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Dr. Mustafa
Neuroanatomy lectures (6)
Dr. Mustafa
Medulla oblongata
It extends from the pons to the spinal cord at the foramen magnum. The
upper part of the medulla oblongata is called the open medulla that forms the
lower half of the floor of the fourth ventricle. The lower part of the medulla
oblongata is called the closed medulla and the basic structure of it is similar
to that of the spinal cord.
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The nuclei:
1- The spinal tract and spinal nucleus of the trigeminal nerve:
It extends throughout the medulla and inferiorly it is continuous with the
posterior horn of the spinal cord. The spinal tract of the trigeminal nerve
is the pathway for the pain and temperature from ipsilateral side of the
face.
The spinal nucleus of the trigeminal nerve has three parts: (from rostral
to caudal parts).
I- oralis. II- interpolaris. III- caudalis.
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2- Hypoglossal nucleus (motor nucleus): it lies in the floor of the 4 th
ventricle (open medulla), near the midline and extends into the closed
medulla. It supplies the muscles of the tongue.
3- Dorsal nucleus of the vagus: it is considered as a visceral efferent
nucleus of parasympathetic activity. It has two important functions:
a- Motor action to the smooth muscles of the gastrointestinal tract
and respiratory tracts (lead to contraction of these smooth
muscles).
b- Secretomotor action to the glands (induce their secretion) of the
GIT and respiratory tracts.
4- nucleus ambiguous (visceral efferent):
It is motor nucleus; it is responsible for supplying the striated muscles
of the pharynx, larynx and the palate.
Nucleus ambiguous gives fibers to glossopharyngeal nerve, vagus
nerve, cranial part of the accessory nerve and recurrent laryngeal
nerve.
5- nucleus of tractus solitaries:
It is sensory nucleus made up of two parts; caudal part and rostral
part.
The caudal part of this nucleus is considered as general visceral
afferent nucleus that responsible for oral cavity general sensation.
The rostral part of the nucleus is considered as special visceral
afferent nucleus that responsible for special sense of the oral region
(taste or gustatory function).
Thus the nucleus of the tractus solitaries has a sensory input from the
oral region.
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Nucleus solitarius receives all of the following:
a. Taste sensation from the anterior two thirds of the tongue via the
chorda tympani.
b. Taste sensation from the posterior third of the tongue via the
glossopharyngeal nerve.
c. Taste sensation from the epiglottis via the vagus nerve.
d. General sensation from the oral cavity and viscera supplied by the
vagus nerve.
6- The gracile and cuneate nuclei:
They are situated posteriorly in the closed medulla, the gracile being the
more medially placed, while the cuneate being the lateral one. Their
efferents pass ventrally and cross the midline as the internal arcuate fiber
and then they will form the medial lemniscus that passes upward to end
in the ventroposterior nucleus of the thalamus.
7- Accessory cuneate nucleus:
It is situated more laterally and involved in the cuneocerebellum tract
(for unconscious proprioception).
8- The olivary nucleus (inferior olivary nucleus):
It is motor nucleus and has a very characteristic shape as crumpled bag
in open medulla. It is considered as the motor coordinate center.
The main afferent (input) to the inferior olivary nucleus is from:
a- Cerebral cortex, b- Basal ganglia, c- Red nucleus, d- Spinal cord.
The olivary nucleus will send efferent fibers (output) to the contralateral
side of the cerebellum (as climbing fibers).
9- Lateral reticular nucleus.
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The 4th ventricle divides the medulla into an open medulla (forming the
floor of the 4th ventricle) and a closed medulla that contain the central
canal of CSF and continuous with that of the spinal cord.
At the closed medulla, there are two types of decussation:
1- The sensory or lemniscal decussation at the upper part of the closed
medulla oblongata.
2- The motor or pyramidal decussation at the lower part of the closed
medulla oblongata.
The tracts of the medulla oblongata will be studied with the figures of
the medulla oblongata:
● at level of olivary nucleus (open medulla).
● at level of sensory decussation (upper part of closed medulla).
● at level of motor decussation (lower part of closed medulla).
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Pons
(It means transverse fibers)
The pons is part of the brain stem that continuous with the medulla
oblongata inferiorly and mid brain superiorly, the lower half of the pons
is related posteriorly to the floor of the 4th ventricle.
The basic structure of the pons:
a- The basal part.
b- The tegmental part.
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The pons has two parts; an anterior or ventral part called the basal part
which has the same structure all over the pons. The basilar part is made
up of: 1- Longitudinal fibers that are represented by the Corticospinal and
corticonuclear fibers.
2- Transverse pontine fibers.
3- This basilar part also contains a number of masses of gray matter
called the pontine nuclei. The basilar part is in continuity superiorly with
the cerebral peduncles of the midbrain.
