Brainstem: Anatomy & Function

BrainstemAnatomy and function of the brainstem. Midbrain. Pons. Medulla oblongata
~ 15 min

In this note, we are going to discuss the anatomy of the brainstem.

Brainstem - anterior aspect
Brainstem - anterior aspect
Brainstem - sagittal section
Brainstem - sagittal section

It has three portions:

  • The midbrain
Midbrain
Midbrain
Midbrain
Midbrain
  • The pons Varolii (or simply “pons”)
Pons Varolii
Pons Varolii
Pons Varolii
Pons Varolii
  • The medulla oblongata
Medulla oblongata
Medulla oblongata
Medulla oblongata
Medulla oblongata

In the sagittal plane, the brainstem can be divided into another three parts:

  • Basilar part
  • Tegmentum
  • Tectum

The brainstem participates in the regulation of vital functions, such as respiration, heartbeat, maintaining blood pressure, etc.

The brainstem is also the location of the cranial nerve nuclei, through which numerous nerve tracts pass.

Midbrain

The midbrain. Its anterior part is formed by the so-called cerebral peduncles.

Cerebral peduncles
Cerebral peduncles
Cerebral peduncles
Cerebral peduncles

In fact, these are thick nerve fiber bundles, between which there is a small depression – the interpeduncular fossa.

Interpeduncular fossa
Interpeduncular fossa
Interpeduncular fossa
Interpeduncular fossa

The posterior part is called the сorpora quadrigemina, which consists of 4 tubercles or colliculi,

Corpora quadrigemina
Corpora quadrigemina

2 superior colliculi,

Superior colliculi
Superior colliculi
Superior colliculi - sagittal section
Superior colliculi - sagittal section

and 2 inferior colliculi.

Each superior colliculus continues into the superior brachium,

Superior brachium
Superior brachium

which is connected to the lateral geniculate body of the thalamus.

Lateral geniculate body
Lateral geniculate body

Each inferior colliculus continues into the inferior brachium,

Inferior brachium
Inferior brachium

which is connected to the medial geniculate body of the thalamus.

Medial geniculate body
Medial geniculate body

The superior colliculi and lateral geniculate bodies are parts of the visual pathway, while the inferior colliculi and medial geniculate bodies are parts of the auditory pathway.

The superior cerebellar peduncles are structures that connect the midbrain to the cerebellum.

Superior cerebellar peduncle
Superior cerebellar peduncle

которые соединяют его с мозжечком (cerebellum).

Cerebellum
Cerebellum

And now let’s discuss the cross-sectional anatomy of the midbrain.

So, the first cross-section is at the level of the upper midbrain, through the superior colliculi.

Here we can see the cerebral aqueduct, also known as the Sylvian aqueduct, which is filled with cerebrospinal fluid.

Cerebral aqueduct
Cerebral aqueduct
Cerebral aqueduct
Cerebral aqueduct

The superior colliculi, through which the section is made, are located here.

Superior colliculi
Superior colliculi

Next is located the oculomotor nucleus

Oculomotor nucleus
Oculomotor nucleus

and the accessory nucleus or Edinger–Westphal nucleus.

Accessory nucleus / Edinger–Westphal nucleus
Accessory nucleus / Edinger–Westphal nucleus

And the oculomotor nerve emerges (or exits) from the anterior aspect of the midbrain, in the interpeduncular fossa.

Oculomotor nerve
Oculomotor nerve
Oculomotor nerve
Oculomotor nerve

In addition, we see the substantia nigra.

Substantia nigra
Substantia nigra

It is a component of the basal ganglia system and consists of two parts: the pars reticulata and pars compacta. The pars reticulata contains inhibitory GABA-ergic neurons, and the pars compacta contains dopaminergic neurons and also melanin, which, in fact, gives the substantia nigra a black color. By the way, the substantia nigra is the boundary between the midbrain tegmentum and its basis – the crus cerebri.

There is one more structure – the red nuclei, and together with the cerebellum, they play a role in motor coordination and maintenance of posture.

Red nuclei
Red nuclei

The mesencephalic nucleus of the trigeminal nerve is also located in the midbrain.

Mesencephalic nucleus of the trigeminal nerve
Mesencephalic nucleus of the trigeminal nerve

The netty areas are called the reticular formation.

