EPSP v. IPSP

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  1. There are more than 20 neurotransmitters at CNS synapses all of which open certain synapses; there are 3 different types of synapses
    1. excitatory – produce excitatory postsynaptic responses (EPSPs), they are depolarizing and typically increase gNa or decrease gK and sum up to cause an action potential; e.g. glutamate
    2. inhibitory synapses – produce inhibitory postsynaptic potentials (IPSPs), they are generally hyperpolarizing and typically increase gCl or gK; e.g. GABA
      1. opening a Cl channel puts a small hole near the soma and the opened Cl channel shunts out the capacitance current of Na killing the summed EPSPs
      2. as the membrane becomes increasingly depolarized, EPSP decreases and IPSP increases
    3. modulatory synapses – modulate the actions of other transmitters, may be depolarizing or hyperpolarizing and often act on second messenger systems; e.g. norephinephrine
  2. when the membrane potential reaches 0, EPSP disappears because of a decreased tendency for Na to enter the cell and an increased tendency for K to leave the cell; when the membrane potential is positive, EPSP changes direction
  3. when the membrane potential reaches –80 mV, IPSP changes direction
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