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No. 23
1. The Brain Stem(2)
Ⅱ) Internal Structure
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The internal structure of brain stem
comprises
nuclei of cranial nerves,
nuclei of non-cranial nerves,
ascending and descending tracts,
reticular formation.
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1. The nuclei of cranial nerves
(1) The quality and sorts
The nuclei of cranial nerves represent
seven kinds of qualities and arranged
medialolaterally six horizontal functional
columns. Three motor columns and four
sensory columns are situated medial and
lateral to the sulcus limitans respectively.
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① The general somatic motor nucleus
It supplies the skeletal muscles evolved
from the myotome, e.g. the extrinsic
muscles of the eye and the muscles of
tongue.
Four pairs of nuclei.
Nucleus of oculomotor nerve,
Nucleus of trochlear nerve,
Nucleus of abducent nerve,
Nucleus of hypoglossal nerve.
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② The special visceral motor nucleus
It supplies the skeletal muscles evolved
from the branchimere, e.g. the muscles of
expression, mastication, pharynx, larynx,
sternocleidomastoid and trapezius.
Four pairs of nuclei:
Motor nucleus of trigeminal nerve,
Nucleus of facial nerve,
Nucleus ambiguous,
Nucleus of accesory nerve.
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③ The general visceral motor nucleus
It is also called the parasympathetic
nervous column.
It controls the activities of smooth muscle,
cardiac muscle and glands in the head,
neck, thorax and abdomen.
Four pairs of nuclei.
Accessory nucleus of oculomotor nerve,
Superior salivatory nucleus,
Inferior salivatory nucleus,
Dorsal nucleus of vagus nerve.
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④ The special visceral sensory nucleus
It receives the first order afferent fibers which
originate from the taste buds.
One pair of nuclei.
Nucleus of solitary tarct.
⑤ The general visceral sensory nucleus
It receives the first order afferent fibers which
originate from the viscera and heart and vessels
in the head, neck, thorax and abdomen.
One pair of nuclei.
Nucleus of solitary tarct.
⑥ The general somatic sensory nucleus
 It receives the first order afferent fibers
which originate from the skin, muscle,
muscular tendon, and most part of oral
and nasal mucous membrane.
 Three pairs:
 Mesencephalic nucleus of trigeminal nerve,
 Pontine nucleus of trigeminal nerve,
 Spinal nucleus of trigeminal.
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⑦ The special somatic sensory nucleus
It receives the first order fibers originate
from the vestibulocochlear organs
(equilibrant and spiral organs) in the
internal ear. Including:
vestibular nuclei,
cochlear nuclei.
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(2) Functional columns of nuclei of cranial nerves
and the their nuclei’ locations and functions
1) The general somtic motor column
① Oculomotor nucleus (Ⅲ)
It is located ventromedially to the central gray
matter at the level of the superior colliculus.
The fibers from the nucleus course ventrally,
leave the brain medial to the cerebral peduncle,
join the oculomotor nerve, supply most of the
extraocular muscles except the lateral rectus and
the superior obliquus, i.e. the superior rectus,
inferior rectus, medial rectus, inferior obliquus,
levator palpebrae superioris.
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② Trochlear nucleus (Ⅳ)
It lies in the ventral region of the central gray
matter close to the midline at the level of the
superior colliculus.
Its outgoing fibers pass laterally and dorsally
round the central gray matter. They reach the
cranial end of the superior medullary velum,
decussating with those of the opposite side and
become the unique cranial nerve which emerges
from the dorsal surface of the brain stem, form
the trochlear nerve, innervating the superior
obliquus.
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③ Abducent nucleus (Ⅵ)
It lies deeply in the facial colliculus.
The fibers from the nucleus of abducent
nerve course ventrally and inferiorly,
emerge through the bulbopontine sulcus,
forming the abducent nerve, control the
lateral rectus.
④ Hypoglossal nucleus (Ⅻ)
 It lies under the hypoglossal triangle and
extends down to the closed part of
medulla oblongata, on each side of the
median plane.
 The fibers originating from hypoglossal
nucleus run ventrolaterally, finally
emerging from the anterolateral sulcus to
become the (rootlets of) hypoglossal
nerve, control the movements of lingual
muscles.
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2) The special visceral motor column
① The motor nucleus of trigeminal
nerve (Ⅴ)
It lies lateral to abducent nucleus.
