The diencephalon along with the midbrain,
pons, and medulla oblongata comprise the brainstem. A clear
understanding of the importance of this area of the CNS requires
that to be familiar with both its external and internal
features. In addition to performing many vitally important
regulatory functions (respiratory and cardiovascular), the
brainstem also serves as a central point of relay between the
cerebrum, the cerebellum, and the receptors and effectors of the
cranial and spinal nerves.
The prominent external features of
the brain stem are illustrated. The cerebrum and
cerebellum have been removed in each drawing in order to
afford an unobstructed view of the brainstem from
anterior, posterior, and lateral perspectives.
The most prominent features of the
anterior and lateral midbrain are the cerebral
peduncles. These broad bundles of descending fibers from
the cerebrum converge to form a V on the anterior
surface, bounded above by the optic chiasm and below by
the superior border of the pons. The mammillary bodies
and the pituitary gland are framed by the two peduncles.
Four prominent enlargements, the corpora quadrigemina,
can be seen on the posterior surface of the midbrain.
The quadrigemina (four bodies) include two superior
colliculi and two inferior colliculi. The trochlear
nerves (IV) emerge from the posterior surface of the
midbrain just below the inferior colliculi, wrapping
around the cerebral peduncles to appear anterolaterally
at the superior border of the pons. The oculomotor
nerves (III) also originate in the midbrain, emerging
anteriorly at the superior border of the pons.
The pons is a distinctively prominent
feature of the brainstem. It appears as a broad band of
transversely running fibers when viewed anteriorly and
from the side. The fibers extend into the cerebellum
behind and appear to be holding it to the brainstem.
Those which wrap laterally to the cerebellum form the
middle cerebellar peduncles.
The pons is bounded superiorly by the midbrain and
inferiorly by the medulla oblongata. The trigeminal
nerves (V) are prominent lateral projections. The
abducens nerves (VI) originate in the pons and emerge
close together at the anterior inferior border of the
pons. The facial nerves (VII), originating in the pons,
and the vestibulocochlear nerves (VIII), originating in
the pontomedullary area, emerge at the pontomedullary
The most prominent anterior features
of the medulla oblongata are the medullary pyramids.
They appear on the anterior surface as two vertically
running rounded eminences which emerge from under the
pons to become continuous with the spinal cord below. In
the lowest portion of the anterior medulla, descending
corticospinal (pyramidal) tract fibers cross over in the
pyramidal decussation. The corticospinal tracts are
often called pyramidal tracts because of the unique
pyramidal shape they give to the anterior medulla as
they descend into the spinal cord.
The olive is a lateral feature of the medulla. Emerging
from the lateral medulla posterior to the olive in
descending order are the glossopharyngeal nerves (IX),
the vagus nerves (X), and the bulbar accessory nerves
(XI). The hypoglossal nerves (XII) emerge from the
lateral medulla anterior to the olive.
Three sulci are visible in posterior view, a single
posterior median sulcus and two laterally placed
posterior intermediate sulci. Two rounded eminences, the
gracile tubercle (clava) and the cuneate tubercle are
observed on either side of the posterior median sulcus.
The fasciculus gracilis leads to the former while the
fasciculus cuneatus leads to the latter. The posterior
intermediate sulcus separates the fasciculus gracilis
and gracile tubercle from the fasciculus cuneatus and
cuneate tubercle on either side.
CROSS-SECTIONAL ANATOMY OF THE BRAINSTEM
As pathways ascend and descend through the brainstem
they often undergo shifts in position which can only be
seen by a careful examination of cross-sectional
anatomy. This is verified by close examination of the
eight representative sections schematically illustrated
in Figs. 9-4 through 9-11. There is no real shortcut or
alternative to "learning" these cross sections. Indeed,
the function of the brainstem as a relay center between
the cerebrum above, the cerebellum behind, and the
spinal cord below is easier to visualize.
As an academic exercise, it is useful to follow the
course of pathways through the brainstem. By doing this
it is possible to observe how the tracts change in
relative position and size as they descend through the
stem. For example, the corticospinal tracts enter the
brainstem in the middle third of the basis pedunculi
(ventral portion) of the cerebral peduncles where they
are widely separated from each other. As they descend
through the pons they move to a deeper position away
from the surface. However, upon entering the medulla
they begin to converge and once again move to the
surface, giving rise to the medullary pyramids. Bundles
of crossing fibers of these tracts can be observed in
the pyramidal decussation in the lower medulla.
