Parkinson’s Disease and Aging

The Nervous System

The nervous system is comprised of two major regions, the central and peripheral nervous system. The central nervous system is composed of the brain and spinal cord, while the peripheral nervous system is composed of the nerves in the body. The brain is found in the cranial cavity, with the spinal cord within the vertebral column. The nervous system helps receive information about the environment, therefore creating an appropriate responses.

These responses can be further divided into three steps, the first being sensation. Sensation is when information is received about the environment, this is known as a stimulus. The sensory information then travels through the CNS and then through the afferent side of PNS. When information comes from sensory receptors like the skin, skeletal muscle, and joints, it is known as somatic sensory information, and finally if this is from blood vessels or internal organs, it is known as visceral sensory information. This processes and transmit sensory information to coordinate bodily function and movement. The next step is response, here the system produces a response because they received a signal from sensory stimulation.

The efferent branch of PNS then carries the signal always from the CNS to that specific organ. When this organ is skeletal muscle, it is called somatic motor, when it is from cardiac, smooth, or glandular tissue, this is called visceral motor. The responses are managed by different systems, if they are voluntary, they are from the somatic nervous system, and if they are involuntary they are from the autonomic nervous system. Finally, the last step is integration, here the stimuli received by the sensory structures are interpreted to the nervous system, and here the information is processed. In the CNS, the stimuli are compared with other stimuli or memories, this can then lead to a recalled response that can be regenerated (Biga, et al.). CNS and PNS Model (Biga, et al.,)

Within the nervous system are two types of cells that make up nervous tissue, neurons and glial cells. Neurons work for the communication that the nervous system creates. These are electrically active and eventually release chemical signals that connect between each other and the target cells. Glial cells, are smaller and are a support for nervous tissue, they maintain extracellular environment, improve signal conduction and protect them from harmful pathogens (Biga, et al.).

When understanding how this system works, there are a few components and pieces that come into play. On the skin, the sensory receptor found is the thermoreceptor, the amount of change that is created in the electrical state happens due to the strength of the stimulus created, this is called a graded potential, if the signal is strong, this creates an electrical signal that travels to the axon, the strength of this signal specifically is called the threshold, and the “electrical signal” is the action potential. The action potential travels along the axon near the receptor, to the axon terminals, then through the synaptic end bulbs of the CNS, and finally this creates a release of the neurotransmitter (Biga, et al.).

When specifically looking at this in the CNS, the neurotransmitter crosses through the synapse, and binds to a receptor protein, when this happens, the cell membrane changes its electrical stage, and begins a new graded potential, if it is strong enough to reach the threshold, it will send the action potential to the synapse, the neurotransmitter is released and binds to the receptor.

The thalamus then sends the sensory information to the cerebral cortex and a stimulus response begins. In the cerebral cortex, this information is processed through the neurons, this triggers an emotional state, response, and memories, and this then begins the plan to develop the most appropriate response. When a specialized signal is sent down the spinal cord to create movement, the upper motor neuron begins this motion in the precentral gyrus of the frontal cortex, this moves down the spinal cord, and into the lower motor neurons to create an action to have muscle fibers contract (Biga, et al.).

Aging in the central nervous system can be seen through multiple dimensions. To start, in older adults, there is reduced brain volume that can be correlated to motor deficits that occur with normal aging. This is seen through atrophy at the somatosensory cortex, and can be presented with increased falls, poor balance, and increased use of visual feedback for motor performance. This can also include issues associated with GAIT and their changes (Seidler, et al., 2010).

When looking at white matter and cortical grey matter impairment and changes, in patients with Parkinson’s disease and those who are unimpaired, there were some studies that found that there were little to no white matter changes. Previous studies have challenged the view that white matter degeneration and grey matter reduction occurred later in disease (Rektor, et al., 2018). Changes in white matter with aging (Inzitari, et al., 2009)

Gray matter is also greatly reduced in older adults especially in the prefrontal cortex since it has increased susceptibility to gray matter atrophy. This can be related to motor performance problems, due to the motor control being more independent in these regions of the brain (Seidler, et al., 2010). A decline in white matter volume is noted but continues more rapidly than gray matter changes. This not only affects quantity but also quality of tasks that require motor skill and performance. Another area to note is the corpus callosum, it is directly affected and has been connected with bimanual coordination and the inhibition of ipsilateral motor cortex during single handed movements.

This can affect day to day activities that require coordination when using both hands (Seidler, et al., 2010). Another area to also note is the cholinergic reduction that has been associated with the decline in the hippocampus, which can cause significant cognitive problems. Other areas noted that are also shown to create motor issues are; serotonin concentration and a reduction in nigrostriatal dopamine transporter that underlies the decreased fine movement control and slowed down reaction to motor function (Seidler, et al., 2010).

