Physiology of the Skeletal Neuromuscular Junction

Question 1

What is skeletal muscle innervated by?

Answer 1

Fast conducting ⍺-motor neurones with myelinated axons and cell bodies in the spinal cord or brain stem

Question 2

What occurs to the myelinated axons of ⍺-motor neurones when they are near the muscle?

Answer 2

The axon divides into unmyelinated branches that innervate an individual muscle fibre

Question 3

What makes up a motor unit?

Answer 3

The motor neurone and number of muscle fibres that it innervates is the motor unit

Question 4

What occurs to motor neurone branches at the neuromuscular junction?

Answer 4

The individual branches further divide into multiple fine branches that end in a “terminal bouton” - a chemical synapse with the muscle membrane at the neuromuscular junction

Question 5

What occurs at the terminal boutons?

Answer 5

Action potentials arising in the cell body are conducted via the axon to the boutons, causing the release of ACh

Question 6

Name key features of a skeletal neuromuscular junction

Answer 6

• Terminal bouton (and surrounding Schwann cell)
  -Synaptic vesicles
  -Synaptic cleft
  -End plate region (sarcolemma)

Question 7

Where do synaptic vesicles release their contents?

Answer 7

Active zones

Question 8

Where are nicotinic ACh receptors located?

Answer 8

Located at regions of the junctional folds that face the active zones

Question 9

Describe the 3 steps of ACh synthesis and storage

Answer 9

1. Choline transporter takes choline into pre-synaptic terminal (symport with Na+)
2. ACh is synthesised from choline and acetyl CoA by choline acetyltransferase
3. ACh is concentrated in vesicles by the vesicular ACh transporter where they are stored until an action potential arrives

Question 10

Describe the 3 events that lead to the release of neurotransmitters into the synaptic cleft

Answer 10

1. Arrival of the action potential at the terminal leads to the opening of voltage-gated Ca2+ channels (triggered by the depolarisation)
2. Influx of Ca2+ into the terminal causes neurotransmitter vesicles to travel to the active zones at the presynaptic membrane
3. These vesicles fuse with the cell membrane (exocytosis) causing neurotransmitter release into the synaptic cleft

Question 11

How many ACh molecules activate each nicotinic ACh receptor?

Answer 11

Two

Question 12

Describe structure of nAChrs

Answer 12

Pentamers of glycoprotein subunits surrounding a central cation selective pore which contains a gate that opens when ACh binds to the exterior of the receptor

Question 13

What happens when the gate opens on nAChrs?

Answer 13

-There is simultaneous Na+ influx and K+ efflux
-Because the driving force for Na+ is greater than for K+ at resting membrane potential influx of Na+ is greater than efflux of K+ - rapid depolarising end plate potential is generated by the simultaneous opening of many nAChRs

Question 14

How is an endplate potential produced?

Answer 14

• The electrical response to one quantum (ACh vesicle) of neurotransmitter is a miniature endplate potential
• Many miniature end plate potentials summate to produce the endplate potential - a graded response

Question 15

How is a contraction initiated?

Answer 15

An endplate potential that exceeds the threshold triggers the opening of voltage activated Na+ channels around the endplate - initiating an “all or nothing” contraction (normally always occurs)

Question 16

How many action potentials are needed to bring about a twitch (contraction) of a muscle fibre?

Answer 16

One action potential in the motor nerve triggers one action potential in the muscle and a subsequent ‘twitch’ (contraction) of the muscle fibre

Question 17

Why are voltage-activated Na+ channels required?

Answer 17

Having a muscle fibre with voltage activated Na+ channels means that the action potential propagates from the endplate over the length of the muscle fibre - contraction

Question 18

What causes the contraction?

Answer 18

-The muscle action potential causes contraction by release of Ca2+ from the sarcoplasmic reticulum
- The action potential travels over the surface membrane of the muscle fibre and down the T tubules which are close to the SR (where Ca2+ is stored)

Question 19

How does release of calcium lead to a contraction?

Answer 19

The release of Ca2+ causes contraction by interacting with troponin associated with the myofibrils

Question 20

What does acetylcholinesterase do?

