<prompt> All of the following are actions of muscarinic antogonists, except :Decrease gastric secretionsDecrease respiratory secretionsContract radial muscles of irisFascilitates AV conduction  <response>: Ans is c i.e. Contract radial muscles of iris o Radial muscles are innervated by sympathetic systems not by parasympathetic muscarinic system Iris muscles that control the size of pupil There are two types of muscles in iris that control the size of pupil: i The iris sphincter or constrictor pupillae circular muscles: These muscles are innervated by the parasympathetic system and cause constriction of pupil miosis. ii The iris dilator or dilator pupillae radial muscles: These muscles are innervated by sympathetic a, adrenergic system and cause dilatation of pupil mydriasis o So, pupil size may be altered by following mechanisms by different ANS drugs: A. Mydriasis dilatation of pupil i Sympathomimetic drugs a, agonists: By contraction of radial muscles dilator. ii Antimuscarinic drugs: By blocking the action of circular muscles pupillary sphincter. B. Miosis constriction of pupil i Parasympathomimetic muscarinic drgus: By stimulating the contraction of circular muscles pupillary sphincter. ii Sympatholytic drugs a, antagonists: By blocking the action of radial muscles iris dilator About other options o Muscarinic antagonists antimuscarinic drugs decrease both gastric and respiratory bronchial secretions o Antimuscarinic drugs facilitate AV conduction see previous explanations. 