Aging and Neurodegeneration

Abstract

Aging and neurodegeneration are closely-related concepts which have been found to lead to
significant increase in disease burden among elderly people of both genders. Aging is affects
both the sensory and nervous systems. Its effects on the sensory system include are reduction
of somatosensory, taste, olfactory, hearing, and visual functions. It also negatively affects the
functioning of the nervous system through its effects on morphological arrangement,
neuronal metabolism, and oxidative imbalance. All these factors are associated with
neurodegeneration. Neurodegeneration is in turn responsible for multiple neurodegenerative
disorders. The most common of these disorders is Alzheimer’s disease. Others are
Parkinson’s and Huntington’s disease. These diseases have debilitating effects on the motor,
sensory, and cognitive functioning of the people that they affect. Given that they lack a cure
and the number of people who are older than 60 is increasing at a rapid pace, these conditions
pose a significant threat to global health.

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Introduction

Aging process is defined as progressive accumulation of changes over time which in
turn are associated with or are responsible for increasing susceptibility to both disease and
death as one’s age advances (Grimm & Eckert, 2017). Aging leads to decline of all organs,
including the brain. This decline is accompanied by increased susceptibility to multiple
diseases. In fact, according to Grimm & Eckert (2017), age is the main risk factor for disease
prevalence in the world.
Closely related to aging in terms of effects on the brain is neurodegeneration.
Neurodegeneration is the progressive loss of function, structure, and even death of neurons.
Aging is a key risk factor od neurodegeneration. The combined effects of aging and
neurodegeneration lead to the development of multiple neurodegenerative disorders, such as
Alzheimer’s disease (Grimm & Eckert, 2017). This paper examines how the aging process
affects the sensory and neurological systems and how it contributes to neurodegeneration
which in turn leads to the development of various neurodegenerative disorders.

Effects of Aging on Sensory System

Aging has been found to have major negative effects on the sensory system. As one
grows older, they start to gradually experience losses in their sensory system. The sensory
system includes somatosensory, taste, olfactory, hearing, and visual functions (Iwamoto et
al., 2013). Even though the reduction of these functions is common in normal aging, the
reduction of the functions can be severe for people experiencing various neurodegenerative
disorders such as Alzheimer’s and Parkinson’s disease.
In regard to taste, age has been found to contribute to a general reduction in salty
sense. However, there are individual variations on how aging affects sour, bitter, and sweet
tastes (Iwamoto et al., 2013). Other sensory functions that decrease with age include

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proprioception and deep vibration sensation. Aging also leads to reduction in superficial
sensation such as pain, temperature, and touch (Iwamoto et al., 2013). Having impaired
sensory system resulting from aging process can lead to deterioration of one’s quality of life
and activities of daily living among the elderly.

Effects of Aging on the Nervous System

Just like other body organs, aging also affects the nervous system. The effects of
aging begin showing when one reaches the age of 50. Due to these effects, the brain gradually
declines thus leading to impairment of attention, motor coordination, learning, and memory
(Castelli et al., 2019). There also multiple methods through which aging affects the nervous
system. These methods include oxidative imbalance, neuronal metabolism, and
morphological arrangement.
Oxidative imbalance
To preserve the structural integrity of the brain and its optimal functioning, there is
need to maintain a tight balance between the antioxidant system and oxidative stress (Castelli
et al., 2019). During aging, this balance is usually damaged because neurons often
accumulate damaged mitochondria and aggregated and impaired proteins (Castelli et al.,
2019). This accumulation is due to oxidative stress. According to Castelli et al. (2019),
problems associated with oxidative stress lead to cognitive impairment while those that are
associated with decreased levels of antioxidants in plasma and brain lead to memory
impairment.

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Figure 1: Effects of aging on Neuronal Cell and the Involved Mechanisms. Source:
Castelli et al. (2018).
Neuronal metabolism
Aging causes impairment of neuronal metabolism. Even though the brain comprises
of just around 2% of the human body mass, it uses more than 20% of its energy (Castelli et
al., 2019). Aging has been found to reduce the amount of energy that is available to the brain.
The reduced amount of energy available to the brain is due to a number of reasons. One of
them is that aging reduces the ability of mitochondria to transduce energy (Castelli et al.,
2019). Aging also reduces the amount of glucose that is available to the brain and impairs
intake of neuronal glucose (Castelli et al., 2019). Thus, due to aging, there is failure in both
glucose intake and its control by the brain.
Studies have shown that decreased levels of glucose intake in the brain are associated
with cognitive impairment (Castelli et al., 2019). However, the impairment is relatively mild.
The extend of cognitive performance depends on the area of the brain that is affected by the
decreased level of glucose intake. For instance, decreased consumption of glucose in the
cortex has been linked to significant drop in cognitive performance.
Morphological rearrangement

