Video: Anterior view of the brainstem

Let me paint you a picture of a medieval market town. It's a loud, hustling, and bustling hub. There's an overseas merchant shouting about his goods and you can hear a chicken clucking somewhere far in the distance. You've probably already figured out that this tutorial is not about medieval markets, but it is about the brainstem, which is basically the marketplace of the brain. It's like a passageway between different parts of the central nervous system, and lots of white matter tracts carrying sensory and motor innervation pass through here.

It also has a huge concentration of neural structures. For example, nuclei, which are like your little market stalls. Starting to make sense, right? Minus the chickens, of course.

Well, what if you were just passing through and just needed to orientate yourself so you knew where you were? You'd want to take a look at the big picture – the landscape – instead of looking at each individual market stall. That's what we're doing today. We're looking at the landscape of the anterior view of the brainstem.

So, before we study the landscape of the anterior brainstem in detail, let's have ourselves a quick overview of the topics we'll cover in this tutorial. Firstly, we'll look at the three structures which constitute the brainstem – the medulla oblongata, the pons, and the midbrain. Here we'll look at the midsagittal section as well to make these structures a little bit easier to place. We'll then run through the structures you see on the anterior aspect of each of the three parts. As we go along, we'll have a look at some cranial nerves as most of them arrives at the brainstem. Finally, we'll finish up with some clinical notes on the anterior brainstem.

As we go through this tutorial, we'll very briefly address some of the functions of the structures we learn about, but we also have more in-depth videos on most of them as well as a separate tutorial on the posterior view of the brainstem. Right. So, before we start looking at the individual parts of the brainstem which you can see highlighted now, let's see where it is located in relation to the skull and the other parts of the brain. To do this, it will be helpful to see both the base of the brain, which gives us an anterior view of the brainstem, and midsagittal views of the brain. You'll notice that superiorly, it is continuous with the forebrain, specifically, the diencephalon, and inferiorly, with the spinal cord. Posterior to the brainstem, you'll find the cerebellum, which plays a vital role in motor control.

The brainstem actually lies in the posterior cranial fossa of the skull and the anterior aspect specifically rests on the clivus. It acts as a link or bridge between different parts of the central nervous system and carries ascending and descending tracts between these structures. It also contains important reflex centers for breathing, consciousness, and cardiovascular activity. Finally, it contains nuclei for cranial nerves three to seven.

Now that we've gone over some general information about the brainstem, we can move on to learn about its specific structures. We'll start with the most inferior of the three parts – the medulla oblongata.

The medulla oblongata is a continuation of the spinal cord. So just like the spinal cord, it has a groove on its anterior surface in the midline called the anterior median fissure. Lateral to the fissure, we find the anterolateral sulcus which is a bilateral groove through which the rootlets of the hypoglossal nerve emerge. It's actually continuous with the anterolateral sulcus of the spinal cord.

On either side of the anterior median fissure, you'll find vertical protuberances or elevations known as the pyramids. The pyramids contain fibers from the corticospinal tracts, which are tracts descending from the cerebral cortex and are responsible for controlling voluntary movement. Because these elevations or pyramids are a result of the transmittance of the fibers of the corticospinal tract through the medulla, these tracts are also sometimes simply referred to as the pyramidal tracts. Laterally, they stretch as far as the anterolateral sulcus, which we just saw on the previous slide.

At the lower portion of the pyramids, we have a small area called the pyramidal decussation. It's here that a lot of the corticospinal tract fibers cross over to the other side or decussate in the medulla oblongata. So, what we end up with is one side of the brain controlling the muscles on the opposite side of the body. Posterior and lateral to the pyramids, we find two little bulges – one on either side. They’re referred to as the olives, and they get their bulging appearance from the inferior olivary nuclei complex housed in the area of the medulla.

So, you've probably already noticed that the brainstem is the place of origin for quite a few cranial nerves. In fact, only three of the twelve cranial nerves don't originate in the brainstem. Of the remaining nine, only one emerges from the posterior surface of the brainstem, so the anterior surface is definitely very cranial nerve-rich.

