Phosphodiesterase hydrolyzes cAMP into inaction, so that the second message depends on continued presence of the ligand. Two neurons are required to reach from the CNS to the target organ. These neurons synapse in autonomic ganglia . All preganglionic autonomic neurons release ACh as their neurotransmitter, which binds to nicotinic receptors on the postsynaptic neuron. All parasympathetic postganglionic neurons release ACh, which interacts with muscarinic receptors on the target organs.
Parasympathetic responses are usually specific and localized.
Sympathetic effects tend to be widespread. Many organs are innervated by both sympathetic and parasympathetic neurons, which usually have opposing effects. The a subunit or the b g complex can then bind to an ion channel.