Muscle Physiology

Question 1

Describe skeletal muscle.

Answer 1

• Cylindrical
• Under voluntary control
• Multinucleated
• Striated

Question 2

Describe smooth muscle.

Answer 2

• Under involuntary control
• Mononucleated
• Non-striated
• Found in hollow organs

Question 3

Describe cardiac muscle

Answer 3

• Under involuntary control
• Mono/binucleated
• Striated
• Branched

Question 4

How are skeletal muscle fibres arranged?

Answer 4

• Muscle composed of groups of muscle fibres
• Fibres bundled into fascicles
• Within muscle fibres are myofibrils

Question 5

Describe muscle fibre cell structure. PART 1

Answer 5

• Cells wrapped in sarcolemma
• Invaginations in sarcolemma - T-Tubules descend into cells and wrap around myofibrils
• Tubules increase SA for ion transfer between ECF and ICF

Question 6

Describe muscle fibre cell structure. PART 2

Answer 6

• SR wrapped around myofibrils - store of calcium ions
• Terminal cisternae at ends of SR
• Point where cisternae meets T-tubules is the triad

Question 7

Describe myofibril structure.

Answer 7

• Striated due to intraccellular cytoskeletal proteins
• Made up of myosin and actin chains
• Functional units are sarcomeres
• Tropomyosin wrapped around actin chains with troponin on its surface

Question 8

What is the relationship between muscle fibres and motor neurons?

Answer 8

• Muscle fibre innervated by single motor neuron
• Motor neurons. innervate multiple fibres
• In areas that require fine control, motor neuron will only innervate few muscle fibres.
• OPPOSITE FOR AREAS THAT DON’T NEED FINE CONTROL

Question 9

Outline what occurs at the neuromuscular junction. PART 1

Answer 9

• Pre-synaptic terminals release ACh which bind to post-synaptic nAChRs
• Ion channels activate causing sodium influx and cause action potential
• Wave of depolarisation induced which reaches T-junction.

Question 10

Outline what occurs at the neuromuscular junction. PART 2

Answer 10

• Wave of depolarisation reaches T-tubules
• Activates VGCCs allowing influx of extracellular calcium into cell
• Channels connected to RyRs on wall of SR
• When RyRs activated, conformational change in RyRs

Question 11

Outline what occurs at the neuromuscular junction. PART 3

Answer 11

• RyR activated and calcium flows into cell
• Rise in intracellular calcium and myofibrils contract
• Removal of calcium through SERCA2 (back into SR) and sodium-calcium exchanger (out of cell in exchange for sodium)
• Myofibrils relax

Question 12

What happens in the cross bridge cycle? PART 1

Answer 12

• When relaxed, actin chains bound by troponin and tropomyosin. Prevent myosin binding
• Myosin bound by ADP, are ‘cocked’
• Calcium binds to troponin C following rise in intracellular calcium
• Myosin binding sites are exposed as troponin released

Question 13

What happens in the cross bridge cycle? PART 2

Answer 13

• Myosin binds
• Conformational change causing movement of myosin head
• Causes movement of Z bands closer together
• ADP released from myosin and ATP binds causing release of myosin head from actin.

Question 14

What happens in the cross bridge cycle? PART 3

Answer 14

• Reduction in intracellular calcium. Troponin reinstated and binding sites blocked
• ATP hydrolysed back to ATP
• Myosin head moved back into cocked position

Question 15

Why does rigor mortis occur?

Answer 15

• Rise in intracellular calcium
• Actin binding sites exposed
• Myosin binds
• No ATP available to remove myosin heads from actin binding sites
• Continuous contraction

Question 16

Describe Type 1 fibres

Answer 16

• ‘Slow twitch’ - used when long-term sustained contractions requireed
• ATP synthesised by oxidative metabolism
• Require high oxygen supply so large amounts of myoglobin to facilitate this

Question 17

Describe Type IIa fibres.

Answer 17

• Utilise oxidative and anaerobic metabolism
• Anaerobic metabolism - fast but inefficient so contraction is rapid
• Oxidative metabolism allows fibres to be slightly fatigue resistant

Question 18

Describe Type IIb fibres.

Answer 18

• Fast twitch - reliant on anaerobic metabolism
• Don’t have large amounts of myoglobin/mitochondria
• Easily fatigable

Question 19

Describe cardiac muscle

Answer 19

• Muscle cells joined by gap junctions
• Allows for spread of depolarisation to neighbouring cells (initiated from SAN)

Question 20

Describe electrical-contraction coupling in cardiac muscle.

Answer 20

• Calcium entry into cell from extracellular spaces at T-tubules
• Activates RyRs - causing calcium release from SR
• Calcium interacts with myosin and actin to cause contraction
• Removal by SERCA and NCX exchanger

Question 21

Describe multiunit smooth muscle cells.

Answer 21

• Exist as separate entities stimulated by their own motor nerves
• Behave independently
• Have swellings called varicosities which are pre-synaptic terminals

Question 22

Describe unitary smooth muscle cells.

Answer 22

• Most common type
• SMCs joined by gap junctions - allowing for passage of depolarising ions from cell to cell
• Only a few muscle cells need to be innervated by autonomic neurons
• Portion of muscle that contracts are functional units

Question 23

Describe autonomic-mediated smooth muscle contraction

Answer 23

• Activation of Gq linked GPCRs
• Increase intracellular IP3
• Binds to IP3 receptors on SR causing calcium release into cytosol

Question 23

What is another autonomic mediated method of muscle contraction?

Answer 23

• Opening of calcium ion channels
• Activates ryanodine receptors in SR - calcium released into cytosol
• Calcium channels close once threshold met
• Activity of neighbouring cells influenced by gap junctions

Question 24

What is the difference between skeletal/cardiac and smooth muscle contraction?

Answer 24

• Smooth muscle contraction - dependent on calcium influx (not sodium)
• Smooth muscle contraction slower - calcium channels open slower than sodium channels