Neurotransmitters will never enter the postsynaptic neuron. They will attach to receptors on the membrane of the postsynaptic neuron, release, and then be dealt with while in the synaptic cleft. In some instances, the neurotransmitter simply diffuses away from the synapse, but in most cases synaptic proteins help to recycle the molecules. Some enzymes will break the neurotransmitter into parts, which then reenter the presynaptic neuron and are reassembled. Acetylcholinesterase is a common example of this type of enzyme. Other neurons use membrane pumps to retrieve neurotransmitter from the synaptic cleft.

Once an action potential has reached the end of the presynaptic neuron, voltage-gated calcium channels are stimulated, allowing calcium to enter the neuron. This influx of calcium stimulates the release of neurotransmitters into the synaptic cleft.

The neurotransmitters then bind to receptors on the surface of the postsynaptic neuron to generate small electrical stimuli. The sum of these stimuli must reach threshold in order for an action potential to be generated.

GABA and acetylcholine are two common neurotransmitters. GABA generally inhibits the postsynaptic neuron, while acetylcholine generally excites/stimulates the postsynaptic neuron.

Acetylcholine activates muscarinic receptors in postganglioinic parasympathetic nervous tissue and nicotinic receptors in skeletal muscle fibers, and preganglionic sympathetic and parasympathetic nervous tissue.

Muscarinic acetylcholine receptors (mAChRs) are found in the G protein-receptor complexes in the cell membrane of certain neurons. The suffix -ergic means exhibiting or stimulating the activity of. For example, dopaminergic receptors bind to dopamine. 

Neurotransmitters are packaged in vesicles, traveling through the synaptic cleft (space between neurons) from the pre-synaptic membrane to the post-synaptic membrane, where they bind to their receptors and influence the post-synaptic membrane potential.

Renshaw cells are interneurons in the spinal cord that release the inhibitory neurotransmitter glycine.

Competitive inhibitors directly compete with ligands for the same binding sites (active sites). Irreversible inhibitors are molecules that bind irreversible to the active site. Covalent modulators are molecules that bind covalently to proteins and changes their activity. pH and temperature may alter the 3D shape of the protein and can be irreversible, while ligand binding, and competitive inhibition is a reversible process that involves the formation of weak, not covalent, bonds.

Cholinergic neurons, whether in the sympathetic or parasympathetic nervous system, release acetylcholine as the neurotransmitter. Nonadrenergic, noncholinergic neurons include some postganglionic parasympathetic neurons of the gastrointestinal tract, which release substance P and nitric oxide. Adrenergic neurons release norepinephrine. 

Norepinephrine is released by adrenergic neurons.

Cholinergic neurons release acetylcholine as their neurotransmitter (in the sympathetic or parasympathetic nervous system).

Nonadrenergic, noncholinergic neurons (which include postganglionic parasympathetic neurons of the gastrointestinal tract) release substance P and nitric oxide.

The depolarization of the presynaptic membrane causes voltage-gated calcium channels to enter from the extracellular space. This causes the synaptic vessels which are docked on the presynaptic terminal to fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft.