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Anatomy of The Brain Stem
Part 2: internal structure
Prepared by
Dr / Amani
Almallah
Internal structure of the Medulla Oblongata
To study the internal structure of the medulla, three transverse section are done
on three levels:
1- In closed medulla at the level of motor (pryamidal) decussation (Level 1)
2- In closed medulla at the level of Sensory decussation.
3- in upper level of open medulla (at the level of Olive).
Nuclei in Medulla
I- cranial nerve nuclei: - Hypoglossal nucleus (S E).
- Nucleus Ambiguus (SVE), Dorsal motor nucleus of vagus (GVE), Nucleus
Solitarius (S.V.A. & G. V. A.).
- Inferior vestibular nucleus (SSA),
-Inferior salivary nucleus (GVE)
- Spinal tract & nucleus of trigeminal (GSA).
These nuclei are Sensory & motor nuclei of the lower 5 cranial nerves:
(Vestibulocochlear (8), Glossopharyngeal (9), Vagus (10), Accessory (11), and
Hypoglossal (12) + addition to deep origin of 7th & 5th CN)
II- Relay Nuclei : Gracile & cuneate nucleus (pass somatic sensory
information to the thalamus).
III- Cerebellar Relay Nuclei:
1- Inferior olivary nucleus: the largest one from 4 olivary nuclei (olivary
complex).
2- Accessory (lateral) cuneate nucleus (relay information from the spinal cord,
cerebral cortex, and the brainstem to the cerebellar cortex).
3- Arcuate nucleus (medial to the pyramid), its axons will form ventral
external arcuate fibers which reach the cerebellum through ICP
4- Lateral reticular nucleus :
Medullary cranial nerves Nuclei
These nuclei are Sensory & motor nuclei of the lower 5 cranial nerves:
(Vestibulocochlear (8), Glossopharyngeal (9), Vagus (10), Accessory (11), and
Hypoglossal (12) + addition to deep origin of 7th & 5th CN)
1- Hypoglossal nucleus (GSE):
Lies in the floor of the 4th ventricle (underlying the hypoglossal trigone) close to
the median plane of the opened medulla. Its fibers entirely motor supply the
muscles of the tongue.
2- Dorsal nucleus of vagus:
Lies in the floor of the 4th ventricle lateral to the hypoglossal nucleus (underlying
the vagal trigone) . Its fibers run in the vagus nerve (X) these fibers supply smooth
muscles of the viscera and inhibitory to the heart. Dorsal nucleus of vagus also
receives sensation from viscera through the glossopharyngeal & vagus.
Medulla Oblongata
3- Nucleus and tractus Solitarius :
Lies in dorsolateral part of the open medulla (medial to the inferior cerebellar
peduncle). Share in three cranial nerves (facial, glossopharyngeal & vagus).
It receives taste sensation from the tongue & epiglottis through facial,
glossopharyngeal & vagus. It also receives general visceral sensation from
glossopharyngeal & vagus.
4- Nucleus ambiguus : It lies in the
lateral part of open medulla , just behind
the inferior olivary nucleus. Its upper
Parts gives fibers to glossopharyngeal
nerve, its middle part gives fibers to
vagus and its lower part gives fibers to
spinal part of accessory.
Hypoglossal N
1 2
D
4
C
3
B
A
Nuclei of the vagus & hypoglossal
1- Dorsal nucleus of vagus
3- Nucleus ambiguus
2- nucleus solitarius
4- spinal tract & nucleus
5- Spinal nucleus and tract of trigeminal
Lies close to the lateral surface of the medulla. It extends upwards to the lower part
of the pons and downward to the upper segments of the spinal cord to become
continuous with the substantia gelatinosa of rolandi (SGR).
It carries pain and temperature sensation from the same side of the face &head.
The nucleus sends efferent fibers which cross the midline and ascend in the
trigeminal lemniscus.
6- Inferior salivary nucleus (GVE):
Lies in the upper most part of the medulla in line with the dorsal motor nucleus of
the vagus. It gives fibers which run in glossopharyngeal nerve as a preganglionic
parasympathetic (secretory motor) which relay in the otic ganglion to parotid.