When noting the many changes within the brain with aging, there are also some factors that do not impact until a lot later in life. Overall for everyone, brain function does decline, this can be seen through the loss of short term memory, ability to learn new things, verbal skills, intellectual performance, and even reaction time or performance tasks becoming slower. This is manifested when the nerve impulses become slower to process, this is can be seen through the decrease in nerve cells that create an additional loss of function.

Redundancy happens as more cells need to function but must compensate for the loss/death of nerve cells that occurred. With this, the creation of new connections are made with the remaining nerve cells. Finally, the production of new nerve cells occurs predominantly post brain injury, this occurs in the hippocampus, and basal ganglia. With these changes, it is noted that other issues arise, like decreased blood flow to the brain, this occurs by about 20% and can ultimately lead to the loss of brain cells prematurely, and potentially impair mental function in the long run (Effects, n.d.).

Parkinson’s disease (PD), is defined, typically as a disorder of voluntary movement, this is now better understood as more of a spectrum within non-motor functions like; autonomic failure, cognitive dysfunction, and neuropsychiatric factors. These factors can all play a role in not only the CNS but also in the involvement of the PNS (Orimo, Ghebremedhin, & Gelpi 2018).

Motor dysfunction that can manifest are; more resistance to passive limb movement, slowness in movement, reduction in amplitude, absence of normal unconscious movement, and decreased voice volume (Ghatak, Trudler, Dolatabadi,& Ambasudhan, 2018). In a study that looked at somatosensory temporal discrimination threshold (the shortest interval an individual recognizes a pair of stimulus during different times), they found that STDT changes in patients with PD can reflect the progressions, but in the early stages of PD, it is similar those of healthy subjects without PD (Conte et al., 2016).

In various studies, it has been found that there is a significant amount of involvement of the PNS within the heart and GI tract in patients with PD, this shows that PD can spread from the peripheral to central nervous systems through the autonomic nerve fibers. This results from abnormalities of basal ganglia function, chronic dopamine loss, and anatomical/biochemical changes to the function of GABAergic and glutamatergic pathways to the basal ganglia (Ghatak,Trudler, Dolatabadi,& Ambasudhan, 2018).By the time patients present to a physician with PD they have lost over 70% of dopaminergic neurons of the substantia nigrapars compacta (Ghatak,Trudler,Dolatabadi,& Ambasudhan, 2018).

Physical activity interventions overall have shown great improvement in cognitive deficits, especially through life span, but can provide great intervention and treatment, post diagnosis. In a study conducted understand the implication of exercise or physical activity in patients with PD, it was found that increased frequency of sessions showed the most improvement, specifically with being more functional and feeling better overall (Caciula , Horvat, & Nocera, 2016). PA provides a prevention component that can increase mortality rates, especially from chronic diseases like cardiovascular disease, diabetes, cancer, hypertension, obesity, depression, and osteoporosis.

A few studies have shown that the risk of PD was reduced for men who played sports in the college and adult life, this has overall been replicated multiple times but something interesting they also found, was that this was not the case for women. Overall, various types of exercise like aerobics, treadmill training, dance, Tai Chi, and yoga have shown an improvement in specific PD motor symptoms if used in conjunction with treatment. Another area that PA helps improve is mood, mood disturbances occur in individuals with PD, therefore creating a lower quality of life, PA has shown that mood in both PD patients and healthy patients has increased. Overall the understanding is that PA is most effective when approaching to improve non-motor symptoms in patients with PD as opposed to targeting motor functions and symptoms (Bhalsing, Abbas, & Tan 2018).

Previously, PA was understood to be seen as a preventative step, but increasingly, studies have shown that they prove to be cost effective therapeutic interventions that promote an overall healthier lifestyle. This is done through increased function and self-reliance that prevents or decreases symptoms like falls, apathy, fatigue, depression, and cognitive function (Bhalsing, Abbas, & Tan 2018).

When looking at PA, a pattern of targeting “the other” symptoms has been quite clear, but there are instances when PA is not possible in a typical fashion that most expect. Other areas to understand and incorporate to target some of these other areas of the disease are things like mindfulness training and yoga. These two specifically have shown to improve coping skills, motor disability, and motor/non-motor symptoms of PD that include: anxiety, depression, cognition, postural instability, and GAIT (Li, Dong, Cheng, & Le, 2016). Much like PA, this training can provide an incentive look at an overall holistic care of patients with PD when the focus tends to lean toward motor function only. Motor function improvements can be relayed and more effective if all aspects of that person’s lifestyle and quality of life are healthy and taken care of.

References

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