Lecture 6 - Synaptic Communication Il End- Plate Potential (EPP) - Vertebrate NMJs are cholinergic - Motor End Plate: The postsynaptic part of the NMJ - EPP (graded potential) gives rise to an action potential o If graded potential reaches the threshold value, the muscle cell will fire an AP (A) o GP gives rise to an AP acetylcholine (ACh) - Graded potential has an in-built safety factor (B) motor axon - Curare: competitive inhibitor of acetylcholine that does not block Na+ and K+ VG channels C Two ways to generate EPP experimentally: intracellular microelectrode `ACh pipette - Nerve stimulation -> apply intracellular microelectrode to the muscle cell muscle fiber neuromuscular junction - lontophoresis -> treatment of adding ach Neurotransmitter release - Action Potenial arrives at presynaptic neurone - Ca2+ entry through Ca2+ (VG) channels in presynaptic neurone - Ca2+ binds to synaptotagmin (Ca2+ sensor) - Vesicle brought close to membrane in response to high conc. of Ca2+ - SNARE complex make a fusion pore - Transmitter released through this pore > diffuses through synaptic clef - Neurotransmitter binds to receptors on postsynaptic clef - Release of Ach activates nicotinic ACh receptor channels o To open a single channel 2 molecules of Ach needed o In response to the binding, Na+ enters the nACh channel and K+ leaves - Outcome = depolaristion of cell -> AP continues! (C) nerve stimulation 5 mv LY ACh iontophoresis r 5 mv L 10 ms time -+ PRESYNAPTIC: POSTSYNAPTIC: vesicle Ca2+ entry synaptotagmin release of transmitter synapse Release of ACh activates nicotinicACh receptor channels SNARE complex Ca2+ ACh 00- Modified from Unwin (1995) voltage-gated Ca2+ channel fusion pore ma from SNARE com ACh Na ENa 0 nAChR K+ 2ACh closed= lopen Ex ACh
Sequence of events during Neuromuscular Transmission: AP in motoneuron D nAChR 00 Na+ channel · ACh esterase - Ca2+ -00 00 00 ACh end-plate potential ACh is degraded quickly by ACh esterase in the synaptic cleft 00-00 00 excitation contraction coupling SUMMARY: - Action potential arrives at the motor neuron terminal where it opens voltage gated Ca2+ channels - Ca2+ entry initiates exocytosis of vesicles containing Ach - ACh binds to nicotinic ACh receptors on the muscle end-plate, causes them to open and the flow of cations causes a depolarization called the end-plate potential - The end-plate potential depolarizes the adjacent muscle membrane and activates voltage- gated Na+ channels, thereby initiating an action potential in the muscle fibre which then contracts due to excitation-contraction coupling. Synaptic/ Ach Reversal Potential (EAch ) - VERY KEY! Reversal potential is the membrane potential at which there is NO NET FLOW of a particular ion from one side of the membrane to another K+ and Na + reversal potential - If membrane resting potential is -70 mV - K+ reversal potential = - 80 Mv - Na+ reversal potential = +50 o Why is K+ RP closer to membrane RP than Na+? o The cell is more permeable to K+ ions and will affect resting