Final answer:
Excitatory neurotransmitters in the CNS act by opening sodium (Na+) channels, leading to the depolarization of the postsynaptic neuron and increasing the likelihood of an action potential.
Step-by-step explanation:
Excitatory neurotransmitters of the CNS usually act by opening sodium (Na+) channels. When an action potential reaches the axon terminal, voltage-gated calcium (Ca2+) channels in the membrane open, leading to the release of neurotransmitters into the synaptic cleft.
These neurotransmitters then bind to receptors on the postsynaptic membrane that are specific ligand-gated channels, causing them to open. In the case of excitatory neurotransmitters such as acetylcholine at the neuromuscular junction, the opening of these channels results in the rapid influx of Na+ ions, which leads to depolarization of the membrane and creates an excitatory postsynaptic potential (EPSP), thereby making the postsynaptic neuron more likely to fire an action potential.
In contrast, neurotransmitters like GABA, which are inhibitory, open chloride (Cl-) channels, resulting in the hyperpolarization of the postsynaptic neuron and making it less likely to fire an action potential. It's important to note that although calcium plays a crucial role in the process of neurotransmitter release, it is the sodium channels that are typically responsive in excitatory postsynaptic potentials.