7-Inferior vestibular nucleus (SSA):
It is one of 4 nuclei for vestibular part of the CN VIII. It lies in the open medulls in
the lateral part of the floor of 4th ventricle subjacent to the vestibular area & close
to the medial side ICP.
8- Cochlear nuclei: two pairs of nuclei, ventral cochlear nucleus lies in front of
inferior cerebellar peduncle while dorsal cochlear nucleus lies behind it
Nuclei of cerebellar relay
1- Olivary nuclei: four in number; Medial, dorsal, inferior & superior olivary
nuclei. Three of them present in medulla, the 4th one (superior) presents in pons.
The inferior olivary nucleus: it is the largest one, a corrugated sac of grey
matter with helius directed medially, lies lateral to the pyramid . It is one of the
extra-pyramidal centers.
Its afferent connections: 1- Cerebello-olivary
3- cortico-olivary
4- spino-olivary .
Its efferent are: 1- Olivo-spinal
2- strio-olivary
5- Rubro-olivary
2- Olivo-cerebellar
Majority of Its axons form (olivo-cerebellar tract) cross the midline to pass
through the ICP to reach the cerebellar cortex of the opposite side.
2- accessory cuneate: lies lateral to the cuneate nucleus in the upper part of the
closed medulla. It receives afferent from lateral fiber of the cuneate (carrying
proprioceptive senation from the cervical region of the spinal cord. Then its
efferent form Dorsal external arcuate fibers which pass in the inferior
cerebellar peduncle of the same side to the cerebellum.
III- Main Tracts of Medulla
1- Pyramidal tract & motor decussation.
2- Gracile & Cuneat tract after their
decussation forming (internal arcuate
fibers). The upwards continuation of
these fibers in the open medula
forms Medial lemniscus.
3- Medial longitudinal bundle (MLB).
4- Spinal tract of trigeminal
5- Tectospinal tr.
6- Spinocerebellar tr. From Dorsal spinocerebellar tract
7- Olivocerebellar tract from (Inferior olive to cerebellum)
8- Reticulocerebellar tract (Reticular formation to cerebellum
Pass through ICP
Medullary tracts can be divided into
Tracts present in the median plane
from anterior posteriorly:
1- Pyramidal fibers
2- Medial Lemniscus
3- Tecto-spinal tract
4- Medial longitudinal bundle
Tracts present in lateral part of the
medulla:
1- Spinal Lemniscus
2- Spinal tract of trigeminal nerve
3- Spino-cerebellar tracts
4- Inferior cerebellar peduncle
Details of the medullary tracts
1- Pyramid is formed by pyramidal tract (corticospinal) fibers before
decussation. fibers decussate to form (level of motor decussation).
2- Gracile nucleus: lies on the posterior surface of the closed medulla close to the
posterior median fissure. This nucleus receives the termination of the gracile tract
(carrying proprioceptive sensation from the lower ½ of the body (Gracile Below
T7).
3- Cuneate nucleus: Lies on the posterior surface of the closed medulla lateral to
the Gracile nucleus. This nucleus receives the termination of the Cuneate tract
(carrying proprioceptive sensation from the upper ½ of the body) Cuneate from
C1 to T7) .
the efferent fibers of both Gracile & cuneat cross to the opposite side to form
internal arcuate fibers, decussate with its fellow to form (sensory decussation) &
ascends to the thalamus (PLVNT) as Medial lemniscus.
Level 2: closed medulla at level of sensory decussation
Sensory decussation lies in closed medulla in a level higher than that of the motor
decussation.
4- Inferior cerebellar peduncle (ICP): large peduncle emerging from the postrolateral aspect of the medulla ascending upward laterally connecting it with the
cerebellum.
Its afferent:1- Dorso-spinocerebellar tr.
3- olivo-cerebellar tr.
2- vestibulocerebellar tr.
4- Dorsal ext. arcuate fibers.
5- Reticulo-cerebellar tr.
Its efferent: 1- Cerebello-olivary tr.
2- Cerebellovestibular tr.
3- Cerebello-reticular tr.
5- Medial longituidinal bundle (MLB): this is a well defined bundle along the
whole length of the brain stem, close to the median plane close to the floor of the
4th ventricle. Its lower end is continuous with the fasciculus proprius anterior in the
spinal cord.
