To find an answer to this, we go all the way to the beginning, conception. After the egg and the sperm cell become one, they will grow from a clump of cells to a fetus by cell division (see from stem cell to fetus). The awareness of the fetus happens simultaneously with the physical development of the brain. During the growth of the fetus, the basic reflexes are formed in the first fourth month (reptile brain) and registration of sensations starting from the sixth month (emotional brain).
The reptile brain (brain stem) is directly connected to the spinal nerves from which our body is controlled. Also, 10 of the 12 nerves of the brain find their origin here, as a result, the brainstem forms the physical link between the brain and the body. The emotional brain that is directly connected to the brainstem has, therefore, a major influence on the brain stem. We notice this because our body shows reactions beyond our conscious control, such as shame, fear, falling in love, etc.
Consciousness means being able to distinguish oneself from the outside world and to have the awareness of one’s own self. Subconsciousness uses the biggest part of our total brain. It is 200 thousand times faster than our consciousness and contains all the information which we received through our senses from the moment we are born. It controls all learned autonomous processes in our body such as walking, body language, expression of emotions, following traffic rules etc.
Our subconscious mind continues to store and process all information received through our senses into a worldview on which all our conscious actions are based. Try this experiment: Picture in your mind the food you like the most. Make it as real as possible by including also the smell and the taste. Take a few seconds to enjoy it. See how it makes your mouth water and your stomach starts rumbling.
While you were trying to understand the experiment your subconsciousness was already sending signals to your mouth and stomach in order to respond to your imagination. To make that happen our emotional and reptile brain are working together to activate the necessary nervous system to accomplish this.
So it’s therefore not strange that we get sick from stress, we can give ourselves a stomach ulcer or we can actually damage our heart through the loss of a loved one. Fortunately, it also works the other way around and we are healthier when we ‘feel good about ourselves’, we can stimulate our healing process.
All things we can consciously learn and influence ourselves can be optimized though time even to the degree that they become an autonomous process. For example: driving a bike. In the beginning it was difficult and we needed all our conscious attention to control the bike but after many hours of practicing and repeating the same exercise, our subconsciousness slowly take over the control and we just have to think what we want and our body will autonomously take the necessary actions.