When your brain is working normally, signals travel along nerves to and from your body and other parts of your brain as electrical impulses. Nerve signals travel across the gaps (synapses) between nerve cells (neurons) with the help of chemicals called neurotransmitters include one called acetylcholine. These signals are essential for brain activity e. g. memory, language and problem solving and controlling movements. In Alzheimer's, the number of nerve cells in the brain gradually reduces and the brain shrinks. Nerve cells can't be replaced so if you have Alzheimer's, memory and thinking get progressively worse as more cells are destroyed. Alzheimer's also causes a reduction in neurotransmitters particularly, acetylcholine, leading to weakened transmission of nerve signals.
Attribution: BUPA