Answer 20

It hydrolyses ACh into choline and acetate at the endplate membrane

Question 21

What happens to choline and acetate?

Answer 21

• Choline is taken up by the choline transporter
• Acetate diffuses from the synaptic cleft

Question 22

Describe in what way acetylcholinesterase is very efficient…

Answer 22

It hydrolyses some ACh molecules even prior to ACh binding to receptors, once unbinding occurs virtually all ACh molecules are hydrolysed

Question 23

Why is it important that acetylcholinesterase is efficient?

Answer 23

Limits rebinding and means the endplate potential is terminated within a few miliseconds

Question 24

Name 2 types of therapeutical agents that bind to acetylcholinesterase

Answer 24

-Anti-cholinesterases (reversibly bind)
-Insecticides and nerve gases used in war/terrorism (irreversibly bind)

Question 25

What are symptoms of neuromytonia (or Isaac’s syndrome)?

Answer 25

-Cramps
-Stiffness
-Myotonia (slow relaxation)
-Fasiculations (muscle twitches)

Question 26

What occurs in neuromytonia? What does repetitive firing in motor neuroneslead to?

Answer 26

-Can be acquired, usually autoimmune
-Antibodies against voltage-activated K+ channels in the motor neurone disrupt functioning resulting in hyperexcitability and repetitive firing
- Repetitive firing → prolonged endplate potential and repetitive potential discharge in skeletal muscle fibres

Question 27

What drugs can you use to treat neuromytonia? (2 examples)

Answer 27

Anti-convulsants (e.g. carbamazepine, phenytoin) which block voltage-activated Na+ channels

Question 28

What are symptoms of Lambert-Eaton Myasthenic Syndrome (LEMS)?

Answer 28

-Muscle weakness in the limbs (may transiently improve upon exertion)

Question 29

How common is LEMS?

Answer 29

-Very rare, associated with small cell carcinoma of the lung

Question 30

What occurs in LEMS?

Answer 30

-Commonly autoimmune
-Antibodies against voltage activated Ca2+ channels in the motor neuron terminal result in reduced Ca2+ entry in response to depolarisation → reduced ACh release

Question 31

What drugs can you use to treat LEMS? Example

Answer 31

-Anticholinesterases (e.g. pyridostigmine) which increases duration of ACh in the synaptic cleft and potassium channel blockers (e.g. 3,4-diaminopyridine) which increase the release of ACh by prolonging the action potential in the motoneurone terminal

Question 32

What are symptoms of Myasthenia Gravis (MG)?

Answer 32

-Progressively increasing muscle weakness during periods of activity (fatiguability)

Question 33

What occurs in MG?

Answer 33

-Commonly autoimmune
-Antibodies against nAChr in the endplate results in reduction in the number of functional channels → amplitude of endplate potential decreases

Question 34

What drugs can you use to treat MG? Examples

Answer 34

-Anticholinesterases (e.g. pyridostigmine) which increase the concentration of ACh in the synaptic cleft
-Immunosuppressant agents (e.g. azathioprine)

Question 35

What is botulinum toxin and where does it act?

Answer 35

An extremely potent exotoxin that acts at motor neurone terminals to irreversibly inhibit ACh release leading to muscle weakness or paralysis

Question 36

How does the botulinum toxin inhibit ACh release?

Answer 36

Enters presynaptic terminals to enzymatically modify proteins involved in the docking of vesicles containing ACh to the presynaptic membrane, which prevents exocytosis

Question 37

What can the botulinum toxin cause? How can it be used positively?

Answer 37

• Botulism (high death rate, recovery takes weeks as anti-cholinesterases are ineffective as therapy)
• Low dose botulinum haemaglutin complex can be administered IM to treat overactive muscles (dystonias) and also ‘botox’ for wrinkles

Question 38

What are ‘curare-like’ compounds? What can they be used for clinically?

Answer 38

Act as competitive antagonists of nAChr so interfere with the postsynaptic action of ACh
- Endplate potential will be reduced to below the muscle fibre threshold potential
- Used clinically to induce reversible muscle paralysis in certain types of surgery (vecuronium, atracurium)