Chapter 3 Visual Summary

Chemical signals transmit information between neurons; electrical signals transmit information within a neuron. The resting potential is a small electrical potential across the neuron's membrane. Review Figure 3.1, Animation 3.2

Different concentrations of ions inside and outside the neuron—especially potassium ions (K+), to which the resting membrane is selectively permeable—account for the resting potential. At the K+ equilibrium potential, the electrostatic pressure pulling K+ ions into the neuron is balanced by the concentration gradient pushing them out. Review Figure 3.2, Figure 3.3, Figure 3.4, Activity 3.1, Animation 3.3

Reducing the resting potential (depolarization) of the axon until it reaches a threshold value opens voltage-gated sodium (Na+) channels, making the membrane completely permeable to Na+. The sodium ions (Na+) rush in, and the axon becomes briefly more positive inside than outside. This event is called an action potential. Review Figure 3.5, Animation 3.4

Following the action potential, the resting potential is quickly restored by the influx of K+ ions. The sodium-potassium pumps maintain the resting potential in the long run, counteracting the influx of Na+ ions during action potentials. Review Figure 3.6

The action potential strongly depolarizes the adjacent patch of axonal membrane, causing it to generate its own action potential, propagating down the axon. Saltatory conduction of the action potential along the nodes of Ranvier between myelin sheaths speeds propagation along the axon. Review Figure 3.7 and Figure 3.8, Box 3.1, Animation 3.5

Like all other local potentials, postsynaptic potentials spread very rapidly but are not regenerated, so they diminish as they spread passively along dendrites and the cell body. Excitatory postsynaptic potentials (EPSPs) are depolarizing (they decrease the resting potential) and increase the likelihood that the neuron will fire an action potential. Inhibitory postsynaptic potentials (IPSPs) result in hyperpolarization, decreasing the likelihood that the neuron will fire. Review Figure 3.10

Neurons process information by integrating the postsynaptic potentials through both spatial summation (summing potentials from different locations) and temporal summation (summing potentials across time). Review Figure 3.11, Animation 3.6

Action potentials are initiated at the axon hillock when the excess of EPSPs over IPSPs reaches threshold. During the action potential, the neuron cannot be excited by a second stimulus; it is absolutely refractory. For a few milliseconds afterward, the neuron is relatively refractory, requiring a stronger stimulation than usual in order to fire. Review Figure 3.6

Synaptic transmission occurs when a chemical neurotransmitter diffuses across the synaptic cleft and binds to neurotransmitter receptors in the postsynaptic membrane. Review Figure 3.12, Figure 3.13, Animation 3.7

Summing electrical activity over millions of nerve cells as detected by electrodes on the scalp, electroencephalograms (EEGs) can reveal rapid changes in brain function—for example, in response to a brief, controlled stimulus that evokes an event-related potential (ERP). They can also reveal a seizure in people with epilepsy. Review Figure 3.16, Figure 3.17, Figure 3.18

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