Reticular formation
Reticular formation

It is basically a system of nuclei and densely intertwined fibers that perform numerous functions: it synthesizes neurotransmitters, plays a role in motor control, maintains wakefulness, and many other things.

Let’s get to the second cross-section, at the level of the lower midbrain, through the inferior colliculi.

So, instead of the superior colliculi, we see the inferior colliculi.

Inferior colliculi
Inferior colliculi

Instead of the nuclei of the CN III, the oculomotor nerve, we see the nucleus of the СN IV, the trochlear nerve.

Nucleus of the trochlear nerve
Nucleus of the trochlear nerve

This, by the way, is the only cranial nerve that emerges from the posterior aspect of the midbrain.

Trochlear nerve
Trochlear nerve

And in this cross-section, we don’t see the red nuclei, they remained in the upper part of the midbrain.

Almost all other structures are similar to those that we considered previously.

All the other structures are different types of nerve fibers or neural pathways. They connect all the elements of the nervous system and allow it to function as a whole. But here we will not consider the nerve fibers. This is really a topic for a separate part of our application because it is actually quite challenging.

Pons

Let’s get a little lower: the pons Varolii, or simply pons, as we said.

Pons Varolii
Pons Varolii
Pons Varolii
Pons Varolii

There is a small basilar sulcus on its anterior surface, in which the basilar artery is located – a significant vessel.

Basilar sulcus
Basilar sulcus
Basilar artery
Basilar artery

The posterior surface of the pons, in particular the lower part, and the upper part of the medulla form an area, or a diamond-shaped depression, called the rhomboid fossa.

Rhomboid fossa
Rhomboid fossa

This fossa is the projection site of a large number of cranial nerve nuclei.

Projection of the cranial nerve nuclei
Projection of the cranial nerve nuclei

At the bottom of the fossa, there is a median eminence,

Median eminence
Median eminence

the caudal part of which protrudes slightly more than the cranial part. This is the so-called facial colliculus,

Facial colliculus
Facial colliculus

which is formed by the genu of the facial nerve at the point where it wraps around the nucleus of the abducens nerve.

Genu of the facial nerve
Genu of the facial nerve

Furthermore, extending from the pons are the middle cerebellar peduncles, which connect it to the cerebellum.

Middle cerebellar peduncle
Middle cerebellar peduncle
Middle cerebellar peduncle
Middle cerebellar peduncle

Let’s discuss the cross-sectional anatomy of the pons. The first cross-section is made through its upper part.

Here we can see two parts: the basilar part

Basilar part
Basilar part

and the tegmentum.

Tegmentum
Tegmentum

So there is no tectum, in contrast to the midbrain.

In the basilar part, we see a large amount of small pontine nuclei. This is the place of origin of the fibers that head towards the cerebellum.

Pontine nuclei
Pontine nuclei

We also see the trigeminal nerve nuclei, in particular, the motor nucleus

Motor nucleus of the trigeminal nerve
Motor nucleus of the trigeminal nerve
Motor nucleus of the trigeminal nerve
Motor nucleus of the trigeminal nerve
Motor nucleus of the trigeminal nerve - cross-section
Motor nucleus of the trigeminal nerve - cross-section

and the sensory nucleus.

Main sensory nucleus of the trigeminal nerve
Main sensory nucleus of the trigeminal nerve
Main sensory nucleus of the trigeminal nerve
Main sensory nucleus of the trigeminal nerve

At this level, the trigeminal nerve emerges from the brainstem.

Trigeminal nerve
Trigeminal nerve

We also see two small formations: the superior olivary complex

Superior olivary complex
Superior olivary complex

and trapezoid body, which are elements of the auditory pathway.

Trapezoid body
Trapezoid body

Additionally, there is the reticular formation, which we previously mentioned in the context of the midbrain.

Reeticular formation
Reeticular formation

The next cross-section is through the lower part of the pons.

There is a bit of a paradox with the already mentioned trigeminal sensory nucleus. Although it is part of the pons, the nucleus is no longer referred to as the chief one, but rather as the spinal trigeminal nucleus.

