1
Neuro-Anatomy
lec: 4 Prof Dr. Al-Hubaity
The Medulla Oblongata
Is the part of the brain stem (also part of the hind brain) extending from
the foramen magnum below to lower border of the pons above. It is 2.5
cm long which lies against the basilar part of occipital bone, being wider
above and narrower below. It consists of two parts, a closed lower half
(traversed by the central canal) and opened upper half, which has no
central canal and form the lower half of the floor of 4
th
ventricle. It shows
the following features:
Anterior median fissure which continues below with the anterior median
fissure of the spinal cord, this fissure is interrupted inferiorly by the
motor decussation (pyramidal decussation).
On each side of the fissure we can see a vertical band of elevated area
known as pyramid, which is formed by the descending motor fibers
(cortico-spinal) coming from the pre-central gyrus, 80%-85% of these
fibers decussate to the opposite side forming the lateral corticospinal tract
(descends to the white mater of spinal cord), while the remaining 15%-
20% of fibers descends as uncrossed fibers to form anterior corticospinal
tract (also descends to the white mater of the spinal cord).
Just lateral to the pyramid, there is an olive like elevation known as olive
(it is produced by the underlying inferior olivary nucleus) .The
hypoglossal nerve (rootlets) emerges from the groove between the
pyramid and olive.
Just lateral to olive there is inferior cerebellar peduncle, connecting the
medulla oblongata with cerebellum. In the groove between the olive and
inferior cerebellar peduncle three cranial nerves emerge (the glosso-
pharyngeal, vagus and cranial part of accessory nerve). The inferior
cerbellar peduncle is around of communication between the cerebellum
and both medulla oblongata and spinal cord.
2
Note
:
between the inferior border of pons and upper part of medulla
oblongata
three
cranial
nerves
emerge
(abducent,
facial,
vestibulocochlear nerves). The abducent emerges between pons and
upper border of the pyramid.
The posterior surface of the M.O
A -
On the upper half (open medulla) which forms the lower part of the
floor of the 4
th
ventricle, it shows the following features:
V shaped depression called inferior fovea with the following triangles and
area:
1- Hypoglossal triangle lies adjacent to the posterior medium sulcus,
it overlies the nuclei of both the hypoglossal and the
glossopharengeal nerves.
2- Vestibular area produced by vestibular nuclei.
3- Vagal triangle: a depressed area between the hypoglossal triangle
and the vestibular area, it over lies the dorsal nucleus of vagus
nerve.
B –
On the lower half (closed medulla) we see:
1- Gracile tubercle and its associated tract or fasculus. The fibers of
the gracile tract arise in the cells of the dorsal root ganglia of the
spinal nerves of the same side (first order neurons) carry
proprioceptive sensation as well as five touch from lower half of
the body.
2- Cuneate tubercle and its associated fasculus contain fibers from
dorsal spinal root ganglia which carry proprioceptive and fine
touch sensation of the upper half of the body.
3
Nuclei in the M.O
1- Inferior olivary nucleus gives rise to:
A) Olivo-cerebellar tract.
B) Olivo-spinal tract.
2- Gracile cuneate nuclei
The fibers of both fascicule terminate at these ganglia, then internal
arcudte fibers passes upward from here. They decussate (forming
sensory decussation) and after the decussation continues as the
medial lemniscns (2
nd
order neurones) which relay (terminates) in
the thalamus.
3- Spinal nucleus and trigeminal nerve which lies medial to spinal
tract of trigeminal nerve, it carries pain and temperature sensation
from the same side of the face. It also receives general sensation
via the 9
th
and 10
th
cranial nerves.
4- Nucleus ambigus which is a motor nucleus which gives rise to the
motor fibers joining the emerging 9
th
-11
th
cranial nerves to supply
the ms of pharynx, larynx and soft palate.
5- Dorsal nucleus of vagus which is a parasympathetic nucleus gives
rise to secretomotor fibers of the vagus nucleus.
6- Inferior salivary nucleus gives rise to seretomotor of
glossopharyngeal which relays in otic ganglion to reach the parotid
gland.
7- Nucleus solitaries receives test sensation from 7
th
, 9
th
and 10
th
cranial nerves.
8- Hypoglossal nucleus and glossopharyngeal nucleus.