Effect of exercise on the improvement of age-induced cognitive impairment: With emphasis on cross talk between the brain and skeletal muscles

Dear Editor-in-ChiefThe brain weighs about 1.36 kg and is mainly composed of blood. Although the brain includes only 2% of the body's weight, it receives 25% of the whole body oxygen consumption, 15% of the cardiac output volume and 2000 L of blood flow per day (Hartmann et al., 1994; Ohta et al., 1992; Xing et al., 2017). The brain has largest and most complex structure of the central nervous system. This organ regulates the functions of human body and stands for the basis of higher neural activities such as consciousness, alertness, learning, memory, intelligence, spirit and language learning. Cognitive function in a person means the brain ability to process, store, and extract information. In addition, this ability is a kind of advanced psychological function such as thinking, memory, and attention. Cognitive function plays an irreplaceable role in our daily life and study. Based on Cognitive psychology, the brain can modify its structure and function according to environmental changes and the experience of different types of exercising, especially exercise training, plays a crucial role in the evolution of the brain (Barnes. 2015; Raichlen et al., 2017). For example, exercise can positively affect synaptic plasticity and synaptic function to promote cerebral cortex, neural network and hippocampus function (an important structure in the memory information processing) (Ding et al., 2006; Li et al., 2019; Loprinzi et al., 2017). Exercise also has a significant effect on brain metabolism. Exercise not only promotes physical health of people, but also prevents mental illness and delays cognitive aging (Robinson et al., 2018).Brain function depends on nerve fibers, number of neurons and synapses. The cognitive performance is also closely related to the number of nerve fibers and synaptic connections, which can be reduced by diseases and aging process. The neurotrophics factors, such as neurotrophic factor (NGF) and brain-derived neurotrophic factor (BDNF) are required to provide nutrients to maintain the structure and function of nerve fibers and synaptic connections (Ivanov, 2014). However, with aging, sharp decrease of dendritic branches, degeneration of glial cells, the reduction of neuron number and contraction of nerves may impede the transmission of electrical signals in the nervous system, leading to the shrinkage of gray matter in the brain and cognitive impairment (Reisberg et al., 2008; Thorin-Trescases et al., 2018).Several studies have shown tha