APHY 101 11.8

The functional redundancy and overlap within the nervous system ensure that individuals can perceive and interact with their environment across many decades.
Aging of the nervous system is a continuous process that begins before birth and progresses throughout the human lifespan.

Developmental Apoptosis:
- Aging of the nervous system originates prior to birth via apoptosis, a form of programmed cell death.
- Apoptosis carves out and sculpts the precise brain structures that remain functional after birth.
- This normal dying off of neurons continues throughout life.
- When brain apoptosis fails to occur properly, pathological conditions result. For example, individuals who die of schizophrenia as young adults possess the same total number of neurons as newborns. These excess neurons may produce surplus dopamine, precipitating hallucinations (the hallmark characteristic of schizophrenia).
Timeline of Axonal Myelination:
- Myelin sheath formation on axons commences during the $14\text{th}$ week of prenatal development.
- At birth, many nerve axons are not yet completely myelinated.
- By the time a child begins to walk, all axons destined to be myelinated have begun sheath development.
- The process of myelination continues through childhood and into adolescence.

Accelerated Neuron Loss and Ventricular Stem Cells:
- By age $30$ , the rate of neuronal die-off accelerates somewhat.
- Pockets of neural stem cells lining the cerebral ventricles maintain the capacity throughout life to give rise to cells that differentiate into neurons and neuroglia.
Overall Brain Shrinkage:
- Over an average lifetime, the total brain mass shrinks by approximately $10\%$ .
- Loss of gray matter is significantly greater than loss of white matter.
Regional Variation in Neuronal Death:
- Neuron loss is unevenly distributed across different regions of the brain:
  - Temporal Lobe: Experiences extensive neuron loss.
  - Brainstem: Very few neurons die in this region.
  - Frontal Cortex: By age $90$ , approximately half ( $50\%$ ) of its original neurons have been lost; however, due to structural redundancy, this deficit does not necessarily hamper overall functional capacity.
Dendritic Changes:
- The number of dendritic branches in the cerebral cortex declines over time.

Indicators of Slowed Neurotransmission:
- Decreasing concentration levels of neurotransmitters.
- Decreasing levels of the specific enzymes necessary to synthesize neurotransmitters.
- Decreasing total density of postsynaptic receptors.
Action Potential Conduction:
- The rate of action potential propagation along axons decreases by $5\%$ to $10\%$ .

General Physical and Functional Signs:
- Noticeable signs of normal nervous system aging include fading memory along with slowed responses and reflexes.
Autonomic Nervous System Decline:
- Decline in sympathetic nervous system function causes transient drops in blood pressure, which may lead to episodes of fainting.
Proprioception, Balance, and Fall Risk:
- By the seventh decade of life, a waning ability of ankle nerves to respond to walking vibrations impairs balance and increases fall risk.
- Multiple compounding factors contribute to impaired balance in older age:
  - Poor eyesight.
  - Anemia.
  - Inner ear malfunction.
  - Side effects of pharmaceutical drugs.
- Due to these combined factors, nearly one-third ( $\approx 33.3\%$ ) of individuals over age $65$ suffer at least one serious fall per year.
Sleep Pattern Changes and Reticular Activating System:
- Age-related shifts in sleep patterns reflect changes in the reticular activating system.
- Older adults generally sleep fewer total hours per night.
- Common sleep disturbances include transient difficulty falling asleep, difficulty staying asleep, and increased body movements during sleep.
- Bouts of insomnia are common, with some individuals sleeping no more than $1$ to $2$ hours per night.
- Electroencephalogram (EEG) readings demonstrate reductions in both stage IV slow-wave sleep and REM (Rapid Eye Movement) sleep.
- These sleep alterations frequently cause excessive daytime sleepiness.

Question 1: How does aging of the nervous system begin even before birth?
- Answer: Programmed cell death, or apoptosis, removes excess neurons in the brain during early development, leaving intact the specific structural components that will function after birth.
Question 2: What are some diseases that affect the aging nervous system?
- Answer: Nervous system diseases that may occur with age include stroke, depression, Alzheimer disease, Parkinson disease, and multi-infarct dementia.
Question 3: What are some of the physical and functional signs of an aging nervous system?
- Answer: Physical signs include a decrease in brain size, loss of neurons, a decrease in the number of dendritic branches, slowed neurotransmission, and a decreased rate of action potential propagation. Functional signs include fading memory, slowed reflexes, decreased sympathetic nervous system function, poor balance, poor eyesight, inner ear dysfunction, occasional drops in blood pressure (which may lead to fainting), and changes in sleep patterns.