Memory, like all brain functions, is controlled by communication between the brain's estimated 100 billion neurons, or nerve cells. Neurons in several areas of the brain are involved in memory, with different aspects of memory handled by separate memory systems -- episodic memory, semantic memory, procedural memory and working memory. Alzheimer's disease gradually damages neurons, slowly degrading brain function, and areas of the brain responsible for memory are among the first to be affected.
The hippocampus crucial to the proper function of episodic memory, and is one of the first brain structures to be damaged in Alzheimer's disease. Episodic memory is the system that allows the learning of new information and the ability to recall recent events. As episodic memory becomes impaired by Alzheimer's disease, affected individuals typically begin to have trouble with short-term memory, forgetting recent conversations and events – such as where they've parked the car or put the car keys – yet retaining the ability to remember past events in great detail.
Typically, semantic memory is the next to be affected by the degenerative effects of Alzheimer's disease. Semantic memory involves the temporal lobe -- which contains the hippocampus -- and areas of the cortex. This memory system controls general knowledge, enabling the recall of facts. This includes the ability to categorize – recognizing and naming objects. Memory loss that affects semantic memory is characterized by the inability to recall the names of common items or difficulty in categorizing items appropriately. For instance, a person with Alzheimer's may not be able to recall the name of a favorite food, or may include elephants or lions in a list of farm animals.
Working memory involves the prefrontal cortex and controls the short-term recall of information, such as an appointment, address or phone number, as well as attention and concentration. As Alzheimer's damage progresses to the prefrontal cortex, impairing working memory, affected individuals may begin to seem scattered, having trouble focusing on tasks or completing activities that involve a series of steps -- losing track in the middle of dressing, for instance, or unable to follow through on tasks such as paying monthly bills or preparing a meal. Additionally, a person who has working memory impairment may be unable to follow conversations or express complex thoughts.
Procedural memory is primarily a function of the cerebellum and governs the learning of skills that become automatic once they are mastered – walking, riding a bicycle or typing, for instance. As a rule, this memory system, if affected at all, is the very last to be damaged by Alzheimer's disease.
When discussing Alzheimer's impact on memory, it is important to note that some age-related changes in memory are a normal, routine occurrence as we grow older, so forgetting an appointment or where you have left the car keys on occasion isn't necessarily a cause for alarm. However, if you or someone you care about seems more forgetful than others of the same age, it is important to have those symptoms evaluated by a medical professional, since early diagnosis of Alzheimer's disease is essential to effective treatment that can slow progression of symptoms and improve quality of life.