The brain also has the limbic system, involving the thalamus, the hypothalamus, the amygdala and the hippocampus.

The hypothalamus regulates the release of hormones in the body, the amygdala regulates the 'fight or flight' response and the hippocampus helps with memory.

Altogether, the brain cells control every aspect of our life from activities like walking and talking to thinking and feeling to having a unique personality.

The basic structural unit of the brain is the neuron. Neurons or brain cells conduct and transmit electrical signals. Millions of neurons are arranged end-to-end to form circuits in the brain. These circuits control all types of activities of the body such as seeing, hearing or even eating and walking. These circuits also control our thoughts and emotions.

The area where two neurons interact is called a synapse. Electrical impulses generated within the neuronal cells jump across the synapse with the help of chemicals known as neurotransmitters. These neurotransmitters allow the electrical impulses to jump from one neuron to another and hence complete the circuits.

Alzheimer's disease first affects the hippocampus and then spreads to the frontal, parietal and temporal lobes of the cerebral cortex. It also spreads to the limbic system, which includes the amygdala.

Alzheimer's disease also affects an area called the basal nucleus of Meynert, which has neurons rich in the neurotransmitter acetylcholine. Alzheimer's disease causes the destruction of the neurons in all these areas.

The formation of neuritic plaques and neurofibrillary tangles are characteristic of Alzheimer's disease. Studies of plaques and tangles in the brains of people with Alzheimer's have given rise to various theories about plaques and tangles.

The amyloid beta precursor protein is a large molecule that continues through the membrane of nerve cells. The enzyme B-secretase, also known as beta-site APP-cleaving enzyme 1, cleaves a part of the molecule on the outside of the cell membrane to form beta amyloid protein. Insoluble clumps of beta amyloid protein, known as amyloid or senile plaques, accumulate in the spaces between neurons.

The neurons contain shuttle vesicles containing neurotransmitters and synaptic proteins from cell bodies to nerve terminals. A protein tau stabilizes the shuttling structures. The tau becomes abnormal and is unable to hold the structures together. These form neurofibrillary tangles within the neurons. The collapse of the transport system may disturb the communication between the neurons. These tangles may cause neuronal death leading to dementia.

In the early stages of the disease, the temporal lobe and the hippocampus are affected, resulting in loss of short-term memory. Patients may experience memory lapses, and may forget familiar words, names and the location of everyday objects. But these symptoms may not be apparent to friends and family and may not be evident during a medical examination.

This stage is followed by a mild decline in memory, which is often noticed by close family. This may be evident during a medical examination. The patient may find it difficult to remember some words or names. Memory lapses begin to affect performance. The patient is unable to recollect a large part of what he read and tends to misplace things. He may find it difficult to plan and organize.

The disease progresses to a stage where a medical examination can diagnose deficits. Patients have decreased knowledge about current affairs and may find it difficult to do complex arithmetic. They may also find it difficult to plan and manage their finances. They may also have diminished memory of the past. Patients may be passive and withdrawn.

In the middle stages of Alzheimer's disease, the plaques spread to the frontal lobe. The beginning of the mid-stages is characterized by major gaps in memory and deficits in cognitive function emerge. Those suffering need some assistance with daily activities.

At this stage, patients may be unable to remember important details, such as their address or telephone number. They may not remember the date or day of the week. They also tend to have problems with simple arithmetic. They may need help choosing appropriate clothing.

Memory continues to deteriorate and personality changes become more significant. At this stage patients become dependant on others for daily chores. They may lose memory of recent events and may be oblivious of their surroundings. Their normal sleep and wake cycle may be disturbed. They may even experience difficulty in remembering the names of well-known friends and family. They may also experience urinary or fecal incontinence. They may become suspicious and suffer from delusions and hallucinations. They may also develop compulsive, repetitive behaviors. They tend to wander and become lost.

During the end stage of Alzheimer's, the neurofibrillary tangles increase in areas like the amygdala and the parietal lobes. Patients become totally bed-ridden and need constant care. They experience a very severe decline in cognitive functions. They also lose their ability to speak or recognize speech and control movement. They need help with daily chores and may suffer from incontinence.

Patients lose their ability to walk without help. They gradually lose the ability to sit without support or hold their head up. The ability to respond deteriorates and muscular rigidity sets in. They lose the ability to smile and swallowing becomes difficult.

The progress of the disease varies. For some people, it may take a decade to reach the final stages, where as for some it may take only 5 years. Most people with Alzheimer's don't die of the disease itself, but of pneumonia, or a urinary tract infection or complications from a fall.

Nearly 6-8% of patients with Alzheimer's develop symptoms before the age of 65. This form of dementia is called early-onset Alzheimer's and tends to run in families. The genetic path of inheritance is much stronger in early-onset Alzheimer's. People with Down Syndrome are particularly at risk. They are often in their mid to late 40's or early 50's when symptoms first appear.

Late-onset Alzheimer's is most common and accounts for nearly 90% of cases. The disease usually occurs after age of 65. The condition may or may not be hereditary.

Familial Alzheimer's disease (FAD) is believed to be completely inherited. In affected families, members of at least two generations are found to have had Alzheimer's disease. This form of Alzheimer's is extremely rare, accounting for less than 1%. It has a much earlier onset, often affecting people in their 40s.

There is no definite diagnostic test for Alzheimer's disease. Diagnosis based on symptoms is almost 90% accurate. Only a postmortem can confirm the diagnosis. Thus it is important to rule out other conditions, such as brain tumors and depression, that have similar symptoms. People with Parkinson's disease also can develop dementia. Many older adults on multiple medications may suffer from impaired memory.

Tests to rule out similar conditions include medical history, physical examination, blood and urine tests, X-rays, imaging techniques such as CT scan, neurological examination and neuropsychological tests.

Intellectual function tests and psychiatric assessment are carried out to evaluate the mental status. This is done by administering simple tests to ascertain the ability of problem solving, language skills, counting, mapping attention span etc.

There is no cure for Alzheimer's disease. The available drugs aim to boost the efficiency of damaged neurons. Drugs can also relieve some of the secondary symptoms such as depression, sleeplessness and agitation.

There are two classes of drugs that are aimed at reducing the cognitive decline. Cholinesterase inhibitors such as donepezil work by increasing the levels of neurotransmitters in the brain. Donepezil is found to delay the onset in people with mild symptoms. Many people who take this drug may find no improvement. Possible side effects include diarrhea, nausea and vomiting.

Memantine is used to treat moderate to severe stages of Alzheimer's. The drug prevents damage to neurons from glutamate, a neurotransmitter. Memantine is sometimes used in combination with a cholinesterase inhibitor. Possible side effects include dizziness. It may also increase agitation and delusions in some people.

Alzheimer's patients exhibit behavioral abnormalities such as aggression, delusions and hallucinations. Anti-psychotic drugs such as olanzapine, quetiapine, haloperidol, risperidone are used to treat these symptoms. Anxiolytic drugs such as lorazepam, oxazepam, zolpidem, buspirone will help relieve anxiety.

Antidepressants such as desimipramine, fluoxetine, paroxetine, sertraline, citalopram are prescribed if the patient shows signs of depression.

When patients wander and lose their way, a card detailing their name and contact telephone numbers should be kept in their pocket.