The parts of the lower brain are collectively referred to as the brain stem, which is the most primitive part of the brain. The brain stem serves as a “highway” that connects the upper brain and the cerebellum to the spinal cord, and through the spinal cord and peripheral nerves, to the rest of the body. The lower brain is responsible for a variety of functions, most notably processes essential to survival.
The brain stem is made up of three distinct parts: the midbrain, the pons and the medulla. The medulla is directly above the spinal cord. The pons is on the front surface of the brain stem. Typically, the pons is divided into two regions: dorsal and ventral. The midbrain is the smallest part of the brain stem and is above and slightly behind the pons. There are several important and distinct regions in the midbrain called “nuclei.”
Each part of the lower brain has specific functions, many of which are important in survival. The midbrain contains components of the visual and auditory systems, with several of its regions being devoted to the control of eye movement. The pons is essential to regulating sleep, breathing, taste, movement and hearing. The medulla contains several centers responsible for vital autonomic functions such as blood pressure and respiration regulation.
Many noteworthy regions within the parts of the lower brain make essential connections with other parts of the brain. For example, the substantia nigra is a nucleus in the midbrain that provides important input to the basal ganglia for the control of voluntary movements. The ventral region of the pons relays movement and sensation information from the cerebral cortex to the cerebellum. Finally, neuronal cell groups in the medulla consist of relay pathways involved in the maintenance of balance, hearing, taste and control of muscles in the face and neck.
Damage to the Lower Brain
If the brain stem is damaged, this compromises the highway between the brain and the body. Damage to the brain stem can inhibit sensory information from reaching the brain, Eric Kandel, James Schwartz and Thomas Jessell write in the book, "Principles of Neuroscience." For example, if you put your hand on a hot pan, the pain signals may not reach your brain and your hand may continue to burn. Damage to the brain stem can also disrupt motor commands the brain sends to the body. As in the case above, if your hand was on a burning hot pan, the brain signal to remove your hand could be slowed or inhibited. Lower brain damage can also impair consciousness, respiration and heart function.
The brain stem is the oldest part of the human brain and it resembles the brain of most reptiles, which evolved millions of years ago when the first reptiles left the seas for land. This is why the lower brain and the cerebellum are sometimes referred to as “the reptilian brain."