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Studies have found that there is no widespread loss of nerve cells during normal
aging. The regions where neuronal losses occur are relatively restricted. It is estimated that
just 10% of the brain’s regions experience neuronal loss due to aging (Castelli et al., 2019).
However, aging has been found to result in significant structural changes of dendrites. In
particular, aging leads to reduction in both number and structure of dendrites. Aging also
results in loss of some dendritic spines (Castelli et al., 2019). Changes in the number and
structure of dendrites have been found to cause significant cognitive decline, especially
memory and attention (Castelli et al., 2019). The changes are also associated with decline in
motor and sensory functions.

Figure 2: Effects of Degeneration on Neuronal Cell and the Involved Mechanisms.
Source: Castelli et al. (2018).

Aging and Neurodegenerative Disorders

The negative effects of aging on neuronal and sensory systems lead to the
development of various neurodegenerative disorders among the elderly. The disorders are
primarily a result of damage to the central nervous system. The exact disorder that affects a
person depends on genetic as well as environmental factors (Hussain et al., 2018). All
neurodegenerative conditions begin mildly and then the situation worsens over time as one’s
cognitive, motor, and sensory functions decline. Eventually, the condition may result in
death. Some of the most common neurological disorders include Alzheimer’s, Parkinson’s,

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and Huntington’s diseases. Unfortunately, despite extensive research on these diseases, no
cure has been found for any of them. Due to this lack of a cure, they remain among the most
feared diseases among older people. According to Wyss-Coray (2016), the number of people
who are older than 60 years is going to double in the next 35 years. Such an increase in older
people is likely to make Alzheimer’s, Parkinson’s, Huntington’s and other neurodegenerative
disorders to be major global health threats.

Figure 3: Neurodegenerative Disorders and Parts of the Brain that they Affect. Source:
Hussain et al. (2018)
Alzheimer’s disease
The condition is the most widespread neurodegenerative disease in the United States
(Hussain et al., 2018). Its incidence is estimated to be 5.4 million with one person becoming a
patient after every 68 seconds (Hussain et al., 2018). Incidences of the condition in other
developed countries are also very high. With life expectancy of many countries rising, the
number of people developing Alzheimer’s condition is also likely to increase.
The condition usually begins as mild memory loss. The symptoms then worsen as the
condition progresses. The loss of memory can become so bad that patients forget how to
perform some of the most basic daily activities such as brushing of teeth and combing of hair.
As the disease progresses, the patients may even forget names and faces of their family
members. Due to these symptoms, persons afflicted with the disease often require permanent

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care until they die (Hussain et al., 2018). Such type of care constitutes a significant societal
burden.

Figure 4: Brain of an Alzheimer’s disease patient at an advanced stage compared to that
of a healthy brain. Source: Bagad & Khan (2013).
Parkinson’s disease
Unlike Alzheimer’s disease whose main symptom is memory loss, Parkinson’s
disease is a movement disorder. It is estimated that 0.3% of people in developed countries
suffer from the condition (Hussain et al., 2018). The condition is caused by reduction in
levels of dopamine. Its symptoms include shaking of neck muscles, legs, arms, and hands. As
the condition progresses, one may develop severe ataxia. At this point, a person suffering
from Parkinson’s disease may completely fail to perform basic everyday tasks.

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Figure 5: Patients with Parkinson’s disease have less dopamine than healthy patients.
Source: National Institute of Environmental Health Sciences (2020)
Huntington’s disease
Huntington’s disease results from intracellular accumulation of Huntington’s protein.
The protein is a mutant (Hussain et al., 2018). The accumulation of aggregates of this mutant
protein leads to apoptosis of the brain cells (Hussain et al., 2018). Therefore, the condition is
a genetic disorder. Its main symptoms are movement problems such as the ones exhibited by
patients with Parkinson’s disease. The disease can also lead to development of dementia just
like Alzheimer’s disease.

Conclusion

Aging process is a significant contributor of neurodegeneration and the conditions
that are associated with it. These neurodegenerative conditions such as Alzheimer’s and
Parkinson’s disease affect, among others, cognitive, sensory, and motor functions. Therefore,
people with these conditions end up losing their ability to perform daily activities, even the
most basic ones such as brushing teeth. Since there is no cure for neurodegenerative
disorders, patients have to be taken care of for the rest of their lives.

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