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.

It has three portions:

The midbrain

The pons Varolii (or simply “pons”)

The 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.

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

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

2 superior colliculi,

and 2 inferior colliculi.

Each superior colliculus continues into the superior brachium,

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

Each inferior colliculus continues into the inferior brachium,

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

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.

которые соединяют его с мозжечком (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.

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

Next is located the oculomotor nucleus

and the accessory nucleus or 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.

In addition, we see the 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.

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

The netty areas are called the 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.

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

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

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.

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

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.

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

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

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

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

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

and the 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.

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

Motor nucleus of the trigeminal nerve - cross-section

and the sensory nucleus.

Main sensory nucleus of the trigeminal nerve

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

We also see two small formations: the superior olivary complex

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

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

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.

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

the motor nucleus of the facial nerve

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

as well as the cochlear

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

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

Medulla oblongata

And now let’s discuss the 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.

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

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

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

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,

connecting it to the 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.

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)

and the 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.

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,

and the dorsal vagal nucleus for the vagus nerve.

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

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

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

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),

CN X (vagus nerve),

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

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.

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

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

Golosary