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A feature that makes postsynaptic integration possible is that in most neurons one excitatory synaptic event by itself is not enough to cause threshold to be reached in the postsynaptic neuron. For example, a single EPSP may be only 0.5 mV, whereas changes of about 15 mV
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Neural Control Mechanisms CHAPTER EIGHT
Neural Control Mechanisms CHAPTER EIGHT
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Time FIGURE 8-29Intracellular recording from a postsynaptic cell during episodes when (A) excitatory synaptic activity predominates and the cell is facilitated, and (B) inhibitory synaptic activity dominates. are necessary to depolarize the neuron's membrane to threshold. This being the case, an action potential can be initiated only by the combined effects of many excitatory synapses. Of the thousands of synapses on any one neuron, probably hundreds are active simultaneously or close enough in time so that the effects can add together. The membrane potential of the postsynaptic neuron at any moment is, therefore, the resultant of all the synaptic activity affecting it at that time. There is a depolarization of the membrane toward threshold when excitatory synaptic input predominates, and either a hyper-polarization or stabilization when inhibitory input predominates (Figure 8-29). Let us perform a simple experiment to see how EPSPs and IPSPs interact (Figure 8-30). Assume there are three synaptic inputs to the postsynaptic cell: The synapses from axons A and B are excitatory, and the synapse from axon C is inhibitory. There are laboratory stimulators on axons A, B, and C so that each can be activated individually. An electrode is placed in the cell body of the postsynaptic neuron and connected to record the membrane potential. In Part 1 of the experiment, we shall test the interaction of two EPSPs by stimulating axon A and then, after a short time, stimulating it again. Part 1 of Figure 8-30 shows that no interaction occurs between the two EPSPs. The reason is that the change in membrane potential associated with an EPSP is fairly short-lived. Within a few milliseconds (by the time we stimulate axon A for the second time), the postsynaptic cell has returned to its resting condition. In Part 2, we stimulate axon A for the second time before the first EPSP has died away; the second synap-tic potential adds to the previous one and creates a greater depolarization than from one input alone. This is called temporal summation since the input signals arrive at the same cell at different times. The potentials summate because there are a greater number of open ion channels and, therefore, a greater flow of positive ions into the cell. In Part 3, axon B is stimulated alone to determine its response, and then axons A and B are stimulated simultaneously. The two EPSPs that result also summate in the postsynaptic neuron; this is called spatial summation since the two inputs occurred at different locations on the same cell. The interaction of multiple EPSPs through ongoing spatial and temporal summation can increase the inward flow of positive ions and bring the postsynaptic membrane to threshold so that action potentials are initiated (Part 4). So far we have tested only the patterns of interaction of excitatory synapses. Since EPSPs and IPSPs are due to oppositely directed local currents, they tend to cancel each other, and there is little or no change in membrane potential (Figure 8-30, Part 5). Inhibitory potentials can also show spatial and temporal summation. Recording microelectrode Inhibitory synapse *c Recording microelectrode Inhibitory synapse *c Excitatory synapses Axon^
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