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Transcript
Synaptic Potentials Postsynaptic potentials develop in the postsynaptic cell’s membrane when neurotransmitter binding to receptors leads to the opening of ion channels. An excitatory postsynaptic potential (EPSP) occurs if the ion movement depolarizes the membrane. If, on the other hand, the membrane becomes hyperpolarized when the ions move, an inhibitory postsynaptic potential (IPSP) is generated. EPSPs and IPSPs are local potentials. EPSP: Opening of sodium- or calcium channels leads to depolarization of the membrane. If there is sufficient depolarization, the threshold potential is reached and an action potential will be produced in the postsynaptic membrane. Since an EPSP depolarizes the membrane, it facilitates action potentials. IPSP: Opening of potassium- or chloride channels leads to hyperpolarization of the membrane. (Since the current is outward for potassium ions, and inward for chloride ions, opening of either of these two channels will cause the postsynaptic membrane to hyperpolarize.) A hyperpolarized membrane has moved farther from the threshold potential and has less probability of producing an action potential. Since an IPSP hyperpolarizes the membrane, it inhibits action potentials. Remember that a neuron synapses with many other neurons. So a postsynaptic neuron can receive signals from many presynaptic neurons simultaneously. Whether or not the postsynaptic cell has an action potential depends on the summation (the additive effect) of all the incoming signals. Each active synapse can result in a local potential (either an EPSP or an IPSP). The net effect of all the local potentials on the trigger zone determines whether or not there is an action potential in the postsynaptic cell. There are two different ways that local potentials can sum to excite the postsynaptic cell to have an action potential. Temporal summation occurs when successive EPSPs at a single synapse occur in rapid succession. The successive potentials occur before the previous ones die out producing an increasing membrane depolarization. Summation can also occur when multiple presynaptic neurons stimulate the postsynaptic neuron at the same time (spatial summation). Each individual synapse lets in a limited number of ions and alters the membrane potential a little. The collective effect of all the synapses allows in enough ions to reach the threshold potential and an action potential is triggered. The interplay between IPSPs and EPSPs is important. Whether an action potential is going to be produced depends not just on the summation of the EPSPs, but on the summation of EPSPs and IPSPs. The algebraic sum of all EPSPs and IPSPs has to be of sufficient amplitude to raise the membrane potential to the threshold for an action potential. What this means is that if the IPSPs prevail, then the postsynaptic cell will be “silent.” One can, therefore, visualize the process of summation as a “tug-of-war” between excitatory and inhibitory currents induced by the binding of neurotransmitters on excitatory or inhibitory postsynaptic receptors, respectively.