For now, we'll have a quick run-through of the cranial nerves emerging at the anterior aspect of the medulla oblongata. First, we have the hypoglossal nerve, or the twelfth cranial nerve, and we've actually already seen this nerve which emerges as a bilateral pair of nerve fibers from the anterolateral sulcus of the medulla oblongata. It sends motor innervation to the tongue muscles.

Moving out laterally, we have another cranial nerve – the vagus nerve – or the tenth cranial nerve. It arises on the lateral aspect of the olives and is responsible for innervation for most muscles of the pharynx and larynx as well as parasympathetic control of the heart, the digestive tract, and the lungs.

Also on the lateral aspect of the olives, but just superior to the vagus nerve, we find the glossopharyngeal nerve. It's the ninth cranial nerve and it carries mixed motor and sensory fibers. It provides sensory innervation to various structures of the head and neck, special sensory innervation for taste on the posterior third of the tongue, and motor innervation to the stylopharyngeus muscle.

We have another cranial nerve in this area; however, it actually arises in the spinal cord, so it's not part of the brainstem. We're, of course, talking about the eleventh cranial nerve – the accessory nerve. You may have heard about the controversy surrounding it because previously it was believed to have a spinal and cranial part; however, now more often than not, the cranial part is attributed to the vagus nerve. The spinal part, which you see here, arises on the lateral aspect of the spinal cord between the anterior and posterior nerve roots of spinal nerves C1 to C6. It carries motor innervation to the trapezius and the sternocleidomastoid muscles.

We have another couple of structures which are not a part of the medulla that are related to it and are useful to know for the sake of orienting yourself. Lateral to the medulla, we find the cerebellum, and the part of it that is now highlighted is called the flocculus. Together with the nodule, it forms the flocculonodular lobe, which is the most inferior lobe of the cerebellum. You need to remember that we're looking at the brainstem from the anteroinferior view here, so the structures which appear most anterior are actually most inferior in the anatomic position.

Just posterior to the flocculus, we have the choroid plexus of the fourth ventricle. It's a layer of modified ependymal cells that produces and filtrates the cerebrospinal fluid. It can be a bit difficult to picture the extent of the choroid plexus and how it relates to the fourth ventricle when you're looking at the anterior aspect. Here you can see its extent along the vertical axis, and from the nodule, it also extends in the horizontal axis, together with the flocculus, which is the section you can see here, and it forms a rough T-shape.

And that guys, wraps up the structures related to the medulla oblongata – which means we can move on to the next topic, the pons.

Before we look at this part of the brainstem, it makes sense to have a quick pitstop at the border of the pons and the medulla, which quite, unsurprisingly, is called the pontomedullary junction. It's marked by a shallow groove called the inferior pontine sulcus here. This is an important area because three pairs of cranial nerves emerge from it. The most medial of the three is the abducens nerve, cranial nerve six, which is one of the cranial nerves controlling the movements of the eye.

Moving out laterally, we have a bilateral pair of facial nerves or the seventh cranial nerves in line with the olives. It innervates the muscles of facial expression and parts of the tongue. Finally, the most lateral and adjacent to the flocculus, we have the eighth cranial nerve or the vestibulocochlear nerve. It sends information of sounds and balance from the inner ear to the brain.

Alright, that's done. Let's now learn about the pons.

So, pons literally means bridge in Latin due to its similarity to a bridge connecting the left and right cerebellar hemispheres. It has quite a distinct striated appearance, which you can see from the transversely running fibers. It's the middle part of the brainstem, so naturally, it connects the medulla oblongata and the midbrain.

Lateral to it, you'll find the temporal lobe, which is one of the four lobes of the cerebral cortex. On its midline, you'll see a vertical depression, and this is the basilar groove. Basilar groove houses the basilar artery which is formed by the confluence of two vertebral arteries. The pons is not particularly complicated when observed from an anterior perspective and the only structure we have left to cover here is the trigeminal or the fifth cranial nerve emerging from its anterolateral aspect. We actually have two parts to the nerve here. The smaller more medial part is called the motor root and the more lateral and chunkier part is known as the sensory root. It's the largest of the cranial nerves giving sensory innervation to the face and motor innervation to the muscles of mastication.

And that is the pons! Alright guys, so we're moving on to the last structure of the brainstem, and that is the midbrain.

