The human brain is one of the most complex and profoundly intricate structures in the known universe. Made up of tens of billions of specialized cells called neurons that communicate with one another via chemical messengers called neurotransmitters (NT's). The brain uses NT's to tell your heart to beat, your lungs to breathe, and your stomach to digest. They can also affect mood, sleep, concentration, weight, and can cause adverse symptoms when they are out of balance. Neurotransmitter levels can become depleted many ways.