All of the following are actions of muscarinic antogonists, except :
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).
Decrease gastric secretions
Decrease respiratory secretions
Contract radial muscles of iris
Fascilitates AV conduction