Section in upper part of closed
medulla
Section in lower part of closed
medulla
6- Spinal tract of trigeminal
7- Spinocerebellar tr. From Dorsal spinocerebellar tract
8- Tectospinal tract.
9- Olivocerebellar tract from
(Inferior olive to cerebellum)
10-Reticulocerebellar tract
(Reticular formation to cerebellum
Internal structure of the pons
Ventral (basilar) part
I- Longituidinal
bundles
III-Pontine nuclei
II-Transverse fibers
Dorsal (tegmental) part
Cranial nerves
nuclei (V, VI,
VII,VIII
four Lemnisci
Medial, trigeminal,
spinal & lateral
Dorsal (or tegmental part):
It forms the upper ½ of the floor of the 4th ventricle. It is continuous above with
the tegmentum of the midbrain and below with the dorsal part of the medulla
oblongata.
Basilar (Ventral) Part:
it is large & composed of transverse fibers, longitudinal bundles & pontine
nuclei.
1- The longituidinal bundles are:
A- Corticospinal (pyramidal) tract: separated by transverse pontine fibers &
collected inferiorly to form pyramid in medulla.
B- Corticopontine fibers: end in pontine nuclei.
II- Transverse fibers: Pontocerebellar fibers, they are axons of pontine nuclei.
Most of them cross the midline to form the middle cerebellar peduncle (MCP)
of the opposite side & end in the cerebellum.
III- Pontine nuclei (Nuclei pontis): group of small cells, their axons form
transverse fibers to form (second order neuron in Cortico-Ponto-Cerebellar
pathway.
Dorsal (tegmental) Part
The most important contents are:
I- Nuclei of the middle 4 cranial nerves (5th,6th,7th, and 8th):
The 4 lemnisci (medial, trigeminal, spinal and lateral)
Cranial nerves nuclei (V, VI, VII,VIII
1-Nuclei of trigeminal nerve
These are four nuclei; 3 sensory nuclei (mesencephalic, main sensory and
spinal) and one motor, which are scattered in the whole brain stem, as follows:
In the midbrain: mesencephalic nucleus
In the pons: motor nucleus and main sensory nucleus.
In the medulla: spinal nucleus
I- Motor nucleus (S.V.E.): its axons form motor root of trigeminal, supplies
the muscle of first pharyngeal arch(the muscle of mastication, mylohoyoid,
anterior belly of digastric and 2 tensors (palate & tympani). It lies the middle of the
pons in line with nucleus ambiguus & facial nucleus.
II- Main (principal) sensory nucleus (G. S. A.): Lies in the middle of the pons lateral
to the motor nucleus. It receives afferent fibers which carry touch & pressure sensations
from the trigeminal area (face & head). Sends efferent fibers cross the middle line line
to join the trigeminal lemniscus.
III- Spinal nucleus (G.S.A.)
Lies in the lower part of the pons,
extending along the whole length of
the medulla & continues with the
substantia gelatinosa of Rolandi
in the spinal cord.
It receives pain & temperature.
The spinal nucleus sends efferent fibers which cross the middle line to join the
trigeminal lemniscus.
IV- Mesencephalic nucleus (G.S.A.): called mesencephalic because it extends into
the mid-brain. It receive proprioceptive sensation from masticatory and ocular
muscles.
2-Nucleus of Abducent nerve (S.G.E.):
It is a motor nucleus, lies in the lower part of the pons near the floor of the 4th
ventricle, close to the midline in series with 3rd, 4th & 12th nuclei.
Its position is known by medial eminence, It is surrounded by the fibers of the
facial nerve forming together a bulge in the floor of the 4th ventricle called the
facial colliculus.
3- Facial nerve (VII) nuclei:
These are 3 nuclei: motor, superior salivary nucleus and nucleus solitaries.
1- Motor Nucleus of facial (S.V.E.):
These motor fibers supply the muscles of second pharyngeal arch (muscles of the face
and scalp as well as the posterior belly of digastric, stylohyoid, stapedius and platysma).
The motor nucleus lies in the lower part of pons & its fibers form a loop around the
abducent nucleus.
The connections of motor nucleus of facial:
1- The upper part of the nucleus is controlled by both pyramidal (corticobulbar) tracts
& its lower part by the opposite tract only.
2- Medial Longituidinal bundle.
