Postsynaptic Potentials

Neural integration is based on the postsynaptic potentials produced by neurotransmitters. Remember that a typical neuron has a resting membrane potential (RMP) of about — 70 mV and a threshold of about —55 mV. A neuron has to be depolarized to this threshold in order to produce action potentials. Any voltage change in that direction makes a neuron more likely to fire and is therefore called an excitatory postsynaptic potential (EPSP) (fig. 12.21a). EPSPs usually result from Na+ flowing into the cell and

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canceling some of the negative charge on the inside of the membrane.

In other cases, a neurotransmitter hyperpolarizes the postsynaptic cell and makes it more negative than the RMP. Since this makes the postsynaptic cell less likely to fire, it is called an inhibitory postsynaptic potential (IPSP)

(fig. 12.21b). Some IPSPs are produced by a neurotrans-mitter opening ligand-regulated chloride gates, causing Cl— to flow into the cell and make the cytosol more negative. A less common way is to open selective K+ gates, increasing the diffusion of K+ out of the cell.

It should be stressed that because of ion leakage through their membranes, all neurons fire at a certain background rate even when they are not being stimulated. EPSPs and IPSPs do not determine whether or not a neuron fires, but only change the rate of firing by stimulating or inhibiting the production of more action potentials.

Glutamate and aspartate are excitatory neurotrans-mitters that produce EPSPs. Glycine and GABA produce

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Threshold-1

EPSP Resting membrane — potential

L- Repolarization -Depolarization

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(a) Stimulus Time

-20 -£ -40 --60 -

Threshold

Resting membrane IPSP potential

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Hyperpolarization

  • b) Stimulus Time
  • b) Stimulus Time

Figure 12.21 Postsynaptic Potentials. (a) An excitatory postsynaptic potential (EPSP). (b) An inhibitory postsynaptic potential (IPSP). The sizes of these postsynaptic potentials are greatly exaggerated here for clarity; compare figure 12.23. Why is a single EPSP insufficient to make a neuron fire?

IPSPs and are therefore inhibitory. Acetylcholine (ACh) and norepinephrine are excitatory to some cells and inhibitory to others, depending on the type of receptors present on the target cells. For example, ACh excites skeletal muscle but inhibits cardiac muscle because the two types of muscle have different types of ACh receptors. This is discussed more fully in chapter 15.

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