Communication Between Neurons

A neuron can communicate with another cell across the synaptic cleft only after an action potential reaches the axon terminal. Vesicles that contain neurotransmitters are stored in the axon terminal. When an action potential reaches the axon terminal of the presynaptic neuron, the vesicles fuse with the presynaptic membrane. This fusion releases neurotransmitters into the synaptic cleft. The neurotrans-mitters quickly diffuse across the synaptic cleft and bind to receptor proteins embedded in the membrane of the postsynaptic cell. This process is shown in Figure 49-4 on the previous page.

The interaction of neurotransmitter and receptor molecules changes the permeability of the postsynaptic membrane by affecting chemically gated ion channels. The opening of Na+ channels in the postsynaptic membrane causes the membrane potential of the neuron to become more positive. If this change in membrane potential is great enough, a new action potential is generated in the receiving neuron. So, the electrical signal continues. However, the release of neurotransmitters may cause the opening of too few Na+ channels in the postsynaptic membrane or may cause the opening of other channels that allow negatively charged ions into the cell. As a result, the membrane potential of the receiving neuron will not become positive enough or may become more negative. No action potential will be generated in the receiving neuron, and the nervous signal will terminate.

Neurotransmitters do not remain in the synaptic cleft indefinitely. Instead, most neurotransmitters are cleared from the synap-tic cleft shortly after they are released. Many presynaptic neurons reabsorb neurotransmitters and use them again. At other synapses, neurotransmitters are broken down by enzymes. The reabsorption or breakdown of neurotransmitters ensures that their effect on postsynaptic cells is not prolonged. Topic: Neurons Keyword: HM61025 Topic: Neurons Keyword: HM61025

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