Nicotinic acetylcholine receptors are therapeutic targets for neurodegenerative disorders, addiction, and mental illness. These pentameric ligand-gated ion channels are prototypical members of the Cys-loop receptor superfamily, which mediate fast neurotransmission throughout the central and peripheral nervous systems. Fundamental questions about nicotinic receptor biophysics and pharmacology remain, due in large part to the limited high-resolution structural information. We propose to determine high-resolution structures of two archetypal nicotinic receptor subtypes using single particle cryo-electron microscopy and investigate structure- based mechanistic hypotheses using molecular dynamics simulations and electrophysiology. Our first target is the muscle-type nicotinic receptor, the founding member of the pentameric receptor superfamily. Mutations in the channel, as well as autoimmune antibodies to the receptor, cause myasthenic syndromes. Our second target is the human α7 nicotinic receptor. α7 is exceptional among nicotinic receptors in several ways: it assembles physiologically as a homopentamer, it is expressed abundantly in the brain but also in non- electrically excitable cell types, it has a high permeability to Ca2+, and it desensitizes in microseconds. The receptor is a target in neurodegenerative disease, addiction and inflammation. We propose to use single particle cryo-electron microscopy combined with mutagenesis, electrophysiology and molecular dynamics simulations to elucidate mechanisms of channel activation, ligand recognition and ion permeation in these two distinctinctive nicotinic receptor subtypes to define general mechanisms and idiosyncratic properties.