The healthy human brain contains tens of billions of neurons, which specialized cells that process and transmit information via electrical and chemical signals. They send messages between different parts of the brain, and from the brain to different parts of the body. Alzheimer’s disease disrupts this communication among neurons; this can result in loss of function and cell death.
The brain typically shrinks to some degree in healthy ageing but does not lose neurons in large numbers. However, in Alzheimer’s disease, the damage is worse, as many neurons stop functioning, lose connections with other neurons, and die. Alzheimer’s interrupts processes vital to neurons and their networks, including communication and metabolism.
At first, Alzheimer’s disease destroys neurons and their connections in parts of the brain involved in memory, which includes the entorhinal cortex and hippocampus. It later affects areas responsible for language, reasoning, and social behaviour, such as in the cerebral cortex Over time, a person with Alzheimer’s gradually loses his or her ability to live and function independently. Ultimately, the disease kills.
Many molecular and cellular changes take place in the brain of a person with Alzheimer’s disease. These changes can be observed in brain tissue after death. Investigations are underway to determine which changes may cause Alzheimer’s and which may be a result of the disease.
The beta-amyloid protein involved in Alzheimer’s comes in several different molecular forms that collect between neurons. It is formed from the breakdown of a larger protein, called the amyloid precursor protein. One form, beta-amyloid 42, is thought to be toxic. In the Alzheimer’s brain, abnormal levels of this protein clump together to form plaques that collect between neurons and disrupt cell function.
Neurofibrillary tangles are abnormal accumulations of a protein called tau that collects inside neurons. Healthy neurons, in part, are supported internally by structures called microtubules; they help guide nutrients and molecules from the cell body to the axon and dendrites. In healthy neurons, tau normally binds to and stabilizes microtubules. However, in Alzheimer’s disease, abnormal chemical changes cause tau to detach from microtubules and stick to other tau molecules, forming threads that eventually join to form tangles inside neurons. These tangles block the neuron’s transport system, which harms the synaptic communication between neurons.
Alzheimer’s is currently widespread all over the world. Hopefully in time, with enough research and tools, we will be able to come up with solutions to fight what’s taking over our minds.