Synaptic Integration

The summation of numerous EPSPs may be needed to produce a depolarization of sufficient magnitude to stimulate the postsynaptic cell. The net effect of EPSPs on the postsynaptic neuron is reduced by hyperpolarization (IPSPs),which is produced by inhibitory neurotransmitters. The activity of neurons within the central nervous system is thus the net result of both excitatory and inhibitory effects.

Unlike action potentials, synaptic potentials are graded and can add together, or summate. Spatial summation occurs because numerous presynaptic nerve fibers (up to a thousand, in some cases) converge on a single postsynaptic neuron. In spatial summation, synaptic depolarizations (EPSPs) produced at different synapses summate in the postsynaptic dendrites and cell body (fig 7.29). In temporal summation, the successive activity of a presynaptic axon terminal causes successive waves of transmitter release, resulting in the summation of EPSPs in the postsynaptic neuron. The summation of EPSPs helps to determine if the depolarization that reaches the axon hillock will be of sufficient magnitude to generate new action potentials in the postsynaptic neuron.

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Action potential

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Release of neurotransmitter from neuron © only ,

Release of neurotransmitter from neuron © only ,

Release of neurotrans-mitter from neurons d and (2)

■ Figure 7.29 Spatial summation. When only one presynaptic neuron releases excitatory neurotransmitter, the EPSP produced may not be sufficiently strong to stimulate action potentials in the postsynaptic neuron. When more than one presynaptic neuron produces EPSPs at the same time, however, the EPSPs can summate at the axon hillock to produce action potentials.

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