Spinal trigeminal nucleus
Spinal trigeminal nucleus
Spinal trigeminal nucleus
Spinal trigeminal nucleus

We also see several other nuclei: the nucleus of the CN VI – the abducens nucleus,

Abducens nucleus
Abducens nucleus

the motor nucleus of the facial nerve

Motor nucleus of the facial nerve
Motor nucleus of the facial nerve

and autonomic (superior salivatory) nucleus of the CN VII – the facial nerve

Superior salivatory nucleus
Superior salivatory nucleus

as well as the cochlear

Сochlear nuclei
Сochlear nuclei
Сochlear nuclei
Сochlear nuclei

and vestibular nuclei, related to the CN VIII – the vestibulocochlear nerve.

Vestibular nuclei
Vestibular nuclei
Vestibular nuclei
Vestibular nuclei

And the cranial nerves VI, VII, and VIII emerge from the brainstem at the border between the pons and the medulla

Abducens nucleus
Abducens nucleus
Facial nerve
Facial nerve
Vestibulocochlear nerve
Vestibulocochlear nerve

Medulla oblongata

And now let’s discuss the medulla oblongata.

Medulla oblongata
Medulla oblongata
Medulla oblongata
Medulla oblongata

On its anterior surface, we can see the anterior median fissure, which continues into the anterior median fissure of the spinal cord. So, it is actually quite long.

Anterior median fissure
Anterior median fissure

On either side of this fissure, there are elevations called pyramids.

Pyramids
Pyramids

In one place, the anterior median fissure is interrupted by the so-called pyramidal decussation – the structure where pyramidal tracts, or pyramidal fibers, pass from one side to the other.

Lateral corticospinal tract
Lateral corticospinal tract
Lateral corticospinal tract
Lateral corticospinal tract
Pyramidal decussation
Pyramidal decussation
Pyramidal decussation
Pyramidal decussation

Laterally to each pyramid, there is one more elevation, called the olive, or the olivary body.

Olive
Olive

On the posterior surface, we can see the posterior median sulcus, which continues into the posterior median sulcus of the spinal cord.

Posterior median sulcus
Posterior median sulcus

In addition, on the posterior surface of the brainstem, the upper part of the medulla and the lower part of the pons form an area, or a diamond-shaped depression, called the rhomboid fossa.

In addition, from the medulla oblongata extend the inferior cerebellar peduncles,

Inferior cerebellar peduncle
Inferior cerebellar peduncle

connecting it to the cerebellum.

Cerebellum
Cerebellum

Let’s discuss the cross-sectional anatomy of the medulla oblongata.

Foremost, there are those structures that create elevations on the anterior surface of the medulla, called pyramids and olives. Pyramids are basically thick bundles of pyramidal tracts, so these are white matter structures. And olives are huge inferior olivary nuclei that have connections with the cerebellum and play a role in motor coordination.

Inferior olivary nuclei
Inferior olivary nuclei
Inferior olivary nuclei
Inferior olivary nuclei

Now, as for the cranial nerve nuclei: there are a lot of them here.

Note that the CN IX, the glossopharyngeal nerve, and the CN X, the vagus nerve, have the same sensory and motor nuclei. These are the nucleus solitarius (or nucleus of tractus solitarius)

Nucleus solitarius
Nucleus solitarius

and the nucleus ambiguous.

Nucleus ambiguous
Nucleus ambiguous

In addition, the nucleus solitarius is also related to the CN VII – the facial nerve, so it receives information from this nerve as well.

Sometimes, the nucleus ambiguous is also indicated as one of the nuclei of the CN XI – the accessory nerve.

This information is somewhat contentious and may not be found in all textbooks, but we will keep it in mind for future reference. Apropos CN XI, there is an additional controversy: the accessory nucleus is situated not in the brain, but in the cervical spinal cord.

Accessory nucleus
Accessory nucleus

Furthermore, the CN IX and CN X have separate autonomic nuclei, which we can see in this cross-section: the inferior salivatory nucleus for the glossopharyngeal nerve,

Inferior salivatory nucleus
Inferior salivatory nucleus

and the dorsal vagal nucleus for the vagus nerve.

Dorsal vagal nucleus
Dorsal vagal nucleus

Moreover, here is the motor nucleus of the CN XII – the hypoglossal nerve, and this is the only nucleus of this nerve.

Hypoglossal nucleus
Hypoglossal nucleus

And of course, we see here the sensory nucleus of the trigeminal nerve, in particular, its spinal part.

