The Neuromuscular Junction

Question 1

what is a neuromuscular junction?

Answer 1

The junction between a motor neuron and a myocyte (muscle fibre)
There are many neuromuscular junctions along the length of a muscle to ensure that all the muscle fibres contract simultaneously ensuring a powerful muscle movement.

Question 2

what does a NMJ allow for?

Answer 2

It allows for rapid and reliable neurotransmission
The strength of synaptic transmission at each NMJ virtually guarantees that all muscle fibres contract rapidly in response to every single action potential conducted.

Question 3

what is the role of Ca2+ in excitation- contraction coupling?

Answer 3

Action Potential in presynaptic terminal
Depolarisation opens voltage gated Ca2+ channels
Ca2+ influx into presynaptic terminal (down concentration gradient)
Ca2+ entry into presynaptic terminal triggers vesicle fusion
Ca2+ sensor is a synaptotagmin – it changes its conformation on Ca2+ binding to it which triggers vesicle fusion
Neurotransmitter released (e.g. Ach) by exocytosis

Question 4

what receptor is at the post synaptic membrane of a NMJ and what is its structure?

Answer 4

Nicotinic Acetylcholine receptor
5 Subunits of proteins
Two binding sites for ACh: So two molecules of ACh bind to confirmational change

Question 5

what happens at the nAChr when ACh binds to it?

Answer 5

ACh binds to nicotinic ACh receptors (ligand-gated ion channel) on postsynaptic membrane
Ligand gated ion channel: When ACh binds a conformational change occurs which opens the pore to allow ion movement

Question 6

what happens at a generic synapse (NOT NMJ)?

Answer 6

An excitatory neurotransmitter is released and will bind to an excitatory ionotropic receptor - this could be an ACh nicotinic receptor
2 molecules of ACh binding to the receptor causing Na+ influx and membrane depolarisation
Small excitatory post synaptic potential (EPSP)

Question 7

what components makes the pre- synaptic neurone of a NMJ a specialised synapse?

Answer 7

Lots of mitochondria
More vesicles containing neurotransmitter than any other synapse in the CNS
Multiple active zones for release of large amounts of neurotransmitter simultaneously
One action potential causes the release of 200-300 quanta, all releasing their 4000 molecules of ACh each (much more than a generic synapse)

Question 8

what components makes the post- synaptic neurone of a NMJ a specialised synapse?

Answer 8

Junctional folds
Very high density of nACh receptors
high density of voltage gated Na+ channels

Motor end plate is equivalent of post synaptic neurone in a generic synapse.

Question 9

what is an end plate potential?

Answer 9

the voltage produced solely by ACh binding to nAChR on motor end plate. They result in an action potential being generated

EPP is very large (compared to most synaptic potentials e.g. EPSP) – Many ACh vesicles released; high density of nAChRs

Question 10

how is EPP different to AP?

Answer 10

EPP and an Action potential are two separate events. AP produce by opening of VG Na+ ion channels

Timing: Pre-synaptic AP to EPP generation in post-synaptic membrane ~ 1 msec

Question 11

how is an AP generated at the NMJ?

Answer 11

The resting membrane potential at the motor end plate is -90mV compared to that of a neuron which is -65mV.
A lot of ACh released - ACh binds to nAChRs on postsynaptic end-plate membrane (2 ACh molecules required to activate receptor)
Thousands of nAChR receptors are activated - channel opens – permeable to both Na+ & K+
Na+ influx is much greater than K+ efflux
This results in a 70mV worth of depolarisation, and the end plate potential changes to -20mV (peak).
This massively exceeds the threshold value so VG Na+ ion channels are able to open (more Na+ influx) - generate an action potential.
This makes the synapse highly reliable i.e. will cause an action potential

Question 12

what is the fate of ACh after its bonded to the nAChr?

Answer 12

ACh binds to nAChR for ~ 1 ms
Released back into synaptic cleft - hydrolysed by Acetylcholinesterase (AChE) into choline and acetate.
Choline can then be taken up by the presynaptic neuron and used to make more ACh.

Question 13

as you move away from the motor end plate, what happens to EPP and AP?

