Researchers have discovered changes that take place in the brains of those who have Alzheimers disease that may cause the memory loss and decline in other mental abilities that occur with Alzheimers disease. While it is not entirely understood why these brain changes occur, scientists have been searching for underlying factors that may lead to Alzheimers disease. Such precursors include an increase in Amyloid ? peptides, a decrease in the neurotransmitter acetylcholine, and the demyelization of the myelin sheath.
Throughout the process of aging the concentrations of acetylcholine decrease resulting in irregular lapses of short-term memory. Once acetylcholine is released into the synapse, a protein (acetylcholinesterase) breaks it down. Acetylcholinesterase is required to ensure that acetylcholine does not stay in the synapse for an excessive amount of time; if it remains in the synapse too long it can impair the brains health. Acetylcholine is important for the functions of many different nerves and is particularly important for parts of the brain involved in memory and learning because they use acetylcholine extensively (Chu et al. 2005).
Notably, acetylcholine levels are lower in people with Alzheimers disease. This suggests that the loss of acetylcholine-secreting neurons may cause some of the symptoms of Alzheimers disease. Alzheimers disease is characterized by the presence of neurofibrillary tangles and neurotic plaques that are formed by ? -amyloid deposits (Quirion, 1993). The accumulation of these ? -amyloid peptides in Alzheimers disease plays a contributing role in triggering synaptic dysfunction in neurons (Lefort et al. , 2012).
It was found that amyloid ? 1-42 peptides (A? 1-42) play a key role in the pathogenesis of the cholinergic dysfunction in Alzheimers disease (Hoshi et al. , 1997). Since cholinergic cells play an important role in memory and learning, any damage done to these cells will become dysfunctional. There is an increase in soluble A? 1-42 that may agitate cholinergic functions, leading to the decline of memory and cognitive functions that are characteristics of Alzheimers disease. Hoshi et al (1997) hypothesized that soluble A? -42 is produced at an early phase of Alzheimers disease and can start effecting cholinergic neurons by repressing acetylcholine synthesis, thus causing a drop in acetylcholine release, modulating synaptic connections, and finally resulting in cholinergic deficits, in which may provoke progressive loss of memory and cognitive function in patients with Alzheimers disease. Therefore, soluble A? 1-42 may have a principal role in the cholinergic dysfunction in the formation of Alzheimers disease by suppressing acetylcholine synthesis.
White matter, or myelin, coats and insulates neuronal axons. Nonneuronal cells called oligodendrocytes wrap around the axon; these oligodendrocytes create the thickness of the myelin coats around the axon and control the speed of electrical impulses that affects processing of information (Fields, 2010). Loss of myelin is associated with Alzheimers disease (Sjobeck et al. , 2005); this demyelization could be an explanation for Alzheimers disease. The white matter consists of millions of bundled axons that connect neurons in different parts of the brain.
Myelin is vital for high-speed transmission and any damage done in such regions can impair sensor, motor, and cognitive functions (Fields, 2010). When demylination occurs, the nerve fibres affected would have inhibited action potentials, which would slow down the signal. Eventually, with increased demylination, that signal could potentially cease to transmit signals altogether and may effect the retrieval of memories and facts of ones own life. Sjobeck et al (2005) noted that the anterior part of the brain is more heavily affected by myelin reduction than the posterior.
The anterior section of the brain contains the frontal cortex, which is involved in thinking, judgement, and higher level functioning. Myelin loss in this section of the brain could severely impact those functions and be a significant factor in Alzheimers disease. Acetylcholine has two receptors: the nicotinic receptor and the muscarinic receptor. The nicotinic cholinergic systems are involved in cognitive functioning including learning and memory (Chu et al. , 2005). Since acetylcholine has such a strong effect on memory and learning, it is notable that acetylcholine impacts these nicotinic receptors.
A subtype of a nicotinic receptors, ? 7, is found largely in astrocytes (found in the central nervous system), and creates inflammation that aids in the deterioration process of neurons. Using immunochemistry it was found that astrocytic intracellular calcium induces neuronal cell death, particularly in the hippocampus (Taektong et al. , 2004). The hippocampus plays vital roles in long-term memory. Any damage done there would impair the hippocampuses functions; therefore, degeneration in this area would cause Alzheimers disease-like symptoms.
While the nicotinic receptor plays a role in the formation of Alzheimers disease, the muscarinic receptor plays a role in the treatment of the same disease. Degeneration of neurons is the main cause of Alzheimers disease, but a high level of cholesterol is a large factor as well. Treating degeneration can be extremely difficult. However, focusing on decreasing the levels of cholesterol can show the same positive effects as treating degeneration for those on the threshold of Alzheimers disease.
Treating cholesterol with M1 muscarinic acetylcholine receptor (M1-MAChR) agonists may provide a practical way to reduce squalene synthetase activity level (high squalene synthetase activity is a key precursor in cholesterol), thus decreasing the chance of receiving Alzheimers disease, and helping those who live with the disease (Zuchner et al. , 2005). In addition, Kojro et al (2010) hypothesized that cholesterol lowering drugs such as statins could have a healing potential in the prevention and treatment of Alzheimers disease.
These studies wanted to find the precursors of Alzheimers disease. Scientists and researchers are searching for not only medicine to combat this disease, but also a way to cure it. While going over the studies, the main signs of Alzheimers disease are having prominent neuron death in the hippocampus, and the gradual death of cholinergic brain cells. The topic of how these brain deficits effect Alzheimers disease is important because the more we learn about how and why this disease can occur the better chance we have to control and perhaps even terminate the disease completely.