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Lecture# 11
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Brainstem
Composed of three parts:
-midbrain: most superior part, Pons, and medulla
Functions:
1. Chanel or conduit for descending and ascending fibers (most of the descending and ascending
fibers pass through the brainstem, ex: corticospinal)
2. Contains Reflexes for vital centers like respiratory center and cardiovascular center
3. Inside the brainstem we have the reticular formation that controls the sleep-awake cycle
4. There are a lot of nuclei for cranial nerves located inside the brainstem (nuclei of cranial
nerves 3-12 are located within the brainstem).
Medulla oblongata
-it’s the conical shaped last part of brainstem
-connects Pons superiorly and spinal cord inferiorly
-lower half of medulla oblongata contains the central canal (this canal is a continuation of the
central canal present in the spinal cord)
-upper half of medulla expands to accommodate the diamond shaped 4th ventricle (recall that the
4th ventricle is located between the dorsal surface of medulla oblongata and the anterior surface
of cerebellum )
- cross sections through medulla oblongata we are going to discuss are one of two types:
1. Open medulla or open section: superior half of medulla, named open because it contain The
4th ventricle (ventricle is a cavity so it’s open)
2. Closed medulla or closed section: inferior half of medulla, named closed because it contain
central canal
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 External features of medulla
A. Anterior or ventral view:
1. The 1st thing we notice is the anterior median fissure (it will continue as the anterior median
fissure in the spinal cord)
2. On both sides of the anterior median fissure or lateral to the anterior median fissure on both
sides we have two projections or swellings, these are the pyramids.
Pyramid: is projection present on the anterior surface of medulla oblongata, if we took a cross
section through the pyramids we will find the corticospinal tract (pyramidal tract) the decussation
of this tract is at the end level of medulla (junction between medulla and spinal cord)
Due to this decussation the left cortex control functions of the right side of our body and vice
versa
3. Postrolateral to the pyramids we find the olives, cross section in the olive shows that it
contains the inferior olivary nucleus ( the inferior olivary nucleus is a structure present inside
medulla but it forms a projection that is obvious on the external aspect of medulla and this is
what we call the olive )
4. Posterior to the olives we have the ICP “inferior cerebellar peduncle” : ICP is a group of fibers
that form the connection between medulla and cerebellum. <<the ICP is more obvious in the
posterior view >>
B. Posterior or dorsal view:
1. Posterior median sulcus (it continues as the post median fissure of the spinal cord)
2. On both sides of the post median sulcus we find two tubercles (medial→ Gracile,
lateral→ Cuneate). These tubercles are the external plugging of the two nuclei (Gracilius and
Cuneatus) which are the site of the 2nd relay of medial lemniscal (posterior white column) tract.
<<Gracilius & Cuneatus are two nuclei present inside the medulla and what we
see as external features of medulla are the two tubercles that represent the plugging of these
nuclei so we
DON’T
call them nuclei when we are looking at them from outside (externally)>>
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Nerves emerging from the anterior aspect of medulla
-Hypoglossal nerve: is one of the cranial nerves, it’s nucleus is present inside medulla oblongata,
it emerges between pyramids and olives
- 3 cranial nerves emerge between the olives (anterior) and the ICP (posterior), these are the
vagus.N, cranial part of accessory.N, and glossopharengeal nerve.
Cross sections through medulla oblongata
1. Section at the level of pyramidal or motor decussation
2. Section at the level of sensory decussation (decussation of posterior white column)
3. Section at the level of olive (section through the olive)
-the following topic is purely a description of each section; that is you need to
Check the slides along with the sheet.
-transition between any section and the subsequent one is gradual, but
Rather new structures start to appear and others disappear gradually.
-There is no typical section that shows all the structures together; that
Doesn’t mean that they are not present together in the same section.
Level of motor decussation:
-the most inferior level (caudal section)
-also called pyramidal decussation
-this is a closed section because it’s through the inferior half of medulla
-decussation means: the fibers on the left side will cross to the right side and vice versa
-the decussation occurs anterior to the central canal
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In this section: <<slide # 12>>
-notice the location of both central canal and central gray matter (anterior to the central canal
the decussation will take place but not at this level)
-notice that the H shaped area will start to appear
-what was mentioned of the abbreviations is the following:
.DMS: dorsal median fissure
.VMS: ventral median sulcus
.P: pyramids
.FC & FG: fasciculus (F) Gracile and Cuneate
.SN5: spinal nucleus of trigeminal nerve
”WE
DON’T SEE THE DECUSSATION IN THIS SECTION BECAUSE THE
FIBERS DIDN’T DECUSSATE YET, THE DECUSSATION IS MORE PROMINENT AT HIGHER
LEVELS.”
