sequesters Ca2+ via SERCA Ca2+ ATPases. Action potential opens dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), releasing Ca2+ from sarcoplasmic reticulum.

Question 6

Describe the process of a muscle contraction stopping.

Answer 6

1. AP stops
2. ACh is degraded by AChE
3. DHPR and RyR close
4. Ca2+ ATPase pumps remove cytoplasmic Ca2+
5. Troponin-tropomyosin complex blocks actin-myosin binding sites
6. Muscle relaxes

Question 7

What is a twitch?

Answer 7

Twitch = excitation-contraction coupling.

Steep increase in tension, slow release as it takes longer to remove Ca2+ from cytoplasmic space.

Twitch is much longer than an AP, so multiple APs can sum up to increase tension.

Question 8

What is the neuromuscular junction?

Answer 8

NMJ is the point of contact where nervous system controls muscles.

Every muscle fibre has 1 NMJ.

Question 9

Describe a spinal reflex.

Answer 9

Neural circuit bypasses brain cord.

Stimulus - > Receptor -> Sensory neuron -> Spinal cord -> Somatic motor neuron -> Skeletal muscle

Known as a myosynaptic reflex - an involuntary contraction.

Question 10

Describe the 3 main fibre types:

Answer 10

Type 1 - Slow contracting, fatigue resistant

Type 2a - Fast contracting, fatigue resistant

Type 2b - Fast contracting, fast fatigue

Question 11

Describe the trade-off between contraction speed/force and susceptibility to fatigue

Answer 11

Slow motor units are for continuous activity (small forces, fatigue resistant)

Fast motor units are for high forces (fatigue quickly)

More myofibres = greater force

More motor units = greater dexterity.