The Brain Stem

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The brain stem is divisible into three continuous parts: the midbrain, the pons, and the medulla. The midbrain is most rostral and begins just below the diencephalon. The pons is in the middle and is overlain by the cerebellum.

The medulla is caudal to the pons and is continuous with the spinal cord.

The brain stem is the home of the origins or sites of termination of fibers in 9 of the 12 cranial nerves (CN).

CRANIAL NERVES

Two cranial nerves, the oculomotor and trochlear (CN III and IV), arise from the midbrain (Figure rV-5-1).

Four cranial nerves, the trigeminal, abducens, facial, and vestibulocochlear nerves (CN V, VI, VTI, and VIII), enter or exit from the pons.

Three cranial nerves, the glossopharyngeal, vagus, and hypoglossal nerves (CN IX, X, and XII), enter or exit from the medulla. Fibers of the accessory nerve arise from the cervical spinal cord.

Vagus Nerve Substantia Nigra

Midbrain

Spinothalamic tract Medial lemniscus Corticospinal tract Corticobuibar fibers

Superior colliculus Nucleus III Red nucleus Substantia nigra Nerve III

Midbrain

Spinothalamic tract Medial lemniscus Corticospinal tract Corticobuibar fibers

Superior colliculus Nucleus III Red nucleus Substantia nigra Nerve III

Spinothalamic tract Medial lemniscus

Corticospinal tract

Main sensory nucleus of V Motor nucleus of V

Nerve V

Upper pons

Spinothalamic tract Medial lemniscus

Corticospinal tract

Note

The descending hypothalamic fibers course with the spinothalamic tract

Main sensory nucleus of V Motor nucleus of V

Nerve V

Spinothalamic tract Medial lemniscus

Corticospinal tract

Lower pons

Spinothalamic tract Medial lemniscus

Corticospinal tract

Nucleus VI Nucleus VII

Vestibular nucleus (VIII) Cochlear nucleus (VIII) Nerve VIII Nerve VII Nerve VI

Spinothalamic tract Medial lemniscus Corticospinal tract

Nerve XII

Upper medulla

Spinothalamic tract Medial lemniscus Corticospinal tract

Nerve XII

Nucleus XII Vestibular nucleus VIII Interior cerebellar peduncle Nucleus solitarius - VII & IX Spinal tract V Spinal nucleus V Nucleus ambiguus - IX & X Inferior olive

Lower medulla

Spinothalamic tract

Spinothalamic tract

Nucleus gracilis Nucleus cuneatus

Spinal tract & nucleus V

Nucleus gracilis Nucleus cuneatus

Spinal tract & nucleus V

Figure IV-5-2. Brainstem: Cranial nerves and Identification of Sections

Brain Stem Nucleus

NEURAL SYSTEMS

Each of the following five ascending or descending neural tracts, fibers, or fasciculi course through the brain stem and will be found at every transverse sectional level.

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