Tropicamide is a muscarinic receptor antagonist that competitively blocks acetylcholine at parasympathetic sites in the eye. By inhibiting muscarinic receptors on the iris sphincter and ciliary muscles, it causes relaxation of these muscles, resulting in pupil dilation (mydriasis) and paralysis of accommodation (cycloplegia). This action facilitates thorough ophthalmic examination and relieves pain from ciliary spasm.