Introduction
Physiological Markers BDNF In reviewing papers, one particular marker which continuously appeared was brain derived neurotropic factor (BDNF). It is significant in relation to measuring cognition because increased levels have been found to advance learning and sustain cognitive functions by increasing cerebral blood flow, improving synaptic plasticity and acting as a neuroprotective agent and improved neuroelectric functionality (Haapala, et al. 2013 ). By challenging the brain with physically demanding exercises enhanced mental performance and learning can be observed. Moreover, as well as increasing levels of BDNF, exercise mobilizes gene expression profiles that would be predicted to benefit brain plasticity processes (Ermutlu et al., 2014).
Angiogenesis Across animal studies it has been established that different brain adaptations can be initiated by different forms of physical exercise. Endurance exercise is shown to generate blood vessels and increase capillary density (angiogenesis) and thereby increase cerebral blood flow (Haapala et al. 2013). Neurotropins such as BDNF have been found to stimulate angiogenesis in both human and animal studies
The growth and development of nervous tissue (neurogenesis) seems to be coupled with angiogenesis. One review by Voelcker-Rehage (2013), related changes in angiogenesis and cerebral blood volume to changes in VO2 max, suggesting that differences in brain activity might be instigated by a greater supply of oxygen. Prolonged endurance exercises, such as cardiovascular and resistance training, require an increase in both glucose and oxygen because of the increased metabolic demand of the active neurons. Therefore, increased physical exercise is closely linked to enhanced blood supply (Voelcker-Rehage, et al., 2013). The work of Bullitt (2009), found the increase of capillary densities indicates a shorter diffusion distance for oxygen from blood vessels to neurons, thus promoting metabolism of neurons. It was concluded that angiogenesis in the cerebral regions is a direct result of amplified exposure to cardiovascular exercise (Bullitt et al., 2009).
Animal Studies Numerous studies have been conducted on the effects of exercise on cognition in animals. Voluntary wheel running is the most commonly used experimentation model used in rodents to study exercise induced nerual changes and beneficial changes in behaviour (Hötting & Röder, 2013). Dogs are also commonly used for cognitive related studies, in particular those examining dementia and Alzeimher’s Disease, as they show a similar tendency to suffer from cognitive decline with age, as humans do (Cotman & Berchtold, 2007).
Animal studies have suggested that exercise induces changes in the nervous system that are both structural and functional. Hötting & Röder listed these changes in 2013 as including; an increase in angiogenesis and andritic spine density, enhanced long term potentiation, an augmented release of growth factors such as BDNF and IGF-1. They also stated that exercise effects neurotransmitter systems such as increasing the levels of serotonin, noradrenaline and acytlycholine. One of the most reported structural changes in studies using animals is the increased rate of neurogenesis within the hippocampus, specifically in the dentate gyrus (Hotting & Roder). This is a particualry important area of the brain in the formation of new memories.
It may be disputed that some of the beneficial changes found in animal studies are due to the enrichment of the animal’s ennvironment which has also been shown to improve cognitive function (Cotman & Berchtold, 2007). Neurons are lost in the hippocampus as animals age but according to Cotman and Berchtold in 2007, beagles living in an enriched environment, which included access to toys, kennel mates and a regular exercise regime, had a notably larger amount of a certain subset of hilar neurons. However, it can be argued that many of the animal studies conducted in this field cannot be equated to the effects found in wild animals or, indeed, humans, as the animals, often rats or mice, are kept in unnatural conditions with little to no environmental enrichment. The introduction of exercise can be considered an environmental enrichment, or a novel activity, and may have an effect on the results (Hatchard et al., 2014).
Type of exercise Animals: The type of exercise is also an important parameter when comparing results. Both voluntary wheel running and forced wheel running have been shown to improve neurogenesis, learning and memory in mice. In voluntary wheel running, the animal may run whenever they please whereas foced wheel running consists of having the animal run on the treadmil within strict experimental designs such as duration, intensity and time (Li et al., 2013). The latter is considered to be a more objective means of measuring results. In 2013, Li et al. also explained that results are also effected by the time of day at which exercise takes place and also the intensity of said exercise. Regular exercise has also been proven to be more beneficial to neurogenesis in animal studies, as it has in humans. In the study conducted by Li et al. mice were divided into groups including; a control group of mice which did not partake in the exericse programme, a group of regular duration runners, a group of irregular duration runners and a group of mice that ran at irregular times during the day. Neurogenesis in the dentate gyrus was also examined in this study. Corticosterone levels were taken to determine stress levels as high levels can counteract neurogenesis and treadmill running has been shown to increase stress levels in many studies (Li et al., 2013). They found that the higher intensity mice (3 hours of running) had higher levels of neurogenic activity in comparison to the lower intensity mice (1 hour of running) which actually had a higher level of cell neurogenesis. The control group of mice showed no changes. It can be concluded from this study that excessive or inadequate levels of exercise have negative effects on neurogenesis. They also found that regular treadmill running was more beneficial to spatial awareness and memory formation (Li et al., 2013).
Human Trials: From these studies it is apparent that certain parts of the brain are activated and enhanced by cardiovascular activity, this leads us on to ask whether other types of physical exercise trigger different parts of the brain. A noteworthy study by Ermutlu (2014), compared different types of exercise on brain electrical activity. Electroencephalography (EEG) scans were recorded in professional dancers, professional fast ball sports athlete and even though these particular exercise types are comparable based on physical effort, they have very different cognitive and sensory motor features due to the divergent skills required. This is strikingly apparent when comparing the two sets of EECs. The plasticity induced differences of physical exercise coupled with the training effects of constant practice of skills over a prolonged period of time in fast ball sports and dancing may have additive effects on brain networks and functions.
