BrainStem
Is that part of the CNS housed within the Cranial Vault. The major regions of the adult Brain are: the Cerebrum, DienCephalon (Thalamus and HypoThalamus), MidBrain (MesenCephalon) Pons, Medulla Oblongata, and Cerebellum.
The Medulla Oblongata, Pons, and MidBrain constitute the BrainStem. The BrainStem connects the Spinal Cord to the remainder of the Brain and is responsible for many essential functions.
Damage to small BrainStem areas often cause death, whereas relatively large areas of the Cerebrum or Cerebellum may be damaged without causing permanent symptoms. All but two of the 12 Cranial Nerves, enter or exit the Brain through the BrainStem. (Also See: The BrainStem)
Medulla Oblongata
The Medulla Oblongata - approx. 3 cm long - is the most inferior portion of the BrainStem and is continuous Inferiorly with the Spinal Cord. Superfically the Spinal Cord blends into the Medulla but internally there are several differences.
Discrete Nuclei (clusters of Gray Matter, composed mostly of cell bodies, surrounded by White Matter) with specific functions are found within the Medulla Oblongata but not within the Spinal Cord. In addition, the Spinal Tracts that pass through the Medulla do not have the same organization as the tracts of the Spinal Cord.
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On the Anterior surface two prominent enlargements, called Pyramids because they are broader near the Pons and taper towards the Spinal Cord, extend the length of the Medulla. The Pyramids consists of Descending Nerve Tracts involved in the Conscious control of Skeletal Muscles.
Near their Inferior ends the descending nerve tracts cross to the opposite side, or Decussate - means to form an X. This Decussation accounts, in part for the fact that each half of the Brain controls the Opposite half of the Body.
Two rounded, oval structures, called Olives, protrude from the Anterior surface of the Medulla Oblongata just Lateral to the Superior margins of the Pyramids.
The Olives consists of Nuclei involved in functions such as Balance, Coordination and Modulations of Sound Impulses from the Inner Ear. The Nuclei of Cranial Nerves IX (Glossopharyngeal), X (Vagus), XI (Accessory), and XII (Hypoglossal) are also located in the Medulla.
Functionally the Medulla Oblongata acts as a conduction pathway for both Ascending and Descending Nerve Tracts. Its role as a conduction pathway is included as part of the description of Ascending and Descending Nerve Tracts.
The functional characteristics of the Reticular System are described later in this chapter. Various Medullary Nuclei also function as centers for several Reflexes (regulation of Heart Rate, Blood Vessel Diameter, Breathing, Swallowing, Vomiting, Coughing, and Sneezing).
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Pons
The portion of the BrainStem just superior to the Medulla Oblongata is the Pons, which contains Ascending and Descending Nerve Tracts and several Nuclei. The Pontine Nuclei, located in the anterior portion of the Pons, relay information from the Cerebrum to the Cerebellum.
The Nuclei for Cranial Nerves V (Trigeminal), VI (Abducens), VII (Facial), and VIII (Vestbulocochlear) are contained within the Posterior Pons. Other important Pontine areas include the Pontine Sleep Center and the Respiratory Centers, which along with the Medullary Respiratory Centers help control Respiratory Movements.
MidBrain
The MidBrain or MesenCephalon, is the smallest region of the BrainStem. It is just superior to the Pons and contains the nuclei of Cranial Nerves III (Oculomotor) and IV (Trochlear).
The Tectum (Roof) of the MidBrain consists of four nuclei that form mounds on the dorsal surface, collectively called Corpor (Bodies) Quadrigemina (Four Twins). Each mound is called a Colliculus (Hill); there are two Superior Colliculi and two Inferior Colliculi.
The Inferior Colliculi:
Are involved in Hearing and are an integral portion of the Auditory Pathways in the CNS. Neurons conducting impulses from the structures of the Inner Ear to the Brain, all synapse in the Inferior Colliculi.
The Superior Colliculi:
Are involved in Visual Reflexes, and they receive input from: the Eyes; the Inferior Colliculi; the Skin; and the Cerebrum. The Superior Colliculi regulate the reflexive movement of the Eyes and Head, in response to a number of different stimuli.
Fibers from the Superior Colliculi project to Cranial Nerve Nuclei and to the Superior Cervical portion of the Spinal Cord where, they stimulate Motor Neurons involved in turning the Eyes (Oculomotor, Trochlear, and Abducens Cranial Nerves) and the Head (the Accessory Cranial Nerve and Superior Cervical Cord Levels).
Impulses reaching the Superior Colliculi from the Cerebrum are involved in the Visual tracking of moving objects.
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The Tegmentum (Floor):
Of the MidBrain largely consists of Ascending Tracts from the Spinal Cord to the Brain and also contains the paired Red Nuclei.
The Red Nuclei are so named because they have a pinkish color in fresh Brain specimens, because of an abundant Blood supply. The Red Nuclei aid in the Unconscious Regulation and Coordination of Motor Activities.
The Cerebral Peduncles:
Comprise that portion of the MidBrain Inferior to the Tegmentum. They consists of descending tracts from the Cerebrum to the Spinal Cord and constitute one of the major CNS Motor Pathways.
The Substantia Nigra:
A nuclear mass between the Tegmentum and Cerebral Peduncles, is a pigmented region of the MidBrain with CytoPlasmic Melanin Granules that give it a dark gray-to-black color.
The Substantia Nigra has interconnections with other Basal Ganglia Nuclei of the Cerebrum and is involved in Coordinating Movement and Muscle Tone.
Reticular Formation
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Scattered like a cloud throughout most of the length of the BrainStem is a group of Nuclei collectively called the Reticular Formation, which receives afferent Axons from a large number of sources and especially from Nerves that innervate the face.
These Axons play an important role in Arousing and Maintaining Consciousness. The Reticular Formation and its connections constitute a system - the Reticular Activating System, which is involved with the Sleep/Wake Cycle.
Visual and acoustical stimuli and mental activities can stimulate the Reticular Activating System to maintain attention and alertness. Stimuli such as a ringing alarm clock, sudden bright lights, or cold water being splashed on the Face can arouse consciousness.
Conversely removal of Visual or Auditory stimuli may lead to drowsiness or sleep. Damage to cells of the Reticular Formation can result in Coma.
The Reticular Activating System is relatively sensitive to certain drugs. general anesthetics function by suppressing this system. It may also be the target of many tranquilizers.
On the other hand Ammonia (Smelling Salts) and other irritants stimulate Trigeminal Nerve endings in the Nose, sending impulses to the Reticular Formation and the Cerebral Cortex to arouse an unconscious patient.
Descending Fibers from the Reticular Formation constitute one of the most important Motor Pathways. Fibers from the Reticular Formation are critical in controlling Respiratory and Cardiac Rhythms and other vital functions.
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