Capítulo 2. Marco teórico 29
2.2 Uso de las redes sociales 42
The role of the cortex in the acute adjustment to exercise was first suspected by Krogh and Lindhard (1913) when an anticipatory rise in both HR and respiration was observed prior to the onset of exercise. Pharmacological blockade studies have since supported the idea that the cortex has an important role in adjusting autonomic cardiovascular variables in a manner related to the rapid HR adjustments at the onset of volitional work (McCloskey and Mitchell 1972, Mitchell, Reeves et al. 1989, Victor, Pryor et al. 1989, Mitchell 1990). Subsequently, neuroimaging methods have enabled more detailed studies using volitional exercise with evidence for involvement of the MPFC and IC in modulation of the autonomic nervous system (Williamson, Friedman et al. 1996, Williamson, McColl et al. 1999, Williamson, McColl et al. 2001, Williamson, McColl et al. 2002, Williamson, Fadel et al. 2006, Wong, Masse et al. 2007).
Considerable change in cortical activation occurs at the onset of moderate intensity exercise where cardiovascular changes appear to be dominated by reductions in parasympathetic contributions. These changes emphasize increased activation within the IC and decreased activation relative to baseline in the MPFC and HC that correlate with HR (Wong, Masse et al. 2007, Norton, Luchyshyn et al. 2013). Importantly, as mentioned previously, these cardiovascular centers are found in regions of the brain most vulnerable to the effects of advancing age (Raz, Gunning et al. 1997, Raz, Lindenberger et al. 2005, Nyberg, Salami et al. 2010, Salami, Eriksson et al. 2012). However, substantial inter- individual differences exist in the rate and extent of age-related cortical atrophy leading researchers to examine the potential modifiable risk factors for improved brain health into senescence.
Physical activity has emerged as a potent stimulus for improving neural health and has been shown to have positive global influences in aging, including spared brain volume (Erickson, Prakash et al. 2009, Erickson, Voss et al. 2011, Niemann, Godde et al. 2014), improved task-related functional responses (Colcombe, Kramer et al. 2004, Voelcker- Rehage, Godde et al. 2010), increased white matter integrity (Johnson, Kim et al. 2012, Voss, Heo et al. 2013, Voss, Weng et al. 2016), and cognitive performance (Josefsson, de Luna et al. 2012). Therefore, the possibility that physical fitness can impact cortical health is of particular importance given that the functional architecture of the cardiovascular control sites, in particular, are negatively altered in advancing age. However, interpretation of the literature remains difficult as studies have previously focused on individuals whose baseline condition, including cognitive impairment or a sedentary lifestyle, represent an effect of such conditions as much as an exercise effect. Further, whether a physically active lifestyle can affect cortical circuitry related to autonomic control and exercise outcomes remains unknown.
A critical aspect of understanding how aerobic activity is protective for brain aging is identifying the fundamental principles by which exercise positively affects brain health. In humans, several studies suggest that physical exercise leads to improvements at both the structural and functional levels in the aging brain (Kramer, Erickson et al. 2006, Hillman, Erickson et al. 2008, Erickson, Prakash et al. 2009) (Voelcker-Rehage, Godde et
al. 2010, Liu-Ambrose, Nagamatsu et al. 2012). Using voxel-based morphometry, Colcombe et al. (Colcombe, Erickson et al. 2003) reported that a higher cardiorespiratory fitness (VO2max) was associated with attenuation of cortical decay to both gray and white
matter in the frontal, prefrontal, and temporal regions in older adults. In another study, Erickson et al. (Erickson, Prakash et al. 2009) performed a region-of-interest analysis in 165 non-demented older adults and found that higher fitness levels were associated with an increased volume of the bilateral HC. Moreover, Voss et al. (Voss, Prakash et al. 2010) observed that 12 months of aerobic training leads to increased functional connectivity in regional connections that support both the default-mode network and the frontal executive network, suggesting that physical exercise has a restorative effect on large-scale brain circuitry. The evidence supporting a positive influence of fitness on the maintenance and delayed decline of cognitive function is well documented (Hillman, Erickson et al. 2008, Josefsson, de Luna et al. 2012, Bherer, Erickson et al. 2013). Evidence from cross-sectional studies indicates that age-related differences in cognitive performance are observed when older adults are compared to younger adults, and are reduced if the comparisons involve higher-fit individuals rather than sedentary older adults (Spirduso 1975, Clarkson-Smith and Hartley 1989, Hillman, Weiss et al. 2002, Renaud, Bherer et al. 2010). In a meta-analytic review of randomized-control trials of aerobic exercise on neurocognitive functions, Smith et al. found that individuals who were randomly assigned to aerobic exercise training showed modest improvements in attention, processing speed, executive function, and memory (Smith, Blumenthal et al. 2010). Such exercise-induced structural and functional changes have been documented in various brain regions, but the plethora of data has supported alterations in the prefrontal cortex and medial temporal lobes. Thus, the regions of the cortex most vulnerable to the precipitous effects of aging are those same regions where physical activity promotes improvements in cognitive performance. It is noteworthy that these same regions are also responsible for cardiovascular control. Nonetheless, little is known about concurrent autonomic consequences, despite similar, and often times overlapping, regional specificity in the cortex.