I

5. Change in ionic permeability of postsynaptic cell 5. Change in ionic permeability of postsynaptic cell

6. Change in membrane potential of postsynaptic cell

H i

O

H 1

H

ch3 1 3

H — C — 1

C

O — C —

C

N — CH3

H

H

H

CH3

Figure 8-3 The sequence of events between the arrival of an action potential at a synaptic terminal and the release of chemical transmitter.

channels of nerve membrane; they open upon depolarization and close again when the membrane potential repolarizes. Thus, when an action potential invades the synaptic terminal, the calcium permeability of the membrane increases during the depolarizing portion of the action potential and declines again as membrane potential returns to normal.

Although the external calcium concentration is low (1-2 mM), the internal concentration of calcium ions in the ICF is much lower (<10-6 M). From the Nernst equation, then, the equilibrium potential for calcium would be expected to be positive. Therefore, both the concentration and electrical gradients drive calcium into the terminal, and when calcium permeability increases, there will be an influx of calcium. During a presynaptic action potential, there is a spike of calcium entry into the terminal, resulting in release of neurotransmitter into the extracellular space. This sequence is summarized in Figure 8-3.

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