Alzheimer's Disease and its Effects on the Central Nervous System

Alzheimer's disease (AD) is a progressive brain disorder that gradually destroys a person's memory and his ability to learn, make judgments, communicate, and carry out daily activities as well as eventually bringing about his or her death by killing neurons. Neurons are the functional units of the Central Nervous System and, when functioning properly, allow the interpretation of sensory impulses, the signaling of motor impulses, logic, memory, speech, and higher human thought processes. It may also result in personality or behavioral changes such as anxiety, paranoia, and hallucinations (National Institutes on Aging [NIA], 2005). Alzheimer's is the most common form of dementia, a group of conditions characterized by a progressive decline of cognitive function. It is also a growing crisis in America. According to the Alzheimer's Association ([AA], 2005), about 4.5 million Americans have AD—about twice as many as in 1980! Furthermore, they estimate that this number will continue to climb to between 11.3 and 16 million by 2050. Already half of nursing home patients have AD or a related disorder.

The central nervous system (CNS) is a complex system composed of the spinal cord and the brain. All the events that lead to AD occur in the brain. The brain is a network of billions of neurons with specialized functions. Four main structures, separated by spaces called ventricles, make up the brain. The largest structure is the cerebrum. The cerebrum interprets sensory input, inaugurates voluntary motion, and controls higher thought; it is made up of a thin, convoluted layer of gray matter called the cerebral cortex, large tracts of white matter that take information between different parts of the brain, and a small number of inclusions of grey matter called basal nuclei. The second structure is the diencephalon made up of the hypothalamus and thalamus. The hypothalamus maintains homeostasis and controls the pituitary gland. The thalamus integrates sensory signals and sends them to the cerebrum for further processing; it also is involved in memory and emotion. The third structure is the cerebellum, which coordinates voluntary motion. Fourth is the brain stem, which controls involuntary body processes and transfers information between the spinal cord and the rest of the brain. Another important part of the brain is the limbic system, a complex network of tracts and basal nuclei that blends mental functions with primitive emotion and is responsible for memory and learning. The limbic system contains the hippocampus, which is the center of memory—especially long-term memory, and the amygdale, which attaches emotions to information (Mader, 2004).

Neurons are the functional units of the CNS that transmit nerve impulses. Every neuron has three parts: a cell body, containing the nucleus and other organelle; dendrites, short extensions that receive signals from sensory receptors or other neurons; and an axon that conducts nerve impulses. Within neurons, axons carry nerve signals by a rapid shift in polarity through a temporary shutting down of the sodium-potassium pump. When this happens, positively charged sodium ions pass into the cell, quickly shifting the overall polarity of the cell. At a certain point of polarization, the sodium channels close and potassium channels open, allowing the neuron to return to its normal state. Nerve impulses are a two state operation, a neuron either conducts an impulse or not. The number of impulses in a certain time determines the intensity value, if any. Every axon branches into many small endings tipped with a swollen area called an axon bulb. Each of these bulbs lies nearly touching the dendrite of another cell at an area called a synapse. Transmission across synapses occurs by a special chemical called a neurotransmitter. The image to the right shows this. When a nerve impulse reaches the end of an axon, channels open to allow the entrance of calcium ions. This causes the exocytosis of vacuoles containing a neurotransmitter. The neurotransmitter diffuses across the gap between the two membranes (called the synaptic cleft) and binds to receptors located on the opposing membrane. This transmits either an excitatory or an inhibitory signal. A neuron has synapses with thousands of other neurons, so it receives many signals of both types every second and integrates them. If it has received enough excitatory signals, it transmits a nerve impulse (Mader, 2004). Unlike most other cells in the body, which regenerate regularly, most neurons need to survive a lifetime and, therefore, have to maintain themselves (Rodgers, 2002).

In Alzheimer's disease, neurons lose synapsal contact, die, and shrivel up. Biologically, the most evident symptom is the brain atrophies and the vacuoles enlarge. The atrophy starts at the entorhinal cortex (a part of the cerebral cortex that touches the hippocampus) then proceeds into the hippocampus and into the cerebrum. This explains why memory is the first thing affected (Rodgers, 2002). There are also detectable changes in the levels of several neurotransmitters. There are decreases in serotonin, acetylcholine (ACh), norepinephrine, and somatostatin, but an increase in glutamate (Alzheimer's disease, 2005). From diagnosis, an individual will live 3 to 20 years, 8 on average. In this time, the individual loses the ability to remember things, perform daily tasks, and eventually move or speak. If the patient does not die from something else, the brain breakdown will eventually cause death (AA, 2005).

