W13 - Neuromuscular Junction Flashcards

1
Q

Explain synaptic transmission at the neuromuscular junction

A

These stages can be influenced by drugs and toxins
1. Synthesis
- from precursor component parts w/ enzymes
2. Storage
i) protect from metabolic enzymes
ii) package quanta (1 vesicle) containing multiple neurotransmitters
3. Release
- Ca+ enters pre-synaptic voltage-gated channels -> binds to protein -> causes exocytosis of vesicles
4. Activation
- changes in excitation of post-synaptic
5. Inactivation
- specialised effectors recognise & re-uptake
- enzyme breaks down neurotransmitters into component parts

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2
Q

Explain how drugs can enhance synaptic transmission.

A

A) Direct stimulation
- the natural transmitter
- analogues
B) Indirect stimulation
- increased transmitter release
- inhibition of transmitter removal

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3
Q

Explain how drugs can inhibit synaptic transmission

A

A) Blocking synthesis, storage or release from pre-synaptic neurons
B) Blocking post-synaptic receptors

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4
Q

Define agonists

A

Drugs, hormones or transmitters that bind to specific receptors + initiate conformational change in receptors
- have both affinity and efficacy
- affinity: ability to bind
- efficacy: ability to initiate a biological response once bound
- the receptor only recognises the precise ligand/agonist, and activates them
- reversible

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5
Q

Define antagonists

A

Drugs, hormones or transmitters that bind to receptors but don’t activate them
- have affinity but lack efficacy
- blocks receptor activation (by agonist)

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6
Q

Explain competitive receptor antagonists

A
  • competes with agonist for agonist binding site
  • block can be reversed by increasing agonist concentration
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7
Q

Classify the two types of nicotinic acetylcholine receptors

A
  • acetylcholine (ACh) is the neurotransmitter at the NMJ
    • pre-synaptic neurons that synthesise and release ACh = cholinergic
    • ACh acts on receptors called cholinoceptors
  • Nicotinic cholinoceptors (nAChRs)
    • gated-ion channel
    • activated by ACh or nicotine (tobacco alkaloid)
    • NOT muscarine
  • Muscarinic cholinoceptors
    • G-protein (GPCRs)
    • activated by ACh or muscarine
    • NOT nicotine
    • slower than nicotinic cholinoceptors
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8
Q

Explain how fast synaptic transmission occurs with transmitter-gated ion channels

A

Eg. Nicotinic acetylcholine receptor nAChR
- Cation conducting channel (Na+ in, K+ out)
- Agonist binding -> rapid conformational change -> open channel
- selective for certain ions
- signalling is extremely rapid (millisec)

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9
Q

Explain how the patch-clamp technique can be used to record the functional properties of single receptors

A
  • glass micropipette forming a tight gigaohm seal (suction) with the cell membrane
  • contains a wire bathed in an electrolytic solution to conduct ions
  • allows to report activity of single receptor, (a dip in graph = opening of channel)
  • pA = 10^-12A (i.e. 1 million millionth of an Amp)
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10
Q

Explain the structure of neuromuscular junction

A

The motor axon is surrounded by α-bungarotoxin binding once identifying the nicotinic receptors
- α-bungarotoxin: protein that binds irreversibly
- shows up red on scans (?)

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11
Q

Explain how mepps at the NMJ are recorded + the mechanism

A
  • i.e. small depolarisations, via a glass electrode (end plate region)
  • release of ACh from vesicles
    • 1 vesicle (multiple of neurotransmitter) = multiple nAChR
      • exocytosis (coordinated to sharp increase in recording)
    • activation of nAChR => cation channels open -> flux of Na+ ions into muscle fibres -> local depolarisation at endplate region
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12
Q

Explain how black widow spider venom influences spontaneous transmitter release

A
  • Eg. Black Widow spider venom (α-LATROTOXIN –α-LTX) influences spontaneous transmitter release
    • 10-15 mins: massive ACh release → muscle spasms
    • 1 hr: ⇒ depletion of vesicles
      • inhibition of exocytosis
        • presynaptic neuron changes shape (expands); acts as if there is an extra membrane layered inbetween, (no exocytosis)
      • distended terminal paralysis
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13
Q

