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You are watching: Which of the following does not describe the process of summation

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.


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The postsynaptic impacts of the majority of synapses in the brain are not almost as large as those at the neuromuscular junction; indeed, PSPs as a result of the task of individual synapses are normally well listed below the thresorganize for generating postsynaptic activity potentials, and might be just a portion of a millivolt. How, then, can neurons in the brain transmit information from presynaptic to postsynaptic cells if a lot of central synaptic effects are subthreshold? The answer is that neurons in the main nervous mechanism are frequently innervated by hundreds of synapses, and also the PSPs created by each energetic synapse can amount together—in space and in time—to recognize the habits of the postsynaptic neuron.

Consider the extremely simplified situation of a neuron that is innervated by 2 excitatory synapses, each generating a subthresorganize EPSP, and also an inhibitory synapse that produces an IPSP (Figure 7.7A). While activation of either one of the excitatory synapses alone (E1 or E2 in Figure 7.7B) produces a subthreshost EPSP, activation of both excitatory synapses at around the exact same time reasons the 2 EPSPs to amount together. If the sum of the two EPSPs (E1 + E2) depolarizes the postsynaptic neuron sufficiently to reach the threshold potential, a postsynaptic activity potential outcomes. Summation therefore allows subthresorganize EPSPs to affect activity potential manufacturing. Likewise, an IPSP generated by an inhibitory synapse (I) can amount (algebraically speaking) with a subthresorganize EPSP to mitigate its amplitude (E1 + I) or can amount via suprathresorganize EPSPs to prevent the postsynaptic neuron from getting to threshold (E1 + I + E2).


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Figure 7.7

Summation of postsynaptic potentials. (A) A microelectrode records the postsynaptic potentials produced by the task of 2 excitatory synapses (E1 and also E2) and also an inhibitory synapse (I). (B) Electrical responses to synaptic activation. Stimulating (even more...)


In brief, the summation of EPSPs and also IPSPs by a postsynaptic neuron permits a neuron to incorporate the electrical information provided by all the inhibitory and also excitatory synapses acting on it at any type of minute. Whether the amount of active synaptic inputs results in the manufacturing of an activity potential counts on the balance in between excitation and inhibition. If the sum of all EPSPs and IPSPs results in a depolarization of sufficient amplitude to raise the membrane potential over threshost, then the postsynaptic cell will develop an activity potential. Conversely, if inhibition prevails, then the postsynaptic cell will reprimary silent. Typically, the balance between EPSPs and also IPSPs changes continually over time, depending on the number of excitatory and inhibitory synapses active at a given moment and the magnitude of the existing at each synapse. Summation is therefore a neurotransmitter-induced tug-of-war in between all excitatory and inhibitory postsynaptic currents; the outcome of the challenge determines whether or not a postsynaptic neuron becomes an active facet in the neural circuit to which it belongs (Figure 7.8).

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Figure 7.8

Events from neurotransmitter release to postsynaptic excitation or inhibition. Neurotransmitter release at all presynaptic terminals on a cell results in receptor binding, which causes the opening or closing of particular ion channels. The resulting conductance (even more...)


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