Final answer:
Negative feedback in the parasympathetic nervous system involves activation of autoreceptors on presynaptic terminals which inhibits further release of ACh, thereby preventing excessive cholinergic transmission.
Step-by-step explanation:
Negative feedback prevents further cholinergic transmission in the parasympathetic nervous system primarily through the activation of autoreceptors on presynaptic terminals. When acetylcholine (ACh) is released from the parasympathetic postganglionic fibers, it binds to muscarinic receptors on the target cells. However, some ACh can also bind to autoreceptors (usually muscarinic type) on the presynaptic neuron itself. This autoreceptor activation inhibits further release of ACh, acting as a negative feedback mechanism to modulate the signal and prevent excessive cholinergic stimulation.
Drugs that affect the cholinergic system can either enhance (parasympathomimetic drugs) or inhibit (anticholinergic drugs) cholinergic effects. Autoreceptors serve to regulate this system by providing a means for the neuron to 'self-regulate' its neurotransmitter output.