Explain a simplified model of “fast” synaptic transmission

A
  1. presynaptic AP
  2. a synchronous localised Ca2+ influx via voltage-gated Ca2+ channels
    • bind to sensor proteins on vesicles (approx. 0.1 ms)
  3. vesicles undergo exocytosis -> release of ACh
  4. activation of nAChR
  5. large depolarisation of the endplate region of the muscle cell (an epp; made up of multiple mepps)
  6. depolarisation is large enough => postsynaptic voltage-gated Na+ channels to initiate an action potential
  7. K+ exiting the cell bring depolarisation back down
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14
Q

Explain how Mg2+ and Ca2+ extracellular solution reduces

A
  • Magnesium blocked the voltage-gated Ca2+ ion channel
    • high magnesium and low calcium = reduced epp (not enough to cross the threshold)
      • no muscle AP = no contraction
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15
Q

State how to calculate quantal content

A
  • the amplitude of the epp is a multiple of the amplitude of mepp (smallest epp amplitude = mepp amplitude)
  • quantal content
    • mean EPP amplitude (mV)/mean MEPP amplitude (mV)
      • i.e. no. of vesicles/stimulus
    • release of a vesicle gives a “quanta” of transmitter
      • each quanta -> mepp via activation of nAChR
        • mepps are spontaneous (w/o nerve stimulation)
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16
Q

Explain how cholinergic transmission can be blocked at the synthesis step

A
  1. From exocytosis, ACh not only activated channels, but also AChE (enzyme) on post synaptic neuron
  2. ACh is broken down into acetate and choline, and there is a reuptake of choline into the presynaptic neuron
  3. The choline acetyl transferase (CAT) synthesises ACh from choline and Acetyl Coenzyme A precursor from mitochondria
  • the reuptake of choline is Na+-dependent
    • can be blocked by hemicholinium 3 => less ACh in each vesicle
    • amplitude of epp and mepp both decreased i.e. no change in quantal content
17
Q

Explain how cholinergic transmission can be blocked at the storage step

A
  • after ACh is synthesised, the transport into the vesicles can be blocked by inhibition of the ACh vesicular transporter
    • by Vesamicol
    • both epp and mepp decreased (no change in QC)
18
Q

State the effects of tetrodotoxin on cholinergic transmission

A
  • produced by bacteria; concentrated in organs of the puffer fish (e.g. liver)
  • causes paralysis of the diaphragm => respiratory failure
  • more potent than local lidocaine
  • blocks Na+ channels (i.e. no AP, no release, no EPP)
19
Q

State the effects of conotoxins on cholinergic transmission

A
  • by cone snail
  • voltage-gated Ca2+ channels blocked
  • decrease in Ca2+ influx -> decrease in release
  • EEP decrease, no mepp change, decrease in QC
20
Q

State the effects of dendrotoxin in on cholinergic transmission

A
  • from green mamba
  • blocks voltage-gated K+ channels -> prolonged AP -> increase in Ca2+ influx -> more release
  • increased epp amplitude, no change in mepp amplitude (QC increased)
  • the synthetic drug 4-aminopyridine (A-AP) has a similar mechanism
21
Q

State the effects of botulinum toxin in on cholinergic transmission

A
  • most potent toxin from clostridium botulinum bacteria
  • respiratory paralysis
  • cosmetically used as botox, can also be used for chronic migraines
  • blocks vesicle fusion by cleaving a vesicular protein required for exocytosis -> decreased release
  • epp amplitude decreases; no change on mepp amplitude
    • QC decrease
22
Q

Describe the function of the organ bath

A
  • Dissect the phrenic nerve hemi-diaphragm preparation & mount in an organ bath.
  • Electrically stimulate the phrenic nerve 1/10 sec & record the resultant muscle twitch on a chart recorder or computer.
23
Q

How does tubocurarine produce skeletal muscle relaxation at the NMJ

A
  • muscle relaxant
  • only in blood supply
  • decreases twitch tension
    • decreases depolarisation -> not sufficient for epp to pass threshold for muscle fibre AP generation
  • competes with muscle nicotinic ACh receptors
  • works post-synaptically, but not pre-synaptically
24
Q

State the conditions of tubocurarine action on a synapse

A