The tegmental part is the posterior part of the pons and it forms the
upper part of the floor of the 4 th ventricle, which separates the pons
anteriorly from the cerebellum posteriorly.
This part is studied in three levels or sections:
1- At pontomedullary junction.
2- At level of the facial colliculus (lower pons).
3- At level of the trigeminal nuclei (middle of the pons).
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1- At pontomedullary junction.
There are group of nuclei at pontomedullary junction (i.e. part at pons
and other part at medulla). These nuclei are:
1- Cochlear nuclei and
2- Vestibular nuclei.
The cochlear nuclei:
They are divided by the inferior cerebellar peduncle into:
Posterior (dorsal) cochlear nucleus and anterior (ventral) cochlear
nucleus. These dorsal & ventral cochlear nuclei are receiving input from
the inner ear through the cochlear nerve. The fibers from the cochlear
nuclei will pass to both ipsilateral and contralateral superior olivary
nucleus (considered as a sensory nucleus for hearing), so the auditory
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pathway has bilateral presentation, and thus, there is no complete hearing
loss, but partial deafness.
The crossed fibers (which crossing the midline) will form a mass of
white matter called the trapezoid body that end into the superior olivary
nucleus. The fibers from the superior olivary nucleus will ascend as a
band called the lateral lemniscus.
The lateral lemniscus will end at the inferior colliculus of the mid brain,
then to medial geniculate body of the thalamus that give output to the
auditory cortex. The medial geniculate body receives auditory
information from both ears but mainly from the opposite ear.
The above system is the auditory pathway.
The vestibular nucleus:
It is divided into four parts:
The superior vestibular nucleus or part – inferior vestibular nucleus –
medial vestibular nucleus – lateral vestibular nucleus which is the largest
part of the vestibular nucleus.
The vestibular pathway begins from the semicircular cannels, utricle
and saccule of the internal ear (the labyrinth) and these fibers will end
into the vestibular nucleus.
The fibers from the vestibular nucleus will pass to the following:
a- The major fibers from the vestibular nucleus will enter the ipsilateral
side of the cerebellum.
b- Some fibers pass to the thalamus then to the cerebral cortex.
c- The vestibular nuclei also have an important connection to the medial
longitudinal fasciculus.
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The medial longitudinal fasciculus is not considered as a part of the
vestibular pathway, but it is important for control the position of the eye
ball and head movements according to the vestibular stimuli or input.
The medial longitudinal fasciculus has the following connections:
a- To the vestibular nuclei.
b- To the spinal cord and especially the cervical segments of the spinal
cord.
c- Nuclei of the 3rd, 4th and 6th cranial nerves that is responsible for the
movements of the eye ball.
The damage of the medial longitudinal fasciculus like in multiple
sclerosis, it will lead to a condition called the Doll's eye.
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2- At level of the facial colliculus (lower pons).
Facial colliculus is the fibers of the facial nerve that emerge from
facial nerve nucleus. These fibers pass posteriorly and medially, then
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hook around the Abducent nerve nucleus, then the fibers will pass
laterally and then pass out from the pons.
The nucleus of the Abducent nerve is the main structure that forms the
facial colliculus rather than the fibers of the facial nerve that emerge
from the pons. Thus, the facial colliculus that situated as an elevation in
the floor of the 4th ventricle is formed of two parts:
- The superficial part that represented by the facial nerve fibers.
- The deep part of the facial colliculus is represented by the nucleus of
the 6th cranial nerve which supplies the lateral rectus muscle of the eye
ball.
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3- At level of the trigeminal nuclei (middle of the pons).
At this level, the four groups of trigeminal nuclei could be seen together,
otherwise they could be seen separately (not together) at other parts of
CNS.
The nuclei of the trigeminal nerve:
These are groups of nuclei that extend on a large area of the central
nervous system.
1- The first group of nuclei of the trigeminal nerve is the main sensory
nucleus and it receives the touch sensation from the same side of the
face.
2- The second group of nuclei of the trigeminal nerve is the spinal
nucleus of the trigeminal nerve that extends from the pons to the
upper part of the cervical region of the spinal cord. It receives the pain
and temperature from ipsilateral side of the face.
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3- The third group of nuclei of the trigeminal nerve is the mesencephalic
nucleus which extend from the pons to the mid brain and it is
responsible
for
the
proprioception
sensation
of
the
temporomandibular joint (TM joint).
The above three nuclei of the trigeminal nerve are sensory nuclei.
4- The fourth group of nuclei of the trigeminal nerve is the motor
nucleus that is responsible for supplying the muscles of mastication
(medial & lateral pterygoid muscles, masseter and temporalis
muscles).
The sensory component of the trigeminal nerve has a ganglion at the tip
of the petrous part of the temporal bone which is called the trigeminal or
semilunar ganglia.
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