Answer 13

EPP decays as you move further away from the motor end plate. Smaller EPPs are being generated as density of nAChRs decreases as you move from the synpase.
However VG Na+ ion channels are present all across the membrane of the muscle fibre
Therefore the size of action potentials remain constant as you move away from the motor end plate.
Red – EPP
Blue – action potential

Question 14

after the NMJ, what happens to the post synaptic AP?

Answer 14

Action potential invades T (transverse) -tubule system.
The T- tubules allows the transmission of the AP to move away from the motor end plate and to separate myofibrils to cause contraction
Sarcoplasmic reticulum is where the calcium needed for contraction is provided from in response to depolarisation

Question 15

what is a DHP receptor?

Answer 15

functions in skeletal muscle essentially as a voltage sensor, triggering intracellular calcium release
The DHP receptor is tethered to the ryanodine receptor (located Ca2+ release channel).

Question 16

what does depolarisation of the post- synaptic membrane cause?

Answer 16

Depolarisation of the membrane causes activation of this DHP receptor
The DHP receptor undergoes a conformation change which activates the ryanodine receptor and causes it to be removed from the sarcoplasmic reticulum. This causes opening of the calcium release channel.
This allows calcium out of the SR which then causes contraction – by the sliding filament mechanism.

Question 17

what happens during repolarisation of the membrane?

Answer 17

Repolarisation of the membrane, the Ca2+ releasing channel becomes closed
Preventing more release of calcium
And calcium which has been released is pumped back into the SR

Question 18

what would a single AP down an axon cause?
how can this be measured?

Answer 18

a twitch

This can be measured by measuring muscle tension and motor neuron activity.
Action potentials need to be fired at a frequency of 40 hertz (allow enough summation) to produce a sustained contraction.

Question 19

why does summation of twitches need to occur?

Answer 19

AP produces a contraction but that decays away, another AP produces another segment of contraction

individual twitches sum up at higher freq APs but still doesn’t produce smooth contraction

a smooth and sustained contraction is only achieved at a higher freq of 40Hz

Question 20

how does summation of twitches occur?

Answer 20

When you increase the frequency of twitches summation occurs and you get a tetanus.
The Tetanus contraction is sustained and smooth.
Our body stimulates muscles at a very high frequency meaning summation occurs much more quickly removing the ups and downs resulting in a smoother contraction.

Question 21

what is a disease that affects the NMJ?

Answer 21

Myasthenia Gravis

Question 22

what is Myasthenia Gravis?

Answer 22

affects 1:200,000
autoimmune disease of nAChR - reduced number of nAChR at NMJ - the synapse fails, becoming unreliable
Causes muscle weakness during a sustained activity
Affects (fatigues) those muscles which are most used e.g. skeletal eye muscles used in oculomotion - early sign is eyelid dropping and vision dropping.
Improves with rest and is better in the morning
Treatment is AChE inhibitors - blocks the enzyme breaking down ACh, so ACh lasts for longer time stimulating, amplifying activity at the synapse which prolongs signal
Medication: neostigmine (contains inhibitor)

Question 23

how does muscle firing and tension compare in someone with Myasthenia Gravis and someone w/o?

Answer 23

compound AP:
1 AP caused by many synapses firing at once
with someone with Myasthenia Gravis some of their individual synapses fail due to reduced no. of receptors being activated by ACh - results in a decrease in size of muscle tension

neostigmine amplifies signal (due to longer binding of ACh to receptor) so the size of AP remains the same and so does muscle tension

Question 24

what is prolonged binding?

Answer 24

Prolonged binding (e.g. due to no enzyme to metabolise or irreversible bind) of an agonist to a receptor (e.g. at a NMJ) will initially generate a lot of action potentials (causing twitching) but then eventually inactivate the voltage gated Na+ channels that mediate action potential firing (due to overactivity) and would thereby cease to function, leading to muscle paralysis.

Question 25

what are the long term effects of prolonged binding?

Answer 25

Prolonged binding can also have long term effect too. This is due to a phase 2 block: the e.g. nAChRs are desensitised due to prolonged stimulation. This could permanently damage the receptor from the membrane, and so the Acetylcholine doesn’t have anything to bind on to when it is released. This is reversible but it takes time as new nAChRs need to be synthesised on the membrane to replace the desensitised ones.
Prolonged action of a drug at a NMJ may be due to a specific enzyme that is meant to metabolise it not being present or mutated both due to a genetic defect.