Integrating Postsynaptic Potentials Summation PDF

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Integrating Postsynaptic Potentials Summation...

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Integrating Postsynaptic Potentials: Summation An individual EPSP has a small effect on the membrane potential, typically producing a depolarization of about 0.5 mV at the postsynaptic membrane. Before an action potential will arise in the initial segment, local currents must depolarize that region by at least 10 mV. Therefore, a single EPSP will not result in an action potential, even if the synapse is on the axon hillock. But individual EPSPs combine through the process of summation, which integrates the effects of all the graded potentials that affect one portion of the plasma membrane. The graded potentials may be EPSPs, IPSPs, or both. We will consider EPSPs in our discussion. Two forms of summation exist: temporal summation and spatial summation (Figure 12–18). Temporal Summation. Temporal summation (tempus, time) is the addition of stimuli occurring in rapid succession at a single synapse that is active repeatedly. This form of summation can be likened to using a bucket to fill up a bathtub: You can’t fill the tub with a single bucket of water, but you will fill it eventually if you keep repeating the process. In the case of temporal summation, the water in a bucket corresponds to the sodium ions that enter the cytosol during an EPSP. A typical EPSP lasts about 20 msec, but under maximum stimulation an action potential can reach the axon terminal each millisecond. Figure 12–18a shows what happens when a second EPSP arrives before the effects of the first EPSP have disappeared: The effects of the two are combined. Every time an action potential arrives, a group of vesicles discharges ACh into the synaptic cleft, and every time more ACh molecules arrive at the postsynaptic membrane, more chemically gated ion channels open, and the degree of depolarization increases. In this way, a series of small steps can eventually bring the initial segment to threshold. Spatial summation occurs when simultaneous stimuli applied at different locations have a cumulative effect on the membrane potential. In other words, spatial summation involves multiple synapses that are active simultaneously. In terms of our bucket analogy, you could fill the bathtub immediately if 10 friends emptied their buckets into it at the same time. In spatial summation, more than one synapse is active at the same time (Figure 12–18b), and each “pours” sodium ions across the postsynaptic membrane, producing a graded potential with localized effects. At each active synapse, the sodium ions that produce the EPSP spread out along the inner surface of the membrane and mingle with those entering at other synapses. As a result, the effects on the initial segment are cumulative. The degree of depolarization depends on how many synapses are active at any moment, and on their distance from the initial segment. As in temporal summation, an action potential results when the membrane potential at the initial segment reaches threshold...