Note: I can’t tell if the Dr is talking about this section in general or about the specific slide (#
12) but she said that “ we can’t see the decussation at this level “
-in this section the Gracile & Cuneate nuclei start to appear, posterior to them we see two tracts
(or fascicule); fasciculus Gracilius and fasciculus Cuneatus, each tract is composed of fibers
related to same name nucleus .
-we also see (at this level) laterally the structures that pass through the lateral white column of
the spinal cord; these are the tracts: spinocereblar, spinothalamic (the arrangement here is the
same arrangement of these tracts when they pass through the spinal cord)
-SN5: spinal nucleus of trigeminal, posterior to this nucleus we find it’s fibers, and this nucleus is
the extension of Substantia gelatinosa (this is an example of gradual transition), it extends
through(we find it in) the lower part of Pons, the whole length of medulla oblongata and the 1st 2
cervical segments of the spinal cord.
Substantia gelatinosa functions to perceive pain and touch sensations, SN5 also perceive these
sensations especially from the face.
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In the caudal section of medulla we can see:
_in the gray matter: Gracile, Cuneate, & SN5 nuclei
_in the white matter:
1- Ascending sensory tracts spinocerebellar (ventral& dorsal)*, spinal lemniscus (spinothalamic
tract), Gracile & Cuneate tracts**, and spinal tracts of trigeminal.
*ventral and dorsal spinocerebellar tracts: carry proprioception
** Gracile & Cuneate tracts: also carry proprioception
2- Ascending motor tracts: pyramidal tract, 2\3rds of the fibers forming this tract will cross to the
other side to form the motor decussation.
Level of sensory decussation:
-also called liminescal decussation
-this level is almost at the middle of the medulla
-fibers emerging from Cuneate & Gracile nuclei cross to the opposite side
-in this section we still see some structures found or seen in the previous section ( pyramidal
decussation level ) those are the following: <<slide #17>>
.AMF: anterior median fissure
.PMS: posterior median sulcus
.P: pyramids VERY PROMINENT AT THIS LEVEL
.ant & post spinocerebellar tracts, and spinothalamic tract
.SN5(spinal nucleus of trigeminal) & it’s tract
.MORE PROMINENT Gracile & Cuneate nucleus
.central canal &central gray matter
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- we DON’T see the pyramidal decussation at this level
 this section (sensory decussation ) differs than the previous one in that, it contains the fibers
that emerge from Gracile & Cuneate nuclei , the presence of the medial decussation, and
appearance of the hypoglossal nucleus.
-the fibers emerging from Gracile & Cuneate nuclei:
. walk anterio-medialy, so their decussation is anterior to the central canal ( like the motor
decussation) although their nuclei are posterior to the central canal .
.they are called the internal arcuate fibers before the decussation, and continue as medial
After the Decussation.
.terminate in the ventropostrolaterla nucleus of the thalamus.
So in this section or at this level:
The gray matter contains: Gracile& Cuneate nuclei, SN5
The white matter contains ascending and descending fibers:
_Ascending fibers: Gracile& Cuneate tracts, spinal tract of trigeminal nerve, spinocerebellar tact
(ventral & dorsal), and spinal leminscal tract .
_Ascending tracts: pyramidal (corticospinal tract) and extra pyramidal tracts *
*medial leminscus (post white column)
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slide # 21:
1: anterior median fissure
2: dorsal median sulcus
3: the triangular structure labeled with # 3 is the hypoglossal nucleus
3: the circular structure labeled with # 3 and post to the hypoglossal nerve is the
inferior olivary nucleus
4: hypoglossal nerve
5: SN5 and at its peripheral aspect we find its tract (# 5 that projects out of the
figure)
6: Gracile nucleus
7: Cuneate nucleus
8: internal arcuate fibers
9: leminscal decussation
10: pyramids
Done by: NOURA ODEH
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