Final answer:
Glycine and GABA cause hyperpolarization of the membrane potential.
Step-by-step explanation:
The opening of chloride ion channels by glycine and GABA causes hyperpolarization of the membrane potential.
When the chloride ion channels open, negatively-charged chloride ions (Cl-) enter the neuron's cell body, pushing the neuron away from firing and making the membrane potential more negative. This has a quieting effect on the brain.
This is in contrast to depolarization, which refers to a change in membrane potential that makes the neuron more likely to fire.