Sensory nucleus of the trigeminal nerve
Sensory nucleus of the trigeminal nerve

The CN XII – the hypoglossal nerve emerges between the pyramid and the olive, in the anterolateral sulcus.

Hypoglossal nerve
Hypoglossal nerve

And lateral to the olive, in the posterolateral sulcus, moving from the rostral to the caudal part of the medulla, the CN IX (glossopharyngeal nerve),

Glossopharyngeal nerve
Glossopharyngeal nerve

CN X (vagus nerve),

Vagus nerve
Vagus nerve

and CN XI (accessory nerve) emerge in that order.

Accessory nerve
Accessory nerve

We can also see the cochlear nuclei

We can also see the cochlear nuclei
We can also see the cochlear nuclei
We can also see the cochlear nuclei
We can also see the cochlear nuclei

and vestibular nuclei of the CN VIII – the vestibulocochlear nerve, but that depends on how the cross-section is done.

Vestibulocochlear nerve
Vestibulocochlear nerve
Vestibulocochlear nerve
Vestibulocochlear nerve

Let’s get a little lower: the cross-section at the level of the middle part of the medulla or the level of the internal arcuate fibers.

We can find two large nuclei in the posterior part: the medial one – the gracile nucleus

Gracile nucleus
Gracile nucleus

and the lateral one – the cuneate nucleus. At this point, you need to remember that these are the sensory centers in the brainstem.

Cuneate nucleus
Cuneate nucleus
Golosary

Brainstem: Anatomy & Function

brainstem
truncus encephali
midbrain
mesencephalon
pons
pons Varolii
medulla oblongata
medulla oblongata
basilar part
pars basilaris
tegmentum
tegmentum
tectum
tectum
cerebral peduncles
crura cerebri
interpeduncular fossa
fossa interpeduncularis
quadrigeminal plate
lamina quadrigemina
superior colliculi
colliculi superiores
inferior colliculi
colliculi inferiores
lateral geniculate bodies
corpora geniculata lateralia
medial geniculate bodies
corpora geniculata mediale
superior cerebellar peduncles
pedunculi cerebellares superiores
cerebral aqueduct
aquaeductus cerebri
oculomotor nucleus
nucleus nervi oculomotorii
accessory nucleus
nucleus accessorius
substantia nigra
substantia nigra
compact part
pars compacta
reticular part
pars reticulata
red nucleus
nucleus ruber
mesencephalic nucleus of the trigeminal nerve
nucleus mesencephalicus nervi trigemini
reticular formation
formatio reticularis
red nucleus
nucleus ruber
mesencephalic nucleus of the trigeminal nerve
nucleus mesencephalicus nervi trigemini
reticular formation
formatio reticularis
basilar sulcus
sulcus basilaris
basilar artery
arteria basilaris
rhomboid fossa
fossa rhomboidea
median eminence
eminentia mediana
facial colliculus
colliculus facialis
middle cerebellar peduncles
pedunculi cerebellares medii
pontine nuclei
nuclei pontis
motor nucleus of the trigeminal nerve
nucleus motorius nervi trigemini
principal sensory nucleus of the trigeminal nerve
nucleus principalis nervi trigemini
superior olivary nucleus
nucleus olivaris superior
trapezoid body
corpus trapezoideum
spinal nucleus of the trigeminal nerve
nucleus spinalis nervi trigemini
trochlear nucleus
nucleus nervi trochlearis
cochlear nuclei
nuclei cochleares
vestibular nuclei
nuclei vestibulares
anterior median fissure
fissura mediana anterior
pyramids
pyramides
pyramidal decussation
decussatio pyramidum
olive
oliva
posterior median sulcus
sulcus medianus posterior
inferior cerebellar peduncles
pedunculi cerebellares inferiores
inferior olivary nuclei
nuclei olivares inferiores
nucleus of the solitary tract
nucleus tractus solitarii
ambiguous nucleus
nucleus ambiguus
inferior salivatory nucleus
nucleus salivatorius inferior
dorsal nucleus of the vagus nerve
nucleus dorsalis nervi vagi
hypoglossal nucleus
nucleus nervi hypoglossi
spinal nucleus of the trigeminal nerve
nucleus spinalis nervi trigemini
gracile nucleus
nucleus gracilis
cuneate nucleus
nucleus cuneatus
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