Alzheimer’s disease (AD) is an irreversible, progressive disorder in which brain cells (neurons) deteriorate, resulting in the loss of cognitive functions, primarily memory, judgment and reasoning, movement coordination and pattern recognition. In advanced stages of the disease, all memory and mental functioning may be lost. The condition predominantly affects the cerebral cortex and hippocampus, which lose mass and shrink (atrophy) as the disease advances.
The cerebral cortex is an extremely convoluted and complicated structure associated with the “higher” functions of the mind—thought, reasoning, sensation, and motion. Each hemisphere of the cerebral cortex contains areas that control certain types of activity. These areas are referred to as the frontal lobe, parietal lobe, temporal lobe, and occipital lobe.
- The frontal lobe, located behind the forehead, is involved with controlling responses to input from the rest of the central nervous system (brain and spinal cord). It is responsible for voluntary movement, emotion, planning and execution of behavior, intellect, memory, speech, and writing.
- The parietal lobe, located above the ear, receives and interprets sensations of pain pressure, temperature, touch, size, shape, and body part awareness.
- The temporal lobe, located behind the ear, is involved in understanding sounds and spoken words, as well as emotion and memory.
- The occipital lobe, located at the back of the head, is involved in understanding visual images and the meaning of the written word.
The hippocampus plays a crucial role in learning and in processing various forms of information as long-term memory. Damage to the hippocampus produces global amnesia.
Plaques and Tangles
The two most significant physical findings in the cells of brains affected by Alzheimer’s disease are neuritic plaques and neurofibrillary tangles. Another significant factor in AD is the greatly reduced presence of acetylcholine in the cerebral cortex. Acetylcholine is necessary for cognitive function.
While some neuritic plaques, or patches, are commonly found in brains of elderly people, they appear in excessive numbers in the cerebral cortex of Alzheimer’s disease patients. A protein called beta amyloid occupies the center of these plaques. Surrounding the protein are fragments of deteriorating neurons, especially those that produce acetylcholine (ACh), a neurotransmitter essential for processing memory and learning. Neurotransmitters are chemicals that transport information or signals between neurons. Neurofibrillary tangles (NFTs) are twisted remnants of a protein called tau, which is found inside brain cells and is essential for maintaining proper cell structure and function. An abnormality in the tau protein disrupts normal cell activity.
Incidence and Prevalence of Alzheimer’s Disease
According to the Alzheimer’s Association, about 5.2 million people in the United States suffer from Alzheimer’s disease—and two-thirds of those affected are women. The Association reports that about 3.2 million American women and 1.8 million American men over the age of 60 have Alzheimer’s. Approximately 10 percent of all people over the age of 65 and as many as 50 percent of those over the age of 85 are diagnosed with the condition, which is the sixth leading cause for death in the United States.