chapter 9.docx - POSTSYnAPTIC EFFECTs OF TRANSMITTER...

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POSTSYnAPTIC EFFECTs OF TRANSMITTER AGENTS (the reversal potential) How chemical transmitters produce their postsynaptic potentials: the chemical transmitter substance in combining with its receptor molecule in the postsynaptic membrane induces a CONDUCTANCE CHANGE in the postsynaptic membrane The combination of the transmitter substance with the receptor molecule opens up channels in the membrane with which the receptor molecule is associated the channels opened are VOLTAGE INSENSITIVE: the opening or closing of the channels creating the action potential are regulated by the membrane potential—the conductance change to a chemical transmitter, such as acetylcholine, is the SAME at one membrane voltage as it is at any other membrane potential—otherwise known as LIGAND GATED CHANNELS referring to the chemical messenger the only factor that causes these channels to open is when the receptor binds the neurotransmitter or its AGONIST the channels producing an EPSP or an EPP are separate from and have quite different properties from those underlying the action potential MAXIMUM MEMBRANE POTENTIAL REACHED BY EPSPs and EPPS if a transmitter agent increased the conductance to sodium by opening channels in the membrane, sodium would enter the postsynaptic cell and depolarize it (pushed by its strong electrochemical gradient). An EPSP (nerve cells) or EPP (muscle fibers) would be created, but there are no POTASSIUM CHANNELS to quickly repolarize the membrane, so what occurs is kind of like a prolonged action potential when excitatory transmitters (ACETYLCHOLINE) are released from the presynaptic axon, the membrane NEVER REACHES the EQsodium (+55) the highest EPP/EPSP that can be evoked is about -17 mV for EPP and 0mV for EPSP. This can be sohwn by recording the endplate while all voltage gated sodium and potassium channels are blocked by TTX/TEA and puffing on ACh from another electrode placed near the axon terminals because the endplate potential is far from sodium EQ, channel is permeable to other ions in addition to sodium. Receptors at excitatory synapses (endplates) are equally permeable to sodium and potassium, so the equilibrium potential reached is between the two eq potentials of the ions to which the channel is permeable (REVERSAL POTENTIAL for that (ACh)receptor) MEASURING THE REVERSAL POTENTIAL FOR ACETYLCHOLINE RECEPTORS AT THE NEUROMUSCULAR JUNCTION Same as voltage clamp in squid gianx axon except MICROELECTRODES are used to measure the membrane potential for the muscle fiber//pass current instead of wires 1. One to measure the potential 2.
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  • Spring '17
  • Buskirk
  • Biology, Transmitter, Chemical synapse

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