The fibers originating from this nucleus
leave the brain between the basilar part
and the middle peduncle of cerebellum,
form the motor root of trigeminal nerve,
innervating the masseter.
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② The facial nucleus (Ⅶ)
It is situated at the ventrolateral part of
the reticular formation, ventromedial to
the spinal tract and nucleus of trigeminal
nerve.
The fibers from the nucleus course
dorsomedially, round the dorsal side of
nucleus of abducent nerve, turn
ventrolaterally, forming genu of facial
nerve, emerge through the bulbopotine
sulcus, constitute the facial nerve,
innervate the activities of facial muscles.
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The upper portion of the nucleus which
innervates the muscles of expression
above the eye fissure (the frontal belly of
occipitofrontalis and the orbicularis oculi),
receives fibers from the corticonuclear
tract of both sides, while the lower portion,
which innervates the muscles of the lower
part of the face, receives the fibers from
those of the opposite side only.
③ The nucleus ambiguus (Ⅸ,Ⅹ,Ⅺ)
 It is placed deeply in the reticular
formation.
 The fibers arising from the nucleus
ambiguous leaving the brain stem behind
the olive, provides the special visceral
efferent fibers for the glossopharyngeal
nerve, vagus nerve and cranial root of
accessory nerve, innervate pharyngeal,
laryngeal, and soft palatine muscles.
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④ The spinal accessory nucleus (Ⅺ)
It is located in the gray matter of the
lower part of the medulla oblongata, and
gives rise to efferent fibers to supply the
sternocleidomastoid and trapezius. In
corresponding to the spinal roots of the
accessory nerve, the spinal nucleus of
accessory nerve may extend downwards
as low as the level of the fifth cervical
segment of the spinal cord.
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3) The general visceral motor column
① The accessory oculomotor nucleus (Ⅲ)
This nucleus is situated dorsally to the nucleus of
oculomotor nerve.
The preganglionic parasymjpathetic fibers travel in
the oculomotor nerve to relay in the ciliary
ganglion, subsequently in the short ciliary nerves
to innervate the ciliary muscle and the sphincter
pupillae.
The oculomotor nucleus and the accessory
oculomotor nucleus constitute the oculomotor
nuclear complex.
② The superior salivatory nucleus (Ⅶ)
 It is situated in the reticular formation.
 The parasympathetic preganglionic fibers from
this nucleus join in the facial nerve, control the
secretion of submandibular gland, sublingual
gland and lacrimal gland.
③ The inferior salivatory nucleus (Ⅸ)
 It is near to the rostral end of the dorsal nucleus
of vagus.
 It sends preganglionic parasympathetic fibers
into the glossopharyngeal nerve, controls the
secretion of parotid gland.
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④ The dorsal nucleus of vagus nerve (Ⅹ)
It lies under the vagus triangle lateral to the
hypoglossal nucleus.
The preganglionic parasympathetic fibers
originating from the dorsal nucleus of vagus
nerve emerge posterior to the olive, join the
vagus nerve, and control the activities of smooth
muscle, cardiac muscle and glands or the organs
in the neck, thoracic cavity and a large number of
abdominal visceral organs.
4) The visceral sensory column
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The nucleus of solitary tract (Ⅶ,Ⅸ,Ⅹ)
It is situated around the solitary tract,
ventrolateral to the dorsalnucleus of vagus nerve.
The upper part (end) of this nucleus receives the
special visceral sensory (taste) fibers, while the
lower part receives the general visceral sensory
fibers.
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The solitary tract constitutes a composite
descending bundle of visceral afferent fibers
consisting of the primary afferent fibers from the
facial, glossopharyngeal and vagus nerves which
enter the brain stem by passing through the
posterolateral sulcus. This tract conveys the
gustatory stimulus and general visceral stimulus
to the adjacent nucleus of solitary tract.
In fact, the nucleus of solitary tract is shared by
these three cranial nerves.
5) The general somatic sensory column
① The mesencephalic nucleus of trigeminal
nerve (Ⅴ)
 It located in the lateral margin of the central gray
matter around the mesencephalic aueduct.
 It is concerned with the conduction of the
proprioceptive impulses of masseter, facial
muscles, and extraocular muscles, from the
trigeminal nerve.
