Chemical synapses & Motor End Plate of the Muscle

Fig. 3.1 AP--> depolarize the presynatpic terminal
---> release of a transmitter substance into the synaptic cleft
---> binding to specific receptor---> open channels
---> affect MP (if depolorize to threshold level---> trigger AP)

Chemical synapses:
1) 30-50 nm distance between pre- and post-synaptic membrane
(adjacent extracellular space: typically 20 nm)
2) cytoplasmic discontinuity
3) ultrastructure: presynaptic active zone, synaptic vesicles, postsynaptic receptors
4) chemical transmitter에 의한 transmission
5) synaptic delay:at least 0.3 ms, usually 1-5 ms or longer
(presynaptic AP, increased presynaptic Ca2+ permeabilty,
Ca2+ influx, release of NT by exocytosis of vesicles,
reaction of transmitter with postsynaptic receptors,
activation of synaptic channels; synaptic currents produces
postsynaptic potential, postsynaptic AP)
6) direction of transmission: unidirectional

mediate either excitatory or inhibitory actions
tend to be involved in behaviors more complex than electrical synapse.
are more flexible (plasticity--> higher brain functions)
amplification of neuronal signals--> generates large PSP
(each vesicle contains several thousand molecules of NTs)

Motor End Plate of the Muscle
two major types of chemical synaptic transmission
differ according to the possynaptic receptors
directly and indirectly (by means of a second messenger)
gating ion channels
Fig. 10-1, Neuromuscular junction
end plates: a motor nerve fiber brances (synaptic boutons)
end on a skeletal muscle fiber
best-known synapses: directly gatged

Fig. 3.2: end-plate potential(EPP):1950년도, Paul Fatt, Bernard Katz가 연구
motor neuron 자극하면서
microelectrode inserted into the muscle fiber
acetylcholine이 neurotransmitter로서 release
nicotinic ACh receptor에 binding
rapid depolarization and repolarization (time constant 5ms)
보통 약 70 mV (much larger than other CNS synaptic potential: 1 mV)
End plate에서 떨어져서 측정: end-plate potential이 작아짐
(because ACh receptors confined in the end plate region).
electrotonic potential의 일종
End-plate current 측정 (voltage clamp).
inflow of the current restricted to the region of the end plate.
away from end plate: current flows back of the muscle fiber.

Fig. 3.3
MP을 voltage clmap로 변화시킴: -120mV---> +38 mV.
end-plate에서 excitatory postsynaptic current(EPSC)측정.
at -10 mV: end-plate current reverses its direction (reveral potential).
relatively nonspecific increase in the membrane conductance for
Na+ and K+. (Takeuchi) (same in excitatory synapses in CNS)
Transmitter-gated channel들은 따라서 votage-gated channel과 다르다.
(large, nonselectivity (Ca2+, Na+, K+, certain orgain cations can
pass through), wile voltage-gated channel: specific for each ions)
End-plate current는 end-plate potential 보다
shorter & fades away within a few msec.
(slow decay of EPP: partly determined by the passive time
constant of the muscle membranes)
depolarization이 증가하면 더욱 shorter.
[Ca2+]o이 낮으면 end-plate potential이 작아진다.
보통 end-plate potential이 30 mV정도 depolrize되면
---> trigger AP---> myofibrils to contract.