CHAPTER 13 Brain and Cranial Nerves

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13.2 Brainstem

LEARNING OUTCOMES

Pons

The part of the brainstem just superior to the medulla oblongata isthe pons (figure 13.4 a ). The pons contains ascending anddescending tracts and several nuclei. The pontine nuclei, locatedin the anterior portion of the pons, relay information from thecerebrum to the cerebellum.The posterior region of the pons contains nuclei for cranialnerves V (trigeminal), VI (abducens), VII (facial), and VIII (ves-tibulocochlear). Other important pontine areas are the pontinesleep center, which initiates rapid eye movement sleep (seechapter 14), and the pontine respiratory center, which works with therespiratory centers in the medulla oblongata to help control respi-ratory movements (see chapter 23).

After reading this section, you should be able to

A. List the parts of the brainstem and describe theirstructural characteristics.

B. Explain the functions of the parts of the brainstem.

The brainstem connects the spinal cord to the base of the brain.The brainstem consists of three parts: the medulla oblongata, pons,and midbrain (figure 13.4). In addition to these anatomical divi-sions, the reticular formation is a functional unit that spans allthree divisions. The brainstem is responsible for many essentialfunctions. Damage to small areas often causes death, because manyreflexes essential for survival are integrated in the brainstem, whereasrelatively large areas of the cerebrum or cerebellum may be damagedwithout life-threatening consequences.

Midbrain

The midbrain, or mesencephalon, is the smallest region of thebrainstem (figure 13.4 b ). It is located just superior to the pons.The midbrain contains the nuclei of cranial nerves III (oculomotor),IV (trochlear), and V (trigeminal).The tectum (tek′t ŭ m; roof; figure 13.5) of the midbrain consistsof four nuclei that form mounds on the dorsal surface, collectivelycalled corpora (k ō r′p ō r- ă ; bodies) quadrigemina (kwah′dri-jem′i-n ă ;four twins). Each mound is called a colliculus (ko-lik′ ū -l ŭ s; hill);the two superior mounds are superior colliculi, and the two inferiormounds are inferior colliculi (see figure 13.4 b ).The superior colliculi receive sensory input from visual, audi-tory, and tactile sensory systems and are involved in the reflex move-ments of the head, eyes, and body toward these stimuli, such as loudnoises, flashing lights, or startling pain. For example, when a brightobject suddenly appears in a person’s field of vision, a reflex turnsthe eyes to focus on it; when a person hears a sudden, loud noise, areflex turns the head and eyes toward it. The superior colliculi alsoreceive input from the inferior colliculi and the cerebrum.The inferior colliculi are involved in hearing and are an inte-gral part of the auditory pathways in the CNS. Neurons conductingaction potentials from the structures of the inner ear (see chapter 15)to the brain synapse in the inferior colliculi. Collateral fibers fromthe inferior colliculi to the superior colliculi provide auditoryinput that stimulates visual reflexes.The tegmentum (teg-men′t ŭ m) of the midbrain largely consistsof ascending tracts, such as the spinothalamic tract and the mediallemniscus, that carry sensory information from the spinal cord to thebrain. The tegmentum also contains the red nuclei, the cerebralpeduncles, and the substantia nigra. The paired red nuclei (figure13.5) are so named because in fresh brain specimens they are pink-ish in color as a result of an abundant blood supply. The red nucleiaid in the unconscious regulation and coordination of motor activi-ties. Cerebral peduncles (pe-d ŭ ng′klz, p ē ′d ŭ ng-klz; the foot of acolumn) constitute the portion of the midbrain ventral to the tegmen-tum. They consist primarily of descending tracts, which carry motorinformation from the cerebrum to the brainstem and spinal cord. The substantia nigra (n ī ′gr ă ; black substance) is a nuclear massbetween the tegmentum and cerebral peduncles containing cytoplas-mic melanin granules that give it a dark gray or black color(figure 13.5). The substantia nigra is interconnected with other basalnuclei of the cerebrum, described in section 13.5, and it is involvedin maintaining muscle tone and coordinating movements.

Medulla Oblongata

The medulla oblongata (ob-long-gah′t ă ), often called the medulla,is about 3 cm long. It is the most inferior part of the brainstemand is continuous inferiorly with the spinal cord. The medullaoblongata contains sensory and motor tracts, cranial nerve nuclei,and related nuclei. Superficially, the spinal cord blends into themedulla oblongata, but internally several differences exist. In themedulla oblongata, the gray matter is organized into discrete nuclei (figure 13.4 b ), clusters of gray matter composed mostly ofneuron cell bodies. This arrangement contrasts with that of thegray matter of the spinal cord, which extends as a continuous massin the center of the cord. Several medullary nuclei function ascenters for vital reflexes, such as those involved in regulating heartrate, blood vessel diameter, respiration, swallowing, vomiting,hiccuping, coughing, and sneezing.Two prominent enlargements on the anterior surface of themedulla oblongata are called pyramids, because they are broadernear the pons and taper toward the spinal cord (figure 13.4 a ). Thepyramids are formed by the large descending motor tracts involvedin the conscious control of skeletal muscles. Near their inferiorends, most of the fibers of the descending tracts cross to the oppo-site side, or decussate (d ē ′k ŭ -s ā t, d ē -k ŭ s′ ā t). This decussationaccounts, in part, for the fact that each half of the brain controlsthe opposite half of the body. Its role as a conduction pathway isdiscussed in the description of ascending and descending tracts(see chapter 14).Two rounded, oval structures, called olives, protrude from theanterior surface of the medulla oblongata just lateral to the superiorends of the pyramids (figure 13.4 a,b ). The olives are nuclei involvedin functions such as balance, coordination, and modulation of soundfrom the inner ear (see chapter 15). Nuclei of cranial nerves V(trigeminal), VII (facial), IX (glossopharyngeal), X (vagus), XI(accessory), and XII (hypoglossal) are also located within the medullaoblongata (figure 13.4 c ). Note that some cranial nerves, such as V,VII, and X, have more than one nucleus in the brainstem and thatsome nuclei, such as the solitary nucleus and nucleus ambiguus, serveas nuclei for multiple cranial nerves.