2- Superior Salivary nucleus (parasympathetic nucleus):
It lies in the lower part of the pons just above the inf. Salivary nucleus & lateral to the motor
facial nucleus. Its efferent fibers run in the facial nerve as preganglionic fibers to relay in the
submandibular &pterygopalatine(sphenopalatine) ganglia.
3- Nucleus solitaries
It lies commpletely in the medulla oblongata, & receives all taste fibers via facial,
glossopharyngeal and vagus nerves.
4-Nuclei of Vestibulocochlear Nerve
I- Vestibular nuclei (S.S.A.)
These are 4 in number (medial, lateral, superior and inferior) situated in the lateral
part of the floor of the 4th ventricle where they produce an elevation called
vestibular area, medial to the ICP. The first three lies in the lower part of pons
while the inferior one lies in the open medulla.
Connections: The vestibular nuclei receive afferent fibers of unconscious
proprioceptive sensation (equilibrium) from the inner ear via the vestibular
division of the 8th cranial nerve while they send efferent fibers to:
1- Spinal Cord (vetsibulospinal )
2- Cerebellum (vestibulocerebellar)
3- Form Connection with other cranial nerve nuclei through Medial longitudinal bundle
II- Cochlear nuclei( S.S.A.):
These are Dorsal & Ventral nuclei, lie on relation to the surfaces of the inferior
cerebellar peduncle, very close to pons.
Connections:
They receive afferent fibers from the cochlea via cochlear nerve.
- Their efferent fibers form Trapezoid body
They send efferent fibers to the following nuclei in the Pons
Superior Olivary nucleus, Trapezoid nuclei & Nucleus of lateral Lemniscus.
The 4 Lemnisci
Definition: they are 4 bands of ascending sensory fibers in the brain stem. Each
lemniscus starts from a certain decussation below & ends in the thalamus above.
1- Lateral lemniscus (the most lateral one) carrying hearing impulses from both
ears mainly the opposite side. mainly from the opposite side. Its fibers are the
axons of the nuclei of the trapezoid body of both sides mainly of the opposite
(3rd order neurons in auditory pathway) to terminate in the inferior colloculus
& Medial geniculate body.
2- Spinal lemniscus : (just medial to lateral lemniscus ) carrying pain, temp., &
crude touch from the opposite side of the body below the head. It is formed by
the union of the lateral and ventral spino-thalamic tracts.
The lateral spinothalamic tract is formed by the axons of the substantia
gelatinosa, while the ventral spinothalamic tract is formed by the axons of the
nucleus proprius of the spinal cord. Spinal lemniscus terminates in PLVNT of
thalamus.
3- Trigeminal lemniscus: (just medial to spinal lemniscus ) carrying pain,
temp., touch & proprioceptive sensation from the face & scalp of the opposite
side. It is formed by the axons of the spinal nucleus of trigeminal nerve which
cross the midline & ascend lateral to the medial Lemniscus (2nd order neuron in
this pathway). Trigeminal lemniscus terminates in PMVNT of thalamus.
4- Medial lemniscus: (the most medial of the 4 lemnisci) carrying
proprioceptive & fine touch sensation from the opposite side of the body below
the head. They are the
axons of the gracile &
cuneate nuclei
(2nd order neurons)
It terminates in PLVNT .
Internal structure of the mid brain
Ventral part (cerebral peduncle)
I- crus cerebri
(Basis peduncle)
III-Tegmentum
II- Substantia nigra
Tectum (Dorsal part)
Two sup. colliculi
two inf. colliculi
• Midbrain has four decussations:
• At the level of inferior colliculus:
- Decussation of SCP.
- Decussation of trochlear nerve.
• At the level of superior colliculus:
- Ventral tegmental decussation of rubrospinal tract .
- Dorsal tegmental decussation of tectospinal nucleus.
Inferior colliculus
Trochlear nucleus
MLB
Lemnisci
Corticospinal tr.
Decussation
of SCP
Corticonuclear tr.
Ventral part (cerebral peduncle):
Formed of two thick pillar separated from each other by a depression called
interpeduncular fossa. The peduncle is consistes of three parts:
1- Curs cerebri: containing descending ( pyramidal &corticopontine ) tracts.
