Neuromuscular

Question 1

Describe the anatomy of a myosin molecule.

Answer 1

2 heavy chains wound together to form Y shape, 4 light chains on Y, each light chain contains ATPase

Question 2

Describe anatomy of actin.

Answer 2

Two wound molecules of F-actin containing myosin binding site (ADP).
ADP covered by tropomyosin
Tropomyosin has 3 types of troponin on it - troponin I, troponin C, troponin T

Question 3

What are the affinities for each troponin?

Answer 3

Troponin I - affinity to actin molecule
Troponin T - affinity to tropomyosin molecule
Troponin C - affinity for calcium

Question 4

How are the binding sites (ADP) on actin exposed?

Answer 4

Increase in extracellular calcium with AP -> binds to troponin C which causes conformational change in tropomyosin -> moves into grooves between F-actin strands, exposing myosin binding site

Question 5

What colour are slow/type I fibres and why?

Answer 5

Red muscle - increased myoglobin

Question 6

What colour are fast/type II fibres and why?

Answer 6

White muscle - decreased myoglobin
Contain lots of mitochondria and glycolytic enzymes

Question 7

What is a motor unit?

Answer 7

All muscle fibres innervated by same nerve

Question 8

What is the treppe effect and what is it caused by?

Answer 8

Increased muscle contraction strength with subsequent contractions
Probably occurs due to increase in cytosol calcium as more is released from the sarcoplasmic reticulum with each action potential, and failure to pump all back into SR after each contraction

Question 9

Why is rate of muscle degradation faster than rate of muscle development?

Answer 9

Probably due to ATP dependent ubiquitin proteasome pathway -> proteolysis

Question 10

Where does the NMJ typically occur in a muscle fibre (end or middle?)

Answer 10

Middle

Question 11

When the nerve is not being stimulated, what holds ACh vesicles to the cytoskeleton of the nerve?

Answer 11

Synapsin protein

Question 12

What causes release of ACh vesicles from the cytoskeleton?

Answer 12

Action potential down nerve triggers opening of voltage gated calcium channels = influx of calcium. This acts on calcium-calmodulin dependent protein kinase -> phosphorylates synapsin protein meaning vesicles are released

Question 13

Why is fatigue of the NMJ rare?

Answer 13

Has a high safety factor, meaning energy in action potential is 3x that needed to generate ACh release (however can get ACh depletion)

Question 14

When ACh binds to its receptor in the subneural cleft, how does this trigger muscle contraction?

Answer 14

ACh binding with receptor triggers opening of an ACh dependent ion channel - large enough for movement of Na/K/Ca (mostly Na) -> sodium enters muscle fibre which creates local positive potential inside muscle fibre (END PLATE POTENTIAL)
Then depolarises channels around it and AP spreads

Question 15

What does acetylcholinesterase break ACh into? Which part is recycled?

Answer 15

Acetate ion + choline.

Choline recycled - reabsorbed by nerve terminal

Question 16

How does muscle action potential differ to nerve action potential (3 ways)?

Answer 16

1. Resting membrane voltage more negative in muscle (-80- -90)
2. Duration of the action potential is longer in muscle (5x as long)
3. Velocity of conduction is slower in muscle

Question 17

What are transverse tubules and why are they needed?

Answer 17

T tubules are extensions of the cell membrane into muscle fibre - needed as muscle fibres are large, change in surface action potential does not spread deeper. Therefore T-tubules help it spread down muscle fibre.
T tubules go through fibres to activate myofibrils

Question 18

What is excitation contraction coupling?

Answer 18

An action potential causing increase in intracellular calcium therefore causing contraction

Question 19

What is stored in the sarcoplasmic reticulum?

Answer 19

Ca2+

Question 20

Name the 2 main parts of the sarcoplasmic reticulum and their functions.

Answer 20

1. Terminal cisternae - abut T tubes so when T-tubules depolarise current flows into the terminal cisternae
2. Longitudinal tubules - surround myofibrils =

Question 21

After muscle contraction, how is calcium moved out of the myocyte cytoplasm? (skeletal muscle)

Answer 21

1. SERCA pump (needs ATP) pumps calcium back into SR
2. Calsequestrin protein in sarcoplasmic reticulum which binds calcium

Question 22

What receptor in skeletal muscle sarcoplasmic reticulum cisternae detects change in voltage, activates and therefore causes opening of calcium release channels?

Answer 22

Ryanodine receptor channels (also called dihydropyridine receptors)

Question 23

What are the 3 types of acquired myasthenia gravis?

Answer 23

Focal
Generalised
Fulminant - acute, rapidly progressing tetraparesis

Question 24

What is the cause of acquired myasthenia gravis?

Answer 24

Autoantibodies against the ACh receptor, leading to destruction

Question 25

How do you diagnose ACh?

Answer 25

Gold standard = demonstrating anti-ACh antibodies in serum (species specific, 98% canine cases detected) Could also consider edrophonium Rx test (not widely available anymore) or neostigmine Rx test. Edrophonium v short acting (mins), neostigmine lasts a few hours. BUT NOT SPECIFIC - may improve with other Dz

Question 26

What is a side effect of acetylcholinesterase drugs?

Answer 26

Precipitation of a cholinergic crisis - bradycardia, respiratory depression, SLUDGE signs

Question 27

What breeds are overrepresented for acquired myasthenia gravis - name 5

Answer 27

Akita Golden retriever GSD German short haired pointer Chihuahua

Question 28

How do you treat acquired myasthenia gravis?

Answer 28

1. Acetylcholinesterase drug (pyridostigmine most common) 2. Immunosuppression - 2nd agent common, pred questionable due to muscle weakness etc

Question 29

What 2 scenarios may anti-acetylcholine receptor antibodies be negative in? What should you do to confirm diagnosis if this is the case?

Answer 29

Very early in disease If treating with steroid Retest in 1-2m

Question 30

How do smooth muscle action and myosin filaments differ from skeletal muscle?

Answer 30

No Z disc - instead have dense bodies from which actin radiates. Myosin in the middle No troponin complex - binding of calcium instead controlled by CALMODULIN

Question 31

Describe 3 differences in smooth muscle contraction vs skeletal.

Answer 31

1. Muscle fibres of smooth muscle can shorten much further as myosin has side polar cross bridges (one side pulls one way whilst another side pulls the other) 2. Prolonged tonic contraction due to slow cycling of myosin heads due to reduced ATPase activity (reduced letting go) and also due to latch mechanism = small amount of energy needed to maintain contraction 3. Stress relaxation/reverse stress relaxation property - can maintain constant pressure despite volume change

Question 32

Where does smooth muscle get its calcium from compared to skeletal muscle?

Answer 32

Very underdeveloped SR therefore most calcium extracellular

Question 33

Once an action potential spreads and intracellular calcium increases, how does this cause smooth muscle contraction?

Answer 33

Calcium binds to calmodulin (regulatory protein) which activates a myosin light chain kinase This myosin light chain kinase enzyme then phosphorylates the regulatory chain of the myosin light chain head -> can then bind to actin

Question 34

How is smooth muscle contraction stopped?

Answer 34

1. Ca pump pumps Ca out of cell 2. Need to switch off the myosin light chain head (dephosphorylate it) -> this is done by myosin phosphatase in cytosol -> removes phosphate

Question 35

How does the resting membrane potential of smooth muscle differ to skeletal muscle?

Answer 35

Less negative -50 to -60 (compared to -80 to -90 in skeletal)