② The pontine nucleus of trigeminal nerve (Ⅴ)
 It lies on the lateral side of the motor nucleus, it
extends inferiorly and continues with the spinal
nucleus of trigeminal nerve. The pontine nucleus
receives the primary afferent fibers of the
trigeminal nerve, correlating to the transmission
of tactile sensibility of head and face. The
descending fibers are the spinal tract of
trigeminal nerve.
 The second order fibers from this nuculeus
decussate to the opposite side to join the
trigeminal lemniscus.
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③ The spinal nucleus of trigeminal
nerve (Ⅴ,Ⅶ,Ⅸ,Ⅹ)
It is directly continuous with the
substantia gelatinosa of the dorsal gray
column of the spinal cord.
It lies on the medial side of the spinal
tract of trigeminal nerve and is
concerned with the mediation of pain and
thermal sensibilities of the trigeminal area.
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The general somatic sensory fibers of
trigeminal nerve transmit pain and
thermal sensibilities, enter the pons, and
then descend into the mudulla oblongata,
constitute the spinal tract of trigeminal
nerve, and terminate the spinal nucleus of
trigeminal nerve. This nucleus receives
primary afferent fibers from the trigeminal,
facial, glossopharyngeal and vagus nerves.
The second order fibers originating from
this nucleus cross to the opposite to from
the trigeminal lemniscus.
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6) The special somatic sensory column
① The vestibular nuclei (Ⅷ)
They are situated deep in the vestibular
area.
They are divided into the medial, inferior,
lateral and superior vestibular nuclei.
The nuclei receive the major afferent
fibers from the vestibular nerve. The
efferent fibers of the complex constitute
vestibulospinal tract, correlating muscular
tension, vestibulocerebellar tract, and
medial longitudinal fasciculus.
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② The cochlear nuclei (Ⅷ)
They are situated ventrolateral and dorsal
to the inferior cerebellar peduncle.
They are divided into the dorsal cochlear
nucleus and the ventral cochlear
nucleus. These two nuclei receive
afferent fibers from the cochlear nerve.
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2. The nuclei of non-cranial nerves
(1) The nuclei of non-cranial nerves in medulla
oblongata
① Gracile nucleus and cuneate nucleus
They are situated in the deep of the gracile
tubercle and cuneate tubercle respectively.
The fibers of the fasciculi gracilis and cuneatus
have ascended ipsilaterally through the spinal
cord, and the majority terminate in their
respective nuclei.
The gracile and cuneate nuclei are the important
relay stations for conducting deep and fine touch
sensatory impulses.
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② The inferior olivary nucleus
It is deep in the olive of medulla oblongata.
This nucleus may be the relay station
between the cerebrum, red nucleus and
the cerebellum.
③ The accessory cuneate nucleus
(lateral cuneate nucleus)
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(2) The nuclei of non-cranial nerves in pons
① The pontine nuclei
They are scattered throughout the basilar part of
the pons, constitute relay stations on the
pathway from the cerebral cortex to the
cerebellum.
The axons of the cells of pontine nuclei form the
transverse fibers of the pons (pontocerebellar
fibers). These fibers run transversely, crossing
the median plane, form the middle cerebellar
peduncle, enter the cerebellar hemisphere of the
opposite side.
② The superior olivary complex
③ The nucleus of lateral lemniscus
(3) The nuclei of non-cranial nerves in
midbrain
① The red nucleus
 The red nucleus is situated in the center of
the tegmentum. It appears pinkish color in
fresh specimen.
 It receives the fibers from the
contralateral side of cerebellum via the
superior cerebellar peduncle.
 The efferent fibers decussate in the
ventral tegmental decussation to
constitute the rubrospinal tract.
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② The substantia nigra
It lies between the tegmentum of the midbrain,
extending throughout the whole length of the
midbrain. The cells of subatantia nigra rich in
melanotic pigment and dopamine
③ The superior colliculus
It serves as reflex center correlating movement
of the head and eyes used to localize and follow
visual stimuli. It may also respond to auditory
stimuli by the afferent fibers from inferior colliculi
and to somatic stimuli by the spinotectal tract.
④ The inferior colliculus
 They are reflex centers for auditory
responses and concerned in the ability to
localize the source of sounds.
⑤ The pretectal region (pretectal nuclei)
 It is the center of pupillary reflex situating
at the junction of the midbrain and the
diencephalons.
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3. The long ascending and descending tracts.
