Neuro 3 - the NMJ

Question 1

What is an NMJ?

Answer 1

A specialised chemical synapse between a motor neuron and a muscle fiber - motor end plate

Question 2

What are the main structure constituting the NMJ?

Answer 2

• pre synaptic terminal “bouton”
• synaptic cleft
• postsynaptic endplate region on the muscle fibre - highly folded membrane

Question 3

What is particular about the innervation of muscle fibers?

Answer 3

they receive input from a SINGLE neuron. This neuron can innervate multiple muscle fibers.

Question 4

What are the steps to initiate muscle contraction? (7)

Answer 4

1. AP opens VGCC
2. Ca2+ enters
3. Vesicle Exocytosis triggered by Ca influx
4. Acetylcholine diffuses across cleft
5. ACh binds to receptor cation channel (nAChr) and opens channel
6. AP triggered and spreads along surface membrane
7. Acetylcholine esterase breaks down ACh to stop muscle fibre response.

Question 5

What is the neurotransmitter for skeletal muscle?

Answer 5

Acetylcholine

Question 6

What happens at rest at the NMJ?

Answer 6

Vesicles release ACh at very low rate causing miniature end-plate potentials (MEPPs) not big enough to cause AP - contraction

Question 7

List disorders of the NMJ and which part they affect

Answer 7

Botulism - toxin disrupts ACh exocytosis
Myasthenia Gravis - autoimmune disorder against ACh receptors
lambert-Eaton myasthenic syndrome (LEMS) - autoimmune disease agains the VGCC - associated with lung cancer

Question 8

What is the structural arrangement of muscle?

Answer 8

Muscle (cells, blood vessels etc) surrounded by epimysium –> fasicle = bundle of muscle cells surrounded by perimysium –> Muscle fiber cell / myofibre; seperated by endomysium –> myofibril; numerous sarcomeres in series –> myofilament –> sarcomere- contract unit
So in muscle fiber, have myofibrils (round in rounds)
myofibres

Question 9

What separates sarcomeres?

Answer 9

Z disk

Question 10

What is A-band composed of?

Answer 10

A-band is dArk and composed of myosin

It is the delimitation of thick filaments

Question 11

What is the I-band composed of

Answer 11

I-band is LIght and composed of actin

It is the space between A band because thin and thick filaments overlap

Question 12

Explain the steps in the sliding filament theory

Answer 12

1. AP propagates along sarcolemma and T-tubules
2. Dihydropyridine receptor in T-tubule opens Ryanodine receptor in Sarcoplasmic Reticuluum
   Releasing Ca2+ (as a response to AP – DHOR senses change)
3. Ca2+ binds to troponin which remoes the blocking action of tropomyosin. Exposes myosin binding site on actin filament
4. Myosin heads form cross-bridges with actin filament  movement
5. Ca2+ is actively transported into SR continuously using ATP
6. When AP stops, free Ca2+ declines. Ca2+ dissociated from troponin. Tropomyosin returns to position and blocks the troponin again. Cross-bridges detach, muscle force decreases  end of movement