CRANIAL NERVES AND
Cranial nerve fibers are classified as general or
special, somatic or visceral, and afferent or efferent.
Special fibers are those which innervate the special
sense organs associated with hearing, seeing, smelling,
and tasting. In addition, they innervate the vestibular
apparatus and those skeletal muscles derived from the
mesoderm of the branchial arches. This latter group
includes the muscles of facial expression and
mastication as well as laryngeal and pharyngeal muscles.
Also included are the sternomastoid and trapezius
muscles. All other cranial nerve fibers are classified
Fibers are further designated somatic or visceral.
Somatic fibers innervate those skeletal muscles derived
from mesodermal somites as well as innervating
structures of ectodermal origin. The latter include the
skin, the eye, the vestibular apparatus, and the inner
ear. Exceptions are the olfactory epithelium and the
taste buds. Even though the olfactory epithelium and
taste buds are of ectodermal origin, the cranial nerve
fibers innervating them are classified as visceral
because of the close functional relationship which the
senses of smell and taste have with the truly visceral
Visceral fibers innervate structures of entodermal
origin including cardiac muscle, smooth muscle, and
glands. Also included here are those skeletal muscles
derived from the mesoderm of the branchial arches. As
previously noted, cranial nerve fibers mediating smell
(I) and taste (VII, IX, and X) are typically included
here rather than with the somatic group. Cranial nerve
fibers are also classified as afferent or efferent,
depending on the direction of their impulse conduction.
Afferent fibers conduct impulses toward the CNS while
efferent fibers conduct them away.
An oddity in the classification scheme arises from the
practice of classifying all proprioceptors as general
somatic regardless of whether they are associated with
somatic or branchial muscles. This leads to the
confusing observation that a muscle can be innervated by
both special visceral efferent and special somatic
afferent fibers at the same time. The muscles of
mastication are an example (Fig. 9-14). The scheme of
cranial nerve fiber classification is presented again in
Alar and Basal
When the embryonic neural tube
closes, a groove remains in each lateral wall which
separates the posterior from the anterior portions. The
former gives rise to the alar lamina or plate, while the
latter forms the basal lamina (Fig. 9-12). Brainstem
sensory nuclei are found in the alar lamina, while motor
nuclei are generally distributed in the basal lamina.
Figure 9-12 is a composite sketch of cranial nerve
nuclei as found in the brainstem from the midbrain to
the medulla oblongata. It is not a sketch of any single
brain stem section but instead represents a construct
intended to show the relative positions of the nuclei
with respect to each other in cross section. Notice that
the efferent (motor) nuclei are located in the basal
plate, while the afferent (sensory) nuclei are located
in the more lateral alar plates. The dividing line is
the sulcus limitans.
Fibers and the Brainstem
It is not too difficult to trace the
emergence of each cranial nerve from the brainstem. A
more difficult task is to appreciate the distinct fiber
types present in each cranial nerve. But unquestionably
the most difficult task of all is to trace the efferent
origins and afferent terminations of the cranial nerve
fibers in the brainstem. These relationships are
illustrated in Figs. 9-13 through 9-15.
Nerve (I): The fibers of this nerve are SVA. They carry
information pertinent to smell from the olfactory epithelium to
the dendritic glomerular zone of the mitral cells in the
olfactory bulb. Mitral cell fibers then conduct smell
information to the olfactory cortex. Damage to these tracts
causes anosmia (loss of the sense of smell).
Nerve (II): The fibers of this nerve are SSA. They conduct
information concerning vision from the ganglion cell layer of
the retina primarily to the lateral geniculate bodies. Damage to
these fibers causes anopsia (loss of vision).
Nerve (III): The oculomotor nerve contains GVE and GSE fibers.
The GVE fibers originate in the Edinger-Westphal nucleus (an
accessory nucleus of III) in the upper midbrain. They represent
the preganglionic parasymapthetic fibers to the ciliary
ganglion. Postganglionic fibers innervate the ciliary muscles,
which control the thickness of the lens, as well as the
sphincter muscles of the iris, which control pupil size. Damage
to these fibers eliminates the pupillary light reflex and
interferes with accommodation reflexes.