The midbrain, also known as the mesencephalon, is the shortest of the three parts of the brainstem which connects the pons and the cerebellum inferiorly to the forebrain superiorly. Its connection to the pons is marked by a small groove called the superior pontine sulcus. The midbrain plays a role in motor control especially of the eye movements as well as contributing to sound and visual processing and thermal regulation.

So, because the midbrain is such a small area, it also doesn't have that many structures on its anterior aspect, which is great for you because it means less studying! The main thing that you can see are the cerebral peduncles – the two halves of the midbrain. Their anterior aspect visible here are called the crus cerebri and they are separated from the posterior part or the tegmentum by the substantia nigra. These two structures are not visible here so we’ll not go into any more detail about them today.

Between the peduncles and the optic chiasm, you'll find a depression called the interpeduncular fossa. Its floor is formed by the posterior perforated substance. It gets its name from the lots of little holes on its surface, which are created by the many small blood vessels which perforate the region and are an important blood supply for the midbrain. The fossa actually contains parts of the circle of Willis, which is an anastomosis of vessels supplying the brain and the origins of the oculomotor nerve, which is the third cranial nerve. These nerves arise from the medial aspect of each of the crus cerebri, and as the name suggests, provides motor innervation to the muscles controlling the movements of the eyeball and the eyelid.

We have another cranial nerve which originates from the midbrain area, and this is the trochlear nerve or the fourth cranial nerve. It's a bit of a funny one actually because although it originates in the midbrain, it arises on its posterior surface making it the only cranial nerve not to originate on the anterior aspect of the brainstem. It immediately decussates and wraps around the brainstem to appear on the anterior surface which is what we're seeing in our image here. It then goes on to send motor innervation to muscles which control eye movement.

Alright, so that's the structures of the midbrain covered, but you've probably noticed there's a few rather peculiar-looking structures medial and superior to the cerebral peduncles. So they're not technically part of the brainstem, but they are useful to know when you're looking at the region, so we're going to start with these little spherical structures medial to the superior part of the cerebral peduncles, and these are called the mammillary bodies and are often categorized as part of the hypothalamus.

The hypothalamus is the region of the diencephalon responsible for linking the nervous system to the endocrine system. The mammillary bodies play a role in episodic memory.

Moving on to the next structure of the hypothalamus, we have the tuber cinereum. It's a convex mass of gray matter in the area of the arcuate nucleus, superior to the mammillary bodies, and superiorly bound by the optic chiasm. Right in the center of it, we find a small raised area called the median eminence. This raised area is the attachment site for the infundibulum of the pituitary gland. It's also known as the pituitary stalk or the infundibular stalk, and it's simply the connection between the hypothalamus and the posterior part of the pituitary gland.

Still in the hypothalamus, we're moving on to structures associated with the second cranial nerve, which is the optic nerve, and we're going to start with the most anterior structure – the optic nerves themselves. They're the first part of the optic pathway carrying the visual information from the retina to the brain, and bear in mind, that the second cranial nerve is one of the few nerves not arising from the brainstem.

The optic nerves from both sides unite their fibers at this x-shaped structure called the optic chiasm. Here fibers from the medial or nasal part of each retina cross over to the opposite side whereas the fibers from the lateral or temporal part of the retina continue on the same side. The anterolateral corners of the chiasm are continuous with the optic nerves whereas the posterolateral corners are continuous with the optic tracts. These tracts continue posterolaterally wrapping around the cerebral peduncles of the midbrain to terminate in the posterior thalamus.

We don't have any structures left to tackle, so let's jump straight in and have a look at these guys here that you can see highlighted on your screen right now, and these are the olfactory tracts. They play a part in the olfactory pathway associated with cranial nerve one which, of course, is transmitting the special sensory information of smell. They carried the fibers from the olfactory bulb and then divide into the medial and the lateral olfactory striae. Medial striae carry fibers to the olfactory bulb of the opposite side via the anterior commissure whereas the lateral striae remain in the same hemisphere and terminate in the olfactory area of the cerebral cortex.

Sandwiched between the medial and lateral striae and the optic tracts, we find the anterior perforated substance. Similarly to the posterior perforated substance, it has a porous appearance. This is due to blood vessels perforating its surface to provide blood supply to various structures of the brain in the related areas.

