NEUR30080 L4 SQ synaptic transmission at NMJ

learning goals

NMJ vs CNS synapse

motor neuron

cell body in the ventral horn

myelinated up until microns of the terminal

schwann cells

myelinating or terminal

the cleft

about 50 nm

filled with basal lamina

junctional folds

in the sarcolemma

one branch

one fibre

motor neuron and fibres

ONE motor neuron innervates many fibres, but each fibre is innervated by exactly ONE neuron

anatomy of NMJ image

NMJ and schwann cells image

oligodendrocytes

on axon in CNS

schwann cell

on axon in PNS

postsynaptic fibre: receptor localisation

• AChRs are concentrated on crests of junctional folds at approx. 10,000 per um2

• among the highest memb protein densities recorded

postsynaptic fibre: Nav1.4 localisation

• voltage-gated Na+ channels are confined to the depths of the folds (not crests)

postsynaptic fibre: intervening distance

• the EPP must spread a fraction of a micron down the wall of each fold to reach that Nav population

postsynaptic fibre: negligible attenuation

• the distance is small relative to lambda, so attenuation of the EPP between crest and fold depth is negligible

postsynaptic fibre: summary

the folds place a high density of Nav1.4 within a fraction of a micron of the receptor array, so endplate current reaches those channels essentially undiminished

EPP

end plate potential

development of NMJ: agrin

the motor nerve terminal releases agrin into the synaptic basal lamina

• agrin binds Lrp4, which activates the associated MuSK receptor tyrosine kinase

development of NMJ: rapsyn and AChR

• rapsyn is recruited to the sarcolemma downstream of MuSK activation

• AChR anchored to the post-synaptic memb (on the muscle fibre)

• the Lrp4-MuSK complex anchors AChesterase in the basal lamina through ColQ

synaptic vessicle image

AP propagation: 7 steps

one pre-syn AP produces one muscle AP

where each step happens image

non-selective cation channel: cooperative binding

• 2 ACh molecules must bind at separate subunit interfaces, a single molecule does not open the channel

non-selective cation channel: non-selective cation conductance

the pore passes Na+ and K+ without meaningful discrimination between them

non-selective cation channel: reversal potential

= approx. 0 mV

the permeability-weighted midpoint between ENa and Ek

non-selective cation channel: driving force

with muscle Vm at approx. -85 mV, the inward driving force is approx. 85 mV

non-selective cation channel image

non-selective cation channel: I = g(Vm - E)

adult isoform of the receptor

brief openings and higher single-channel conductance

embryonic isoform of receptor

prolonged openings and lower single-channel conductance

which receptor isoform is this?

embryonic

which receptor isoform is this?

adult

developmental switch

gamma is replaced with e as the synapse matures, and the substitution reverses following denervation

distinct from the neuronal receptor

the alpha7 receptor (from lecture 1) is a related but structurally distinct (different) protein

embryonic and adult receptors image

the sequence image

spontaneous activity at the resting endplate

• at rest, the endplate shows small spontaneous depolarisations - miniature EPPs (MEPPs)

• when the nerve is stimulated, the memb potential is depolarised. The EPP triggers an AP when it reaches threshold

• recordings turned out to be similar to the responses at synapses elsewhere in the NS. MEPPs and EPPs are thus particular forms of excitatory post-synaptic potential (EPSP)

spontaneous activity at the resting endplate image

electrophysiology: ACh image

every response is multiple of same unit

1. lower external Ca2+: release becomes unreliable, so responses fluctuate

2. peaks evenly spaced: at 1x, 2x, 3x the size of one spontaneous blip

3. therefore, discrete packets: not a continuous stream of NT

a quantum: the unit

a quantum: the unit - quantal size and content

quantal size and content table

Q:

Ca2+ is trigger

• conc dependence: reducing Ca2+ diminishes EPP, raising it increases EPP

• pharmacological manipulation: The Kv blockers prolong the pre-syn spike, admitting more Ca2+ and releasing more quanta

• quantal content as readout: most pre-syn manipulations act on quantal content rather than size

calcium in, transmitter out

safety factor

myesthenia gravis

passive transfer

myesthenia gravis vs lambert-eaton syndrome

learning outcomes