The lateral surface of the crus is crossed by Optic tract, Posterior cerebral artery,
superior cerebellar artery & basal vein and Trochlear nerve
2- Substantia nigra: thin layer of pigmented grey matter between the crus &
tegmentum. It is functionally related to basal ganglia (extrapyramidal system)
3- Tegmentum: is the posterior part of the cerebral peduncle continuous below with the
tegmentum of the pons. It contains:
I- 4 lemnisci (Medial, trigeminal, spinal & lateral)
II- Medial longituidinal bundle.
III- RED Nucleus
IV- cranial nerves nuclei (3rd & 4th )
V- Superior cerebral peduncle decussation (dentato-rubral fibers).
VI- Ventral & dorsal tegmental decussation.
Three decussations
- Ventral tegmental decussation formed by rubrospinal tract, lies in the upper part of the
mid brain at the level of superior colliculus.
while the dorsal tegmental decussation formed by tectospinal tracts, lies behind the
ventral tegmental decussation at the level of superior colliculus.
Red nucleus: is an important (large ovoid mass) relay station in extrapyramidal system.
Lies in the medial part of the tegmentum dorsomedial to S.N at the level of superior
Colliculus .
Connections:
Afferent :1 – Dentato-rubral from contra-lateral dentate nucleus of cerebellum.
2- Fronto -rubral (corticorubral) fibers from precentral gyrus.
3- strio-rubral fibers from globus pallidus.
4- from subthalamus .
5- from hypothalamus
6- tectum.
Efferent fibers : 1- Rubrospinal tract: fibers arise from the red nucleus descends to the
anterior grey column of spinal cord.
2 - Rubrobulbar tract to motor nuclei of facial , trigeminal nerves .
3- Rubro-reticular to the reticular formation along the brain stem.
4- Rubrothalamic , Rubrotectal (to superior Colliculus) , Rubro nigral ( same side),
Rubro- olivary ,Rubro-cerebellar.
Occulomotor nerve nucleus (GSE):
Lies ventral to the aqueduct at the level of superior colliculus. This nucleus supplies
most of the extraocular muscles except lateral rectus and superior oblique.
Edinger Wetphal nucleus, (GVE)
The axons of the Edinger-Westphal nucleus form preganglionic fibers which relay in the
ciliary ganglion. it is parasympathetic nucleus for sphinter pupillae and ciliary muscle.
Trochlear nerve nucleus (GSE): : Lies ventral to the aqueduct at the level of
inferior colliculus. The nerve crosses in the midline posterior to the cerebral
aqueduct. It supply the superior oblique muscle
NB: The trochlear nerve is the only cranial nerve which emerges on the back of
the brain stem, and its fibers cross to the opposite side.
Mesencephalic Nucleus of Trigeminal Nerve GSA
It is one of the sensory nuclei of the trigeminal nerve which ascends in the
midbrain, & is concerned with proprioceptive sensation from the face.
Lies in the central grey matter, lateral to the aqueduct, throughout the whole
length of the midbrain. It receives proprioceptive fibers from the muscles of
mastication, orbit, face and tongue.
-Inferior colliculus: The inferior colliculus is a reflex center for hearing & May play
a role in localization of sounds
It is connected with the medial geniculate body by the brachium of inferior colliculus .
-It is formed of central nucleus surrounded by nerve fibers derived from lateral
leminiscus .
Afferent : - It receives cochlear afferent fibers from the lateral Lemniscus.
2- Auditory cortex (temporal cortex through brachiun of the inferior colliculus) .
Efferent : 1- to inferior Colliculus of
opposite side through intercollicular
commissure.
2- to superior Colliculus of same side
through which it is connected to
tectospinal and tectobulbar tracts.
3- MGB (of both sides)Through
brachium.
The superior colliculus: lies in the upper part of the mid brain. Function as a center for
visual reflexes. Contain a centre for control of conjugate vertical eye movements.
is connected with the lateral geniculate body by a ridge of fibers called brachium of
superior colliculus.
Connections of the Superior Colliculus
The superior colliculus is a reflex center for vision.
It receives afferent fibers from: 1- Retina vis brachium of superior colliculus.
2- Occipital cortex via optic radiation
3- Inferior colliculus of the same side.
4- Sup. Colliculus of the opposite side.
It sends efferent fibers to : 1- Spinal cord (Tecto-spinal tract).