Okay, so for our clinical notes today, there aren't many notable conditions related to just the surface anatomy of the anterior surface of the brainstem as most conditions in this region are related to the nuclei and tracts within the brainstem, which is something we only mentioned in passing. So, instead, we're looking at brainstem death, or what happens if the structure as a whole, shuts down. The cause can be pretty varied so trauma, tumors, and vascular incidents can all cut off blood supply to the brain which can cause brainstem death in as quickly as six minutes.

The brainstem holds centers controlling the cardiovascular and respiratory systems which explains why it is so essential for survival. So, it is important not to confuse brainstem death with other causes which can simulate the same symptoms like drug effects, hypothermia, and metabolic disturbances – for example, glucose imbalance caused by diabetes.

Brainstem death is quite interesting because, by law in some countries, it is considered equivalent to brain death where no recovery is expected – for example, in the UK. In the US, however, the law very clearly states that brainstem death is not equal to brain death so the affected individual could not be taken off life support.

Okay, guys, we've covered all the topics we had planned in this fairly straightforward tutorial, so let's have a little recap of the surface anatomy of the anterior brainstem.

We started by looking at the position of the brainstem in the posterior cranial fossa and its communications to the spinal cord, the cerebellum, and the forebrain. We then moved on to learn the surface features of the three divisions of the brainstem – the medulla oblongata, the pons, and the midbrain. Starting with the medulla oblongata, we first saw the vertical grooves on its anterior surface, the anterior median fissure, and the anterolateral sulci.

In the anterolateral sulci, we found rootlets of the hypoglossal nerve, and lateral to the anterior median fissure, we saw the medullary pyramids containing fibers of the corticospinal tracts. On the inferior part of the pyramids, we found the pyramidal decussation where a lot of these fibers cross over to the other side of the body. Lateral to the pyramids, we found the olives containing olivary nuclei.

We also saw a couple of structures related to the medulla oblongata, but not actually part of the brainstem. Lateral to the medulla, there was the flocculus of the cerebellum, and just posterior to it, the choroid plexus of the fourth ventricle.

We moved to the next structure to superior to the medulla oblongata which is the pons and we saw that the pontomedullary junction was marked by a shallow groove called the inferior pontine sulcus before we moved on to the pons itself. We saw that it has a distinctive striated surface and appears similar to a bridge connecting the two cerebellar hemispheres. On its midline, we found a shallow basilar groove which is the location of the basilar artery.

We swiftly moved on to the midbrain – the most superior of the three parts of the brainstem. We learned that it connected to the forebrain superiorly and inferiorly to the cerebellum and the pons. This junction with the pons is marked by the superior pontine sulcus.

The main structures of the midbrain were its two bilateral hemispheres called the cerebral peduncles, and we learned that their anterior most part is often referred to as crus cerebri. Here, we also looked at some structures related to the midbrain. We saw the interpeduncular fossa defined by the peduncles and the optic chiasm with posterior perforated substance forming its floor. We also saw the mammillary bodies, the tuber cinereum, and infundibulum of the pituitary gland, all of which are part of the hypothalamus.

Although the cranial nerves were scattered all throughout this tutorial, now it might be useful to see them all in one place, so we'll start with the cranial nerves not emerging from the brainstem. Most superiorly, we have the olfactory tract associated with the first cranial nerve which terminates in the cerebral cortex. We saw its medial and lateral olfactory striae bordering the anterior perforated substance. Moving on, we looked at the optic nerves, chiasm, and tracts associated with the second cranial nerve which are structures of the hypothalamus.

Moving inferiorly, we have cranial nerves emerging from the midbrain – the oculomotor nerve and the trochlear nerve, which is actually the only nerve emerging on the posterior surface of the brainstem that wraps around its sides and is visible on the anterior view.

Moving on to the pons, we have the trigeminal nerve from its anterolateral surface and three nerves at the pontomedullary junction – the abducens nerve, the facial nerve, and vestibulocochlear nerves. Finally, at the medulla oblongata, we saw the glossopharyngeal, the vagus, and the hypoglossal nerves. The eleventh cranial nerve. the accessory nerve, is the only nerve arising at the spinal cord.

And, finally, we looked at brainstem death for our clinical notes.

Alright, thanks for sticking with us throughout this tutorial, and see you next time!