What is Alzheimer’s disease?


Alzheimer’s disease (AD) is a chronic neurodegenerative disease and the main form of dementia. The earliest symptom is difficulties in remembering recent events, or short-term memory loss. Along with the progression of the disease more symptoms will arise including problems with language, disorientation, mood swings and behavioural issues. AD is histopathologically characterized by amyloid beta (Aβ) depositions in extracellular plaques and by hyperphosphorylated tau protein in intracellular tangles. In addition, progressive atrophy of neuronal tissue in the brain, especially in the hippocampus and several cortical areas, is found. Another pathogenic event that occurs early on in the disease development is synaptic dysfunction.



Figure 1: Representation of a healthy brain (left) versus an Alzheimer brain (right). Loss of volume, shrinkage of the hippocampal area and cerebral cortex are found, with severely enlarged ventricles.



What is a synapse?


Synapses are the communication points between neurons. They are essential for learning and memory formation. In response to increased or decreased activity, synapses have the ability to strengthen or weaken, this is called synaptic plasticity. This plasticity plays a key role in learning and memory. The human brain consists of approximately 90 billion neurons, and each neuron has approximately 10.000 synapses. All these neurons communicate together using their synapses to pass electrical or chemical signals to another neuron.




Figure 2: Schematic figure of a synapse consisting of pre- and post-synaptic terminals. The content of the synaptic vesicles (SV) are released from the presynaptic terminals and bind to receptors on the dendritic post-synaptic membrane.