BDNF has been shown to be the principal signalling molecule in the hippocampus and cortex of the brain using animal studies. It contributes to neuronal survival and synaptic plasticity and long term potentiation, which is important for learning and memory. Evidence has suggested that exercise can have impressive effects on BDNF levels. One week of wheel running in rats has been shown to increase levels by 1.5 times (Cotman & Berchtold, 2007). BDNF levels were also shown to be a reliable measurement in animal studies as its results were found to be independent of housing conditions (Hatchard et al., 2014). This may eliminate some of the effects of environment enrichment. In a study where mice were expected to locate a platform, a test of spatial awareness and memory, BDNF levels were measured in mice undertaking an exercise routine. Those such mice were shown to locate the platform more often in comparison to the non-exercising, control mice. Another group of mice were then treated with a BDNF signal blocker (TrKB-IgG chimera) and these mice showed no improvements in spatial awareness and memory levels (Cotman & Berchtold, 2007). It can be concluded from this that BDNF levels are an accurate evaluation of the beneficial changes in cognition.
In conjunction with animal studies, levels of BDNF, and other neurotropins such as IGF-1, have been measured after exercising in humans and been found that the peripheral BDNF level increases in humans after an acute bout of physical exercise and returns to baseline within a few minutes to several hours (Hötting, 2013).
Conclusion Interestingly it has been shown that new neurons will die if adequate learning opportunities or novel experiences do not accompany the increase in cardiovascular activity (Hötting, 2013). With this in mind, the significance of regular exercise in conjunction with an enriched environment becomes more apparent.
Treatment of Cognitive Impairment Overall, it is clear that the benefits of physical exercise are unanimous. However, few studies have examined the effects of chronic exercise on cognition. This could be useful in a variety of ways for human athletes and animals in sport.
Aerobic exercise (AE) can be defined as “planned, structured repetitive physical activity for extended periods and at a sufficient intensity to improve or maintain physical fitness” (M.Mackay-Lyons, , 2013) Parameters of AE include, duration, intensity, frequency, mode and the timing of exercise onset after the stroke occurred. (M. Hasan, 2016). Intensity level can be measured using perceived levels of exertion, heart rate or respiration rate (M. Hasan, 2016) Cognition is affected by aging, especially in terms of memory and attention. Executive function (EF), a set of behavioural processes including time management and the ability to pay attention, depends on the prefrontal cortex. The cortex reduces in size with age. Bonavita (2016) associates better cognitive performance amongst older people who have active rather than sedentary lifestyles. Liu-Ambrose (2007) highlights that cognitive impairments impede motor learning. It is important to understand the positive impact of exercise on cognitive impairments as this can help to increase independence, reduce disability and prevent institutionalisation amongst the elderly (Patel, Coshall, Rudd et al, 2002). Mahmadal Hasan states that after stroke, „cognitive health and the recovery of complex motor skills are intertwined”. Commonly used measures of cognition in Irish and Australian hospitals include the MMSE and the Montreal Cognitive Assessment. These are questionaires… Forms of cognition important to rehabilitation include : Working Memory, Attention, Executive function (mahmadul hasan) Studies using neuroimagery show that there is a link between being fit in terms of cardiorespiratory and aerobic health, and brain structure and function (especially in pre-cortical areas and areas where neural circuits related to EF are located) (SOURCE). Timing Rand et al’s (63) study suggests that while aerobic exercise may help early into recovery, 8 weeks after the end of a 6 month treatment, there were no significant improvements to note. 65 suggests that longer tratment time reports more consistant benefits. In this study a 12 week home exercise program combining 30 mins of AE with 90 mins of cognitive exercises improved cognition in terms of the MMSE. Level of Intensity Mahmudul Hasan compared the intensity of various program in his systematic review. They found that programs of low intensity did not show significant improvements on cognitive outcomes such as working memory. This study found that moderate intensity and combination programs resulted in more substantial improvements in memory and learning. Moderate intensity exercise includes treadmill walking, overground walking and stationary cycling sessions, ranging from 20 to 60 minutes depending on patients levels of perceived exertion (1((compare to animal studies section)) Marzolini (2013) states that while there is little evicence to prove that AE alone will improve executive function in stroke patients, as a combination therapy, the results are very promising. Combination therapy means other interventions used for stroke rehabilitation, including physiothrapy, speech and language therapy and occupational therapy. Gaps in research Lack of control groups in many of these studies as it would unethical to deprive a patient of treatment for the sake of research limited the findings of these studies.
Reference: Bullitt, E., Rahman, F.N., Smith, J.K., Kim, E., Zeng, D., Katz, L.M., Marks, B.L., 2009. The effect of exercise on the cerebral vasculature of healthy aged subjects as visualized by MR angiography. American Journal of Neuroradiology 30, 1857–1863.
Ermutlu, N., Yucesir I., Eskikurt G., 2014. Brain electrical activities of dancers and fast ball sports athletes are different. Cogn Neurodyn (2015) 9:257–263
Haapala, E., 2013. Cardiorespiratory Fitness and Motor Skills in Relation to Cognition and Academic Performance in Children – A Review. Journal of Human Kinetics volume 36/2013, 55-68 Hötting, K., Röder, B., 2013. Beneficial effects of physical exercise on neuroplasticity and cognition Neuroscience and Biobehavioral Reviews 37 (2013) 2243-2257
Voelcker-Rehage, V., Niemann, C., 2013. Structural and functional brain changes related to different types of physical activity across the life span. Neuroscience and Biobehavioral Reviews 37 (2013) 2268-2295