Brain Stem
The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem. The structure emerges from the ventral surface as a cone that connects the brain to the spinal cord. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the adjacent cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates.
The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.
Midbrain
One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. It is separated into the tectum and tegmentum, from the Latin words for roof and floor, respectively. The tectum is composed of four bumps known as the colliculi (singular = colliculus), which means "little hill". The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. Fibers between the inferior and the superior colliculi give auditory input that stimulates visual reflexes to a loud sound, etc.
The superior colliculus involved with visual reflexes combining sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a vision, sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not!
The tegmentum is continuous with the gray matter of the rest of the brain stem. Throughout the midbrain, pons, and medulla, the tegmentum contains the nuclei that receive and send information through the cranial nerves, as well as regions that regulate important functions such as those of the cardiovascular and respiratory systems.
Figure 12. The brain stem comprises three regions: the midbrain, the pons and the medulla.
Pons
The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. The bridge-like white matter looks like a little pooch on the anterior side of the brain stem area. Gray matter in the interior (tegmentum region) of the pons contains neurons receiving descending (motor) input from the forebrain that is sent to the cerebellum, mostly to coordinate movements.
Medulla (me'-dull-a)
The medulla is the region known as the myelencephalon in the embryonic brain. The initial portion of the name, "myel," refers to the abundant white matter found here, which is continuous with the white matter of the spinal cord. The tegmentum of the midbrain and pons continues into the medulla because this gray matter is responsible for processing cranial nerve information. A diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, such as general brain activity and attention, especially to stay awake during long lectures.
Figure 13. The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.