2- Brain stem (tectobulbar) tracts.
3- MLB.
4- superior colliculus of the opposite & inferior colliculus of the same side.
5- to cerebellum (tectocerebellar) tracts.
Pretectal nucleus : a group of nerve cells present posterolateral to superior Colliculus .
It is involved in light and accomodation reflexes .
Afferent : retino –pretectal fibers from the optic nerve for light reflex .
From the visual cortex for accomodation reflex .
Efferents : from pretectal nucleus→ ipsilat. and contralateral Edinger whestphal
nucleus of oculomotor via post. commissure → preganglionic fibers→ oculomotor
nerve→ ciliary ganglion→ short ciliary nerves→ constrictor pupillae muscle for direct
and indirect light reflexes.
- Motor nucleus of oculomotor for accomodation reflex.
Medial Longitudinal Bundle (M.L.B.)
It is an important associative coordinating longituidinal bundle in all levels of the
brain stem.
It is situated in the most posterior part of the brain stem close to the midline
consisting of both ascending & descending fibers
It extends from the floor of the 3rd above (just above the midbrain very close to the
pineal body) to the lower end of the medulla below.
It begins where it gets afferent fibers from the following 2 nuclei:
Interstital nucleus of Cajal & Nucleus of the posterior commissure
It runs throughout the whole length of the brainstem, & ends below by becoming
continuous with the anterior intersegmetal tract of the spinal cord.
The fibers of this bundle are mainly cochlear vestibular.
The M.L.B. gives off efferent branches to the following nuclei:
Medial longitudinal bundle
- Nuclei of the 3rd, 4th& 6th nerves: to the muscles of the eye
- Nucleus of the 5th nerve: to tensor tympani muscle
- Nucleus of the 7th nerve: to muscles of the face &stapedius muscle
- Nucleus ambiguous: to muscles of the larynx & soft palate
- Nucleus of the 12th nerve: to muscles of the tongue
The M.L.B. has the following main functions
Coordination of the movements of the eyeball (3rd, 4th& 6th nerves) in response to
impulses from the vestibular and cochlear nuclei
Coordination of the movements of the lips(7th nerve), tongue (12 nerve) and soft palate
and larynx (10th& 11th nerves) in speech
Coordination of the movements of the lateral and medial recti muscles of the eyeballs
of both sides, in conjugate movements of the eyes
Reticular Formation
It is a diffuse mixture of nerve cells & fibers which interlacing in a networklike form along the length of brain stem. It forms a ground substance through
which the different nuclei, fibersbundles & tracts are situated in the brain stem.
In medulla, occupies region dorsal to inferior olivary nucleus.
In pons, occupies region of tegmentum dorsal to trapizoid body & lemnisci .
In midbrain, occupies region of
tegmentum dorsal to decussation of SCP
& red nucleus on the inner side of
lemnisci.
Functions of reticular formation:
1- control somatic motor activity
Through reticulospinal.
2- control somatosensory activity: on the spinal cord & brain stem sensory
nuclei. Some reticulospinal fibers terminate in substantia nigra of rolandi. They
involved in suppression of pain sensation so their stimulation lead to analgesia.
3- Vasomotor activity:
4- Conciousness, sleep & wakefulness: it responsible for maintaining state of
wakefullness. Sleep centers are said to be in the reticular formation &
serotonergic fibers also involved in sleep activation mechamisms since its
inhibition causes insomnia.
5- Biological rhythm:
6- Memory & emotion:
7- Control of endocrine function:
Afferent connections:
1- Spinoreticular from spinal cord
2- Cerebllaroreticular fibers from cerebellum.
3- Tectoreticular from tectum
5- Nigroreticular from substantia nigra
4- Rubroreticular from red nucleus
6- Strioreticular from corp. striatum
7- from thalamus & hypothalamus
8- From limbic system.
Efferent connections: to
1- Lateral (Medullary) reticulospinal tr. 2- Medial (Pontine) reticulospinal tr.
3- Reticulocerebellar
4- tectum & red nucleus of midbrain
5- Motor cranial nerve nuclei especially (GVE) column.
6- Thalamus, hyopthalamus & sunthalamic nucceus.
7- corpus straitum
8- Different part of limbic system
9- cerebral cortex