(1) The ascending tracts
①The medial lemniscus
The second order fibers arise from the gracile and
cuneate nuclei go anteromedially, wind around
the ventral side of central canal, forming the
internal arcuate fibers, and then bend medially
to reach the median plane, where they decussate
with the corresponding fibers of the opposite side,
forming the decussation of medial lemniscus.
After decussation, the fibers ascend along the
two side of median line, constituting the medial
lemniscus.
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In the medulla oblongata: posterior to the
pyramid.
In the pons: penetrating the trapezoid body.
In the midbrain: dorsolateral to the red nucleus.
The medial lemniscus terminates in the ventral
posterolateral nucleus of thalamus.
This tract connect deep and fine touch sensation.
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②The spinal lemniscus (spinothalamic tract)
After the anterior and posterior spinothalamic
tracts ascend to the medulla oblongata, we
definite the two tracts as spinal lemniscus.
It lies dorsolaterally to the inferior olivary nucleus
and, As it ascends to the pons and midbrain it
lies dorsolaterally to the medial lemniscus,
terminating in the ventral posterolateral nucleus.
The spinothelamic lemniscus connects superficial
sensation of somatic trunk and limbs.
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③The trigeminal lemniscus
The axons of the cells of the pontine and spinal
nuclei of trigeminal nerve, cross the median
plane to form the trigeminal lemniscus
ascending with the medial lemniscus to the
thalamus.
It lies dorsolaterally to the medial lemniscus,
terminates in the ventral postomedial nucleus of
thalamus.
The trigeminal lemniscus relates to the
conduction of the tactile and pressure impulses;
and the pain and thermal impulses from the skin,
mucous membrane, cornea, conjunctiva and the
meninges through the branches of the trigeminal
nerve.
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④The lateral lemniscus
The efferent fibers from the cochlear nuclei run
medially in the ventral part of the tegmentum,
intersecting with the vertical fibers of medial
lemniscus to form trapezoid body. Then the fibers
cross the median raphe and decussate with the
corresponding fibers of the opposite side to form
the principal ascending auditory pathway, the
lateral lemniscus.
It lies dosolaterally to the medial leminicus and
terminates in medial geniculate body.
It connects auditory impulses.
⑤ The medial longitudinal fasciculus
⑥ The ventral spinocerebellar tract and dorsal
spinocerebellar
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(2) The descending tracts
The pyramidal tract:
The axons originating from the giant pyramidal
cells (Betz cells) and other pyramidal cells of
various sized which are located in the precentral
gyrus and anterior part or paracentral lobule
constitute the strong pyramidal tract.
In the course of descending this tract pass
through the posterior limb and genu of internal
capsule, the middle 3/5 of bottom of cerebral
peduncle, the basilar part of pons, and the
pyramid of medulla oblongata.
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The pyramidal tract includes the corticonuclear
tract and corticospinal tract.
The corticonuclear tract (corticobulbar tract)
terminate in the cranial nuelei of brain stem and
the corticospinal tract in the anterior horn of the
spinal cord.
In the lower part of the pyramid, about 85% of
fibers of corticospinal tract cross the median
plane and continue down the spinal cord in the
lateral funiculus as the lateral corticospinal
tract. The uncrossed fibers retain their
ventromedial position and descend in the anterior
funiculus of the spinal cord as the anterior
corticospinal tract.
The pyramidal tract concerned with controlling
the voluntary movements of skeletal muscles.
4. The reticular formation of brain
stem
 In the brain stem, there is an extensive
field of intermingled gray and white
matter collectively termed the reticular
formation.
 According to the traditional view, the
reticular formation forms the central core
throughout the brain stem; lying laterally
between posterior and anterior horns;
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(1) The main nuclei of the reticular formation of
brain stem
1) Cerebellar reticular formation nuclei
2) Raphe nuclei
3) Medial (central) gigantocellular nuclei
4) Lateral parvicellular (small—celled) nuclei
(2) The functions of the reticular formation
The reticular formation is an important
integration center for the vital activity; its major
functions may sum up as follows:
Somatomotor control,
The activation of the behavioral arousal,
Visceromotor control.
Ⅲ. Transverse sections of brain stem
1) The transverse section through the
medulla ablongata at the level of the
pyramidal decussation
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2) The transverse section through the
pons at the level of facial colliculus
3) The transverse section of the
midbrain through the superior
colliculus