What It Is
Dementia is not a single disease with a single cause, but a collection of symptoms characterised by the progressive loss of cognitive abilities. This deterioration
within the brain affects memory, understanding, behavior, judgment and ultimately the ability to perform the simple tasks of
everyday life. People whose decline has
led to a diagnosis of dementia before the age of 65 are said to have early-onset dementia, and they may represent up to 5 per cent of the UK's total cases of dementia;
but the vast majority of people with dementia are diagnosed after the age of 65 and this is referred to as late-onset dementia. Although it used to be regarded as just
a normal inescapable part of ageing, late-onset dementia is, in fact, a chronic condition in which the overall rate and pattern of decline can vary considerably
between individual patients, and the brain.
Alzheimer's disease is responsible for around 60-70 per cent of all cases. The disease interferes with the proper healthy functioning of parts of the brain, preventing
nerve cells (neurons) within the brain from communicating with and responding to other brain cells, and the cells affected eventually die. A more detailed summary of Alzheimer's Dementia
can be found HERE
It is not yet clear exactly how the disease works, but it is normally associated with two strange structures that appear in microscopic form inside ageing brains: amyloid plaques
and neurofibrillary tangles. The plaques are chiefly made up of a protein fragment called beta-amyloid, which in certain circumstances clumps together to form these insoluble
plaques. The tangles within brain cells are mainly formed from another proteins called tau, the normal role of which is to stabilise vital parts of the cell structure.
In Alzheimer's disease this tau separates and forms abnormal twisted tangles with other tau filaments, which disrupts the ability of one neuron to transmit messages and
nutrients to other neurons. We do not know whether the plaques and tangles are a cause of the disease or a product of the disease since both abnormal structures have been
found in the brains of people without dementia symptoms.
Neurons. Neurons are cells that underline the functions of the central nervous system, including the brain and spinal cord, and the peripheral
nervous system consisting of ganglions and nerves outside the brain and spinal cord. Neurons vary in shape but they share the common characteristics of electrical excitability.
Neurons process chemical and electrical signals and transmit these signals to neurons and other types of cells. Neurons have several major compartments, including soma,
dendrite, and axon. The shapes of these compartments varies depending on the functionality of the neuron
The Soma of the neuron houses the nucleus of the neuron, which contains the genetic materials. Processes including transcription and translation of genes, assembling
and modification of proteins take place in the soma. The
Dendrites, a tree like compartment arise from the soma, receives input from extracellular space, including chemical
or electrical signals from other neurons. The tree like structure of the dendrite enables the neuron to receive and integrate inputs from a relatively large neighborhood.
The Axon is a long and slender structure arises from the soma. It is the output structure of a neuron. The initial segment of an axon, where the axon connects to the soma,
is the place where action potentials are most often initiated. The axon carries action potentials away from the soma and transmits signal to other cells. A
Synapse is a
structure that permits the signal transduction from one neuron to the target cell (notice that the target cell could be the neuron itself). A chemical synapse consists of presynatpic terminal, postsynaptic terminal and synaptic cleft. When the
action potential reaches the presynatpic terminal, generally located on the end of an axon, it triggers the release of
Neurotransmitters into the synaptic cleft. These neurotransmitters then bind to the receptors on the postsynaptic terminal, generally located on the soma or dendrite of the target cell, which completes the signal transduction.