Final answer:
Various drugs affect pupil size by altering sympathetic or parasympathetic activity. Drugs that stimulate the sympathetic system or block parasympathetic activity, such as amphetamines and anticholinergics, cause dilation, while those that have a depressive effect, like barbiturates and opioids, tend to constrict the pupil. Pilocarpine can reverse mydriasis by stimulating the parasympathetic system.
Step-by-step explanation:
The impact of various drugs on pupil size is determined by their interaction with the autonomic nervous system, specifically the sympathetic and parasympathetic systems which control pupillary dilation and constriction. Barbiturates and ethanol/isopropyl alcohol can lead to pupil constriction due to their depressive effects on neural activity, including the autonomic nervous system. Amphetamines, cocaine, and cyclic antidepressants typically cause pupillary dilation (mydriasis) by stimulating the sympathetic nervous system or blocking reuptake of norepinephrine.
Anticholinergics like atropine and scopolamine, present in plants like belladonna, block the parasympathetic response, leading to pupil dilation which historically was considered cosmetically appealing. On the contrary, pilocarpine is a muscarinic agonist that promotes pupil constriction by stimulating the parasympathetic system, reversing the effects of drugs like phenylephrine that induce mydriasis. Opioids generally induce pupillary constriction (miosis), except for some like meperidine. However, during drug withdrawal, opposite effects can be observed, leading to pupillary dilation as a rebound effect from the cessation of these substances.
Antihistamines and anticholinergics have similar effects in dilating the pupils by inhibiting the parasympathetic system, while dopamine, a neurotransmitter essential for motor function and reward, has varied effects on the pupil size depending on its interaction with different receptors in the eye.