Below the cerebrum is the area of the brain called the brain stem. The brain stem is made of the pons, the midbrain, and the medulla oblongata. Another associated structure that surrounds the brain stem and works closely with all three structures is called the reticular formation, which consists of over 100 neural networks and performs a variety of functions including motor control, pain management, cardiovascular control, consciousness, sleep, and the ability to automatically analyze and separate important visual and auditory input from background stimuli.
The structures of the brain stem are small, accounting for less than 3 percent of the brain's total mass. The tissues of the brain stem are mostly gray matter surrounded by a layer of white matter. These collections of neurons work to produce automatic physiological processes. The brain stem keeps a person alive even when they are not consciously thinking about certain activities. The physiological processes include respiration, digestion, and circulation of the blood. The brain stem is highly innervated, containing 10 of the 12 pairs of cranial nerves. These nerves serve as a pathway between the brain and the lower areas of the body.
Midbrain and Pons
The midbrain is a structure in the brain stem located between the pons and the diencephalon. Several nerve fibers connect the midbrain with both the cerebrum and the cerebellum. Located in the center of the midbrain is a collection of gray matter neurons that are connected to the amygdala, which controls many human emotions. This gray matter also helps suppress pain sensations, as well as regulate fear and anger.
Located on the surface of the midbrain are four projections that process sensory information. The superior colliculus contains visual reflex centers that cause the head and the eyes to work in conjunction when looking at an object. The inferior colliculus contains auditory relay centers that connect to the ear and respond to sounds. The inferior colliculi are responsible for producing a startled reaction when a loud sound surprises a person. Also found within the midbrain are two structures called substantia nigra that have a high melanin pigment concentration, which is needed for the formation of neurotransmitters, the chemicals that transport nervous signals across gaps between neurons.The pons is the part of the brain stem located between the midbrain and the medulla oblongata. It serves as a relay center, transmitting signals from different parts of the nervous system to the brain and back to the effector organs. The pons has several neural pathways that are oriented in different directions. Pathways that are positioned longitudinally connect the cerebrum with the spinal cord. Other nerve fibers are oriented laterally and connect the pons with the cerebellum. There are several cranial nerves that come from the pons and help with the functioning of the medulla oblongata.
Medulla Oblongata and Reticular Formation
Located at the base of the brain stem is the medulla oblongata. The medulla oblongata is continuous with the spinal cord and controls several involuntary functions, such as breathing, swallowing, and heart rate. The area just above where the medulla oblongata meets the spinal cord is a group of nerve fibers that crosses over at a point called the decussation of the pyramids. The nerve tissue forms an X shape, meaning that nerves from the left side of the body merge into the right side of the brain, and nerves from the right side of the body merge into the left side of the brain. This allows each cerebral hemisphere to control the voluntary movements of the opposite side of the body.
The main function of the medulla oblongata is to control involuntary physiological processes that maintain homeostasis. There are three important functional areas of the medulla oblongata:
- Respiratory center: This area maintains breathing rate and controls how deeply a person breathes.
- Cardiovascular center: This area controls heart rate, as well as the rate of contractions of the heart muscle. It also constricts and dilates blood vessels.
- Miscellaneous center: This area is in control of other activities, such swallowing, coughing, and sneezing.