Glossary of Terms
Acetylcholine A chemical that acts as a messenger to transmit signals in the brain from one neuron to another; this chemical is decreased in AD brain tissue.
AD Alzheimer's disease.
Age-dependent penetrance A type of "penetrance" (see Penetrance) that depends on age; that is, the trait or disease shows itself at a more-or-less defined age.
Age of onset The chronological age at which a disease is first noticed because of symptoms.
Agnosia Loss of the ability to perceive normal stimuli of the senses, for example, hearing, smell, sight, taste, and touch.
Agraphia Difficulty in writing (e.g., poor penmanship or legibility in writing).
Alexia Difficulty in reading.
Alzheimer's disease A disease characterized by dementia and