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Physical Exercise and Neuroplasticity

Examining the role of exercise on long-term brain function.

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Neuroplasticity is the brain's ability to form new neural connections throughout life, influenced by intrinsic or extrinsic stimuli. Merriman-Webster defines neuroplasticity as the "capacity of neurons and neural networks in the brain to change their connections and behavior in response to new information, sensory stimulation, development, damage, or dysfunction."1

It's believed that an American psychologist and philosopher, William James, presented the first theory of neuroplasticity about 120 years ago in his book Principle of Psychology. However, the first documented use of the term Neuroplasticity was in 1948, by Polish Neuroscientist Jerzy Kanjorski.

In more recent years, Michael Merzenich, professor emeritus at the University of California, San Francisco, has made major landmarks in the field of neuroplasticity. His research in the field of neuroplasticity has led to many practical applications for improving people's daily lives. He developed software to help people with learning disabilities overcome their impairment through brain training exercises. Fast-For-Word is one such scientific learning tool that helps correct learning deficits related to a number of temporal processing disorders.

As we know, the temporal lobe plays a role in hearing and auditory memories, speech learning, vision pathways, emotions and sense of identity. It is also the seat for the hippocampus, an important part of the brain, in relation to neuroplasticity. The hippocampus is located in the medial temporal lobe, and in recent years there's been much a talk about hippocampal neuroplasticity and memory. I will discuss the relation between the two in latter segment of this article.

Postit Science is another company that has not only developed software to improve cognitive functioning in people with learning disabilities, but focuses on a number of cognitive areas that may normally decline with aging -- i.e., auditory and visual functioning. These tools have proven to be a boon for our aging population to help in memory and cognation.1

Neuroscientist Dr. Edward Taub helped further the study of neuroplasticity. He has created one of the most technologically advanced methods for treating people that have had strokes or other neurological disorders that cause severe physical impairments or paralysis. His method, termed constraint-induced (CI) movement therapy, is a therapeutic approach to the rehabilitation of movement after stroke, multiple sclerosis (MS), and traumatic brain injury (TBI). In simple words, CI therapy consists of a family of treatments that teach the brain to "rewire" itself following an injury to the brain,1 the groundwork of which is based upon neuroplasticity.

It is now clear that neuroplasticity plays a significant functional and ameliorative role across a wide spectrum of brain diseases, as well as in our normal aging and health. However, neuroplasticity may sometimes also contribute to impairment. As an example, deaf people may suffer from a continual ringing in their ears (tinnitus) -- the result of the rewiring of brain cells starved for sound. For neurons to form beneficial connections, they must be correctly stimulated. This is where physical exercise plays a major role.

Physical Activity and Neuroplasticity

It has long been known that physical activity has beneficial effects on cardiovascular and our overall physical well-being. In recent years, more scientific evidence has been published regarding the correlation between physical exercises and the executive functions of memories in humans. In other words, maintenance of cardiovascular fitness is necessary for long-term effects on cognition. Neural systems that are known to have a high capacity for change appear to be the first that are enhanced by exercise.

A recent study published on PubMed, in March 2012, reported significant improvement in episodic memory (a category of long-term memory that involves the recollection of specific events, situations and experiences), perceptual speed, executive functions, and spatial reasoning after six months of exercise. The experimental group included 68 sedentary men and women between the ages of 40 and 56 years, who were randomly assigned to one of the two exercise training programs: aerobic endurance training (cycling), or non-endurance training (stretching/coordination). Both these groups exercised twice a week for six months. Additionally, a sedentary control group was tested.

The results of this study reported improved memory in both the cycling and the stretching/coordination group compared with the sedentary control group. The improvement in episodic memory correlated positively with the increase in cardiovascular fitness. The stretching/coordination training particularly improved selective attention compared with the cycling training.

Again, a lot of literature in the medical world has documented the beneficial influence of physical activity engendered through aerobic exercise on selective aspects of brain function. Human and non-human animal studies both have shown that aerobic exercise can improve a number of aspects of cognition and performance. An article published in the Journal of Neuroscience in August 2001 examined the positive effects of aerobic physical activity on cognition and brain function at the molecular, cellular, systems and behavioral levels, and found that exercise might not only help to improve physical health, but might also improve academic performance.3

The point of paramount significance is to understand that the most consistent positive effects of physical exercise have been reported on executive functions associated with the frontal lobe of our brain -- the part associated with emotions and judgments. The frontal lobe controls the processes called "mentalizing," upon which our socialization is based.

Mentalizing is the ability to understand another's mental processes. It also controls the functioning of sequencing events -- i.e., the ability to plan a series of movements needed to perform a multi-step task, as simple as making a sandwich or dressing up in the morning or at bedtime, both of which are altered or impaired when the brain is diseased, as in Alzheimer's, or injured, as in TBI. Following an exercise ritual would not only benefit the physical limitations these diseases pose, but canalso revamp the cognition and memory skills of the individual combatting these diseases.

