Alzheimer's disease is a neurodegenerative disorder that affects the brain, leading to the disruption of communication among neurons, loss of function, and cell death. The healthy human brain contains tens of billions of neurons, which are specialized cells responsible for processing and transmitting information through electrical and chemical signals.
Neurons are in constant communication with neighboring brain cells. When a neuron receives signals from other neurons, it generates an electrical charge that travels down its axon and releases neurotransmitter chemicals across a tiny gap called a synapse. These neurotransmitter molecules bind to specific receptor sites on the dendrites of nearby neurons, triggering chemical or electrical signals that either stimulate or inhibit activity in the receiving neuron.
Communication between neurons often occurs across networks of brain cells. In fact, scientists estimate that a single neuron may have as many as 7,000 synaptic connections with other neurons within the brain's communication network.
Metabolism, the process of breaking down chemicals and nutrients within a cell, is critical to the healthy function and survival of neurons. Neurons require energy in the form of oxygen and glucose, which are supplied by blood circulating through the brain. The brain has one of the richest blood supplies of any organ and consumes up to 20 percent of the energy used by the human body, more than any other organ.
Repair, Remodeling, and Regeneration
Unlike many cells in the body, neurons have evolved to live a long time, with some neurons surviving for more than 100 years in humans. As a result, neurons must constantly maintain and repair themselves. They also continuously adjust their synaptic connections, strengthening or weakening them based on the amount of stimulation received from other neurons. Additionally, adult brains may generate new neurons through a process called neurogenesis. Remodeling of synaptic connections and neurogenesis are important for learning, memory, and potentially brain repair.
Other Key Cell Types in the Brain
While neurons are a major player in the central nervous system, other cell types also play a crucial role in maintaining healthy brain function. Glial cells, which outnumber neurons by about 10 to 1, surround and support the function and health of neurons. Various forms of glial cells, such as microglia, astrocytes, and oligodendrocytes, have specific functions in protecting neurons from damage, clearing debris from the brain, and regulating the brain's delicate balance.
How Alzheimer's Disease Affects the Brain
In Alzheimer's disease, damage is widespread, leading to the dysfunction, loss of connections, and death of neurons. The disease initially affects brain regions involved in memory, such as the entorhinal cortex and hippocampus, and later progresses to areas responsible for language, reasoning, and social behavior. Over time, many other regions of the brain are damaged, resulting in a gradual loss of independence and ultimately, fatality.
One of the key molecular changes observed in the brains of individuals with Alzheimer's disease is the accumulation of beta-amyloid protein. Beta-amyloid is formed from the breakdown of a larger protein called amyloid precursor protein. In Alzheimer's, abnormal levels of beta-amyloid clump together to form plaques that collect between neurons, disrupting cell function .
Another characteristic feature of Alzheimer's disease is the presence of neurofibrillary tangles. These tangles are abnormal accumulations of a protein called tau that form inside neurons. In healthy neurons, tau binds to and stabilizes structures called microtubules, which help transport nutrients and molecules within the cell. In Alzheimer's, abnormal chemical changes cause tau to detach from microtubules and form tangles, blocking the neuron's transport system and impairing synaptic communication.
Chronic inflammation is another aspect of Alzheimer's disease. It may be caused by the buildup of glial cells, particularly microglia, which fail to clear away waste, debris, and protein collections, including beta-amyloid plaques. This failure to clear debris and perform their protective functions leads to chronic inflammation and further damage to neurons .
Vascular issues, such as beta-amyloid deposits in brain arteries, atherosclerosis, and mini-strokes, may also contribute to Alzheimer's disease. Reduced blood flow and oxygen to the brain, as well as a breakdown of the blood-brain barrier, can result in inflammation and the accumulation of toxic proteins. This complex interplay between Alzheimer's-related changes and vascular problems further exacerbates the disease.
Loss of Neuronal Connections and Cell Death
As Alzheimer's disease progresses, neurons throughout the brain become injured and die, leading to the breakdown of connections between networks of neurons and significant brain atrophy. By the final stages of the disease, there is a widespread loss of brain volume, contributing to the severe cognitive and functional decline observed in individuals with Alzheimer's .
In conclusion, Alzheimer's disease disrupts key biological processes in the brain, such as neuronal communication, metabolism, and repair. It leads to the formation of amyloid plaques, neurofibrillary tangles, chronic inflammation, and vascular issues. These changes ultimately result in the dysfunction, loss of connections, and death of neurons, leading to the progressive cognitive decline and functional impairment characteristic of Alzheimer's disease.