The GSE fibers originate in the oculomotor nucleus in the upper
midbrain. They innervate the inferior oblique as well as the
superior, medial, and inferior rectus muscles of the eye. Damage
to these fibers results in external strabismus and ptosis of the
Nerve (IV): The fibers of this nerve are GSE. They originate in
the trochlear nucleus of the lower midbrain. They innervate the
superior oblique muscles of the eye. Damage to these fibers
causes the eyes to look slightly upward.
Nerve (V): The trigeminal nerve contains SVE and GSA fibers. The
SVE fibers originate in the trigeminal nucleus located in the
middle pons. They innervate the muscles of mastication
(branchiomeric origin). Damage to these muscles causes paralysis
of the jaws.
GSA fibers fall into two groups, those from proprioceptors and
those from exteroceptors. Proprioceptive fibers have their cell
bodies in the mesencephalic nucleus of V and terminate in the
principal sensory nucleus of V in the pons. Exteroceptive fibers
from the skin of the face and head as well as the teeth and the
mucous membranes conduct information to the principal sensory
nucleus of V. Damage to these fibers causes anesthesia in the
Nerve (VI): The fibers of this nerve are GSE. They originate in
the abducens nucleus in the lower pons and innervate the lateral
rectus muscle of the eye. Damage to these fibers causes internal
strabismus and double vision.
Nerve (VII): The facial nerve is composed of SVE, GVE, GSA, and
SVA fibers. The SVE fibers originate in the facial nucleus of
the pons and innervate the muscles of facial expression. Damage
to these fibers causes facial paralysis. The GVE fibers are the
preganglionic parasympathetic fibers to the submaxillary
ganglion. They originate in the superior salivatory nucleus of
the pontomedullary region. Postganglionic fibers innervate the
submaxillary and sublingual salivary glands.
The GSA fibers conduct information from the skin of the external
ear region to the spinal tract and nucleus of V. The SVA fibers
conduct information from the taste buds on the anterior
two-thirds of the tongue to the solitary tract and nucleus.
Nerve (VIII): The fibers of this nerve are SSA. SSA fibers from
the organ of Corti hair cells conduct auditory information to
the cochlear nuclei of the pontomedullary region. SSA fibers
from the vestibular apparatus hair cells conduct information
concerning equilibrium to the vestibular nuclei in the same
Nerve (IX): The glossopharyngeal nerve is composed of GVE, SVE,
GVA, GSA, and SVA fibers. The GVE fibers originate in the
inferior salivatory nucleus. These are preganglionic
parasympathetic fibers to the otic ganglion. Postganglionic
fibers innervate the parotid salivary glands. The SVE fibers
originate in the nucleus ambiguus and innervate the pharyngeal
muscles (branchiomeric origin). GVA fibers conduct information
from the pharynx and posterior third of the tongue. These fibers
also innervate the carotid sinus baroreceptors and carotid body
chemoreceptors. Signals are conducted to the solitary tract and
The GSA fibers conduct information from the skin of the external
ear region to the spinal tract and nucleus of V. SVA fibers
carry information from the taste buds on the posterior third of
the tongue to the solitary tract and nucleus.
Nerve (X): The vagus nerve is composed of GVE, SVE, GSA, GVA,
and SVA fibers. The GVE fibers originate in the dorsal motor
nucleus of X and innervate thoracic and abdominal viscera. These
are the parasympathetic fibers of the vagus nerve. The SVE
fibers innervate the muscles of the larynx and pharynx
(branchiomeric origin) and originate in the nucleus ambiguus.
The GSA fibers carry information from the skin of the ear region
to the spinal tract and nucleus of V. GVA fibers conduct signals
from the aortic baroreceptors and chemoreceptors as well as
other thoracic and abdominal viscera to the solitary tract and
nucleus. Taste cells in the fauces send signals over SVA fibers
to the solitary tract and nucleus.
Nerve (XI): The fibers of the accessory nerve are SVE. There are
two components to this nerve, a bulbar component arising from
nuclei within the brain stem and a spinal component arising from
nuclei in upper cervical levels of the spinal cord. The SVE
fibers which arise from the nucleus ambiguus of the medulla
innervate the muscles of the larynx and pharynx (branchiomeric
origin). SVE fibers arising in the spinal accessory nucleus in
the upper cervical levels of the cord innervate the
sternomastoid and trapezius muscles (also of branchiomeric
Nerve (XII): The fibers of this nerve are GSE. They originate in
the hypoglossal nucleus of the medulla and innervate the muscles
of the tongue.