Hormones

Let's now take a look at some of the hormones influenced by physical activity, which then affect our memory, learning and/or other functions of brain. In this field, the two most highly publicized neurotransmitters are serotonin and beta-endorphins.

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Serotonin is found in the pineal gland, the digestive tract, the central nervous system, and blood platelets; however, about 80% of our body's total serotonin is in the gut -- in the enterochromaffin cells, where it regulates intestinal movements. The rest is synthesized in the serotonergic neurons in the central nervous system.

Serotonin plays an important part in the regulation of learning, mood, sleep and vasoconstriction. It is believed that serotonin levels change with the seasons -- serotonin transporter protein, a brain chemical that lowers serotonin levels around brain cells, is more active during the winter months, which means the serotonin levels are lower in winters. Researchers from the University of Toronto and the Medical University of Vienna believe that this mechanism explains why some people feel the blues more during the winter. It is theorized that increasing one's exercise regime during winters may have a positive effect on levels of serotonin and thus keep the winter blues at bay.

Beta-endorphins are also part of the mood-regulating chemicals that reduce pain and can even induce euphoria. In fact, it is now even believed that the increase in beta-endorphins when exercising is the true cause of the euphoric state called the "runner's high." However, there is little scientific research to prove that beta-endorphins have a positive effect on depression.

Estrogen, Hippocampus and the Memory Link

As previously mentioned, the hippocampus is located deep within temporal lobes, and is involved in memory forming, organizing and storing. It is a limbic system structure, particularly important in forming new memories and connecting emotions and senses such as smell and sound to memories. The hippocampus appears to be necessary for making new memories, and if you could not live in the present, you'd be stuck in a past of old memories. This is commonly seen in Alzheimer's patients.

The hippocampus also seems to be involved in severe mental illness such as schizophrenia and severe depression -- in both of these conditions, the hippocampus appears to shrink. Another interesting factor to consider is the link between estrogen and the hippocampus. It has been suggested in various studies that estrogen stimulates brain cell growth in the hypothalamus and hippocampus. This leads to the belief that estrogen plays a direct role in promoting brain activity in regions that are believed to be responsible for cognition and memory; however, more research is being done in this field and concrete evidence still lacks to dominantly support the positive role of estrogen on memory and cognition.

More and more recent studies have suggested that the hippocampus is one of the few places where neurogenesis (generation of new neurons) occurs in adult mammals. Throughout life, mammals, including humans, produce a steady stream of neurons in this area of the brain; however, the process appears to be less active in people exposed to high levels of stress. This makes one wonder why forgetfulness becomes more prominent under stress, and how regular physical activity/exercise helps to decrease stress and thus improve one's memory.

Role of Physical Exercise

Experiments have shown that the hippocampus displays a remarkable angiogenic (formation of new blood vessels) and neurogenic (formation of new neurons, even in the adult brain) plasticity, and rapidly responds to changes in physical activity.

Earlier it was considered that neurogenesis only occurred during the embryonic and developmental stages. This view has greatly changed since the discovery of adult neurogenesis, as mentioned earlier, in two brain regions -- the hippocampus and the olfactory bulb. Recently, it has been suggested that altered hippocampal neurogenesis is related to the pathophysiology of mood disorders and the mechanism of antidepressant treatments.

Normal aging causes changes in the hippocampus that may lead to cognitive decline in older adults. In a study published in the Journal of Neuroscience, it was found that a decline in neurogenesis of aged mice was reversed to 50% of young control levels by running, thus signifying the positive effect of exercise on increased hippocampal neurogenesis -- thus improved learning and memory.

Henriette van Praag et al investigated whether voluntary wheel running would benefit mice that were sedentary until middle age (19 months of age). Specifically, young and aged mice were housed with or without a running wheel. After 1 month, learning skills were tested in the Morris water maze. Aged runners showed faster acquisition and better retention of the maze than age-matched controls. Thus, voluntary exercise decreases some of the deleterious morphological (obesity) and behavioral consequences of aging.3

A growing number of studies support the idea that physical exercise is a lifestyle factor that might lead to increased physical and mental health throughout life, even as we age, and not just if we exercised in our prime years. Thus the old saying, "better late than never."

References

1. http://www.merriam-webster.com/dictionary/neuroplasticity
2. http://www.positscience.com/
3. http://www.uab.edu/citherapy/
4. http://www.jneurosci.org/ 2011 Aug. doi: 10.1523/JNEUROSCI.2266-11.2011.
5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1360197/

Meenakshi Malhotra is lead physical therapist at Crestview Rehab in Brownsville, TN.



     

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