Also notable in AD are the presence of beta-amyloid plaques and neurofibrillary tangles. Beta-amyloid plaques are dense, mostly insoluble plaques of proteins and other molecules that form outside and around neurons. They form when beta-amylase is irregularly snipped from amyloid precursor protein (APP) (Rodgers, 2002). Recent discoveries have uncovered that the formation of beta-amyloid is the result of an irregularity in the enzyme secretase. Neurofibrillary tangles form from a protein, tau, which normally stabilizes the structure of microtubules. In AD, however, extra phosphate groups attach to tau causing it to withdraw from the microtubule and clump together inside the neuron. Without the tau, the microtubules disintegrate which cripples the neuron and eventually causes it to die (Rodgers, 2005).

The underlying cause of AD is unknown, but all the symptoms are a result of the loss of synapsal contact and cell death. Most believe a combination of genetic, lifestyle, and environmental factors contribute to AD. There are two forms of AD, early onset and late onset. Early onset is the result of three genetic mutations: a mutation on chromosome 21 that causes abnormal APP production, a mutation on chromosome 14 that causes the production of an abnormal protein presenilin 1, and a mutation on chromosome 1 causes the production of presenilin 2. Presenilin is part of the irregular secretase complex responsible for beta-amyloid. For late onset Alzheimer's disease, there is only one known genetic component, a protein, produced by the gene apolipoprotein E (ApoE) on chromosome 19, which binds quickly and tightly with beta-amyloid. ApoE comes in three alleles, one of which increases the risk of AD. It is still unknown how beta-amyloid and tau are responsible for AD, but they obviously play an important part in the process. Others theorize that part of the cause of AD is oxidation damage from free radicals randomly oxidizing important structures within neurons. There also may be strong connections between AD and other diseases. Studies have shown that people with equal amounts of plaques and tangles but no stroke damage show significantly fewer signs of dementia then stroke victims with similar plaque and tangle counts (Rodgers, 2002). There are also strong ties between AD and diabetes by way of the insulin degrading enzyme. This enzyme, which when it fails, causes low blood sugar in some diabetes patients, is also responsible for the degradation of beta-amyloid. (Rodgers, 2005)

There are currently four medications approved by the FDA for the treatment of AD. All of them treat symptoms, none slow or reverse the disease's progression. Three of the drugs are Acetylcholinesterase (AChE) inhibitors. AChE is an enzyme that breaks down the neurotransmitter ACh, so by inhibiting AChE, ACh levels rise. While increasing ACh levels does temporarily slow mental decline, it does not slow the underlying cell death. Drugs of this type include Donepezil (Aricept®), Galantamine (Razadyne ®), and Rivastigmine (Exelon®). A more recent development is the drug Memantine (Namenda®), an uncompetitive low-to-moderate affinity N-methyl D-aspartate receptor antagonist. It works by regulating glutamate and thus avoids excitotoxicity, a process by which an excess of glutamate attaches to receptors causing an excess of calcium ions that triggers apoptosis—programmed cell death. Because it is a different type, patients can take Memantine in conjunction with one of the others (Alzheimer's disease, 2005). Possible future treatments include the development of a vaccine to make the human immune system target beta-amyloid and the stimulation of stem cells in the recently discovered process of nuerogenesis (neuron creation in certain areas of the brain) (Rodgers, 2005).

Alzheimer's is a terrible disease that disrupts the workings of the central nervous system that allows humans to think, learn, and feel. It makes people forget who they are, who their family members are, how to work, how to speak, and how to live. According to polls, 1 in 3 people know someone who suffers from it, and this number will continue to rise (AA, 2005). Though it works through complicated processes that escape scientists understanding, they discover more every day. Though no one has yet discovered a cure, several treatments diminish symptoms. Each day brings us closer to that illusive day when people can cure or prevent this disease.


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