A literal network in your brain
The reticular formation (RF) is an organized and highly complex network of nerves and tracts in the brain. Some of the largest serotonin release sites in the brain are also located in the RF. Also, some of the areas in the brain which produce the most dopamine and acetylcholine are located in the RF. All three parts of the RF are responsible for sending and producing neurotransmitters responsible for sensory perception and motor activity. Motor activity here includes movements of the face and head, but also the movement of internal organs. The three parts are the ascending RF, descending RF, and the brain stem RF.
The ascending RF plays a role in alertness. It mediates alertness level for consciousness, circadian rhythm (the alertness aspect, unlike the chemical aspect: see melatonin). The descending RF helps suppress pain in emergency situations. It also helps maintain posture, balance, and motor movement. In the medulla and pons, the brain stem RF neurons coordinate motor activity in the face and mouth for eating, facial expressions, and eye movement. While the descending RF is involved in suppressing pain, the brain stem RF modulates and transmits pain.