Practice Passage Test - 2 | Practice Passages for MCAT PDF Download

Passage 

Long-term potentiation (LTP) is a crucial cellular process that underlies learning and memory. It involves the communication between two neurons, where the release of the neurotransmitter glutamate by a presynaptic neuron binds to receptors on a postsynaptic neuron. This binding leads to an influx of sodium and calcium, which activates genes and plays a significant role in memory formation. The initial receptor activated by glutamate is the AMPA receptor, while the NMDA receptor is blocked by extracellular magnesium until there is a change in membrane potential.

Neurodegenerative disorders like Alzheimer's disease are known to disrupt LTP, resulting in memory deficits. In Alzheimer's patients, the hippocampus, a brain region involved in memory acquisition, experiences the loss of essential neurons. One potential mechanism for this neurodegeneration involves calcium-mediated toxicity, which occurs due to excessive neuronal excitation caused by glutamate.

Chronic stress is another contributing factor to cell loss in the hippocampus. It leads to elevated levels of corticosteroids, such as cortisol, which can impact neuronal activity in the brain. This has led to the formulation of the "Glucocorticoid Hypothesis of Aging." Normally, the intact hippocampus inhibits the stress axis (hypothalamic-pituitary-adrenal axis), which is responsible for releasing cortisol during stressful situations. If the hippocampal region is compromised, it may result in the lack of inhibition of the stress axis, leading to further cortisol release and creating a cycle of excessive steroid release with each stressful event.