Up until the 1960s, scientists believed that the number of neurons and connections synaptic cells in the human brain were limited and that, once damaged, these brain cells could not be replaced. But that idea was disproved when researchers began to see ample evidence for the birth of neurons, or neurogenesis, in the adult brains of humans and other animals. Now, there is strong evidence that aerobic exercise can contribute to neurogenesis and play a key role in helping to reverse or repair damage to neurons and their connections in both mammals and fish.
Research (which you can read at this link) shows that one of the main ways in which these changes occur in response to exercise is by increasing the levels of a protein. It has been shown that neural plasticity, or the brain's ability to change, stimulated by this protein, increases cognitive function, including learning and memory. Swimming has been recognized for its cardiovascular benefits as this practice involves all major muscle groups, the heart has to work hard which increases blood flow throughout the body. This leads to the creation of new blood vessels, a process called angiogenesis. The increased blood flow can also lead to a large release of endorphins, hormones that act as a natural pain reducer throughout the body. This surge causes the sense of euphoria that often follows exercise. Swimming also improves short- and long-term memory, and to understand and pinpoint the duration of beneficial effects, researchers have trained some rats to swim for sixty minutes a day five days a week. The team then tested the rats' memory by swimming them through a radial-armed water maze containing six arms, one of which with a hidden platform. The rats had six attempts to swim freely and find the hidden platform. After just seven days of training, the researchers saw improvements in both short and long-term memory, based on a reduction in the mistakes the mice made every day. Researchers have suggested that this increase in cognitive function could provide a basis for using swimming as a way to repair the damage to learning and memory caused by neuropsychiatric diseases in humans.