a. The dendrites receive the impulse and transfer it to the nucleus. The nucleus will then cause a change in the permeability of the membrane surrounding the axon. Potassium, which is normally present in high concentrations within the axon, will diffuse out. Sodium, which is usually present in high concentrations outside the axon, will rush into the axon. This exchange of potassium and sodium is called depolarization. As these electrolytes change positions, an electrical charge is set up and the impulses will travel down the axon until it reaches the terminal bulbs. When the impulse reaches the terminal bulbs, it will cause a release of neurotransmitters stored there into the synaptic cleft. Once in the synaptic cleft, the neurotransmitters will diffuse across the synapse to the dendrite of the postsynaptic neuron causing it to depolarize (see Figure 5-4).
b. Once the postsynaptic neuron has depolarized, the neurotransmitters must be removed from the synaptic cleft to prevent further depolarization. This is accomplished by two means. The neurotransmitter is either reabsorbed into the terminal bulb or an enzyme destroys it. This process ends the impulse.
c. Before the neuron can depolarize again, the electrolyte sodium and potassium must resume their original positions. The sodium pump theory states that before the neuron can depolarize again the sodium is pumped out and the potassium is pumped back in (repolarized).
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